I have an old Caterpillar 112 motor grader, serial # 3U3042. The pony motor will only run with the choke on. I have cleaned the carburetor several times but it still will not run unless the choke is on. I know it's a dumb question but, am I missing something? Also the pony motor will not start after it has warmed up. I can take out one of the spark plugs and the spark plug will ignite the fumes inside the cylinder, but if I put the spark plug back in, it will not fire a lick. I let it cool off and it fires up right away. Anyone have any suggestion?

Change to NGK AB6 plugs just to try.

Check the spark when it won't run again, should still be throwing a bright spark about a quarter inch to the frame, if not, then the coil might be breaking down when it's hot, condenser could be weak, points could need re-gapping etc.

On the choke, Look on the side of the carburetor bowl and you will see a soft plug driven in the end of a drilled passage across the bottom of the bowl from the main jet to the metering well on the other side with the threaded plug. This passageway across the bottom of the bowl can be clogged. The soft plug can be drilled and pulled with a tiny screw so you can clean the fuel passage properly. Replace it with a BB or lead shot sealed with a gasket sealer that gasoline won't touch. The removable metering well above the threaded plug has tiny holes in it that can be clogged.

Also important to check the passageways into the venturi once you have the bowl out of the way. I believe the 9B4838 gasket between the bowl and the carburetor venturi has concentric circles of material there and sometimes the center circle will have been lost.

Good luck!

Hello ccjersey,

Thanks for the suggestions. I have installed new points and condenser and have set points to manual specs. No new plugs though, but will get some. I don't know how to check the coil, but will check the spark as you suggested.

You have given me several ideas about the choke problem and I will let you know what happens after I check them out.

Thanks again

johnstarnes

Hello ccjersey

I checked my parts manual and quess what? The parts manual showed that the gasket between the bowl and the carb venturi does have concentric circles. I dismantled the carburetor and sure enough the inner circle was missing. I quickly carved a gasket out of some on hand gasket material and reassambled the carburetor. Installed it on the pony motor and with crossed fingers hit the starter button. BINGO! The pony motor fired right up and the choke worked just fine. I let it run for a few minutes until it warmed up and then turned it off. Hit the starter again and it fired right up again. I repeated this about 5 times, and each time the pony motor fired right up and ran just fine. Then I decided to crank the main engine. No problem, it cranked up and ran perfectly. After it ran for a few minutes, I shut it down and tried to fire up the pony motor. ZERO! Now it want hit a lick. I have checked both plugs and they are sparking. Checked gas to carburetor and its getting gas. After things cool down the pony motor will once again fire up. Weird!. I haven't had the opportunity to get new plugs yet. I live in a remote area, about 20 miles from nearest parts store. Will have to wait until next trip to town. Until then, any other suggestions? Thanks so much for your help.

johnstarnes

hello johnstarnes ,if it runs ok 5 times until you started the main when no go ,maybe there is a bear/loose wire ,
check key and wiring for damage that may cause a earth leakage to earth stopping the spark ? ,just an idea ,bruce oz

Hello bruce oz,

I haven't tried that yet, but I will first thing Monday. It's really weird how the pony motor will run and start just fine until I start the main engine and then it will not fire at all until everything cools down.

Thanks for the suggestion and I will let you know what happens.

johnstarnes

I,m only guessing but could it be the coil breaking down in the mag. when it gets warmed up.

Hello SJ,

Maybe, I don't know much about magnetos and coils, but the problem never occures until after the main engine has been run for a short period. The pony engine will start and run just fine until I start the main engine and then it will not run again until everything cools down. Is there some way to check the magneto/coil? Thanks for your input.

johnstarnes

I wonder if your spark is still good once the main engine has run and gotten everything hot? If it is, maybe we are barking up the wrong tree!

Any chance the compression would be low once the diesel got it hot? I'm thinking valve clearances here, gets good and hot, valves expand and clearances disappear and valves no longer close properly. It would be a subjective thing, but if you could pull it through a time or two both before and after running the diesel and get a feel for the difficulty turning it over? Or just do a compression test.

Good Morning ccjersey,

I have some new plugs coming tomorrow, but couldn't get NGK AB6 so got Autolite 386, NAPA said they would be just as good, maybe, I don't know. Anyway I will try them and see what happens.

I have thought about the valves, but I have run the pony engine for extended periods and it has reached maximum temperature and still runs and starts OK, but just as soon as I start the main engine and only run it for a very short time the pony engine will not start. I haven't done any compression tests yet, but will try the pull through test both before and after running the main engine as you suggested.

And again Thanks,
johnstarnes