'71 Dieseling- need higher octane?
Team Owner
Joined: Jan 2007
Posts: 53,648
Likes: 878
From: Southern Cal Ca
St. Jude Donor '22
Use manifold vacuum and adjust mixture for highest reading.
lowering rpms as you do it.
Look close at the throttle plates.
too much gap will uncover the main orifices.
screw the secondary adjustment open and close the primary plate gap and still get the right idle.
14 sounds like a decent advance.
might do the 50 rpm lean drop and see if it helps with run on.
might do a 50 rpm drop on the rich side too.
how are the plugs looking?
I guess it could be high compression and rich mixture causing the run on
or carbon hot spots.
Could just be the nature of your beast and an idle solenoid would help.
Sorry just arm chairing here.
Race Director
Joined: May 2007
Posts: 11,334
Likes: 136
From: Out of Site...Out of Mind. Corvette: anything else is just transportation.
St. Jude Donor '09 thru '20
Had the same problem, along with other stuff, out of the Q-jet on my '75. Now that I have a BG Demon on, problems solved.
Just my opinion but I think it has a lot to do with the Ethanol additive that's in the gas.
Just my opinion but I think it has a lot to do with the Ethanol additive that's in the gas.
Melting Slicks
Joined: Mar 2005
Posts: 2,838
Likes: 10
From: Austin TX
As one or two pointed out above, dieseling is almost always caused by the primary throttle blades too open at idle. The TB's are open enough to uncover the transition slots which means fuel is still being sucked into the engine after shut down.
Are you running ported (as GM usually did) or manifold vacuum to the distributor's vacuum advance? If ported, switch to manifold, which will raise your idle speed, then you can adjust (lower) your curb idle at the carb which will close the throttle blades and cover the transition slots.
There's other good reasons you should be running manifold vacuum besides this.
If you're already running manifold vacuum, you'll need to crack the secondary throttle blades at idle which again, will raise your idle speed then you can lower the curb idle adjustment which closes the primary throttle blades and covers the transition slots.
Good luck!
Are you running ported (as GM usually did) or manifold vacuum to the distributor's vacuum advance? If ported, switch to manifold, which will raise your idle speed, then you can adjust (lower) your curb idle at the carb which will close the throttle blades and cover the transition slots.
There's other good reasons you should be running manifold vacuum besides this.
If you're already running manifold vacuum, you'll need to crack the secondary throttle blades at idle which again, will raise your idle speed then you can lower the curb idle adjustment which closes the primary throttle blades and covers the transition slots.
Good luck!
Last edited by 73, Dark Blue 454; Jul 14, 2009 at 12:09 PM.
