Motor idle "surging?"

28Annie

28Annie

Member
Wondering if anyone else has this happening: Car starts & runs great, idles down after warming up just fine. While out driving & waiting at a traffic light the engine idle slowly rises up even after retarding the spark & throttle levers all the way up. If I hit the accelerator it will idle back down for a few seconds but then starts to creep up again. Is this a float issue?
 
2 things I can think of. Easiest would be that the idle control screw has vibrated enough that it's in just a wee bit too much. Enough that as things settle down and the carb levels off to idle it's too high. The other thing could be a minor vacuum leak. When it happens, turn the fuel control knob at the end of the choke out 1/4 turn or less. If it levels off to what you need, look for a leak in the wiper system (if you have a vac wiper) or the carb/manifold gaskets.
 
See if you can replicate the behavior with the car sitting in the driveway with the hood open. I'm thinking the problem is with the linkage relaxing to its preferred setting--maybe sticky throttle assembly or weak spring. If this is what's going on, you should be able to watch the throttle position shift by close observation.

Steve
 
On the possibility of a crack, they tend to show themselves more once things have heated up to running temps... any of the previously mentioned are distinct possibilities! Steve S, That's a good one, I hadn’t thought of that one!
 
With the rod unhooked from the carb,make sure that you have to pull it at least a 1/4" forward to hook on carb,& also make sure you get full travel on throttle plate.
 
Vacuum leaks tend to make the idle go up to a high level then drop off sometimes to letting the engine die. If the engine doesn't die the idle will again go up to high then drop off to very low. Sounds like the spring is not holding the throttle down or the linkage is out of adjustment or both.
 
James Rogers said:
Vacuum leaks tend to make the idle go up to a high level then drop off sometimes to letting the engine die. If the engine doesn't die the idle will again go up to high then drop off to very low. Sounds like the spring is not holding the throttle down or the linkage is out of adjustment or both.
Click to expand...

Jim

I visited you website for the first time today. I like your engine storage carts. You offer some neat stuff.

I am still betting on the sticky needle valve or float. The fuel bowl gets full and the engine speeds up. the fuel bowl gets low and the engine slows down. Most vacuum leaks are steady and the engine runs lean. I can usually hear a vacuum leak unless it is on the connection from the manifold to the block. Poor gas mileage is another indication of a vacuum leak but I have never checked mpg on the Model "A".

I hope he tells us what he finds.
 
The weekend got away from me with family obligations so none of the (excellent) suggestions have been tested out just yet. I will definitely report back on what the solution ends up being. Thanks again for the help!
 
If the gav is opened too much, it will cause the engine to lope at idle, almost like a full race cam. There will usually be black smoke from the exhaust because the engine is running too rich.
 
You must log in or register to reply here.
Back
Top