Sticking throttle when hot

[Voyager]

Question about an occasional sticking throttle in my Superformance SPO 2920 and 2010 Keith Craft street crate 408.

The car is 13 years old and has ~ 5500 miles.

When driven is hot conditions (>90F)… After a couple red lights, the radiator fans come on at an indicated 100C and then sometimes the throttle begins to stick. RPM fails to drop between shifts and slowly creeps up at slow speeds.

I have to push in the clutch and tap the throttle to get it back to normal.

Everything seems to work perfectly while cold in the garage. Only when it’s very hot.

Thoughts?

[Tommy]

I'd start by going to a stronger throttle return spring to see if that helped.

[Randall Thomas]

Most likely the nylon bushing in the cross shaft pivot is galling/sticking and needs to be disassembled, cleaned, the shaft polished and the bushing reamed and reassembled using Poly Eaze which is a sticky lube that will prevent future moisture and debris from re-entering.

[jhv48]

Here’s a way to test it.
When everything is good and hot, park the car. With the engine running, open the hood and slowly add throttle. Then slowly let it off. If the rpm’s stay high, close the carburetor throttle manually to see if it is hanging up and not returning to the stop. If it’s not closing the throttle completely, then try a stiffer return spring. You can take the air cleaner off and look at the throttle blades to see if they are closing all the way.
If everything works as it’s supposed to, then you might have a hanging distributor weight that won’t allow the distributor to return to idle setting. Look for corrosion in the distributor.

[Voyager]

Thanks for the ideas. I'll try them out.

[patrickt]

This is a perfect opportunity to add a second throttle return spring, which you should have had all along....

[Fenmyrch Fie]

I had to disassemble and clean it due to a similar issue

[Voyager]

Originally Posted by patrickt

This is a perfect opportunity to add a second throttle return spring, which you should have had all along....

Added second spring about 10 years ago. Maybe I need a stiffer spring. I'll see what I have available.

Also cleaned linkage once or twice. Will do it again. Only 5500 miles on car.

Still wonder why it only does it when it's very hot outside (>90 degrees in traffic).

[MaSnaka]

Had a similar problem. Not so much from heat or a hot day. Come to a stop light or stop sign and the RPMs would hang up and a soft tap on the throttle would return to normal idle. I thought the advance timing in the dist. were sticking so cleaned and lubed with wd4o, didn't solve the issue. Found the lower throttle plate of the carb was worn at the shafts that control the butterflies. A little bit of play was allowing air to enter below the butterflies or maybe not allowing the carb to settle all the way back to idle. So I replaced the throttle plate and problem is solved. The carb has some miles maybe 30K and 15 years old. 2 springs. I hope this helps. Good luck.

John

[MaSnaka]

Had a similar problem. Not so much from heat or a hot day. Come to a stop light or stop sign and the RPMs would hang up and a soft tap on the throttle would return to normal idle. I thought the advance timing in the dist. were sticking so cleaned and lubed with wd4o, didn't solve the issue. Found the lower throttle plate of the carb was worn at the shafts that control the butterflies. A little bit of play was allowing air to enter below the butterflies or maybe not allowing the carb to settle all the way back to idle. So I replaced the throttle plate and problem is solved. The carb has some miles maybe 30K and 15 years old. 2 springs. I hope this helps. Good luck.

John