I have a 1960 F100 that wants to get a stumble off idle when hot outside. Truck never runs hot coolant temp wise but when the outside temp is high 80s and above I get a weird stumble off idle like the accelerator pump isn't squirting. 289, weiand intake, holley 600cfm vac sec with Phenolic spacer, stock ignition with an AOD trans. The truck has never died or failed to start it is just a very annoying stumble. If I sit at a stop lightwhen I go to take off I have to feather the throttle to clear it then it takes off like a champ. If I matt it when cruising shortly after a stop light it will fall on its face for a second then go fine. My initial thought was vapor lock. The fuel line under the truck never gets above ambient temperature and the line under the hood from the pump is routed away from the engine and wrapped in DEI heat shield. It is definitely heat related. Maybe a taller spacer? A few holes in the core support? Any ideas?
Sometimes a bowl venting issue can cause a problem like this. But I don't know if Holley carbs were particulary prone to this kind of behavior. Has the carb been rebuilt recently? And of course there's always the mysterious heat related ignition gremlins. . .
The carb came off of a running driving car that had zero issues. I would think if it were ignition it wouldnt clear up after moving for a mile or so after a stop light. It's really a sitting still then taking off issue
I had a similar problem on my 57 T-Bird. It ran well at all speeds but had that annoying off-idle stumble once warmed up. I found the fuel level to be a bit low, easy to set on a Holley.
Mine is only when the outside temp is high. The truck runs perfectly even once warmed up on cool days or at night. Only when the ambient temperature is above 85⁰ or so.
Make sure the lever arm to the accelerator under the front bowl is not only in contact with the throttle link but at least a thread or 2 into it. An ez fix with two 3/8”wrenches.
Would a loose set up on the accelerator pump be exacerbated by heat? When is cool out it run absolutely perfectly.
Phenolic spacers are cheap and good insurance against boiling fuel in the bowls. I run em on everything.
Is it definitely lean when it stumbles? Usually a lean stumble may pop back through the carb whereas a rich stumble will act like it's loading up and stuttering. Since heat makes it worse it may be rich?
No I dont know. This is the direction I am starting to lean tho. When the fuel boils it causes it to load up then the pump shot is just too much fuel causing it to fall on its face till it clears up. I just don't know what to do to help the situation.
It's a tough one to cure. Sometimes a little lower float level helps, and a fuel return line (with .030 orifice)from carb inlet area back to fuel pump inlet side (or to tank is even better) can help. A drop base air cleaner makes it worse,traps heat on the carb.
I drove the truck home from work a few min ago 98⁰ over 100⁰ heat index. Ran it with the AC off and it didn't miss a beat. So now to determine if it is the extra heat from the AC, the lowered idle speed, the extra load on the engine or a combo of those.
Think I would look into rerouting the fuel line between the pump and the carb. I suspect there is a location on that fuel line where the heat is collecting while the vehicle is not in motion. One of those digital thermometers might locate the hot spot on the fuel line. A 5-10 degree warmer spot could be all it takes. The AC would just add a few degrees to the entire under hood temp and probably compounds the problem.
Check the float level! [if it is a carb issue] This engine could be mounted at a slightly different angle to the previous one. But I am going to suggest you have a timing issue! Where is the Vacuum advance connected? [Manifold or Ported?] Most of the cars I've remedied that had an off idle stumble had the vacuum advance connected to manifold vacuum. Move it to ported vacuum and re-time the engine and it improves [circled in red] If you engine idles smoother with manifold vacuum , it indicates there is not enough initial advance. Most SBF's were set at 6° BTDC at idle and 30° BTDC total [12° advance in the dizzy] If you hooked up the vacuum to the manifold you couid get another 8-12° advance at idle [total 14-18° at idle] This is because maximum vacuum could be at idle [AND it will idle smoother] As soon as you touch the gas [under load] the vacuum drops And the timing retards from 14-18° back to 6° causing the stumble. Set the idle timing at 12- 14° at Idle and it will idle smoother [36-38° total] AND connect the vacuum advance to "ported" so you get vacuum advance when cruising
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