Carburetor Question (To Jet or to Go Up In CFM)

[Marshy]

So here's the thing... You'll never figure it out unless you troubleshoot. You always start with the easiest/most obvious things. Checking timing is a must IMO. Its also not the only thing to check but its the easiest so get that out of the way.

Your original questions were
1) do I need to rejet my Edelbrock?
2) should I move to a 650 CFM?
3) if theres anywhere else I should check for problems please let me know

The answers were;
1) possibly need a rejet although Edelbrock does not recommend the 600 cfm carb for an engine with cam duration over 220 @ 0.050" lift and you are running a cam with 243* duration. That in itself could make it difficult to resolve your initial problem. It was also pointed out that with a 1200 rpm idle you are now on the power circuit of the carb and the idle screws are no longer at play. That alone means you cent even begin to dial that cab in properly without getting initial timing and idle speed set properly.

2) Edelbrock actually recommends their 800 cmf carb given the large cam duration you have.

3) As we all keep saying, proper timing is the first step. Get the initial timing at about 10-12 degrees at 650-750 rpm then adjust the idle mix screws. If your ignition timing is too low then it will have a hard time idling and you will have to compensate by turn your idle speed screw (not mix screws) in more which opens the main butterfly to get your rpm up. This combined with the retard timing will also make it want to keep firing when you shut it off.

So, start with timing then move to the carb. Good luck.

[cadillac_al]

I think that cam is made to idle rough. I'd say it will be trial and error with the timing to get it to run as good as it ever will. I'm sure the low compression 1986 engine isn't helping. You may get it to sound bad ass though. Keep tinkering.

[Marv D]

Agree. The "thumper' cam will never idle right
BUT, a 243°@ 0.050" is far from HUGE in a 454. The Edelbrock RPM small blockcam is only 7° less than that, intended for a 350" motor, and will idle at 750-800RPM all day long. Another 100 cubic inches would gobble up 15+° and never know its there.

The fact that you have issues with TWO carbs,, you should listen to the fella's here and check elsewhere. Timing,, absolutely if you don't KNOW where the timing is you need to find out/
My first suspect in a backfireing, poorly running motor is the cam has lost an exhaust lobe or 3.

Get the timing set properly and if you still have issues, that neither carb will cure,, time to cut a oil filter open to see how much iron debris is in it,, , and then pull a rocker cover and see how many lobes are wiped.

[KyleSeal]

As I am repeatedly saying: I cannot get it to run long enough or at constant enough RPM to get a timing reading. I put a 5 psi fuel regulator on it and it fires up easier now. I'm going to try turning the distributor a little each time firing up until I find a spot that is okay.

[cableguy0]

Put the engine at 3k rpm and see where the timing is. You will need an advance timing light but you need one anyway if you want to set the total timing properly.

[KyleSeal]

Put a new 750 double pumper, a 4776 not a 4779-9 750, but a regular street 750. Fired up. We're just adjusting it down now.
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[KyleSeal]

Tuned that 750 DP I purchased, drilled the butterflies and threw a tiny powervalve in it, corrected the MSD box wiring, put a new distributor and spark plugs in it and its driving around again. Another issue I had was a bad alternator. New one fixed it.

[Captainfab]

A 4776 is a 600 cfm double pumper, not a 750.

Originally Posted by KyleSeal

Put a new 750 double pumper, a 4776 not a 4779-9 750, but a regular street 750. Fired up. We're just adjusting it down now.
Posted via Mobile Device

Did you ever determine what your timing is?

Originally Posted by KyleSeal

Tuned that 750 DP I purchased, drilled the butterflies and threw a tiny powervalve in it, corrected the MSD box wiring, put a new distributor and spark plugs in it and its driving around again. Another issue I had was a bad alternator. New one fixed it.