Sorry in advance for the long post. History: Car ran perfect -350/300 motor. Before installing headers, I pulled the intake, pulled old-school Mallory 2664501 dual point distributor (with no vacuum advance) and heads to replace valve stem seals and check valves & seats. Dissassembled heads, cleaned and lashed valves and reassembled with new valve stem seals. Then reinstalled everything taking great care to ensure no vacuum leaks, etc.(Vacuum at manifold is around 13). Adjusted valves and installed headers.

Now I have a bad hesitation at low RPM that feels like a fuel (too much) related issue. It's especially bad in stop & go traffic - like it's loading up. It idles PERFECT at 700 RPM and seems fine under no load at all.

So, I replaced all plugs, checked compression (all good), rebuilt carb (Holley 670 Street Avenger), replaced power valve, changed filters, adjusted timing (got 36 degrees "all in" at 3,000 RPM), adjusted mixture, floats, repaced accelerator pump - Still have the same problem. Once I get past the initial hesitation, it runs strong, but the hesitation is pretty bad.

It seems unlikely that the problem I have since the head and header work is something unrelated to what I did since in ran perfectly prior to that work. Regardless, I did the other work above to rule out the unlikely and I still have the problem so I need your expert advice, opinions, suggestions, etc to help solve the problem.

At this point I'm open to any and all ideas cuz it's driving me crazy. Please help.

 

Not sure if this helps but I had a similar issue on a different car. I don't have all the particulars but it seems like a driveability issue in lower RPM ranges with the conversion from closed throttle to partial throttle changes?

I had an 85 mustang with a 600cfm holley on it. It suddenly developed the condition I listed above.

Turned out that my accelerator pump circuit in the holley was blocked by a little piece of gasket material. There circuit is in the bottom of the front metering plate and the opening is very small where the accelerator pump gets its feed from at the bottom of the float bowl.

If you open the throttle just enough for the primary plates to open do you get fuel from the two little squirters from the accelerator pump on the carb?

If not you will get a bad hesitation in around town type driving.

 

If I got this right, you did heads, distributor, and manifold work- put on the headers.... then got a hesitation. As a result, you did further work including carb. If the hesitation was there after the initial bit of work, then the issue must be with either heads, dist, or manifold IMHO. You have 36 deg in at 3000 or so, but where is the timing at low / idle?

 

Check the adjustment of the accellerator pump linkage. This was my problem.

 

 

make sure you connected the vacuum advance to the correct vacuum port.

 

Good thinking, but my accelerator pump is working well and adjusted properly. Any other ideas?

 

I completely agree with your logic that the initial work I did caused the problem

The distributor is set for about 16 degrees of centrifical advance (no vacuum advance). At idle, I'm running about 24 degrees which would indicate that I'm getting about 4 degrees of advance at 700 rpm. Sounds right to me - your thoughts?

 

The accelerator pump is adjusted and working properly. Any other ideas?

 

that 670 is known for being lean.
headers are known for making it run lean.
together it dies
most guys upjet the 670(put rear jets in front)
if the mpg is acceptable, then you r done.

 

Could that cause the hesitation? I thought about that and the Holley tech said he thought it would be worth a try. Any suggestions on size?

 

Have you checked your plug wires for arcing?

cc

 

if you are saying the dizzy is set at 16 deg adv then that would be 32 at the crank so you are running only 4 at initial or am i reading this wrong...sounds like you dont have enough initial.....

 

Not sure I understand timing well enough to answer your question. Using a dial-back timing light, I determined that I'm running about 4 degrees at 700 rpm on the crank and 36 at 3000 - does that answer your question?

 

It still sounds like a carb. with not enough accelerator shot to cover an off idle stumble. At what RPM does this start and then end? mds...

 

So you have 4 degrees of advance with the vacuum can unplugged at 700rpm, but when you plug it in, you get an additional 20 degrees for a total of 24degrees at 700rpm? That is probably too much at idle, and your vacuum can is probably connected to a manifold vacuum source vs. ported like it should be.

I am not an expert with timing, but I have been jerking around with it a lot lately, so I have some knowledge.

Since all your idle advance is basically coming from the vacuum can, when you accelerate, vacuum drops, and you loose your advance, causing the stumble.

Connect the vacuum can to a ported source off the carb, set your timing, with the can disconnected, to whatever the manual says (should be better than 10 degrees w/o vacuum can), and see what happens.

Just a thought.

 

So WHY did you mess around with the carb and everything else? All that did was introduce a whole mess of other possibilities! I suggest you find and read the paper that Lars did on timing and put your learning cap on before you go any further. You can keep shopping here for ideas or you can apply some logic and troubleshooting ability to solve the problem. Good luck and I hope you resolve the issue.

 

Look to your ignition system.

There is nothing that changes in the fuel system going from no load to under load that will cause a hesitation. There is in the ignition.

 

that causes a bog. 4
do a quick test at 16 base.
just prove it fixes it, don't drive any distance.
(then alter the stop later)

 

It does not have vacuum advance.