idle speed slowly increases

Grumpy

The Grumpy Grease Monkey mechanical engineer.
Staff member
Never experienced this one before. I recently upgraded my fuel system (to great results apart from this intermittent issue) to a 255lph EFI fuel pump and Aeromotive bypass regulator (the one designed to work with a carb and EFI pump). Fuel pressure is rock solid at 6.5 psi. Holley Street HP 750VS carb. Feed and return lines are both 3/8" with no restriction based on the rock solid fuel pressure.
Every once in a while when I'm sitting at a light, the RPMs will increase with no input from me. I've seen as much as 1700rpm or so before throwing it in neutral and blipping the throttle, which cures it. It's only done it a couple times, and totally at random. There doesn't seem to be any surging going down the road. The car runs better than it ever has, as I was clearly having fuel delivery issues before with the mechanical setup. Where I used to get a decent second gear scratch on a manual shift, it will now kick the back end sideways. Definitely putting more power down.
Since you need more fuel to increase RPM, my guess is I'm getting a random leak from the carb, like fuel is getting pushed past the boosters. The only thing I have changed is from a mechanical pump to an electric EFI pump, and all related plumbing. I did have to pull and clean both needle and seat assemblies since the initial prime on the EFI pump was over 20 psi. This was after backing the adjustment screw off what I thought would be enough. WRONG. I found some rubber hose fragments lodged in both assemblies, but they looked and operated fine otherwise. Could a little piece of something be causing this issue?


engine rpms increase as air flow increases
, adding additional fuel tends to cause fouled plugs and rough idle,
changes in ignition advance may make the car hard to start or run with less power,
but unless the engine can consume more air the idle speed won,t vary a great deal.
have you considered a rather simple test... adding an additional throttle return spring, as a temp test to see if adding additional resistance, it provides from the carb opening from the at idle set location prevents this from occurring?
carb and in some cases transmission shift linkage that goes to a carb must be free from binding.
if it does you've almost limited the cause to a binding or badly adjusted linkage ,
yes it might be a P.I.T.A. as the additional throttle resistance might be a problem but as a temp test it can provide you with info and facts
throttlss.jpg
 
Back
Top