Fast idle when warm
 

Purchased a 31 roadster few months back and after replacing rotor/cap/points/condenser/wires/plugs I have it starting easily and running smoothly. However, after it is warmed up, about 50% of the time at a stop sign it idles fast. I can slow it down just a little bit by retarding the spark. I have removed the idle needle and cleaned it. Any suggestions?

Re: Fast idle when warm
 

Oil all the places of movement on the accelerator linkage, especially the top two oil pathways. They may be gummed up and not allowing the linkage to return the carb's accelerator arm to "home". Perhaps the heat from the warmed up engine makes this worse. Oiling and working the linkage arm back and forth should help.
Also check the manual hand throttle arm for binding near the starter. When they gum up, they can affect the free movement of the main arm.
Marshall

Re: Fast idle when warm
 

You mentioned that you cleaned the idle needle? I think you might mean the idle air adjustment needle. That needle screw adjusts the air / fuel adjustment at idle ( rich/ lean) and does not adjust the actual idle stop. The idle stop screw is shown in the picture
There are many things that can intermittently cause the idle to be slightly raised.
As Marshall mentioned tight linkage pivots or a misadjusted manual hand lever can cause it.
A weak or broken return spring on the linkage
I also had a worn throttle shaft cause int higher idle..
Disconnect the throttle linkage from the carburetor and feell the strength of the return spring it should have a strong return tension as well as for free linkage movement.
If all this checks out when the idle sticks again try manually pushing the throttle against the idle stop screw. If it goes lower then replace the throttle shaft with an oversize one.
I hope all this makes sense

Re: Fast idle when warm
 

Thank you for the helipad- it hadn't occurred to me to look beyond the carburetor but that makes sense. Will proceed along that path.

Re: Fast idle when warm
 

Also when you remove the link from the carb to check for free movement of the linkage, be sure to check for a bent arm on the right side of the linkage. This is the long thin forged arm on the other end of the carb link. Mine was bent and holding the carb throttle slightly off the idle screw. I bent it a half inch forward and all was well. It can easily be bent cold.

Also, the two uprights from the linkage mounting bolts should curve forward to clear the firewall. I've seen them bent and cause binding because the two holes aren't in a straight line anymore. This part is also forged and can be bent cold.

Re: Fast idle when warm
 

Also you may take a close look at the throttle linkage rods for ware. I always put a dab of grease inside the two ends of the rods to help control ware in the ends. It helps for smooth operation.

Re: Fast idle when warm
 

Dad had this trouble a while back, this solution worked like a charm.

Re: Fast idle when warm
 

Tom W. brought up another good point. While you're checking the long accelerator arm for free travel and angle, have someone sit in the driver's seat and push the accelerator to the floor while you watch the action of the carb's throttle arm. Because carbs and linkages have been changed/altered over the years, often times the long arm has been bent incorrectly for the carb du jours that is now on the engine. What happens is full throttle cannot be attained by flooring the linkage by foot. The carb's butterfly arm does not bottom out against the stop peg, as it should = 3/4 throttle. I see this quite often when people complain that their Model A is slow, has a low top speed or doesn't seem to have the power it should. It can often be traced to this simple adjustment in order to restore the full travel arc of the accelerator linkage and carb butterfly shaft. This condition usually occurs when Tillotsons and Zeniths are swapped back and forth. The distance between the ball on the carb's throttle arm and the accelerator's long arm ball is different, unless someone has bent one or the other.
Anyway, to fix, remove the throttle linkage connector rod at the carb (with the spring-loaded bulbous ends) and allow the long accelerator linkage arm to return to its "neutral" position. Then pull the connector rod until it meets the carb's throttle arm. If it must be pulled a long way to meet, you've already run out of travel. Take two vise grips and clamp one about half way up the accelerator linkage's long arm to hold it while you use the other clamp to bend the lower portion of the arm towards the floorboards (rearwards). Start with a minor bend and check the connection and butterfly shaft travel. If full throttle still hasn't been attained, bend the long arm some more. You just don't want to bend it too much to start. Little increments is better. Once full throttle has been achieved, check that the idle speed is still correct. You'll probably have to tweak the idle speed adjustment screw on the butterfly throttle arm a little because of the new angle of the accelerator linkage's long arm.
Marshall