I just swapped out my C4 for a toploader 4 speed and now my car "bucks" whenever the RPM's drop below 1300 in any gear. The lower the gear the worse the buck is, it so bad in second that it looks like I hydrualics on the car! What could be causing this to happen? It is a rebuilt wide ratio toploader, new center force dual fiction clutch, 3.80 gears, 347 ci engine. Any help would be appreicated.
My car bucks pretty bad under 1000 RPM and im guessing it's because of the size of camshaft.I'm assuming you have a pretty stout cam in that 347.
Thanks for the reply. All the other manual trans vehicles i have driven have all been stock so the bucking at that high of an RPM is new to me. Cam specs are 236 duration at .050 with .528 lift solid flat tappet. I guess that could be considered that stout...
Did you make sure that the flywheel and pressure plate surfaces were clean (oil/grease free) Have you double checked the clutch adjustment.
The flywheel and pressure plate are both new from center force, I wiped them down with brake clean before installing the clutch disc and pressure plate. I played with the clutch adjustment some today with no change in the way it acts. I am using a cable clutch and was told to go to zero lash then another one to two turns, I did a turn and a half. Was this right?
The "bucking" may be a result of slight (or more) pressure on the accelerator and then as the car jumps forward your pedal is slightly released the car slows and pressure increases on the pedal making it jump forward again - This kind of cycle can become more violent as it proceeds. Try supporting your foot better and / or adding spring pressure to the return of the linkage. Remember that as the car jumps forward your body rocks back away from the controls and as it slows you rock forward toward the controls. Similar things can happen in aircraft if the yoke is pressed forward - the plane begins to dive and the pilot is rocked back which pulls the yoke back and the plane rises which rocks the pilot forward and the plane begins to dive again - it is called "pilot induced porpoising". Your car is just accelerating and decelerating causing the foot pressure to counteract the previous movement.
Stephen, you might try bumping the timing a couple degrees up or down and see how much it affects your performance. My car does the same thing and that helped quite a bit.
Stephen; might be clutch chatter. If it continues, have the flywheel resurfaced, even if it is new, it still could be warped or the surface not smooth. Also, take a look at the pressure plate, see if a spring is broken or anything else is amiss. Good luck!
I took a few degrees of timing out it today, it made a world of difference. Still bucks a little when it dips below 1000 RPM but it near as violent as it was. Thanks for all the replys.
I have another issue, in any gear at higher RPM (about 3500 and up) the car gets a high fequency vib. especially in the shifter knob. Any thoughts on what that could be.
And you've never had it before the swap? Does it do this while sitting still? If not I would say the engine is eliminated. If it only does it while driving it must be from a rotating assembly. Could possibly be a u-joint. I would check the front joint closely. Does the yoke have much movement in the back of the tranny such as a worn bushing?
I just went out to see if it was doing in neutral and it is, doesn't seem quite as bad but it is still a hgh feq. vib. With the automatic in it, I always felt like it had a vib as the RPMs went up but it wasn't as bad as it is now. I also had a B&M cable shifter in it so it was bolted to the floor with a cable to the trans, less vibration transfer.
When you swapped the auto for the stick, did you install a pilot bushing in the back of the crank? If not, the transmission imput shaft is dancing all around. Just a thought.
