Carb Info

[Jetaholic]

Here's a post from SoCalJetBoats from "speedymopars" that I found rather interesting. Something I never gave any thought to, but yet it makes perfect sense. See what you guys think.
When you goose it with a Jet, you probably will have a hesitation / bog for a second. This is becuase a jet boat needs very little accelerator pump, but most carbs DP's are designed to hit some kind of limiter (IE the torque convertor or prop). The acc pump covers up the fact the engine can't rev to WOT potential and the fuel takes a while to get flowing from the mains. With the engine effectively unloaded due to the jet's design, the pump shot can rev the engine quite high, high enough to pull on the pri/sec mains almost immediately.
In a car, the lower the stall, the bigger the pump shot for a given carb. My 12 second van with 1800 stall, needs a decent size pump and squirter in the primaries, and a HUGE shot in the secondaries so it doesn't bog off the line.
Thoughts? Opinions?

[Moneypitt]

Any time a carb bogs or hesitates at the hit there is something wrong with the way the carb is set up. Jet boat or John Deere, makes no difference, if it bogs and recovers it aint right.........MP

[bp]

Here's a post from SoCalJetBoats from "speedymopars" that I found rather interesting. Something I never gave any thought to, but yet it makes perfect sense. See what you guys think.
Thoughts? Opinions?
i'd say there's some really good drugs involved...

[YeLLowBoaT]

Any time a carb bogs or hesitates at the hit there is something wrong with the way the carb is set up. Jet boat or John Deere, makes no difference, if it bogs and recovers it aint right.........MP
this... sounds like some one ether needs to change the cam or the nossle.

[Jetaholic]

this... sounds like some one ether needs to change the cam or the nossle.
That's what I was thinkin'...
However...and correct me if I'm wrong but...my way of thinking is...
Depending on the actual stroke of the pump (I know that sounded dirty but bear with me :D)...for example...
Let's say the pump shoots out 25cc of fuel...
In my way of thinking...
The pump will shoot out a total of 25cc of fuel regardless of shooter size...
However...
A smaller shooter size will shoot out a total of 25cc of fuel over a longer duration...
Whereas..
A larger shooter size will shoot out a total of 25cc of fuel over a shorter duration...
Correct? Or no?

[YeLLowBoaT]

sounds right to me.

[Jetaholic]

sounds right to me.
So does DCB...:D

[speedymopars]

That's what I was thinkin'...
However...and correct me if I'm wrong but...my way of thinking is...
Depending on the actual stroke of the pump (I know that sounded dirty but bear with me :D)...for example...
Let's say the pump shoots out 25cc of fuel...
In my way of thinking...
The pump will shoot out a total of 25cc of fuel regardless of shooter size...
However...
A smaller shooter size will shoot out a total of 25cc of fuel over a longer duration...
Whereas..
A larger shooter size will shoot out a total of 25cc of fuel over a shorter duration...
Correct? Or no?
should I respond to this thread ???? :-)
The original conversation is over here....
http://www.socaljetboats.com/index.php/topic,2171.0.html
The answer is yes, you are correct....

[Jetaholic]

should I respond to this thread ???? :-)
The original conversation is over here....
http://www.socaljetboats.com/index.php/topic,2171.0.html
The answer is yes, you are correct....
Even though you're a Mopar guy (Mostly Old Parts And Rust :D)...this definitely makes sense.

[speedymopars]

Hey, Ford's are just F'd Over Rebuilt Dodges! :D
http://members.rennlist.com/mountain914/Acronyms.html
Now you know what all of the acronyms mean :D

[Moneypitt]

That's what I was thinkin'...
However...and correct me if I'm wrong but...my way of thinking is...
et's say the pump shoots out 25cc of fuel...
In my way of thinking...
The pump will shoot out a total of 25cc of fuel regardless of shooter size...
However...
A smaller shooter size will shoot out a total of 25cc of fuel over a longer duration...
Whereas..
A larger shooter size will shoot out a total of 25cc of fuel over a shorter duration...
Correct? Or no?
A 25 CC shooter delivers 25 ccs of fuel in TEN pumps. The carb is just set up wrong, not even close.......Too many people think they are Holley Gurus and mess up carbs so bad it takes a good carb person and alot of correction testing. Properly set up 4 brls will not "feel" the secondaries at all. Granted, these carbs are few and far between anymore with 950s on 454s.........MP

[Jetaholic]

Well if people who knew anything about carbs would post up and reveal their secrets we wouldn't be inquiring and asking these questions now would we?

[Moneypitt]

Well if people who knew anything about carbs would post up and reveal their secrets we wouldn't be inquiring and asking these questions now would we?
The 25 CC shot kinda gave it away. No Holley secrets anymore, there are countless BOOKS about Holleys and carburators in general. There it is in black and white, pages with words on them. Formulas about how much fuel an engine can use. Reccomendations for CFM size per engine displacement. Very few ever read the info, or use it correctly. A gazillion HP, (dyno'd), N/A'd, Hydrolic liftered, with water in the logs, on PUMP GAS needs a 1050 dominator with 105 jets, adapted to an Edlebrock RPM manifold.........
If you're going to play with these things, educate yourself, the only secrets are info you never bothered to learn...........MP

[1968Droptop]

Question; Does a supercharger affect the amount of carburation a certain cu.in. engine requires ?

[speedymopars]

Holleys are actually 30 and 50 cc pumps. And yes, they are measured at 10 full strokes. The pump cam will make the pump do a little to whole lot less than the rated setting, and there are many to choose from. You can make your 50cc pump have less shot volume than a 30cc pump by changing the cam. There are many things to do also, not only the cam, but the screw location, the pump arm clearance, etc etc..
But, the analogy was correct, it really didn't matter that the values were technically wrong. Sometimes it's better not to complicate things when first explaining it.
Getting to accuracy, a .25 shooter is the orfrice size, not the pump capacity. All it does is to regulate the duration of the shot (and hence volume at a given instant) from the pump cam and diaphram size, and make sure the fuel is injected into the airstream during the throttle opening transition. Most primary shooters that we would use on our big blocks (no matter what the brand), would be from a .25 to a .37 depending on cam, manifold, carb CFM, exhaust, etc. 31 is the most commonly used primary squirter on Holley carbs from the factory.
This started as a theoretical discussion on what to expect when you take an automotive carb and bolt it on an engine. We haven't defined a carb, what the problem would be, only what *might* happen. Normally, you don't need as much pump shot as holley provides on a jet boat, much the same way that an engine in a car will not need as much pump shot when it is sitting in Neutral and the engine is reved to redline. Doesn't mean it won't work out of the box, just that there is some performance to be gained by playing with it.
The intake manifold plays a big part in how big of a carb your engine can handle, a dual plane manifold, choking off 1/2 the cylinders to 1/2 the barrels would have no problem taking a 950 CFM carb on a 5000 RPM 454. Conversely, I know I could make it work well on a single plane too by changing the curve a little bit and playing with jets and pumps.
Holleys charts are too conservative for serious tuners - but they are a good starting place. They show an average engine size with an average fuel curve, and give you a range of carbs to chose from. Staying within the reccomended range will assure you will run and have decent performance without too much hassle.
However, they ignore camshaft duration and lift (which significantly affect airflow), head port volume and CFM (again which significantly affect airflow), and the intake manifold which significantly changes the charge velocity. Good simulation programs will tell you the airflow requirements - I am talking the several hundred dollar ones, not the $49 desktop dyno program. You really need to know accurate numbers for your engine though.
(one of my race 440's engine on holleys site says I need 950-1050 CFM - but my calculated airflow requirements are 1610 CFM)
When dealing with a supercharged application, determine what the boost level is going to be, and size it from that. A 454 running 7 lbs of boost is equal to 681 ci. (7 lbs is 1/2 atmosphere so 454 * 1.5 = 681)
That still doesn't say how much air your engine is going to need though. You still need
* manifold / port CFM at cam lift
* area under the curve (rollers are going to need more than flat tappets)
* number of cylinders
* RPM
* manifold type
If you are looking to get mileage, go small. If you want top RPM, go big. Either way Moneypit is correct, you should never feel the secondaries, only hear them.
To me, bigger is better (to a point) - I don't want to lean out on the top end like a too small of carb will do. I also like mech secondaries over vaccum. I like annular nozzles over doglegs or straight boosters. I like 4 corner idle circuits. Dual power valves.
I am saying I like the 9381 (830) and the 8896 (1050) and 80673 (1150) Holleys the best.

[Moneypitt]

Not really looking to start a pissing contest here but,
[quote]Getting to accuracy, a .25 shooter is the orfrice size, not the pump capacity. All it does is to regulate the duration of the shot (and hence volume at a given instant) from the pump cam and diaphram size, and make sure the fuel is injected into the airstream during the throttle opening transition. Most primary shooters that we would use on our big blocks (no matter what the brand), would be from a .25 to a .37 depending on cam, manifold, carb CFM, exhaust, etc. 31 is the most commonly used primary squirter on Holley carbs from the factory. [quote]
What he meant to write was .025, (twenty five thousanths).......to .037, or .031........
The average jet boat can use a 650......750........780......MAYBE an 800, but the best bang for the buck in an average down the river jet will be the 650. Use an 800 or an 850 if you like to change your oil quite often........MP

[speedymopars]

Not really looking to start a pissing contest here but,
[quote]Getting to accuracy, a .25 shooter is the orfrice size, not the pump capacity. All it does is to regulate the duration of the shot (and hence volume at a given instant) from the pump cam and diaphram size, and make sure the fuel is injected into the airstream during the throttle opening transition. Most primary shooters that we would use on our big blocks (no matter what the brand), would be from a .25 to a .37 depending on cam, manifold, carb CFM, exhaust, etc. 31 is the most commonly used primary squirter on Holley carbs from the factory. [quote]
What he meant to write was .025, (twenty five thousanths).......to .037, or .031........
The average jet boat can use a 650......750........780......MAYBE an 800, but the best bang for the buck in an average down the river jet will be the 650. Use an 800 or an 850 if you like to change your oil quite often........MP
Yep, you got me. I put a point in there when I shouldn't have. No pissing contest here, just a good discussion.
My 455 picked up 500 RPM going to an 850 DP from a 750, so I disagree with your assessment. I am also curious what does the carb size have to do with changing your oil? Are you insinuating that the 850 will put more fuel into the chamber therby contaminating the oil? If so, that is in the booster and jetting / curve, not the CFM of the carb side.

[pw_Tony]

Spray down the oil off the cylinders?

[Moneypitt]

[QUOTE]My 455 picked up 500 RPM going to an 850 DP from a 750, so I disagree with your assessment. I am also curious what does the carb size have to do with changing your oil? Are you insinuating that the 850 will put more fuel into the chamber therby contaminating the oil? If so, that is in the booster and jetting / curve, not the CFM of the carb side.[QUOTE]
OK......IMO, you are doing a dis service to carb novice jetboaters. Suggesting, or implying, THEY are going to see any improvement at all if they switched to an 850 is just wrong. Remember these are mostly mild 454/455 engines. To over carb to that extent WILL polute the oil, and possibly eat the bearings. Why?....Because they read on HB how you gained 500 RPMs by switching to an 850 without any additional information......I stand by my statement, a 650 is plenty for a "down the river" jet with a 454/455......I would use a 650 spread bore DP ,(4165), great response down low, and some features inside the 4150s don't have.....Please remember most of these guys are tuning ski boats, not racers..........MP

[steelcomp]

Speedy...your assesment of the need for a squirter on a jet being less, because it revs quicker makes no sense as a generalization. In general, the faster an engine is going to rev, the more fuel it is going to require and be able to use. If it's going to rev almost instantly, it'll usually suck up everything you can throw at it with an acc pump. If there's a bog at the hit, it typically needs more fuel. If it's bogging due to too much fuel, then things are way out of whack. Of course, this is more about applicataion than anything.
Example: my jet, 467, fairly good hp, but two big ol 1050's with four 5cc squirters (I just can't bring myself to call them 50cc) and .028 nozzles, there was zero hesitation no matter how hard I mashed the throttle. Engine would go immediately to the rev limiter. Now I was told those carbs would be too big, and I would be better off running smaller 4150's, but that's what I had, and they were flawless. That's a lot of raw fuel being dumped at the hit. Try that on something that has to pull a gear, or has a substancial load on it, it'd probably fall right on it's face.

[speedymopars]

[QUOTE]My 455 picked up 500 RPM going to an 850 DP from a 750, so I disagree with your assessment. I am also curious what does the carb size have to do with changing your oil? Are you insinuating that the 850 will put more fuel into the chamber therby contaminating the oil? If so, that is in the booster and jetting / curve, not the CFM of the carb side.[QUOTE]
OK......IMO, you are doing a dis service to carb novice jetboaters. Suggesting, or implying, THEY are going to see any improvement at all if they switched to an 850 is just wrong. Remember these are mostly mild 454/455 engines. To over carb to that extent WILL polute the oil, and possibly eat the bearings. Why?....Because they read on HB how you gained 500 RPMs by switching to an 850 without any additional information......I stand by my statement, a 650 is plenty for a "down the river" jet with a 454/455......I would use a 650 spread bore DP ,(4165), great response down low, and some features inside the 4150s don't have.....Please remember most of these guys are tuning ski boats, not racers..........MP
What spread bore DP is available? You have some old stock? The only spread bore that is still available is the 0-80552 (marine) and 0-80555C (auto).
IMO, you are doing a dis service to carb novice jetboaters. Suggesting, or implying, THEY are going to see any improvement at all if they switched to an 850 is just wrong. Remember these are mostly mild 454/455 engines. To over carb to that extent WILL polute the oil, and possibly eat the bearings. Why?....
Yes, please tell me why. I'd love to hear this. If it is true that a 454/455 can't handle an 850 without killing the bearings, well that would explain why Chevy parts are cheap. Supply and Demand. Score one for MOPAR and FORD!

[Moneypitt]

First, you failed to mention your 467 probably has some of the best cylinder heads west of the Atlantic Ocean, 2)The timing is performance tested to perfect, 3)and it aint exactly an average ski boat..........Ray

[Jetaholic]

I'm thinking that the main thing here is pump shot duration.
At a lower WOT RPM, it takes longer for the main jets to start delivering fuel, necessitating the need for a longer duration pump shot. Whereas if your WOT RPM is much higher, the engine will pull harder on the main jets, causing them to supply fuel much quicker, which would require a much shorter shot duration.
Another thing in regards to speedymopars and his experience with a 750cfm - 850cfm carb swap. He's not saying that by swapping to the 850 from the 750 that you will pick up 500 RPMs. He's stating that that's what it did for him. He's merely posting HIS experience with it. Honestly, it all depends on the motor combo...apparently his motor wanted more. However you may do a 750-850cfm swap and see no change...or maybe it's a change for the worst...either way it all depends on the combination.

[steelcomp]

First, you failed to mention your 467 probably has some of the best cylinder heads west of the Atlantic Ocean, 2)The timing is performance tested to perfect, 3)and it aint exactly an average ski boat..........Rayshhhhh...damn it Ray!:D
That part really dosen't matter. the point is, the faster an eninge revs, the more fuel it can "tolerate", and having a standard squirter on a jet isn't going to cause a bog just due to squirter size.

[speedymopars]

Speedy...your assesment of the need for a squirter on a jet being less, because it revs quicker makes no sense as a generalization. In general, the faster an engine is going to rev, the more fuel it is going to require and be able to use. If it's going to rev almost instantly, it'll usually suck up everything you can throw at it with an acc pump. If there's a bog at the hit, it typically needs more fuel. If it's bogging due to too much fuel, then things are way out of whack. Of course, this is more about applicataion than anything.
Example: my jet, 467, fairly good hp, but two big ol 1050's with four 5cc squirters (I just can't bring myself to call them 50cc) and .028 nozzles, there was zero hesitation no matter how hard I mashed the throttle. Engine would go immediately to the rev limiter. Now I was told those carbs would be too big, and I would be better off running smaller 4150's, but that's what I had, and they were flawless. That's a lot of raw fuel being dumped at the hit. Try that on something that has to pull a gear, or has a substancial load on it, it'd probably fall right on it's face.
If you put those carbs on a race car with a stall and gear, do you think you would need more pump or less? The acc pump only covers the gap until the mains come on line, and without the air column stalling in the carb, the mains come on quicker, therefore you would need a smaller squirter and pump capacity on the jet
You have fairly small squirters - what pump cam is in the carbs? That has more to do with how much fuel is dispensed than the theoretical max of the pump capacity. Already you are much smaller than any of my car 4150s - typically 50 cc pump and 45 squirter (at least a 37).
That is my only point - comparitively, you need a smaller shot that what a car that had the same engine would require.

[Moneypitt]

Yeah, I've got a couple of old 4165 650s......Gave away the MOPAR T quads though, they were real junk.........This CFM deal is a no brainer, no winner..There is, however, some Math that kinda spells it out. I think I did see a couple of Fords at the last race, no MOPARs though, and the Fords were in jets.......Again, IMO, your post about picking up 500 RPMs by going from a 750 to an 850 is misleading to some here. In my mind, I just figure how messed up the 750 had to be to be such a slug. Chances are a different 750 may have done the same thing.........But we'll never know, will we?...........MP

[steelcomp]

If you put those carbs on a race car with a stall and gear, do you think you would need more pump or less? The acc pump only covers the gap until the mains come on line, and without the air column stalling in the carb, the mains come on quicker, therefore you would need a bigger squirter and pump capacity.
You have fairly small squirters - what pump cam is in the carbs? That has more to do with how much fuel is dispensed than the theoretical max of the pump capacity. Already you are much smaller than any of my car 4150s - typically 50 cc pump and 45 squirter (at least a 37).
That is my only point - comparitively, you need a smaller shot that what a car that had the same engine would require.Again, in theory your comments are correct, but your stated effect isn't. Been doing Holleys for 30+ years, so I understand the dynamics. Jets present a few unique circumstances, and one of them is the fact that there is so little part throttle load on the engine. (I say part throttle because the transition occurs under part throttle, even if you're mashing it) That reads high vacuum, which means more fuel available earlier from the main circuits. All true. HOWEVER, what little fuel the squirter adds once the main circuit is on line (especially when going to full throttle immediately) isn't going to cause a bog. Maybe a big puff of black smoke, even a stumble, but not a bog. A bog is a lean condition.

[Sleeper CP]

shhhhh...damn it Ray!:D
That part really dosen't matter. the point is, the faster an eninge revs, the more fuel it can "tolerate", and having a standard squirter on a jet isn't going to cause a bog just due to squirter size.
Yeah, what he said. That would be my experience over the year's from a 460,512 and 565. And non of them bogged when we mashed the throttle regardless of th impeller size.
Sleeper CP
Big Inch Ford Lover:D

[cyclone]

don't make it so complicated. if your engine bogs out of the hole when you mash the go-pedal, its highly likey that there's not enough fuel. forget how fast you think the engine revs compared to car.

[Jetaholic]

don't make it so complicated. if your engine bogs out of the hole when you mash the go-pedal, its highly likey that there's not enough fuel. forget how fast you think the engine revs compared to car.
The whole purpose about my carburetor inquiries is the fact that I have a brand new out of the box Holley Marine 750DP...nothing on it has been messed with yet, and this spring I'll be out on the lake testing and tuning. However I would like to know if I need to go up or down on the adjustments (more fuel or less fuel).
However, I'm wanting to tune it in such a way that gives me the best possible fuel economy I can achieve with the motor running at its best performance. Why use more fuel than the motor actually needs? I realize these are jet boats and they are not known for good fuel economy, but at the same time that doesn't mean that I just have to throw all the fuel I can at it to achieve the best performance. Giving the engine more fuel than it requires to perform it best is just a waste of fuel.
And because my carb is brand new, I want to really know what the hell I'm doing when I start tuning and changing stuff on it...and whether I'm too rich or lean so I know which way to go on the adjustments.

[Moneypitt]

don't make it so complicated. if your engine bogs out of the hole when you mash the go-pedal, its highly likey that there's not enough fuel. forget how fast you think the engine revs compared to car.
Can we all say INJECTION, the mechanical light switch for fuel delivery....MP

[Jetaholic]

Can we all say INJECTION, the mechanical light switch for fuel delivery....MP
Trust me if I could afford that I'd have it already. :D

[steelcomp]

The whole purpose about my carburetor inquiries is the fact that I have a brand new out of the box Holley Marine 750DP...nothing on it has been messed with yet, and this spring I'll be out on the lake testing and tuning. However I would like to know if I need to go up or down on the adjustments (more fuel or less fuel).
However, I'm wanting to tune it in such a way that gives me the best possible fuel economy I can achieve with the motor running at its best performance. Why use more fuel than the motor actually needs? I realize these are jet boats and they are not known for good fuel economy, but at the same time that doesn't mean that I just have to throw all the fuel I can at it to achieve the best performance. Giving the engine more fuel than it requires to perform it best is just a waste of fuel.
And because my carb is brand new, I want to really know what the hell I'm doing when I start tuning and changing stuff on it...and whether I'm too rich or lean so I know which way to go on the adjustments.In that case you might as well go to the dyno. That's the only place you're going to get that kind of info. Best you can do is without changing ANYTHING, put that carb on and run it. Do the best to get your timing where you think it should be, install a new set of plugs, and read them. You may have to go a couple jet sizes one way or the other, but that won't impact any mileage that you could even measure. Set your AFR screws with a vacuum gauge if you can't do it by ear. If you don't know how to properly read plugs, have some one with experience help you. If you have good crisp throttle response, and the boat runs smoothly, idles well, etc., leave well enough alone. The amount of fuel you think you might save by changing accelerator pump settings and the likes will be burned up ten fold in testing. If your jetting is WAY rich (which I can tell you it most likely won't be) you'll maybe see some improvement with re-jetting, but like you said...this is a boat, and the best mileage you'll get out of it will be controlled by your right foot. You can't throw all the fuel at it you can, and have the best performance. You'll have the best performance when the tune is right, and the "mileage" will be what it is.

[Jetaholic]

You'll have the best performance when the tune is right, and the "mileage" will be what it is.
That's exactly what I'm after...that "perfect" tune. And yes you're correct...the mileage will be what it is. I've just seen some people go over rich and claim "it's better to be rich than lean", which is correct if you really don't know what you're doing so you don't lean the motor out. But I'm lookin' for not too rich, not too lean...that "perfect" tune.

[speedymopars]

Again, in theory your comments are correct, but your stated effect isn't. Been doing Holleys for 30+ years, so I understand the dynamics. Jets present a few unique circumstances, and one of them is the fact that there is so little part throttle load on the engine. (I say part throttle because the transition occurs under part throttle, even if you're mashing it) That reads high vacuum, which means more fuel available earlier from the main circuits. All true. HOWEVER, what little fuel the squirter adds once the main circuit is on line (especially when going to full throttle immediately) isn't going to cause a bog. Maybe a big puff of black smoke, even a stumble, but not a bog. A bog is a lean condition.
Hey, thanks for agreeing with me (and I completely agree with you in both your posts). If you get a puff of black smoke, you are too rich, and the pump isn't right. We can deal with semantics later (bog/stumble), but I think we can agree in a perfect world the pump shot should end immediately after the mains are flowing as per design.
Funny how this thread got out of hand. It started on another board with the question "If I bolt this carb on my engine, what problems may I have" to which I answered that you *may* need to decrease the pump shot but otherwise it should work.
Many messages and another board later , I'm being called out that an 850 on a 455 didn't give me more RPM. On an olds 455 with a performer dual plane and a .490 230 hyd cam, it sure did. YMMV
I will be doing a test and tune this spring when my setup is together - Stock dual plane, Edelbrock single plane, and Weiand tunnel ram. Each tuned to the best it can do and compared. I have 4 hole, open, and blended spacers in everything from 1/4 inch to 3 inches. If I can find a Edelbrock performer RPM, I'll run that too, but I don't have one handy. I will open spacer the dual plane, and 4 hole the torker for that in the meantime. Donations are accepted.
X1 500 mech Carter, 600 vac holley, 750 Vac holley, 850 DP Holley. And just to piss some people off, I'll do a 1000 CFM thermoquad too (I'll leave the cover at home!).
X2 450 CFM mech Holleys (can't call them DP's they don't have a secondary acc pump)
X2 830 CFM dp Holleys.
I expect the 2x450's on the tunnel ram to give me the best all around power, and the 1000 TQ on a torker with 2+ inches of open spacer to give the next best performance but with the best mileage -but we will see. I definately will run whatever gives me the best all around performance.
BB Mopars have an advantage in this area - 8 bolts and the intake manifold is off and swapped. No water, no oil, no gaskets to contend with. Same day quick swap for all 3 manfolds is very possible. I like to keep consistancy.
If you guys want here is your chance to put it out there - what combo will work the best? I'll give you any additional info you want. Here's your chance to get bragging rights and say "I told you so" .....

[Moneypitt]

Economy and efficienticy go hand in hand. Quite surprisingly so does performance. An efficient engine is the best of both worlds, performance and economy. IF you have money for dyno time, save it and by the injection. A few things you need to accuratly tune on the water. 1) a GPS..2) a fuel pressure gauge...3) a vacuum gauge.....)4 a tach with a memory....The gauges are for reference, the gps for top speed, the tach for max rpm.....Seat of the pants won't give you the necessary info to dial it to perfection......WRITE EVERYTHING you do DOWN. Keep a log of the changes and the results........Oh yeah, different days, on the same water can be very different when it comes to air/fuel ratios...Low pressure, high presure, heat, humidity all play a role in how your engine accepts the diet you're trying to feed it..........As Scott said, don't change anything in the carb until it gives you a reason to..AND THEN change ONE THING AT A TIME....Don't re jet and re set the timing....ONE AT A TIME, record the results, try something different, record the results.....Get the "homework" picture??......The major difference in jetboat performance is timing curve, not carburation, but that is a whole different story...............Happy boating.............Ray

[steelcomp]

That's exactly what I'm after...that "perfect" tune. And yes you're correct...the mileage will be what it is. I've just seen some people go over rich and claim "it's better to be rich than lean", which is correct if you really don't know what you're doing so you don't lean the motor out. But I'm lookin' for not too rich, not too lean...that "perfect" tune.Like I said, without a dyno the only info you have for fuel mixture is the plugs, so getting the "perfect" tune isn't likely, but you can get close enough. There are aftermarket AFR meters you could play with, but that's just getting carried away IMO.

[cyclone]

The whole purpose about my carburetor inquiries is the fact that I have a brand new out of the box Holley Marine 750DP...nothing on it has been messed with yet, and this spring I'll be out on the lake testing and tuning. However I would like to know if I need to go up or down on the adjustments (more fuel or less fuel).
However, I'm wanting to tune it in such a way that gives me the best possible fuel economy I can achieve with the motor running at its best performance. Why use more fuel than the motor actually needs? I realize these are jet boats and they are not known for good fuel economy, but at the same time that doesn't mean that I just have to throw all the fuel I can at it to achieve the best performance. Giving the engine more fuel than it requires to perform it best is just a waste of fuel.
And because my carb is brand new, I want to really know what the hell I'm doing when I start tuning and changing stuff on it...and whether I'm too rich or lean so I know which way to go on the adjustments.
ridding a bog in the holeshot isn't going to affect how much fuel the engine uses while your out cruising around.
If you dont feel like studying a carb book and ripping your new carb apart to find out what's in it then i'd take it to a good tuner and tell them what you have for a engine. they'll set the carb up right and then you can make small adjustments from there.

[Sleeper CP]

That's exactly what I'm after...that "perfect" tune. And yes you're correct...the mileage will be what it is. I've just seen some people go over rich and claim "it's better to be rich than lean", which is correct if you really don't know what you're doing so you don't lean the motor out. But I'm lookin' for not too rich, not too lean...that "perfect" tune.
So you are headed to the dyno :confused: At least that way you have the known baseline. Anything else is just a guess? If you can find a good price on a A/F monitor that you could get to work on your boat as Steel stated that would be the next best thing.
Sleeper CP
Big Inch Ford Lover

[bp]

ridding a bog in the holeshot isn't going to affect how much fuel the engine uses while your out cruising around.
If you dont feel like studying a carb book and ripping your new carb apart to find out what's in it then i'd take it to a good tuner and tell them what you have for a engine. they'll set the carb up right and then you can make small adjustments from there.
very very true. the dyno is an excellent tool, but it won't tell you whether or not you'll have a bog once on the water, nor will it tell you the rpm where you'll have a bog if you have one (and the rpm is important to know to resolve it).

[cyclone]

very very true. the dyno is an excellent tool, but it won't tell you whether or not you'll have a bog once on the water, nor will it tell you the rpm where you'll have a bog if you have one (and the rpm is important to know to resolve it).
Huh? who said anything about a dyno? bob maybe you're referring to a different post?

[smiggley]

Ill just chime in real quick just to add that im running an 850cfm 4165 and I think its the perfect setup for what your looking to do. I have nice holeshot thru to WOT. Mashing it, whatever, its always crisp throttle response. Best of all I never have to dizick with it. I can also stay off the secondaries when putting around. Itll stay on plane but just barely....

[454 Squirt]

Out of curiosity, isn't a dyno basically a big water pump? Heard from someone awhile back that that's all it is.

[VAMI]

Dynos do use water for a brake system.Water is let in against the brake milliseconds at a time so the engine dosent bog or run away.Some 700-800 inch engines can tear up and burn the absorbtion unit from not getting enough water.Also there are electric brake dynos too,not as popular.

[VAMI]

This will mess up all thoughts,when I worked for Nikens brothers Dodge pro stock team we dynoed a comp elem v6 the had 2 1050s and made in the high 600 hp range thats under 300 inchs of engine.If an engine cant move air and have good port velocity,carb signal and someone puts a huge carb on it,its gonna stumble. I do understand most these jets dont go over 5500 and these drag engines go to 9500.On my boat two 750s with four 50 cc sqt never stumbles has a 7200 chip and it hits it for a split sec until the pump loads.