bucking at speed

[pedal2themetal45]

HI All
I think it may be carb related.. I've tried disconecting vacuum advance, retarding the timing, adjusting the carb. Stuff I have done to resolve the problem.
I have rebuilt carb, new plugs, adjusted valves, cleaned the cap/rotor (durspark II), watched under the hood at night to see if moved rpm up and down could hear the miss but no arcing going on from wires.
But still the thing still bucks and jumps when you step on it or at speed.
any and all suggestions well be appreacated.
Thanks
tim

 

[JackFish]

Just wondering if it misses when you hold the rpms steady at about 3000 in neutral?

 

[Soldmy66]

Fuel filter.

 

[rbohm]

HI All
I think it may be carb related.. I've tried disconecting vacuum advance, retarding the timing, adjusting the carb. Stuff I have done to resolve the problem.
I have rebuilt carb, new plugs, adjusted valves, cleaned the cap/rotor (durspark II), watched under the hood at night to see if moved rpm up and down could hear the miss but no arcing going on from wires.
But still the thing still bucks and jumps when you step on it or at speed.
any and all suggestions well be appreacated.
Thanks
tim

8) have you tried advancing the timing? have you checked for vacuum leaks?

 

[pedal2themetal45]

HI
it does have a miss at about 3000 when held steady. It seams to be worse after it heats up, its not bad when its cold. It could be maybe a vacuum leak with the vacuum wipers. It has the old duel vacuum advance but the top one is plugged off and it is using the one out the end of the pod.
I don't know it does have an exhaust leak (that might be heating up the carb/intake since it worse when it heats up). I haven't tried advanceing the timing just retarding it a little.
OH it does have the dura spark II dizzy.

 

[woodbutcher]

Hi pedal to the metal.Sounds like what we used to call"surging".Usually fuel system related,IIRC.Check out fuel delivery system from end to end.Correct any problems,and go from there.Next if the FS is ok go to the ignition system.Go over both with a fine tooth comb.
Good luck.Have fun.Be safe.
Leo

 

[MPGmustang]

bucking at speed usually is related to the vacuum canister... too aggressive or not working. it also mean loose or broken springs.

 

[tri-power 250]

This might sound weird, but here goes. My Mav was bucking at speed, it really didn't matter what speed, I also smelt gas. I looked for leaks all over, then the other day I was working under the car and noticed there was a rubber fuel line that connected the steel fuel line from the tank to the steel line going to the fuel pump. That hose had deteriorated but didn't appear to be leaking. As my luck has been going, I decided to change it out with fuel injection rubber line. Just because it is high quality. The bucking went away. I am thinking the fuel pump was sucking air in and causing a lean condition.
I hope this helps.
Ken

 

[MPGmustang]

This might sound weird, but here goes. My Mav was bucking at speed, it really didn't matter what speed, I also smelt gas. I looked for leaks all over, then the other day I was working under the car and noticed there was a rubber fuel line that connected the steel fuel line from the tank to the steel line going to the fuel pump. That hose had deteriorated but didn't appear to be leaking. As my luck has been going, I decided to change it out with fuel injection rubber line. Just because it is high quality. The bucking went away. I am thinking the fuel pump was sucking air in and causing a lean condition.
I hope this helps.
Ken

My dad had a simular issue, but found a pin hole in the hard line where it went throught the frame on his 55 packard. "sucking air" he found it by being wet after he installed an electric pump near the tank to 'assist' the mech. he didn't expect to find a pin hole.

 

[AzCoupe]

Which carb are you using?
What is the vacuum line hook up to (manifold, carb, or other)?
Have you check the fuel pressure, at idle and at higher rpms?
Have you checked the fuel filter, float level, and the needle & seat?
Does it surge when it's cold, only when it's warmed up, or both?

 

[pedal2themetal45]

HI
Its been running fine for years and just started this.. carb is a stockish YF dizzy vacuum line is to base of carb plugged into outside plug of dizzy and the top nipple plugged off.
Only when it heated up. it runs fine when cold
I think I found the problem fuel related. when It is missing the filter was empty and after sitting for a few minunets it has fuel in it. so I'm going to change the pump and filter and blow thru the line to see if its clogged.
thanks all
tim

 

[ludwig]

Check for contamination plugging the filter in the tank. If there is crud, it blocks the intake. When there is no suction, the crud falls off and everything is fine. Then the acceleration and increased flow suck it back up to the intake filter.

 

[69.5Mav]

Check for contamination plugging the filter in the tank.

Ditto!

My 72 Mav a a huge amount of crud in it. The car would drive about 1.5 miles then stall and rfuse to start on till it sat for a couple od hours. Had a radiator shop boil out and seal the and it was as good as new. Think alcohol broke the crud loose, after that no amount of filter changes could keep up with it.

 

[ludwig]

Think alcohol broke the crud loose,

What made me think that it was tank contamination is that your fuel filter is empty when warm. No resupply in the line.

 

[pedal2themetal45]

OH MAN I sure didn't want to drop tank. . I did have rust issues when I first put it together so I changed the tank out and had tbe inside sealed but it has been about 10Years.. I well watch for empty filter after I change it and the pump..
thanks Lud and others that replied.
tim