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kevencyr · Post by **kevencyr** » Sun Oct 18, 2015 6:00 pm

ok, I've been trying to find out what is wrong with this JD, we got a backfire in carburetor problem, did everything people told me on this forum. brand new firing system, overhaul carburetor, still the same problem. works OK when cold, has soon its coming hot, its starting to backfire in carburetor. anybody could help me please. really need to fix that thing. thanks

gregjo1948 · Post by **gregjo1948** » Mon Oct 19, 2015 4:53 am

I'd check compression when first started and then when it starts firing thru the carb. It sounds like you may have an intake valve issue. Check the bushing in the distributor for wear. If it's worn excessively, it alters the points, which changes when the plugs fire. If firing too soon, (advanced) it is firing on the end of the intake stroke. I really don't know why it matter if it's cold engine or up to running temp...

vtjoe · Post by **vtjoe** » Mon Oct 19, 2015 6:46 am

check valve clearances maybe one intake is TOO TIGHT

boler76 · Post by **boler76** » Mon Oct 19, 2015 10:11 am

When it is running and backfiring, try holding spk plug wire off but close to the top of plug and see if it is firing consistently or missing one and then firing the next, try this on both plugs one at a time. Just a thought

Stan Disbrow · Post by **Stan Disbrow** » Mon Oct 19, 2015 1:51 pm

Hi,

Cold OK, warm backfiring would have me looking at intake valve clearances. They close up as they warm.

Of course I would then take the time to set all the valve clearances. I mean, once the valve cover is off, it is worthwhile to check everything under it.

Stan

mapaduke@yahoo.com · Post by **mapaduke@yahoo.com** » Mon Oct 19, 2015 4:58 pm

Intake valve

kevencyr · Post by **kevencyr** » Mon Oct 19, 2015 6:23 pm

ok, got more info from my dad,he did put some new point, rotor, distributor cover, leads and spark plug in new. compression check cold, above 100 psi, valve clearance check and are ok. what he's telling me is that its work good at idle when its cold, after 30 minutes about, it will run good when working with it, but as soon he put it in idle to go do something, its start backfire and sometime almost stop. if he goes back full throttle and work, run great. the carb is overhauled. any anything. thanks

gregjo1948 · Post by **gregjo1948** » Tue Oct 20, 2015 4:06 am

If there is an auto advance in the distributor, that's what I would suspect is not functioning correctly. Just because the carb was "rebuilt", doesn't mean it isn't worn out. Check for possible vacuum leaks. The carb might be a little too lean... try adjusting...

yellowcrawler · Post by **yellowcrawler** » Tue Oct 20, 2015 7:34 pm

Also check the venturi in the carburetor for a good seal, it can suck air in, causing a lean condition. To test, try closing the choke when it backfires.

harryrg · Post by **harryrg** » Wed Oct 28, 2015 4:35 am

what direction does the distributor turn in my 40c clockwise or counter
clockwise .....had to repair oil pump new piston rings and rod bearings
now get it started

gregjo1948 · Post by **gregjo1948** » Wed Oct 28, 2015 5:29 am

Leave the distributor cap off and crank it over while watching the distributor shaft.

original possum · Post by **original possum** » Thu Oct 29, 2015 4:14 pm

40's are famous for backfiring if they get a too-lean mixture. They don't fire on one stroke because the mixture is too lean, and then the missed fuel ignites on the next exhaust stroke. Look for any way it can get too much air or too little fuel. The throttle butterfly shaft is often overlooked, and the felt washers on them deteriorate, allowing lots of air in.

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