Idle / Timing Question

[scouttster]

I'll be driving along and as long as my foot is on the gas she runs/rides fine.. As soon as I let my foot off the gas, the car jerks or surges in a forward motion - nothing to dangerous or anything, but just irritating. What could this be? Idle issue? Timing? She does idle up and down a bit and I have played with the idle, but have yet to figure it out.. Anyone else experience this? Thankyou

[Neu]

Mine does the same thing, I think. It's just a light jerk when your foot comes off the gas? I think that's common for carbed cars

[scouttster]

I let off the gas in 3rd gear and jerks up and down, like its following the idle of the car.. Not sure what it could be. Thanks for the reply.

[Neu]

Like a hard one? Mine does it, but it isn't usually too hard/bad. It shakes the shifter up a little bit. I always assumed it was because of the drivetrain moving when the carb cuts the gas to the engine.

[sdoan]

Do a search on "driveability" (spelling?). I found some good tips in there.

Not sure if this applies but 2 things I would check, distributor vacuum diaphragms for leaks and that the EGR valve and the EGR modulator are working correctly. My recollection was that bypassing or plugging (I forget) the EGR modulator got rid of the symptom, but that the real problem was a vacuum leak in the dist diaphragm (which was an the same vacuum source).

[takza]

Motor mounts?

[scouttster]

Good idea, Never thought of the motor mounts. I'll have a look at them today.
Thankyou

[Petros]

almost sounds like it could be a vacuum leak in the idle system. check with the car running and spray carb cleaner on all the hoses and components, when engine speed changes you found where the leak is. When the car is returning to idle it gets more air, and has some extra fuel from just being at full throttle.

This also works with propane gas, when the raw gas is sprayed where the leak is the engine will speed up.

Something else to check, there is an electric servo that cuts off the gas supply when the throttle closes to prevent unburnt hydrocarbons in the exhaust when you down shift or between shifts. If this is not working you will be getting a bit more gas when the throttle first returns to closed position. The test is in the FSM, you just unplug it and apply 12v to the right contact and listen for a "click" in the servo.

Good luck.

[takza]

There is also a diaphram that helps to keep the engine from stalling when you let off the throttle...might be out of adjustment?

[scouttster]

I did take the carb and manifold off and cleaned both. I re-used existing gaskets. Not owning a torque wrench, I did the best I could with tightening them down. I have a feeling its a vacuum leak through the manifold or those carb gaskets. Driving the car around the block a few times and the first or second time it doesn't do it, but as the car comes up to temp this jostling, bucking, gets more profound. When the gas is pressed, its no problem, but as I let the gas off - this bucking and jostling and cycling in idle occurs and its much more profound. Does this sound like it could be the symptoms to a vacuum leak? I have ordered all new gaskets and will borrow my friends torque wrench next week. Thankyou for the help.

[Petros]

Yes, it could be vacuum leak(s). Also check all the hoses and vac operated devices for leaks. Easiest way is to use carb cleaner or raw propane gas, spray it around the carb base, all hoses and componets (a little at a time). When the engine speed changes, you found where the leak is located.