If you suspect that the carb is too cold and it has a spacer, try a spacer of a different material and possibly a thinner one.  I dirve mine year round, even when below freezing, without the heat riser and restricted gaskets and the only time I had a significant problem was with a B-intake , 1" aluminum spacer and Edelbrock carburetor.  The problem seemed to be carb icing, which occurred only below 40 degrees or so.  The engine would eventually stall and after a few minutes of sitting with heat soaking into the carb, it would start effortlessly and be fine from then on.  A change to a 3/8" thick gasket from Edelbrock took care of that and I didn't have the problem after that change.  Note that there was never a problem when above about 40 degrees.




Lawrenceville, GA

I run 91 octane premium in all my classics. This is the only gas up here that is supposed to be ethanol free as our 87 is E85 and our  E89 midgrade is of course a blend of premium and E85. I have tried going one jet size up from stock setting and all I saw was cruise AFR go more rich. I have also tried a power valve change to a 9.5 just to see as my cruise is 13+. I think you might have hit on one of my issues when you mentioned the heat riser as when I built the car I gutted the old one and used it as a spacer. I just assumed the cold blooded nature was natural. I am not ruling out my metering block either. Has anyone played with the quick fuel universal metering block and any thoughts on whether it would work on a little 390 cfm carburetor? 

Have you tried increasing the primary jet sizes?  You don’t mention what fuel you are running but ethanol laden fuels do make the engine run leaner.  And the 87-89 octane fuels have more ethanol content than the 91-93 ethanol laden gasoline.  It would be easy enough to just increase the primary jetting 3-4 numbers and see if that either resolves or at least diminishes your hesitation issue.  If you are running 87-89 octane gasoline, you can try the premium grade to see if that helps to alleviate the problem.  If in doubt about the ethanol content in the gasoline, it’s a relatively easy check.  The testers for that can either be purchased or home made as no special chemicals are needed for the test, just plain water.

Lorena, Texas (South of Waco)

Now that is a thought. I have the truck intake gaskets with the restrictor and no heat flapper on the exhaust pipe. It does have a bit more of a issue when cold, but even on a hot day fully warm it still stumbles. 

You have 21/4" pipes on it. Do you still have the flapper valve on one exhaust outlet?  Have you blocked the heat crossover in the intake?  In my opinion you need that exhaust heat in the manifold to prevent a stumble on take off.  On the race track you don't want or need that heat.

John - "The Hoosier Hurricane"

You may have a problem in the metering block (emulsion tubes), it's easier to try a different metering block than to repair it, however there are 2 small pressed in cups that have to be removed, drill a small hole in them insert a metal screw (not to far) and pull it out, you then can see inside and try cleaning, when done solder the hole in the cup and press it back in.

The carburetor isn't "too small" you can rule that out, at least for the issue described. It should work well. For steady flat ground cruise I jetted the stock 2100 to run in the high 15s for AFR, with a good hot spark and plenty of vacuum advance this is optimal for clean burning plugs and at least a nod towards reasonable economy. On heavy acceleration it drops to 12.5 which is plenty rich for acceleration.

19" of manifold vacuum at 3000' ASL is excellent, so a logical question would be what flavor power valve are you running?

A 7.5 or 6.5 would be suitable with the numbers you describe. It is important to note that while the main metering jets are not part of the power circuit as such, jet size most definitely affects the AFR that is seen on power acceleration. The power valve only affects when the extra fuel is added, not how much. The PVCRs or power valve channel restrictors determines this, they are more or less fixed jets. The usual tuning strategy or "fuel curving" is to determine jetting first. The old school way was simply to jet down till "lean surge" or misfire was encountered and then jet back up 2 jet sizes. Then it's a matter of making sure that the power circuit is plenty rich under load. The wide band O2 AFR gauges takes the "hair shirt" element out of this operation.

The "transition circuit" or T slot is part of the idle circuit or just off it. Although it's called the idle circuit it is in play up to around 35 or 40 mph, if the idle mixture screws are too rich it can lead to a not so intuitive condition where the idle RPM is too high thus exposing too much of the T-slot. A lot of city driving is at idle or just off idle, so it is important that the idle mixture setting is set closely. A "lean bog" or "rich bog" is sort of similar in effect, the latter will be accompanied by a puff of smoke out the back.

Yes it is a point distributor and I have checked grounds. I also used a hand vacuum pump at idle to see if movement of the breaker plate caused the miss.

Float levels have been also take from stock setting to way high to just below stock to try to see if it helps.

Valve lash is in spec and I tried opening it up to see if I had a sticking valve/ poor adjustment.