Help! F4 wont idle

I have tried everything to get my 55 F-head to idle properly. It swells up and down in a rhythmic pattern, going from near dying to fairly swiftly. I have eliminated fuel delivery issues, have rebuilt the carb 2 times, have cleaned and checked all vaccuum items, timing, ........ everything. Compression is 120 across the board. I have now found that the only way to get it to idle is to place my hand over the carb air intake and restrict it almost 90%. This seems very odd to me. It seems to be taking in too much air. How do I rectify this problem short of stuffing a sock in it? LOL. The only thing I have been wondering about is the carb base gasket. There are 2 types, on with notches for the little bottom hole in the carb and one that has no notches. Can this make that much of a difference. Help please! It is getting old! THX, CJ.

 

Hey,
I had a problem with my solex carb. It was sucking air past the throttle plate shaft. Took me a long time to figure it out
Gary

 

Try spraying some carb cleaner around the base and gasket areas. If your engine changes than you have a leak in the area you are spraying.

 

I'd try replacing the carburetor with a Known Good unit.

I have seen a similar problem with a rebuilt YF that had some of the wrong parts put in it.

 

I have a similar problem but not as bad so I am interested in what you find out on this. In my case I have a 66 F head with YF carb and I find it is very sensitive to the fuel mixture screw adjustment. I have to ajdust it hot to get it to idle properly otherwise the idle is rough and ramps up and down.

Following up on the other comments, you could try squirting heavy oil around the outside of the gaskets, carb base and throttle rods, if its sucking air the oil will block it off temporarily and you should note a change in engine speed. .

 

The brits call this behavior "hunting." Can have some weird causes, but I'm assuming it runs well other than idle??? No squirrels in the muffler?? (I can still smell the roasted acorns...)

The two bolt flange that mounts the YF/YS carb to the manifold is often over-tightened. That bends the ears down, and warps the sealing surface allowing vacuum leaks. The more it leaks, the more they tighten it... and so on...

Another symptom of a vacuum leak is stumbling on sudden acceleration from idle.

Check the carb base for flatness. It can be flattened by rubbing on some wet-or-dry abrasive paper placed on a true surface.

Good luck,

Pete

 

I have checked for vaccuum leaks, in every way known to man, like 100 times. Nothing. My base is fairly new and I did sand it flat to ensure flatness. The air fuel mixture screw does nothing ...... Ever! At least not with this problem currently existing. It does run fine other than on idle. It does not stumble at all. Any other thoughts? :? Maybe I can just hire someone to ride around under the hood and keep their hand over the carb! I will try a known good carb when I find one. CJ

 

Maybe a tiny piece of hidden crud somewhere in the passageways of the idle circuit in the carb? Or a damaged screw or seat? Maybe take the screw out and spray some carb cleaner through then blow it out with air. How does gas get into the idle circuit? Might be clogged in that area?

You've probably already tried all this, just grasping here. I haven't worked on a YF yet.....

 

Replace the carb with a known good unit. Borrow a carb if need be just to test it.

YF carbs are basically good units, it is just the maintenance done to / on them by prior owners that gives the problems.

A known good unit will go a long way to defining what the problem is.

 

Thanks for all your help. I guess I will have to find a temporary donor. CJ

 

That is the gasket I have on that doesnt work now. I find it hard to believe that it is a gasket issue. An old jeep mechanic mentioned that those carbs can have internal vaccuum leaks. I like the idea of donor carb at this point. CJ

 

Is the thottle plate shaft bore wore? i have seen this missed alot. it creates a varible vac. leak, resulting in "hunting". just a thought to check.

 

Nothing is worn out, unfortunately.

 

Alright, I am gonna take the village idiot award. I hope this experience can help someone elso out in the future. While rebuilding my carb 2 times, I realized that I had taken out the low speed idle jet (brass) and set it aside while I cleaned the rest of the carb to no end. Well......... I never bothered to clean the jet out! Pulled it out, soaked it for 2 minutes, poked a small wire (soft) through the hole, sprayed it all clean and put it back in. Idles like a champ! I recommend this to anyone having idle problems. You can get to it without taking the carb off the Jeep, just take off the top part and unscrew the jet. Be very careful not to damage the bore of the little guy. Geez I can't believe I am so stupid some times!! CJ

 

Not to worry, just your turn is all. Some good info for the next guy having that trouble....

 

We've all been there, done that, don't sweat it.