Bucking Mustang !!

[Spanish Dave]

Hi everyone .. I was wondering if anyone could help me to tackle this problem I have with my 65 fastback. the car is virtually undrivable due to bucking and jerking in any gear at less than 2200 rpm In traffic I have to constantly change down and usually finish up in first or second gear to keep the revs up .. above 2200 it pulls really well and will cruise at 2500 3000 revs all day with no probs .. you can see the engine specs from my sig. I thought it may be the cam as it is a bit on the wild side and gives power from 2500 rpm, but below this it should still be streetable but not a lot of grunt when floored at lower revs. Before this cam I had an edelbrock performer RPM cam which looking back the car was not too different than with this cam. I tried a holley street avenger 670 cfm with vac secondaries and the car seemed slightly better with that .. but still not streetable. It is only pulling around 5 to 6 in. of vaccuum at idle.
I really want to get this sorted out and would like advice on the logical steps to take.
Thanks in advance.

 

[Spanish Dave]

I have been doing a bit of research on this problem and think the power valve may be the problem .. or at least part of it. I have a 6.5 installed that came with the carb (Barry Grant Speed Demon) which must be open all the time at idle (and at 1500 - 2000 rpm) and be dumping excess fuel into the engine. Would you think that this would be a good staring point ? ... maybe a 2.5 ? or should I block it off completely ? I want to do any changes one at a time, because I believe this is the best way to keep a track on things.

 

[tos]

Mechanical or electric fuel pump? Do you know your fuel pressure?
Also is it possible you could have a broken motor mount?

 

[Spanish Dave]

Thanks for the reply ... it is an edelbrock mechanical fuel pump .. I haven't measured the pressure. The motor mounts are fairly new and I have just checked them a couple of days ago ... I have the g/box out and am in the process of fitting a new clutch .. the old one was a centerforce dual friction kit , but most of the friction segments on the flywheel side of the disc had come unglued .. with a bang ... I don't know what they stick them on with but I think rivets would have been a better idea.
This is what is prompting me to try and get the car sorted and order any parts I may need from Summit on one delivery .. if you follow.
I am going for a McLeod Super StreetPro clutch .. any views on this one?

 

[rbohm]

start with the basics here;

1: check for vacuum leaks

2: check your timing

3: go through the ignition system

i think once you go through these procedures, you will find that your bucking goes away most likely.

 

[Spanish Dave]

start with the basics here;

1: check for vacuum leaks

2: check your timing

3: go through the ignition system

i think once you go through these procedures, you will find that your bucking goes away most likely.

Thanks for the advice ... at the moment the g/box is out .. so I can't check these ... but I will when it is all back together and running. The cam is timed straight up with 18 deg of initial and 38 all in at 3000 rpm. The plugs are always black when I pull them which leads me again to the power valve dumping excess fuel.
I have checked for vacuum leaks (with WD40) in the past, but found nothing. You are right, vacuum leaks would cause this type of problem, but I would have expected the plugs to be lighter due to lean running ... but whatever ... carburetors are strange animals !! I did change the front main jets to 68 down from 72 to see if that would make a difference, but totally disregarded the power valve.