I finally have almost all the fine tuning done and about ready to go to the body shop. I am getting 17 mpg now and hope the remaining fine tuning gets me closer to 20 mpg. I still have that annoying jerking problem. When driving around 30-40 mph, the car jerks. It is definitely the engine because when parked and opening the throttle about 1/4 to 1/3, the engine races up and down. I have treid everything, timing, vacuum leaks. The only thing I can think of is perhaps the vacuum advance can is set too sensitive? Would that be a cause. It does this no matter what distributor or carb I have had in it. At the very least, it would be nice if it would at least jerk in time with whatever is playing on the stereo. Any help is greatly appreciated. Oh yes, it is a 225 with a holley 1920 carb, Holley electric fuel pump and everything else pretty much stock.

What's your fuel pressure?

D/W

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If it ain't broke, fix it!

Grab the engine damper and rotate the engine back and forth and "feel" how much timing chain slack you have. A slack chain will give the symptoms you describe.

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Yeah....Im the one who destroyed this rare, vintage automobile.....

The fuel pump is supposed to deliver 6-8 psi although I have not measured it. It did the same thing with a mechanical pump though. I have always seen the filter with plenty of fuel. The timing chain and sprockets are new too. I know it is possible it could also be defective. I will check it out. Is it possible it is vacuum related, like the advance can?

I'll throw out a guess:

Vacuum leak at the throttle shaft on the carb, but only at part throttle?

If you can rev to the point the engine stumbles and keep it there, point an unlit propane torch at it.

Alternatively, point the propane torch at the throttle shaft and rev the engine. If the stumble does not appear, you have found the problem.

$0.02

-S/6

8 psi may be too much for this carb, but you said it did it before the electric pump, so hmmmm. I was going to suggest that you might try lowering the float level or regulating down to 4-6PSI. If you suspect the vacuum advance, disconnect and plug at the carb and test drive without it functioning Also, are you sure you've ruled out a vacuum leak? Sure seems like a likely victim of one to me...

D/W

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If it ain't broke, fix it!

I am also thinking it has to do with vacuum. I have checked and checked for vacuum leaks with a propane torch and even with water but have never found one. This is really aggravating. I will try again and also try some of the other suggestions. Thanks everyone.

Two things to think about: The stock mechanical fuel pump only puts out 3.5-5psi....so perhaps the electric pump is to high. Second, are the bearings in the distributer shaft in good shape, or is there any play in the shaft? This will cause some wander in the timing. Still run points?

This is a new distributor but the old one ran the same way as it did with the mechanical fuel pump. It is electronic ignition. I have done more to try to isolate this than seems possible. Often when something like this happens, it ends up being something right under my nose that I have overlooked. I would also add that it seems to be just a bit less jerky when the engine is warm than when cold. Also, every so often when it is jerking, I may get a single backfire. This happens maybe once every few weeks. That was what caused me to think that maybe now it is the vacuum can on the distributor. Thanks everyone.

Backfire thru carb or exhaust? If carb, then it sounds like you are describing a textbook lean condition. Did you say you tried other carburetors? If not, I'd suspect it.

D/W

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If it ain't broke, fix it!

I do tend toward lean (carbs, alas not me). However, I suspected that and adjusted the mixture with a vacuum gauge this time. It is about 1/2 turn out from where it stumbles. I believe the backfire is in the exhaust by the sound of it. I have tried about 3 different 1920's and 3 different 1945's all with the same result. I will double check everything mentioned here and see if I can finally drive the gremin out. Thanks again everyone.

...all the different carbs you tried probably rules out carburetion (but I'd note that the idle mixture has little to do with cruise or overall mixture).

Do you have vacuum accessories hooked up? (heat/ac controls/actuators, power brakes etc? If so have you tried temporarily disconnecting and pluging the source? I assume you've got good plugs/wires/cap.

D/W

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If it ain't broke, fix it!

I haven't isolated all the vacuum yet. I did, however, put in check valves and a reservoir for the vacuum controls for AC/heat, etc. They weren't there when I got the car so the actuators constantly opened and closed. Those vacuum lines run directly from the intake manifold vacuum tree. Also a check valve in brake booster that runs from the manifold. I will probably clamp off the vacuum advance hose with a surgical hemostat and see how it behaves and just do that on the other lines one at a time. All the ignition components are new and no problems detected there. Except the one I posted on a month ago where the dwell/tach tool reads backwards. I checked every wire on the harness and they are correct so that is still a mystery but I suspect vacuum more since it happens at a specific range of engine speed, whether driving or parked. When I put the latest distributor in, I did back off the vacuum can to the most responsive setting. Maybe that is an issue? Also, I probably need to hook the vacuum gauge to each line and check how it behaves at the various speeds. This is really frustrating and the jerking action is pretty strong and therefore annoying. Thanks again.

Try driving with the vac advance blocked at the carb. Your dwell/tack reads backwards?? What does that mean, does the needle drop down below zero? Are you sure you are hooking it up right? Also, just be be sure, you do have coil (-) going to the points, right?

D/W

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If it ain't broke, fix it!

It's digital so when I open the throttle, the rpm drops. It works fine on my other cars. I hook the black wire to ground and the red to the neg post of the coil. It is electronic ignition. I traced all the wires per the diagrams and it is correct. Maybe it is time for an exorcism on this car. It has certainly had its share of strange things. It was in sad shape when we got it, very neglected and jury rigged by people who were obviously clueless. However, for $250 I guess it was a good deal. The body is basically sound, needs some dings removed and a new paint job but no rust. I am going to try the vacuum advance today.