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MissFiring and backfire

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8N Doug in Ohio

06-30-2003 04:30:34




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I was bushhogging yesterday, the 1950 8N was running fine (a little hard to start), after about a half hour she started missing and backfiring. Barely made it back to the barn. Any ideas what would cause this ? The tractor has never done this before, and since it has been restored, it is used very little.




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Rob

06-30-2003 17:22:19




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 Re: MissFiring and backfire in reply to 8N Doug in Ohio, 06-30-2003 04:30:34  
See if this helps:
There is a quick test you can do from the seat. Warm the tractor to normal operating temp. If it won’t warm up or maintain a constant operating temp then the tstat may be wacked. Put the tractor in high gear and get her just rolling forward a little bit and at idle and then quickly pull the throttle to 1/2 speed. Do this with your good brake partially on. You want a heavy load on the engine as you give it a lot of gas in high gear. You want to make it pull really hard. If the clutch slips then you’ll have to adjust or replace it.
1. If it backfires the fuel mix is too lean or you have a shorted ignition (pull the spark plug wires one at a time and watch for a regular spark without a miss) or sticky valves (you can hear a sticky valve).
2. Your timing is ok if you hear a constant slight pinging (spark knock). 3. If it doesn't ping set the timing and make sure the mechanical advance is free to operate. I hear that if the wrong screw is used on the points it can interfere with the mechanical advance weights. 4. If it pings excessively then set the timing. If that doesn’t correct the pinging then pull all the plugs, clean em up, and set the gap. While you have them out take a compression test on each cylinder with the carb throttle full open. You are looking for 90psi minimum at starter cranking speed. If you get normal compression then you need to check and set that timing! If you have 140+psi pull the head and clean the carbon off it. Make sure the valve stems are not gummy. Low compression says ‘fix my guts’ or it’s a new rebuild that isn’t broken in yet.
5. If the engine pulls evenly and there is no indication of late spark or excessive carbon, check the system for a lean mixture.
6. If the engine pulls unevenly then check the spark at the spark plug boots.
Hope that fixes you up or at least gets you headed in the right direction.

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Dell (WA)

06-30-2003 06:35:01




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 Re: MissFiring and backfire in reply to 8N Doug in Ohio, 06-30-2003 04:30:34  
Doug..... .you write..... "(a little hard to start), after about a half hour she started missing and backfiring"..... .this sounds like the classic frontmount ignition coil overheating problem. The insulative tar inside the coil melts from heat and some of the turns short out and you start getting weak sparkies.....which cause "missing and backfiring".

Amaizingly enuff, once the coil/tar cools off overnite, you can restart your tractor in the morning as if there was "no problem" the day before. Once that tar melts, you'll always have that problem unless you find out what causes the coil to overheat.

The hard starting might be your point gap slipped. should be 0.015..... ....Dell

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