Carb tuning...........................

[FoMoCo]

This will probably be long! OK have had the boat out many times(just got done with 2 year resoration) and messing with the carbs each time(2 600cfm vac sec holleys, with rear metering block kits, and the kit that ties the secondaries(with the tightest spring) together, 78 jets pri, 82 in sec.)Fixed one Vac leak alreadybetwwwn the two parts of the tunnel ram. I cant get the damn things right! Either its WAY TO LEAN and runs at WOT(with 76 pri, 78 sec, or its still lean and stumbles and misses when the secondaries come on with (78 pri & 82 sec). Its funny because its not putting out any black smoke like my car would if it was running FAT! DO I maybe have another VAC leak or maybe the carbs are just way to small! Also when I can floor it with the 76,78 jets and it will only hit like 5200 rpms and not going as fast as it does when I have 78 and 84 jets that only pulls 5100 rpms because I can not floor it! Don’t get me wrong its still faster than I thought it would be because I gpsed it with the bigger jets at ¾ throttle or at least where it would not stuble and I got it up to 74 mph (not legged out, I didn’t want it to run to lean)without a droop or a place diverter! So I am going to try a Torker 2 intake and an 800dp holley Carb that my friend is letting be borrow this weekend to see if there is any difference so I can find out for sure if its all about the carbs. Should I be running 2 750 DP’s or what!? Once again the motor is :
DOVE Block 429 BBF Bored .040
C9VE Heads with 2.19, 1.76 valves
Heads bowl ported, and exhaust hump removed from exhaust ports
10.8:1 compression with TRW Forged pistons
Cobra-jet rods with ARP rod bolts
Balanced rotating ass.
ARP Main studs, ARP head studs
Comp cams 280 AR Solid Roller (.634 lift 236@ .050 dur and 110 lobe sep)
Comp Cams Roller rockers with 1.73 on the intake and 1.8 on the exhaust
Wieand Tunnel Ram with 2 600 Holleys
MSD pro billet dis with digital plus 6 ignition box.
Rewarder shorty headers

[Racing Ray]

Are you running any type of spacers? One of the best ways to find a vacuum leak is with a squirt bottle full of water. Spray it all over the intake and base of the carbs. As it sucks in some water the rpms will drop.
If the TR has a large plenium it will kill some signal to the carbs and when the secondaries open you get more air and not more fuel causing a bog, this is where a 4 hole spacer comes into play, by adding length to the venturies it will help to pull more fuel.

[triple x tx]

vac. secondary holleys dont work to good on tunnel rams for some reason,dump the 600s for either 660 center squirters or 650 double pumpers remove the powervalves and install blockoffs and as a general rule go up 10 sizes bigger than came stock in the primary but be sure the secondary is jetted the same as the primary for example ....a stock 650 dp with powervalve in front(powervalves are not in the secondary tha is why stock carbs are always fatter in the secondary) and 71 primarys and 80 secondarys remove the powervalve install blockoff and put 80 or 81s in the primary and secondary and check your plugs to see if you are running lean or fat and go up or down if nescesary BE SURE TO KEEP THE CARBS SQUARE JETTED if your to lean go up a notch in both carbs primary and secondary....p.s. a better way to check for vac. leaks is to get a can of carb cleaner,while the engine is running at idle spray the carb cleaner around the base plates and anwhere else you suspect a vac leak if the rpm rises you got a leak...hope i didnt confuse you

[fryzll]

I personally like checking vacuum leaks w/ water because of the safety factor AND it wont mess up any painted surfaces. It also seems like carb cleaner gets pulled through the venturies easier (if the scoop isnt on when your spraying it), confusing a vacuum leak w/ just carb cleaner down the carb. I to would say yank the spacers to see what happens. If you want to see if it makes a difference, try locking in the secondarys with a self tapping screw in the linkage (I know its not right, no baggin please http://free.***boat.net/ubb/biggrin.gif ) so basically you have manual secondarys. This made a huge difference on my buddys 383 with a 4160 750 cfm (in a Suburban). With vac. it didnt have crap for low end but with the screw in (to make them manual) it had tons more on the bottom. I know its kinda strange sounding but it really works. Hope it helps.
Chris

[Racing Ray]

I no longer recommend carb spray as it is flammable and has been known to be detramental to ones health when on fire from an erreonous spark. It does work better but care needs to be taken.
Vacuum secondaries work fine on my sons jet boat. Squareing the carbs will work however for a ski boat secondary transition provided by the power valve results in better part throttle operation. On my drag cars I do square them but it is either idle or WOT and secondary transition does not come into play.
Secondary jets are bigger to help with this transition from part throttle to full throttle.

[ChetCapoli]

Hey fomoco,
What power valves do you have in your carbs?
Chet

[FoMoCo]

I put 8.5 power valves in the carbs, with no change!

[ChetCapoli]

fomoco,
why did you have in it before the 8.5? I'm thinking in the area of 4.5 and at least in the 80's on the jets.
chet

[FoMoCo]

It had 6.5 power valves stock! I need to check the vacuum again since I reajusted the valve lash. it sounds a lot different now at idle.

[triple x tx]

i run 2 660 center squirters with the powervalves blocked off and the squared off with 82s...the reason i square up is because 660s secondarys open the same time the primarys do...as far as vac leaks go a vac guage will tell you if you have one (THE READING WILL BE VERY ERATIC) if you decide to run powervalves a good rule of thumb is to take a vac reading at idle whatever the vac pressure is divide by two and run yhat powervalve..for example if you have 9lbs. vac at idle divide by two =4.5 so you run a45 powervalve...

[76Tahiti460]

Ok...I also run the exact set-up you do, 660 center squirters, but what is "Squareing up?"

[LVjetboy]

Squaring means running same jets in primary and secondary. Squaring up means using all jets matched to the higher secondary (or larger) size. Usually done after blocking off the power valve. The power valve does the equivalent of increasing jets 6-10 sizes depending on vacuum signal.
My guess is blocking the power valve and rejetting may work good for full throttle race applications, but for most lake jet use, may just use more gas and hurt part throttle performance.
jer

[Rat Raft]

I keep reading over & over that Vacumn secondary carbs don't work well on tunnel rams. That is a misconsecption brought on by the "bigger is better syndrome" that has affected hot rodders ever since some one put 2 stromburg 97's on a flathead and soon they were running 6 or 8 of the little three bolted critters. Remember that a vacumn secondary carb works exactly the same as a double pumper does once the throttle plates are wide open. The vacumn secondary carb can respond better than the dp if its set up right. We want to be able to stand on the throttle from a idle and have the engine reach full rpm without missing or stumbleing. The double pumper does this by having accelarator pumps in all 4 venturies to deliver fuel until the boosters start pulling fuel through the main system. For this to work right the air velosity through the venturis & boosters must be high to create a vacumn. When this happens the air pressure drops below the ambient air pressure on the float bowls and fuel flows through the main jets & into the venturi via the boosters. These carbs need to be very close to the ideal size for everything to work right. That is why Holley makes them in 50 cfm incrediments. On the vacumn secondary carbs only the primaries have accelarator pumps. These cover for the main system until the main system picks up fuel. As the velosity increases a passage leaks some of the vacumn to below the secondary diaphram and this pulls the secondarys open. This can be done either very quickly if the carb is the correct size and you use a weaker spring or very slowly if the carb is too big or a stiff spring is used. These carbs need to be close to the right size too but its not as critical. Hense Holley doesnt make as many sizes. One problem with these and also some of the DP carbs is Holley sets them up for single carb use. They have to be tweaked for 2 carb settups. Since you have twice as much throttle plate area the main system comes in later. You have to cover for it by using bigger squirters and sometimes larger accelarator pumps. You also have to tie the secondary diaphrams together so they will each receive equal vacumn. Then you have to play with the springs. If everything is right you should have smooth power from a idle to full rpm and the same rpm as a DP. If these carbs are much too big the air velosity through the venturis will be too low and the signal to the secondarys may not be enough to open them. This makes the carb somewhat self adjust for being too big. If you force the secondarys to open anyway there may not be enough of a vacumn drop to pull fuel into the main system and the engine will go lean. This is also what happens when you use too large a dp but you have no vacumn control on the secondarys so you just hold the petal down and wonder why it doesnt run right.

[LVjetboy]

I think you're right RR about dp's and the "Bigger is Better Syndrome" or may a better fit in the "Urban Legend" category.
Was going to mention it but already beat it to death on the other site :) Maybe another one of those urban legends like bigger oil pans keep your oil cool type dealies. From experience I do know my secondaries aren't opening much on my TR with twin 715's at 5k, and figured this was because it didn't NEED more flow for the displacement and rpm I'm running. But, always willing to offer my boat as the floating test lab, next time out I'll force the secondaries open (against their better judgement) and see if I gain in top speed thrill factor...will let everyone know the results as always.
I'd also truely love for GS to do a lake test of different size carbs (600-850) and vs versus dp secondaries (complete with mph and rpm data) on a BBC and report the results. Could be the ultimate "Is Bigger Really Better?" jet boat article. So far no bites on this tech article...Greg?
jer

[skeepwerkzaz]

The best thing to use to check for vac leaks is a propane torch. You don't light it, you turn it on a little, and wave it around all of the suspect places, and when a leak is found, the engine speeds up.
Skeep

[cheyenneNY]

here s the math on carb cfm =[cubicinchs x rpms divided by3456] x volummetric effciency% =cfm