idle adjustment, rough idle demons (1 Viewer)

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Joined
Feb 20, 2011
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17
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150
Location
san diego
It was running so smooth after refreshing all of the gaskets and then all of the sudden it went back to not idleing without the choke and dying at stoplights again. Part of the back story is here:

https://forum.ih8mud.com/threads/confused-by-rough-idle.801259/

I spent many hours over the last few days searching mud threads for "rough idle", "idle circuit", "vacuum leaks", "idle adjustment". I know more but still can not figure it out.

timing is on the BB
vacuum off of intake is steady 17"
fuel is in center of float glass

I checked (disconnected and plugged) the big potential vac leaks without any change
the EGR is disabled
all other smog stuff seems to check out.

Now I am finally suspecting the carb and the idle mixture. The carb should be in good shape as I was frustrated a few years back and purchased a brand new one through SOR (maybe 50k on it). I can keep it running at about 950 rpm so I pulled the top of the air cleaner off and stared to play with the idle mixture. In or out did not make a difference, I took note of its original location and now have left it there. When I look into the carb I can see fuel coming out of the 1st main nozzle, I would suspect this is what is keeping it running. What confuses me is when I disconnect the solenoid valve it will kill the motor and I thought this was connected to the idle circuit.

Sorry for the long story, I try to read (search) more and post less but this is starting to get the best of me.
 
It is probably a vacuum leak from something you didn't check. 17 inches isn't that great because you are idling at 950 RPM. Whack the top of the EGR with the handle of a screw driver to see if that will stop it leaking.
 
I did mess with the valve a bit. Connected it to a hand vac pump and cycled it a few times, I could hear it click shut. I also pulled some vacuum with it running and it killed the engine. What I can not be sure of is if or how much is getting by the valve. I agree that the vacuum leak seems to be where I should be looking, I will start through the systems again.
 
Tried a few more tests.

Egr disabled, thumped on with a screwdriver, not stuck but no way to tell if there is some bypass

VCV for evap plugged

PCV line to carb base plugged

Charcoal vent to carb plugged (don't think this one could affect anything)

opened up idle mix screw and lowered idle screw to get it roughly running at about 650 rpm. If I restrict the carb opening even slightly the vacuum goes from 15 to just under 20 and the RPM goes up to 950. The main issue is I can still see fuel dripping out the first main nozzle with the throttle valve just barely cracked. I am getting close to pulling the carb expecting a big crack...

Any guesses? suggestions? thanks.
 
Dripping fuel out of the primary main nozzle at idle is a sign of either a vacuum leak or no fuel at the idle port in the carb (plugged, bad solenoid). You don't need to pull the carb to check for a crack in the bottom of the manifold. Just open the throttle all the way and look in with a flash light. You can open the secondary manually after you open the primary all the way.
 
Pin_Head, thanks for the help. I did look down through the carb but can not see anything unusual. I kept thinking the idle circuit as well so I pulled the needle, blew it out with compressed air (understanding this is just the end of the passage). I know some fuel is getting through because when I close the needle all the way off it will kill the engine. I think I will pull the solenoid and try to check the passage and make sure the clicking sound is actually doing something.
 
If it stalls when you close the idle mix screw port, then it is not a huge leak. If you replaced the manifold gasket check it for leaks by spraying carb cleaner or propane gas around the gasket. You may need to re-torque it. Check all other sealing surfaces for leaks.
 
I was hoping to get to it today but somehow when I started it up this evening it ran way worse... sounds like it's missing under 1500 rpm. Thought I was on to the little stuff. I keep thinking I am dealing with the idle circuit and the air ports below the butterflys but tonight threw me for a curve. Maybe dirt moving around in in the carb? This may now get pushed of to the weekend.
 
Read the post above yours.
 
If I restrict the carb opening even slightly the vacuum goes from 15 to just under 20 and the RPM goes up to 950.

@salvage60 , @Pin_Head , and @tag3 ...I recall reading that doing this (restricting air into the carb opening by covering the throat of the carb with both hands) is technique used to diagnose a vac leak, but I'm having a difficult time (conceptually challenged) understanding the cause and effect.

If I understand correctly, a properly functioning system (no vac leak) will show no change in idle response whether the carb opening is restricted or not.

Then, an improperly function system will either cut out (idle will drop and engine will die) or idle will improve (as observed in OP's symptoms).

Is this correct, or do I have the logic wrong.

Then, whatever the correct logic, what is the physical diagnosis between the cause (vac leak or other problem) and the effect (response in idle under restricted/unrestricted conditions)?
 
I have googled this without much luck. I would assume rpm rise with air restriction means there is air getting in somewhere, time to check the torque of a few things.
 
The reason that choking it makes it idle better when you have a manifold vacuum leak is that it increases the vacuum high up in the carb, which pulls fuel out of the main jet.

It has no effect when there is no vacuum leak because all the air is coming through the carb and the vacuum is high enough below the throttle plate to pull enough fuel from the idle port.
 
I got back to it today. pulled the carb, all checks out. Re torqued the intake / exhaust manifold, all still tight except for the back two. I pulled the insulator and the lower bonded gasket came off in pieces, pretty sure this was the leak. Not wanting to wait for a new one I cut new paper gaskets for the top and bottom and put the thing back together again. I can now feel the idle circuit working, I could keep it running at under 500 rpm. Adjusted the mixture per the lean drop method and all seems well. What is interesting is the vacuum never seemed to get much higher at idle, just a hair under 18" I think I will run it for a week and then check the adjustments again. If I learned anything it is to double check the torque and customize a 12mm wrench to make carefully tightening the carb easier. Thanks much for the help.
 

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