OC-3 Carburetor question

Jason Epperson · February 02, 2006, 01:08:08 AM

I have a problem with the zenith carb on my OC-3 with the IXB-3 Hercules engine. It runs good at fast idle but it will not idle smoothly at slow idle. I have to have the idle adjusting screw all the way turned in to get it to even run at all at slow idle. I have taken this carb completely apart several times and soaked it in new carbutetor cleaner for almost a week. I can get air and canned carb cleaner through all the passages. I don't know what to do next. The throttle shaft does have some slop in it. Would this cause this engine not to idle? Any ideas?

walter hudson · February 02, 2006, 03:14:50 AM

does the engine black smoke when it is running ? does the idle mixture screw have any effect on the idle ?is your fuel supply clean ? does the mag have a good blue spark ? have you checked the compression on the engine ?low compression and weak valve train can cause it not to idle.
walter

Jason Epperson · February 02, 2006, 04:01:10 PM

Quote from: "walter hudson"does the engine black smoke when it is running ? does the idle mixture screw have any effect on the idle ?is your fuel supply clean ? does the mag have a good blue spark ? have you checked the compression on the engine ?low compression and weak valve train can cause it not to idle.

The engine does not black smoke at all. If I let the engine idle slow and turn the idle screw off of its seat it will start to run rough and die. I just cleaned the fuel tank out and put sealer in it along with a new sediment bowl and in line filter so the fuel should be clean. The mag will jump a 1/2 inch gap so it should be good. I ran a compression check on it a couple of years ago and I believe they were all between 90-100 psi.

Blake Malkamaki · February 02, 2006, 05:36:38 PM

Hi Jason... make sure you don't have any manifold leaks. How is your float level?

Blake

walter hudson · February 03, 2006, 01:18:22 AM

have you checked the spark plugs ? if too lean is your problem they should be clean and white. if you do not have the air breather connected your mixture will be to lean . if the choke is left partially shut ,do you get a smoth idle ? check for manifold leaks by spraying WD40 around the intake and see if the engine speeds up. try advancing the timing a couple of degrees and see if that smooths the idle .
good luck
walter

Jason Epperson · February 03, 2006, 03:44:37 PM

Quote from: "walter hudson"have you checked the spark plugs ? if too lean is your problem they should be clean and white. if you do not have the air breather connected your mixture will be to lean . if the choke is left partially shut ,do you get a smoth idle ? check for manifold leaks by spraying WD40 around the intake and see if the engine speeds up. try advancing the timing a couple of degrees and see if that smooths the idle .
good luck
I haven't taken the plugs out to check them but I will. The air cleaner is connected and makes no difference if it is off. I sprayed carb cleaner around all of the manifold mounting points and the only time it made a difference was when I sprayed where the throttle shaft is so I guess that might be the problem area. Running with the choke partly closed makes it run rougher.

RAB · February 03, 2006, 09:36:17 PM

First thing to consider: is the idle mixture screw an air or fuel adjuster. That way, you know whether mixture is richening or weakening as you back the screw out.
All other things being correct, I would look at the float valve (may not be seating completely, may be closing too early/too late) if your carburation is too rich, lower valve with a washer (if appropriate) to avoid bending the float parts. My feeling is that the throttle plate would have to be quite sloppy to affect the engine too much but you could grease it heavily ( high temp water pump grease is good) to ascertain if it isolates the problem temporarily.
Checking plugs for colour is useful when the engine has been at operating temperature and under load for a period. A cold engine at idle will tell you little and it would need to idle for some time to change the colour from normal running.
One of those old mixture plugs (Gunson?) were really useful in days before the exhaust analysers took over. You could try it in different cylinders to find if mixture varied (a leak somewhere) and watch to see what happened as adjustments were made. Don't know if they are available any longer though.
Does the mag produce it's half inch spark at slow speed or was it checked at higher rpm. Can make a difference.
Regards, RAB

walter hudson · February 04, 2006, 02:20:26 AM

the throttle shaft being a little loose should not be a problem ,if your mixture is too lean at idle closing the choke should have given a better idle.your problem may be somewhere else . be sure none of your plugs are fouled .when starting and stopping a engine alot without working the crawler ,they foul easily.if you are confident the carb is corrrect ,check the timing . a compression test will not tell the condition of an engine . a cylinder leak down test will give more useful info. the engine must hold the comp.', without buying' another tool ,take the hose for your comp. guage and put compressed air in the cylinders with the valves shut and see where' the air leaks out .if you' get much leakage by the valves this' could cause your rough idle .
walter hudson

Jason Epperson · February 04, 2006, 08:26:12 AM

I had the crawler runnning again this evening and I disconnected the air cleaner at the carb. I noticed that instead of a constant suction sound that I heard a fluttering sound coming from the intake part of the carb where the air breather hose would attach. I would guess that maybe the intake valves might be leaking by and causing this. The crawler runs good except it just won't idle the way I think it should. Maybe I am being too picky. I have spent alot of time and money on it and want it to be right.

John Schwiebert · February 04, 2006, 10:07:29 AM

Walter has given you good advice. A vacuum gauge is also a good tool to use and will help tell the condition of the engine or if adjustment will take care of it. J.

walter hudson · February 05, 2006, 12:28:48 AM

it has been my experience that these engines will not idle smoothly at a really low rpm unless everything is in good condition.the valvetrain is very important .the most common problem is the exhaust valves and valve seats , due to rain getting in the manifold and the effects of unleaded gas since the valve seat area is not hardened . i know that getting everything perfect can empty your wallet guickly.
good luck
walter

Jason Epperson · February 05, 2006, 01:36:10 AM

I didn't get a chance to work on the old girl today. It only got into the low 30's today and I don't have any heat in the shop. I think I have an adaptor somewhere that I can screw in the spark plug hole and connect it to an air hose so that I can do a leak down check like you suggested. I appreciate all the ideas that you guys have given me. Hopefully it will warm up a little so I can get back to it and my other project (clutch and TA in a 1086 IH).