Flat head hesitation off idle - Advice??
 

I've had continuing problems with hesitation off idle on a 239 flathead with a Holley 94 (7RT) under no load conditions. Overall poor power as in can't go the third under 30 MPH without lugging. Once the RPMs are up, no bog. Manifold vacuum is 18", at least 3" to the distributor when rev'd up but nothing at idle - no timing change if the vacuum line is connected or not. Tried a couple of carbs (no guarantee any were all that good). Current carb seems OK - accelerator pump working etc but no difference in the bog on any settings - cold, average, or hot weather settings. Starts well and idles like a champ until I hit the gas, then the big bog. Dwell angle is good, timing is set initial 2 degrees BTC. I'm getting some advance as RPMs go up, maybe a couple of degrees but not too much (no markings except TDC so at best a guess for movement and initial timing). I seem to recall pretty substantial advance visually when goosing the 302 in my 71 Mustang but that was 20 years ago and not a flat head. I also had a good difference with the vacuum line connected and disconnected, No difference on the truck. Should I replace the vacuum advance unit on the distributor next or does anyone else have a better idea?

See if vacuum applied to the vac advance unit does anything -- you can pop the distributor cap and watch the points plate. The flathead style advance system is very different from the 302's. But I'd also suspect the low speed system in the carb is plugged, from your description. Is it running hot? Is the timing set to the dot on the crank pulley with no vacuum? That gives you 2 deg. (it is not a TDC mark)

The vacuum advance does move when vacuum is applied - unscientific - can suck 10" on the gauge and minor sucking on the vacuum line does move the plate. Still when rev'd, I see very little movement on the timing. It is very close to the dot at idle.

Ross hit it right on. Temp will tell you a lot about timing and fuel mixture. If it is hot, something is not right. Check to make sure the time plates move inside the dizzy. Just pull it out, marking where it was, hold the bottom end and turn the whole dizzy. The plates should move. You can also hook up a vaccum pump and see the same thing, should move the plates if the vaccum is properly working. Have you noticed any change in idle. Higher, lower, different in any way. Swapping carbs won't really help you. You need to tune one carb to one engine and get it right. Unless one of them is leaking, or obviously not working.

This article might give you some pointers and places to look.

Holley 94'a and Vacuum Distributor

One other question. Could the springs inside be broken. That is really the only way if the plates move to not have advance working. Unless you have a vaccum leak.

I'm thinking either the tubing from the carb to the dizzy is leaking, or there's a vac leak at the base of the carb.

Tuned the current carb to the engine - backyard style to max vacuum and eliminate any missing through the exhaust. It is smooth and consistent at idle. Runs rougher when goosed with no load. Applying vacuum (sucking on the line) did move the plates - I know saying that it sucks it too easy. The temp does not appear to be running too hot. Half on the temp gauge (no clue if the sender and guage combo are accurate) and more importantly, ran in a parade at low speed, beat up the clutch at less than 5 mph for over two hours but no temp problems. It does run cooler when actually driving but not all that different in my experience.

Ross,
Ran a new vacuum line to the distributor - getting more advance on the light but still bogging seriously off idle. Maybe it is the carb - old, e-bay rebuilt and a carb from early ford v-8 sales all do the same thing, Absolutely no difference. Three with the same problem is hard to believe...

Sounds like possibly fuel delivery. If the distributor advance was not working it won't bog like that. Look down the carb and see if you are getting a nice steady stream of gas from the accelerator pump and not just drops. Tried different gas?

Good delivery from the accelerator pump. Nice pulse, not just a few drops. New filter and fuel pump (gaskets leaking on the old one - replaced before it was time to roast marshmallows). I really am stumped.

Hmm maybe coil. I had a 69 firebird do the same thing would idle fine but wouldn't rev just stumble and run rough.

I just checked. At idle, there is a strong consistent spark from all eight wires when the boot is held against the cylinder head bolts. A coil is cheap and I can try that but any other ideas? Removal of each wire did slow the engine down equally.

If you are running on 6v, I'd bet on the condenser being bad. It is real hard to find a quality 6v condenser that will last, and when they die, they can give symptoms like you describe. Even if you were 12v it could be that. You could disconnect the condenser for a test, and see if it helps. Also check the gap on the points, even if timing is right on, if gap is too small, the spark will jump across it.

agree with all the above mentioned Posts. Still if the guy is running a Chandler Groves Holley he might want to remember that there three positions on the aceleator pump. I actualy, I wound up resetting the pump stroke on mine once as I lived in Richmond Va, Phoenix Arizona and of course Dallas Texas with the truck. Im back in Virginia now and I think its on the second hole .
Oh a match book cover is the same almost as a 015 gap guage too ! Ed Shaver

I am running the 6V. I replaced the capacitor last night - massive difference in the power but still left the hesitation off idle. Now is is much easier to hear where the problem comes. Idle smooth, stumble-hesitate through about 1500 RPM then fine and strong the rest of the way up. I'm going to rebuild the best of the carbs I tried and see what that does. It really does seem to be fuel as it goes from the idle circuit to the power circuit.

Thanks for all the help.