the engine in this oc3 was overhauled with new pistons, rods, and bearings, new carb, magneto, plugs and wires. the engine starts and runs like new for about 30 mins. under load. it then loses power goes pap pap(not a backfire)and dies. it will not restart for a period 15 to 20 mins. even with a 12 volt battery spinning the engine. after cooling, the engine will again start and run like new for another 20 or 30 mins. then again dies. i am using 92 oct. fuel, with no ethenol. the new carb. has been replaced with an orginal rebuilt from zimmerman. the condition remains the same. timing of the engine is exact and the spark is strong during the no start period. Plugs appear to burn on the black side. fuel feed is by gravity through a relatively staight tube. vent on fuel tank is open. over choking causes overflow from carb. exhaust is relatively clean even if over choked under load. our local mechanics prefer to talk about other things. the little piece of junk is otherwise in mint condition but is pushing for an early burial unless maybe you could help. is a vaporlock possible within the carb? i could use any help you may have. thank you' r.walters e-mail
roberly@toast.net
Check the fuel lines again...I have had a couple of instances where small bits of something or other have gradually closed off supply...when I thought it was a million other things...like a wheat berry in the tank closing off the exit, or a small piece of rubber in the fuel line.
How about the air cleaner..unlikely, but rodents can get into them?
Does it still die if the rad cap is off? Just wondering, in case you have a block or head crack?
These things are so frustrating...good luck.
Have you tried pulling a plug right after it dies to see if they are wet?
There is a condenser in the mag that could cause your type of problem. It is the same as used with points and a coil. I would try changing that first.
I used to have the same problem....goes real good then quits after half an hour...then not start...then wait half an hour and away she goes again. I replaced the mag and she dont cut out anymore.maybe that is the problem, I have been told the mag gets hot and starts to lose its spark after some years. stew
Since you have a good spark - (is the spark a good blue color?) and the carb floods if you over choke it then they seem to be OK. What is the water temp? Can you put a thermometer in the radiator to check the temp?
Does this happen only on very warm days?
If so, it could be just another old machine falling victim to modern gasoline. I have experienced a similar problem with my Farmall Super M - and several other tractors suffered from it at our steam show this past weekend. What happens is the fuel boils in the fuel line or carburetor due to a combination of warm ambient air and hot air from an engine or radiator - especially if the engine is in a confined space caused by side panels, or if hot air is constantly blowing on the fuel line or carb.
From my experience, an engine will run fine as long as it is under load and the fuel is continuously flowing. But, if shut down, or idled for a duration of time, the fuel flow slows way down allowing the fuel to absorb heat and vaporize in the line, blocking flow.
Better quality of fuel may help - not because the engine requires higher octane - but because of specifications the fuel is made to. My theory is that today's gasoline has a lower vapor pressure, allowing it to evaporate at a lower temperature. Since almost all modern vehicles employ an in-tank fuel pump that pushes the fuel through the line (unlike the old pumps on the side of the engine that pulled the fuel, thus creating a vacuum), evaporation in the lines is no longer an issue. But, when this fuel is used in an old system (even gravity feed), fuel vaporization can cause lots of problems.
Stop at your local airport and pick up a can of aviation gasoline. It's made to much tighter specifications and should not cause the problem.
Blake