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What Causes Backfire?

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JKC

04-11-2003 18:54:20




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I have a 1950 ZAU. Haven't had it running for the last 6 months, as I was rebuilding the carb. Got things back together a couple of weeks ago, started the tractor and she sounded great! Let her run for about 15-20 minutes with no problem. A few days later, started it and drove it around for about 30 minutes..... again, no problem. Several more days have past and I tried to start it tonight..... had a HUGE backfire, and flames shot out of the exhaust. Tried again, same thing. Scared the heck out of me and my son. I am at a loss as to what would cause this. Please help.

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Howard H.

04-13-2003 08:34:57




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 Re: What Causes Backfire? in reply to JKC, 04-11-2003 18:54:20  
Hi JKC,

I used to run a lot of MM irrigation engines and a sticking valve CAN cause that, too.

A neighbor asked one day if we had been shooting coyotes with a high-powered rifle near his place.

I hadn't and didn't think any more about it until I got up on the hill to check my HD-800. It was running fine and pumping water for all she was worth, but the air filter canister ducting (4 inch pipes and rubber fittings) were blown off, along with half of the carburetor (a twin diaghram Impco).

I don't know how it was running so well with only half a carb, but it was. Then it dawned on me - the backfiring was so intermittent that I never heard it (we checked the irrigation wells twice a day), but it was so loud that the neighbor a mile away could hear it...

I don't know if that is your situation, but that incident sure left an impression on me!


Howard H.

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Kelvin

04-12-2003 19:14:15




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 Re: What Causes Backfire? in reply to JKC, 04-11-2003 18:54:20  
The basic idea is that unburned fuel and a spark are getting in the exhaust manifold at the same time.

Had a 403 once that did that (and blew a hole in the exhaust manifold) because one of the exhaust valve seats was missing a chunk.

Simpler idea:

Did you (or anyone else) by any chance remove any spark plug wires?
Firing order is 1,3,4,2.



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Kevin Moschenrose

04-12-2003 21:04:39




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 Re: Re: What Causes Backfire? in reply to Kelvin, 04-12-2003 19:14:15  
The timing can also be off just a little causing the spark plug to fire late and be delaided enough to let the fuel get into the exhaust manifold also.



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JK C

04-12-2003 19:37:55




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 Re: Re: What Causes Backfire? in reply to Kelvin, 04-12-2003 19:14:15  
The firing order solution came to me first, but I realized the plug wires had not been moved in any way. I have a question: How does unburned fuel get into the exhaust manifold? Do you think I could have a valve problem? Does the amount of fuel going to the carburator affect this in any way? Thanks for your help.



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rick

04-13-2003 07:14:53




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 Re: Re: Re: What Causes Backfire? in reply to JK C, 04-12-2003 19:37:55  
Seeing as you recently tinkered with the carb, I would suspect that as a possability. I would think if your mixture was too rich, (too much fuel to air) then you'd have slow flame propegation in the cylinders, so when the exhaust valve opened, there was still unburned fuel in the cylinder, when it gets pushed into the exhaust, it finds the right mixture, and completes its burning, thus a "back fire, or "after fire" you may have a stuck float or wrong mixture. let us know what

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