Pulsing in 4th gear at crusing speed
 

69 C10 307 with 700R4. In 4th at 65 the car pulses like I have the timing set to fast or someone is very light pumping throttled pedal as they cruse. Timing set at 4 degrees BTDC like manual says. Dosent do it in 3 rd. New HEI dist and wires. New Edlebrock Performer 4 barrel. Stock other than headers. Truck runs great except for pulsing. Dont think it did this before I installed new Performer but I cant be sure. This truck is a interior gutted rebuld and just finally started driving to try to get all bugs out of it.

Re: Pulsing in 4th gear at crusing speed
 

Is it rpm related like the engine reving up and down? You can install an rpm gauge and monitor it but also check the vacuum advance and all the vacuum hoses to make sure there's no vacuum leaks anywhere.

Since it doesn't do it in other gears, I suspect it's something in the transmission but since I'm not a tranny person, I don't know what.

Re: Pulsing in 4th gear at crusing speed
 

Does the trans have a lockup torque converter? It the trans t/v cable set correctly with the new carb.? http://www.cpttransmission.com/tech_tvcable.htm

Re: Pulsing in 4th gear at crusing speed
 

Bump your timing up to 12 degrees. That'll smooth it out.

Re: Pulsing in 4th gear at crusing speed
 

Be SURE that the TV cable is set correctly first. Then disconnect the lockup temporarily to be sure that is not it. I'm betting it is one of those.

Re: Pulsing in 4th gear at crusing speed
 

Having the TV cable set correctly is very important. The TV cable regulates how much hydraulic pressure the clutches see in a specific gear. An improperly set TV cable will not create a pulse in 4th.
The converter locking up in 4th would not really feel like a pulse. It would feel more like a shift.

I would look more toward your carburetor, fuel pressure, timing, timing advance mechanism.

Like mentioned before I would first turn up the advance to 10-12 degrees advance. A vacuum gauge will also help to see if that is the problem. If the vacuum gauge moves a bunch when it is "pulsing" you will know there is a problem when under load. If the gauge does pulse then it could be timing or fuel pressure or carb tuning problem.

I wonder if it would do it under the same type of load in another gear.

Just my two cents

Re: Pulsing in 4th gear at crusing speed
 

Anything above about 50-52 total advance @ 2500 can cause a surging effect at cruise.

Re: Pulsing in 4th gear at crusing speed
 

If all else fails, check your driveline u-joints. Make sure they are in good shape, and make sure the yokes are phased properly. I have had many strange vibrations, noises, and motions that have all been attributed to a bum u-joint or out of phase yokes.

I also agree with bumping the base timing up to 10-12 degrees. Even if this is not the problem, your truck will run better.

Re: Pulsing in 4th gear at crusing speed
 

Thanks to all for your comments. The truck was originally set at 12 degrees BTDC and when I noticed the pulsing I set back to 4. Still pulsing. Reset TV cable. No change in pulsing. U joints are all new and drive line balanced. Confident there is no drive line problem.
HOWEVER, in the process of trying to get as much road noise ( wheel bearings, ujoints etc) out of the truck as possible prior to painting I discovered today that the rear end is causing all the nosie. Sounds really shot and creates a whup de whump and low speed. I wonder if it could be so shot that it is causing the pulsing in 4th. 4th is really low. Must be 1 to 1 or lower. Would a really bad rear end cause the pulsing in 4th?