The Stovebolt Site Forums The Shop Area Engine Shop Unexpected Engine Rev at Stop in Gear

Hey guys,
I have a ‘57 3200 with a rebuilt 350, topped with Carter AFB Competition Series 9635S, pushing a TH350 tranny. On occasion, it surprises me at a stop sign with an engine rev of probably 1000 rpm. A quick shift into neutral drops it back to normal. Does this sound like a vacuum problem or could it be transmission related?

Hy DES57, welcome, I can honestly say that never in all my years have I heard of such a symptom. I wouldn't put something like this past a confusilator controlled engine, but a carbureted one? You say when you shift the transmission into neutral the engine returns to idle, no load on the engine and the r.p.m. goes down??? Does this occur as you're coming to a stop or after you're actually stopped? I'll be very interested to follow this thread.

It occurs while stopped for 10-20 seconds with my foot firmly placed on the brake and my hand on shifter, ready to throw it in neutral. I have a three mile drive to town at 45 mph so it’s typically warmed up. Just this week I discovered an open vacuum connection so I plugged that but haven’t had a chance to see if that made a difference. I have the door panels off to upholster them myself. This past spring I had a/c, ps, tilt column, parking brake, and exhaust installed. After four years, this is my first year to actually start driving it. Just building a daily driver. Any suggestions appreciated.

It occurs while stopped for 10-20 seconds with my foot firmly placed on the brake and my hand on shifter, ready to throw it in neutral. I have a three mile drive to town at 45 mph so it’s typically warmed up. Just this week I discovered an open vacuum connection so I plugged that but haven’t had a chance to see if that made a difference. I have the door panels off to upholster them myself. This past spring I had a/c, ps, tilt column, parking brake, and exhaust installed. After four years, this is my first year to actually start driving it. Just building a daily driver. Any suggestions appreciated.

power brakes? that could change the vacuum when you come to a stop, which could change idle a bit but I would not think that much. It may be that he vacuum leak you fixed will solve your problem. I would take it for another test drive and see!

Update: Ran some errands today and it still revved up at the first traffic signal. My fixed vacuum leak didn’t change anything. I’m wondering if something from the transmission is affecting it. PO rebuilt engine and transmission. Also, no power brakes.

Is there a vacuum line going to the transmission?

Check your motor mounts. The engine could be shifting around enough at a stop to pull on the throttle linkage.
Jerry

There is a vacuum hose from Carter 9635s carb to the TH350 transmission. Motor mounts appear tight and not cracked. Took another drive today and it did not lunge/surge. A little cooler out at only about 35 F. When I get time I’ll see if it will stay running with the warm idol dropped a tad. No tach, but I’m guessing it’s about 900 now.

Check to see if the diaphragm to which the vacuum line is attached is not ruptured. You can use a Mitey-vac or your mouth on the hose. It should hold vacuum until next Tuesday if its good.

I would consider what HRL suggested regarding motor mounts. Not that they are loose but where they are located. I have seen some conversions that are using the "bird leg" front mounts and a rear mount for the TH350 transmission. With this configuration there is an extreme distance between the mounts and can cause the engine to torque over to one side which may cause the throttle linkage to bind up.
If you are using the "center" mount position on the engine with the rear mount it is likely OK and would look for another cause.
Fred

Hey Guys,
I’m sorry I haven’t posted lately. Haven’t driven the ’57 for three weeks. I have not tested the vacuum yet either, as Carl suggested. Surprisingly, my 26 year old grandson threw out the idea of the torque converter between engine and transmission. He’s way more mechanical than I am. I’ll have to research that. You gents have any thoughts about the torque converter? I’m not even sure if the old truck has one.

Any chance your brake linkage is rubbing on accelerator linkeage somewhere?

Ed

Sorry it’s taken me so long to post a resolution to this problem. As colder weather has begun here in Illinois, and I found need to drive my 3200 numerous times, I was experiencing idle kick down problems and began the hunt for the cause. As I started with idle mixture screws and subsequent movement of the throttle linkage by hand, I noticed the linkage hanging up a bit. I initially thought the gas pedal rod through the firewall was binding a bit, but further investigation revealed the culprit. The bottom of the accelerator rod lever was touching the firewall. I adjusted the accelerator rod length at the carburetor and subsequently reduced engine idle and all problems solved. Thanks for you interest, help, and patience!