Relationship between Carb-size and Timing



How does the size of a carb(s) on a particular engine affect what the timing should be set at?

300 I-6, Mallory Unilite Distributor, Accel Blaster Coil, Holly 650, Offy Intake, EFI manifolds.
When you run long duration cams with certain types of carb, there can be serious part throttle and driveability problems.

Two examples:-

1. A little 2.0 or 2.3 Pinto/Mustang OHC that gets a 2-barrel non-staged Holley 350 cfm carb. If camshaft duration is just slightly too much, the gas mixture from the carb is reverted, and causes a fuel standoff situation which is very unpleasant at certian engine revs and throttle openings. I've looked into this a little, and have come to the conclusion that this is because Holley carbs often have less of a signal, or venturi restriction, than equivelent 2-bbl Rochestor or Carter carbs. This means the gas is less inclined to flow from the carb to the engine. If you check the throttle diameter verses the venturi diameter, Holleys are less waisted, which is great for power, less good for driveablity.

If overlap or duration is reduced, the problem ceases. This underpins the rule that it's very easy to over-cam and engine, hard to under-cam it.

2.Where this fuel standoff occurs on a little four pot Ford isn't an issue to us here, but on a 200 cube engine with a 350 or 500 cfm 2-barrel carb, it sure is.

Experienced car tuners such as Aussie7Mains and others have seen first hand that a 500 Holley on a 250 can suffer drive problems, even though they are fine at wide open throttle. The signal with a modified cam can cause problems, but not with a stock cam. Wierd huh? Check this old link from the Aussie forum. Could be because the Aussie 2V has lots of intake runner volume? This may not be a problem with the log Aussie and US 250's?.

I can tell you from first hand experience with 250-2vs that a 500 is TOO big with a cam, ok on a stocker, 350 is OK on 200ci.

P.S. I run a 500 cfm Holley with no venturi on my cross-flow 250...I cut it out and added a variable venturi Impco gas carb on it. No fuel stand off, even with the huge off idle vacuum of a hi-intensity aftermarket cam with stock duration but high 50 thou lift duration figures. But LP gas is a different fuel.

Hope this helped!!!!
8) carb size and ignition timing do not go hand in hand. however fuel curve and ignition timing can. changing the carb size does not change the fuel curve. that can only be done by jetting changes, air and fuel jets.