On my 239 flathead it will idle just fine then all a once the rpms will go up a couple of hundred and then it will idle back down to normal.I adjusted the idle mixture screws and seemed to help a lot,but still seems to do it every once and awhile.Any ideas on what else I should check?Thanks,Russell

 

I know little about flatheads but the symptom you described is commonly caused by a vacuum leak in most engines. That's what I would check for first.

 

What is an easy way to check for vacuum leak?

 

A likely sources would be the base gasket of the carburetor or the intake manifold but it could also be a bad fitting or cracked vacuum hose (like for the wipers). There is something you can spray around the possible sources and if the engine temporarily speeds up that is likely where the leak is. My old memory won't allow me right now to tell you what to spray it with. Someone will chime in here soon though and help me out. It might be starter fluid but I can't remember (I have oldtimers disease).

 

starter fluid/ether.

 

I agree that this is the quickest way to detect the leak. If possible use a straw from WD40 can so you can get a more precise spray in the suspect areas.

Have you placed a vacuum guage on the truck to see if you are within the correct range?

You also indicate that you adjusted the Idle mix screw on the carb and it helped alot. Which way did you adjust leaner or richer?

 

I adjusted in until truck ran rough and vacuum started dropping then I backed out about 1/4 turn on one screw.On the other screw it don't seem to effect the idle and the screw is ran just about all the way in.I don't understand this!

 

The idle adjustments don't vary much from what I've seen. Turn them all the way in - careful to not crush the needles into the seats - then open 1 1/4 to 1 1/2 turns.

An old trick if you have junk in the carb orifices is to remove the air cleaner, rev it to between 1500 and 2K rpm and slap a piece of board on the carb top until it stalls. Tends to flood the passages and will often clear the crud. Course it's not recommended from a safety standpoint, but nothing that works is recommended these days..........

 

My Dad used to occasionally cross up a couple of plug wires to intentionally make a motor backfire through the carb-instant passage cleanout! A little risky, though.

 

If it's running okay with one idle screw all the way in and the other at only 1/4 turn my guess is that it's getting air from some place else (like the base gasket or some other place on the carburetor). Like Tim said the idle mixture screws should be open 1 1/4 to 1 1/2 turns...

 

Just a quick note here:

While checking for vacuum leaks using starting fluid, carb cleaner, wd-40, or other flammable materials be a little careful that you don't have some bad plug wires. A misplaced spark can lead to some exciting times in the garage (I'd rather not go into the details on why I'm bringing this up.....eyebrows do grow back though)

Bobby

 

Now, I don't care who you are...that's funny, right there.

 

I would have to agree with these guys. Although I am not 100% certian with these OE carbs but I am guessing that one screw is air and the other is fuel. You likely have the air screw turned in all the way and fuel screw leaned right out. This would only further confirm that a vacuum leak is what you seak....