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-   -   Possibly too big or a carb for 350? (http://www.hotrodders.com/forum/possibly-too-big-carb-350-a-174820.html)

lizardking7750 03-19-2010 08:53 PM

Possibly too big or a carb for 350?
 
I have a Holley 750 duel line street dominator on my 1981 Camaro with a pretty much stock 350 with a small cam and Edelbrock intake and headers. When I rev it up fast it sputters and backfires, and when I rev it up slowly it does it after i let off of it. It does need a new header gasket on the drivers side, has a rip, could that be the problem?

DoubleVision 03-19-2010 09:11 PM

Yep, it could be the issue. If you use paper header gaskets soak them in water 24 hours before you use them. This makes them swell and hold moisture. But the best tip is use stock replacement small block chevy exhaust manifold gaskets, these are made of meshed steel and don`t burn out easy. They are also slotted so you can just drop them in without removing the header bolts. After you get your header gaskets fixed and if the problem persists it`s likely in the tune. Get back with us once you have the headers back in order.

lizardking7750 03-19-2010 09:12 PM

OK, thanks Ill do that tomorrow after work.

32Essex 03-19-2010 09:14 PM

what kind of ignition do you have and what condition is it in. Have you taken the cap off of the dist. and checked the rotor and looked for cracks in the cap. have you looked at the wires, what kind of condition are they in ? check things out, tell us what you have done so we'll know where your at.

lizardking7750 03-19-2010 10:29 PM

The ignition system is practically new, hei, new wires and plugs, inside of cap and rotor is very clean with no cracks or damage. Has a Holley "RED" fuel pump with a fuel cell.

Irelands child 03-20-2010 08:24 AM

Yep, a 750 is too big for a 350 without lots of mods. Use this formula to calculate a good size

Size=CIDxRPM/3456xVE **

Size=350x5000/3456x.80

Size = 405 CFM !!!!!

Of course this isn't real life nor realistic, but it does show that you are way over carbed. Your almost stock 350 just doesn't have enough in the way of being a vacuum pump to make this carb efficient in allowing enough air into it and atomizing the fuel/air mixture well enough. Of course, good heads, a 'big' cam, a few more cubic inches could make this carb work - at higher rpm's, that is :D .


**CID = Cubic inc displacement
RPM = Revs per minute
3456 = a standard constant
VE = Volumetric Efficiancy (.80 or 80 percent is very good for a stock or mildly modified engine)

The biggest carb I would use on this engine would be a 600cfm and when modified, a 650cfm might work well. If you need a perspective, my old F350 with a 460 engine had as OEM, ~a 500cfm Holley. I tried a 780 Holley, but it ran best with a 600 cfm unit, either a Holley 0-1850 or later, an Edelbrock.

Exhaust gaskets wont make an engine run that badly, just sound crappy. Of course, ignition systems if set up "off" will and electric fuel pumps set at too high or low pressure can cause big problems. One question - GM's use full manifold vacuum - do you have it plumbed there? Ported vacuum will not allow enough initial advance (Ford's are opposite and use ported)

fast68 03-20-2010 06:26 PM

how is a 750 too big when stock Q jets are 750 and 800 cfm

and used on all the v-8s and even on 1985 4.3L engines

maybe you mean its jetted too much

but not too much CFM

lizardking7750 03-20-2010 09:22 PM

I had a Q-jet lying around and swapped that on there and the sputtering and backfiring went away, but unless I rev it up pretty high it just fires up revs up a few hundred rpms over idle and then dies. When its dying I can't do anything about it even if I touch the gas pedal. It does run fine when i have the throttle open a good bit. Maybe the carb just needs to be rebuilt? The advance was ran off the carb, but after everything Ive heard I think Ill run it off the manifold.

Irelands child 03-21-2010 07:43 AM

1 Attachment(s)
Quote:

Originally Posted by fast68
how is a 750 too big when stock Q jets are 750 and 800 cfm

and used on all the v-8s and even on 1985 4.3L engines

maybe you mean its jetted too much

but not too much CFM


The primaries on Quadrajets are tiny in comparison, allowing a nice airflow through. The secondaries are huge and only open as demand increases. Holleys have large primary and secondaries to make up their cfm ratings. As far as operational design - apples and oranges

Take a look at the attached picture of a generic M4MC Q-jet. Sorry about the quality. Here's pretty good site about them as well - take a look at their first statement on Design :http://www.carburetor.ca/carbs/tech/...jet-index.html

lizardking7750 03-22-2010 07:50 PM

I rebuilt the Holley after all, it was pretty gummed up and dirty, I also put new gaskets on and set the carb and now the problems have went away. Thanks for all the help.


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