loss of performance

[fxinc]

I just added a highrise manifold in hopes of getting some more rpm's/hp. I am getting a new carb but bolted on the 4barrel Quadabog I had to see if there was any difference. Well there was, lost 500 rpm and 18mph. Is this do to the carb? The engine runs great, good throttle response, but loss of performance. If so, what size carb should I get? Any help would be great.

[Infomaniac]

You should select the manifold to compliment the RPM's you already have.
Does this manifold make it's best power at that RPM?

[Sangster]

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[Moneypitt]

What else changed?? did you have to pull the dist.? Timing? Is the manifold compatable with the spread bore Q jet? Vacuum leak at the base?

[Jungle Boy]

Moneypitt:
What else changed?? did you have to pull the dist.? Timing? Is the manifold compatable with the spread bore Q jet? Vacuum leak at the base? That's what I thought first - Timing? Did you re-time the engine? Did you check to see that the carb is opening up all the way?

[Dennis Moore]

What intake manifold did you install?

[fxinc]

Cast iron GM production highrise(Mercruiser part#17620), spreadbore and squarebore compatible. I will recheck the timing. Would the engine still run(smooth, strong, no skips or backfires) if the timing was way off, like skipping a tooth?
[ July 28, 2003, 07:38 AM: Message edited by: fxinc ]

[Dennis Moore]

You picked the best manifold, it should give you at least a 10 - 15 horsepower increase above 4,500 rpm. Must be something else wrong. My guess would be a vacuum leak.
Sincerely
Dennis Moore

[HOSS]

aLet me get this straight. You had a 2 and installed a 4. You now have a loss of power. You also stated that you installed a cast iron intake. My first question is why? The next thing is Quadrabog! Get it right. Next question is who advised you on selecting a stock intake? Although better than a 2bbl intake why? A Performer is about 120.00 tops! And yeah I already know that the stock cast intake will out perform the Performer. But with weight in mind it doesn`t. Not to mention I KNOW you blocked off the heat risers.
Check your oil. See if its immulsified. Water mixed with it. Then throw that ****in` Quadrabog in the trash( I have a drainage ditch behind my house). Unless you plan to bring it to a SPECIALIST to go through it. And I don`t meen the guy around the block that has cars and is always under the hood on something. You know, the same guy who sold you on this shit you have now.
A stock motor will ALWAYS respond better with a 4 bbl setup as opposed to a 2bbl. You lost power?
I doubt its a vacuum leek because it would run lean. REAL lean. For it to cause a loss of rpm it would be big. A small vacuum leak would actually increase rpm but be slower out the hole. Remember that lean makwes power but it also causes heat! Secondaries not opening sound like the real deal Holyfield to me. On a Quadrabog carb the primaries are actually smaller than a 2bbl carb. You can save gas with a 4bbl quadrabog as opposed to a 2bbl depending on wether or not you open the secondaries.
The Quadrabog is a great carb. Just not too many people know how to mess with them. I`ve done my thing with them. Metering rods, baffle secondary springs/setiings(different holes). But you need to know what the **** your doing cause they are as tempermental as a female. Has to be just the right hole that the rod is in or backfires will occur. :p

[fxinc]

no, it was a 4B to begin with. I was in the process of getting a new carb(wanted peoples opinions of what I should get, Holley-750cfm or above, or?) I put the Quadrajet(4B) on to see if there was any difference. The loss of performance was the result. There's no vacuum leaks, fuel pressure is good. I will recheck the timing and the throttle linkage.

[FastRat1]

my 2 cents...i agree with Dennis..a gd manif..not alum..but ok..i agree with Hoss..chit can the quadrabog..put a 750CFM DP p/n 4779 (mech sec) on it and it will run great rite out of the box
Ron

[Dennis Moore]

When I was working at Hallett Boats we took a boat in on trade that belonged to one of the Isky Camshaft employees. The 454 had, of course, an Isky camshaft and a tricked out Holley 750 double pumper. We ran the boat/engine/drive on the dyno and it made some pretty good power. The manager at Hallet wanted to put it back to stock for resale, the Holley had the choke air horn milled off so we installed a used Quadrajet that we had sitting on the shelf. We literally lifted off the Holley and set the Quadrajet in its place. Didn't even disconnect the rig from the dyno. The engine made the same amount of horsepower as it did with the Holley.
Off the shelf Quadrajet carb, with the stock Mercruiser jetting! If the carb flows 750 CFM and the air/fuel mixture is correct, the brand of carburetor doesn't make any difference to the engine!
Sincerely
Dennis Moore

[1FastHallett]

my question would be does the cam and head setup suport a high rise manifold? they willl decrease low rpm power and increase upper rpm power. second, i just put a victor jr intake and an 850 speed demon carb and got all kinds of upper end power but my motor is set up for high rpm running. is yours??

[fxinc]

Well, the problem seems to be the timing, it was severely retarded(much like me when it comes to tuning). I'll be taking her out to play with the timing today, any suggestions on where I should start degree wise?
at sea level, aspirated, basically stock config 350.

[Dennis Moore]

Stock is 8 degrees advance (at idle). That should give you 32 degrees total at 3500 rpm. I would adjust it to 10 degrees advance giving you 34 degrees total.
I bet it will really fly now!
Sincerely
Dennis Moore

[fxinc]

That did it, thanks to all. Glad I found this site, lot's of good info/help.