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I have a 1985 F150, 300 I6, 4 sp manual, that the previous owner deleted all smog from. I installed an Offy C-Series intake, Quick Fuel 450 cfm carb and a DUI distributor. I adjusted air/idle mixture with a vacuum gauge to about 18 inches of mercury, adjusted initial timing to 9° btdc, and I’m using full manifold vacuum for the advance. I have tried both full manifold and ported, the full manifold definitely runs smoother overall. Truck starts beautilly on choke, around 1500 or 1600 rpm, when it warms up I can kick it down but it will only go to about 1000 rpm. Curb idle screw is backed all the way out. I drive and the idle goes back up to around 1500 rpm, and if I kick it down again it will drop. I cannot seem to get that cycle to stop. I know the it’s off the fast idle cam because I checked. But could it need a choke adjustment? I checked for vacuum leaks as well, none to be found. It’s driving me crazy and I’ve been trying everything. Less timing, more timing, ported vacuum, full vacuum. Nothing helps. Would appreciate any input.
Does your carb have a power valve? If the valve’s diaphragm has blown out, it will let extra fuel into the carb throat at idle.
Has the engine ever run well as presently configured? If it had been but then backfired, that can blow out the power valve.
Have you verified the timing marks are accurate? The outer sleeve of the crankshaft pulley can slip and throw the marks way off.
I’ve never been able to get the idle right since installing this config. It runs great, has decent power, just can’t get the idle to rest below 1000 rpm and can’t get it to 1000 without kicking down every time I come to idle. If I change air/idle mixture it’s a dog. It’s very odd to me.
Pull the throttle cable off the carb and see if it will go lower, I bet it will.
Others have had the cable pull on the carb and not let the idlle go down.
Bending the bracket dose the trick.
Dave ----
How about the choke? Maybe it’s not opening fully on a consistent basis.
Two quick ways to check, both with the engine warmed up:
1) Pop off the air cleaner cover and make sure the choke plate is vertical.
2) Rig the external choke linkage in the full open position with a piece of wire or similar. Make sure the throttle linkage is not affected. Go for a test drive.
Pull the throttle cable off the carb and see if it will go lower, I bet it will.
Others have had the cable pull on the carb and not let the idlle go down.
Bending the bracket dose the trick.
Dave ----
I did think it might be the cable and I bent the bracket a little. But I did not check idle without having it connected so I’ll give that a whirl.
How about the choke? Maybe it’s not opening fully on a consistent basis.
Two quick ways to check, both with the engine warmed up:
1) Pop off the air cleaner cover and make sure the choke plate is vertical.
2) Rig the external choke linkage in the full open position with a piece of wire or similar. Make sure the throttle linkage is not affected. Go for a test drive.
I will definitely verify that. That’s a good point.
Pull the throttle cable off the carb and see if it will go lower, I bet it will.
Others have had the cable pull on the carb and not let the idlle go down.
Bending the bracket dose the trick.
Dave ----
Nailed it! Unreal that I overlooked the easy stuff 🤣 Cable was just a smidge too tight. I was going to check it the other day but I was in a hurry and didn’t. Thanks!
Good to hear that was the issue.
I have seen that posted before and why I said it.
Sometimes the bracket can be bent to give it slack and others think had to make a flat plate to move the bracket closer to the carb.
As you found it dose not have to move much.
Now I have not heard if anyone has had issues with full opening of the throttle or if they even checked?
Dave ----
Good to hear that was the issue.
I have seen that posted before and why I said it.
Sometimes the bracket can be bent to give it slack and others think had to make a flat plate to move the bracket closer to the carb.
As you found it dose not have to move much.
Now I have not heard if anyone has had issues with full opening of the throttle or if they even checked?
Dave ----
Thankfully I put a brand new Lokar throttle cable on it and it has a nice easy adjuster. The fact that the dumb thing would kick down had me thinking it had to be something misadjusted. Glad it was simple and she’s money now.
