What would cause idle to settle slowly

[DJCarbine]

Car just got GT40 heads and a performer RPM cam, otherwise mostly stock 66 289.

The problem I have is that the idle seems to settle VERY slowly. It easily revs to 2-3k in neutral and sounds great, but the idle goes back down very slowly to 1100 where I set it. It just seems to settle a lot slower than with the stock heads and cam.

Is this normal with a larger cam? I was thinking vacuum leak at first but couldn't find a source of leak

 

[brianj5600]

It does sound like a lean condition.

 

[enferno]

what carb are you running? maybe something changed in the carb when you did the swap? did it sit for long?

 

[dennis112]

Vacuum leak, poor adjusted idle circuit, or a mechanical bind of the throttle linkage can all cause a slow return to idle.

Spray a little carb cleaner along the intake manifold-to-head gaskets and the carb base plate.

 

[66 Tiger]

I jsut had a similar problem after my head, intake, cam, & carb swap & in my hastiness to get everything up and going I overlooked re-installing the throttle-return spring.

After checking EVERTHING else, and I mean EVERYTHING ( timing, choke fast idle, idle mixture, vaccum leaks, etc ) - I discovered I didn't install the spring & upon re-installing it, the problem for the most part disapeared... ( I still have some fine tuning left - maybe a different size power-valve as per Holley tech recomended with the cam that I have )

 

[DJCarbine]

I have timing adjusted, I removed the choke, the throttle return springs arnt the issue because I am manually holding the linkage and flicking it back and forth.

I asked a friend who has a few cars all with cams in them, he said he has the same experience, so I am thinking maybe its partly due to the larger cam in there. I'll mess around in the garage some more today and see if I can't figure it out

 

[302 coupe]

why do you have it idling so high? That might be some of your problem. With that cam, it should be fine at 700 or 800 rpm.

 

[DJCarbine]

why do you have it idling so high? That might be some of your problem. With that cam, it should be fine at 700 or 800 rpm.

When I put it to ~700 it will slowly start to die untill I rev it again, and it will again idle at ~700 and slowly die in the next 10 seconds

 

[enferno]

When I put it to ~700 it will slowly start to die untill I rev it again, and it will again idle at ~700 and slowly die in the next 10 seconds

sounds like vacuum leak. start the car up, grab some starting fluid (ether) and spray it around the base of the carb. if you get an increase in rpm, that's where your vacuum leak is, if it's significant enough. make sure your gaskets/spacers are all ligned up propperly for your carb.


again, what kind of carb do you have? sometimes if you don't have the pcv/brake booster ports in use or plugged, you can get a leak.

 

[302 coupe]

sounds like a problem with the carb tune. What type of carb are you running?

 

[fasttback]

When I put it to ~700 it will slowly start to die untill I rev it again, and it will again idle at ~700 and slowly die in the next 10 seconds

Yea I agree with 302coupe, with that cam the idle does seem high. Sounds more like idle circuit problem and/or vacuum leak. Have you adjusted the idle screws, throttle plates, timing, and checked for possible vacuum leaks? Are you by-passing the idle circuit by opening the plates too much at 1100 rpm?

 

[DJCarbine]

Just took the carb off and re-torqued the intake manifold bolts. Supposed to be ~25 ft/lb, they were a lot more loose because the gasket compressed. I also tightened up connections on the carb, and adjusted the float levels. In the process of putting the carb back on and hooking it up I flooded it, and on the first crank a small carb fire lit up. I ran out and blew it out, I have a screwdriver in the throttle plate to air it out, I'll go start it again in a few minutes and see if that fixed it.

 

[351carlo]

Well why did it backfire through the carb?

As a note for the future. When the carb lights up like that, just start the car again and it'll go away. No need to blow it out.

 

[DJCarbine]

It didn't backfire through the carb, it just kinda... popped very quietly. The intake was saturated with gas, and the carb was flooded pretty good. The flame in the carb was barely an inch, and the spark timing is dead on so I know its not off as far as being advanced or retarded too much. I hope the flame was because it was just really really flooded

 

[iskwezm]

Mine used to idle about 8-900 with that cam

 

[DJCarbine]

I pulled the intake manifold and put on new gaskets and such, so I know its not a vacuum leak. It is a little better, I have it idling around 950, timing is set @ 16* BTDC, and its got a nice lope to it. I think its alright, keep in mind this is with the stock manifolds and a performer 289 intake. May have something to do with it.

 

[enferno]

I pulled the intake manifold and put on new gaskets and such, so I know its not a vacuum leak. It is a little better, I have it idling around 950, timing is set @ 16* BTDC, and its got a nice lope to it. I think its alright, keep in mind this is with the stock manifolds and a performer 289 intake. May have something to do with it.

even with that, you should have better throttle return than that.

i'd still double check for vacuum . .

 

[DJCarbine]

I'll get a small video uploaded and linked tomorrow, it still doesn't seem to want to idle below 1k, nor does it want timing any more retarded than 15-20 degrees BTDC. I really have to crank the idle screw to get it to idle, and I think thats past the idle circuit.

 

[DJCarbine]

Throttle shaft vacuum leak. Getting a new carb, already shot the old one with a 3 inch slug from my mossberg M500.