View Full Version : Slow throttle return
MillerCobra
03-23-2020, 04:27 PM
Hi everyone,
Have a problem that is getting worse. F5 Cobra, completed in 2015, 427, Quick Fuel SS-750 and having a very slow throttle return, sometimes getting stuck. Bump the pedal and it re-sets, or I have to put my foot under the pedal and pull the throttle closed. All unacceptable. I have two video's showing the problem. Linkage is off on both videos. I have sprayed carb cleaner at all the moving parts, but this keeps coming back. Any thoughts?
https://youtu.be/yREUqRMukNY
MillerCobra
03-23-2020, 04:28 PM
Second video, engine off, crisp return.
https://www.youtube.com/watch?v=P5yatYtIg1I
i.e.427
03-23-2020, 04:50 PM
I suspect the high idle adjustment. Try this.
https://www.youtube.com/watch?v=4BTiXcMHdXM
RoadRacer
03-23-2020, 04:59 PM
That's fascinating - looks totally different if engine is running or not. FWIW, my 650dp was a little sticky until I put the double spring on and it instantly overcame the issue. I use one of those inner/outer double springs (https://quadrajetparts.com/return-springs-dual-p-2185.html) (not this exact one, but looks like this). But you have two springs.
Can't wait to hear from the experts what this is!
skidd
03-23-2020, 05:24 PM
Sticky Throttle Cable? If you have found that pulling up on the gas-pedal also resets the stuck throttle, my gut leads to the throttle cable, or perhaps even the pedal pivot.
When I first installed my cab, I tried doing the big giant S curve to the linkage. I hated it, despite having the extra long throttle cable. My throttle would never return as smooth as I wanted, without having to put on a ultra heavy spring.
I mounted my carb backwards, and routed the cable in a nice clean gentle "U" shape (linkage is now on the passenger side). Night and day difference.
MillerCobra
03-23-2020, 06:06 PM
Checked, removed the linkage and still the problem. Thanks for the help, good points.
Maybe the secondaries are hanging just a tad, keeping the primaries from closing all the way. I've seen the vacuum secondaries cause problems like this. Can't tell from the video. Try holding the secondaries tightly closed and see if it has any affect.
Railroad
03-23-2020, 06:25 PM
What's on the other end of the primary throttle shaft?
Is this a hot and cold engine issue.
Maybe base plate is torqued down, where heat creates a bind.
Loosen the 4 corner bolts and retighten evenly.
Any chance the vacuum is pulling a ragged gasket into the throttle bore?
Railroad
03-23-2020, 06:26 PM
Maybe the secondaries are hanging just a tad, keeping the primaries from closing all the way. I've seen the vacuum secondaries cause problems like this. Can't tell from the video. Try holding the secondaries tightly closed and see if it has any affect.
Secondary throttle shaft is not moving, in #1 video. Linkage is moving freely also.
MillerCobra
03-24-2020, 03:50 PM
Followed your advise, removed the carb to see if a gasket was rubbing, it was not. Tightened it down very carefully according to the installation manual and a torque wrench. Seems to be working now, engine ran great. I will take out again tomorrow. Mystery on why under power the sticking and hot engine off no sticking. Called Holly and tech said maybe a vacuum leak somewhere, gasket maybe. Maybe I just had it torqued incorrectly.
Railroad
03-24-2020, 05:19 PM
Good deal! I would look at the fast idle screw on the auto choke if you have one.
I have seen them set real close to the fast idle cam and drag on it, even after engine warms up.
Simple fix, back out about 1/4 turn.
Avalanche325
03-25-2020, 03:15 PM
I had the exact same problem. Sometimes mine would stick at 1200 - 1800 rpms. I checked everything with the carb and cable.
The culprit was........the distributor! The advance weights were not returning all the way. Take the "football" apart and clean it. That helped, but didn't quite fix it. What did fix it completely was putting one heavier spring on.