1964 SS Impala 409/340 hp jerks bad

[Billmac409]

Ive been trying to figure out why my car jerks and bucks below 3k rpms. I have to push in the clutch to prevent being thrown out of my seat. I thought I had a burnt flywheel but when examined looked new. I had it resurfaced anyway and replaced the clutch and pressure plate. I next went to the ignition system and and checked the timing. The engine runs smooth at idle and thru the rpm range with no load. I replaced the cap, rotor, wires and plugs. Also the coil with pertronix flame thrower. No change. It uses a crane ignition module instead of points. I checked the voltage to it and it was 12.4. There is no ballast resistor. I would replace the module, but above 3k , it runs excellent. No jerks or nothing. I put my timing light on the balancer and see the timing advance as I increase the rpms. Anyone have any ideas what i can check next? Much appreciated.

 

[La Hot Rods]

Does it do it when your coasting or when you still have some throttle in it?

 

[Billmac409]

Both, but not as bad when off the throttle

 

[La Hot Rods]

What carburetor are you running?

 

[Billmac409]

Edlebrock … 1406

 

[La Hot Rods]

Have you tried any different step up springs?

 

[Billmac409]

Edlebrock … 1406

 

[Billmac409]

No, it used to run good, so i didnt think I should change them.

 

[427John]

Have you tried any different step up springs?

It does sound like a lean surge or buck doesn't it? If it ran well before I would be looking for a little trash lodged in where the metering rod enters the jet.Unfortunately to see it you have to remove the top of the carb and that may dislodge it and move it so you won't see it there when you look. I would pull the gas out and clean out the carb with carb cleaner and air and then put it back together an see if it still does it.

 

[62BillT]

I'm half kidding, but what kind of Cam do you have in there?

 

[Billmac409]

Stock cam

 

[32witha409]

Is it better in a higher gear like 3rd or 4th. My car will buck as it bounces back and fourth in the drive train slop in 1st and 2nd at 1500 or lower rpm.. Mostly because of the cam

 

[Billmac409]

I have the stock cam…. Starting out in 1st driving normal, the car will buck you out of the seat. Tires will squeel as the engine surges. You cant even give it more throttle , will be worse. I have to push in the clutch and shift to 4th then it runs ok. It idles at 800 rpm and is smooth as silk. Once in a while you can hear the exhaust pop( not a backfire). With no load on the engine, it runs perfect. No vibration s and fast throttle response. I hand throttled it slowly from 800 to 3200 rpm . No missing, smooth all the way.

 

[32witha409]

I'm wondering if you have a bad engine mount or if the engine is moving in such a way that the throttle linkage is being yanked opening the carburetor.

 

[62bubble]

Hate to tell you this, but sounds like the same problem I experienced with my 62, 283 engine. Took it to a local engine builder and he had to replace two bend push rods. Worked fine after that....If , down the road, you find that this indeed is what is causing your problem, please post and let us know... Hope not.

 

[boxerdog]

I'm kinda with John on thinking it's gotten lean somehow, not sure how that might have happened. In CA, I'd have tried another batch of gas before anything else and then made a big (smaller) jump on the needles to see if it told you anything. And check any hoses connected to the carb.

 

[Billmac409]

I have the stock cam…. Starting out in 1st driving normal, the car will buck you out of the seat. Tires will squeel as the engine surges. You cant even give it more throttle , will be worse. I have to push in the clutch and shift to 4th then it runs ok. It idles at 800 rpm and is smooth as silk. Once in a while you can hear the exhaust pop( not a backfire). With no load on the engine, it runs perfect. No vibration s and fast throttle response. I hand throttled it slowly from 800 to 3200 rpm . No missing, smooth all the way.

 

[Billmac409]

Thks for your all your suggestions on my problem. I will check them out. As far a the gas, i use sunoco 93, add lead additive, and octane boost every other fill up. I tried shell 93 last time and no difference. I forgot to mention, if i throttle up to 2000-2500 rpms and pop the clutch, the car runs perfectly. Only when driving like going to church does it act up, and it will make you hit the dashboard it jerks so bad, if that helps.

 

[Don Jacks]

What is the tach doing while all this is happening? If it mimicks the engine operation,your issue is in the primary ign.circuit somewhere.Start at the bulk head connector and end at the pick up coil wires in thedist.

 

[64ss409]

What is your timing at idle with the vacuum advance disconnected? And what is it when you reconnect?