carb issue on 1978 j.d. spitfire - Snowmobile World : Your #1 Snowmobile Forum
 
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post #1 of 7 (permalink) Old 10-08-2012, 08:37 PM Thread Starter
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carb issue on 1978 j.d. spitfire

i recently picked up a '78 john deere spitfire (kohler 340 ac). after making sure the engine would run, i dismantled everything in the engine compartment for cleaning and inspecton. i put it all back together today and i'm having carb issues. it will start easily and idle, but when i give it throttle, it does not rev up. it simply continues to sit at idle. it is a slide type mikuni carb. the throttle cable is working properly. when i open up the throttle i can hear the difference as the slide goes up. i have cleaned and re-cleaned everything in the carb to my knowledge. (in fact if i had a $ for every time i took the carb off this afternoon...) it acted like this when i first got it, but it corrected itself and i wrote it off as not being run for a while. it seems as if it's lacking fuel delivery in a bad way. anybody have the answer im looking for? i would greatly appreciate any help. i'm one of the few snowmobilers in central illinois, and i've got to get my machines ready before the first snow, because that may be all we get here! thanks
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post #2 of 7 (permalink) Old 10-09-2012, 07:06 PM
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1978 Check fuel lines for leaks, check the pulser line to the fuel pump if this sled has a pump or it is in the carb, research this sled to see if it came with that carb. I have never worked on a J D Spit fire. Check the fuel filter it could be the problem. Make sure the fuel supply line and the return line are not reversed.
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post #3 of 7 (permalink) Old 10-09-2012, 09:25 PM Thread Starter
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thanks for the input. i'm now beginning to think that it could be an electrical or timing problem. the engine will start and idle, but still will not rev. i think that it is delivering fuel into the cylinders, but the fuel is not being burnt. the plugs are wet when i check them, and they have carbon build-up. it is also discharging an oily residue from the exhaust, which im assuming is unused fuel. i squirted fuel into the open end of the carb while it was at idle, and it bogged out and died. it runs at a very choppy idle. i even tried switching the plug wires around, and it acted the exact same. this thing ran properly for me a couple of weeks ago, what am i missing?
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post #4 of 7 (permalink) Old 10-10-2012, 06:17 PM
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I have not worked on a spit fire. The pistons did they have an arrow on them? Usually the arrow points to the exhaust. If the pistons are on backwards it will act like this.I do not know about JD's. If this is a slide carb is there a choke puck in it and does it sliding freely and clean. A choke puck not completely dropping will also cause this. A yamaha has about 2 mil play in the cable when the puck is released. Make sure there is play in the cable when the cable is released. Is this a free air dual carb? VM 28? When you took the motor appart did you remove the mag?
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post #5 of 7 (permalink) Old 10-11-2012, 12:04 AM Thread Starter
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ok, here's the story now. i checked the compression today just out of curiosity. both cylinders were right around 100. anyway, when i put the plugs back in and started it, it ran just fine. sounded good and the throttle was responsive. i had expermented with different plugs yesterday but had no luck. now a few starts and a few hours later, it's acting the same way. it starts and sits at a choppy idle, but will not rev. i am really leaning towards an ignition problem at this point. is there a reliable way to test/check the coil and cdi box? i plan to see if a coil from my other machines will interchange. by the way, this is a single carb mikuni vm 28 or 30(?). i'll check on the choke tomorrow, but i really haven't changed it. i did not tear down the engine when i went over the sled, it ran for me before i started. guess i shoulda left it dirty. let me know what you think about the coil and cdi box. thanks
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post #6 of 7 (permalink) Old 10-11-2012, 05:06 PM
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I have found that cdi's work or do not. Coils, I have had work cold and when they heat up they stop working, cool down and start working again. I have seen mice plug or semi plug the exhaust pipe. This will cause what you just described. The exhaust cannot escape and will back up into the motor. Make sure all your connections are tight and no wires are chewed by mice. Keeping things clean is a good idea, you may have found a loose connection or a loose ground which may be a bigger problems later. Hope this helps
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post #7 of 7 (permalink) Old 10-12-2012, 06:56 PM Thread Starter
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after some switching around of coils and cdi boxes from another machine, i concluded that they were not the problem. a close inspection followed, and revealed a ground connection that was not proper. after i took care of that, the engine is once again running fine. i appreciate your feedback. thanks

Last edited by aaaaj; 10-12-2012 at 07:16 PM. Reason: adding image
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