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John Deere H backfires

P

Pat Huebsch

New User
I have a hand start JD H that was rebuilt in 2013. The tractor ran good but now it backfires through the carburetor when I try to start it. The compression is good, fresh fuel, valve clearance good. Changed the carburetor and magneto with ones off a running tractor and still have the same results. Is it possible that the manifold or manifold gasket is bad causing a vacuum leak? Any ideas?
Pat
 
Just a little diagnosis info...a lean mixture makes them backfire when running and/or accelerating but flooding and/or being too rich can cause them to backfire when trying to start since they DO fire on exhaust also. Any time you get an over abundance of raw fuel pushed into the exhaust, eventually it will fire off. One thing that will cause this is a bad wire or carbon fouled plug also as it will only fire intermittently and in the strokes it doesn't, it is loading fuel into the exhaust and eventually it fires and ka-bang ! Just something to check since you eliminated the carb/mag and it did run / start fine before. Doesn't take but a black coloring on plug insulator to do this since when starting, you are choking enough to "wet" this carbon. Wet carbon is very good conductor and it will follow it instead of jumping gap.
 
Sorry, i just re-read your post and it did say "backfires through carb". Obviously your timing is on. Only one other way fired combustion gas can get back to carb and that would be through an intake valve not closing all the way.
 
Today I made a plate to cover the intake where the carb bolts to. I welded a 1/4 inch pipe fitting to it so I could connect my air compressor to it. I applied 10 psi to it a found a significant leak in the front center of the manifold where it meets the head. I am assuming the the vacuum leak is the cause of my problem. Time to disassemble.

Pat
 
Shortly after my last post I removed the manifold and took it to a machine shop to have it resurfaced. Reassembled with a new gasket. Tractor runs.
 
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