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Wood-Mizer LT40 with Onan 24 backfiring problem....help

Started by GlennG, October 17, 2006, 12:38:56 AM

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GlennG

   I need some help cause I`m stumped. My buddies LT40 with an Onan 24 hp engine will backfire( violently ) on a semi frequent basis. It backfires only on 2 conditions .

Condition 1 is when the mill is warm , the engine is running and the ignition is swithced to off.

Condition 2 is when the mill is warm and the engine is off but the ignition switch has been selected to run just prior to engaging the starter.

Condition 2 is going to give someone a heart attack. Compete silence then ...BOOM!

The mill runs great, both plugs are tan , I see no evidence of exhaust leaks. The engine idles , accelerates and runs strong.

Anyone ever had a problem like this? I`m baffled

Thanks in advance
Glenn


woodbeard

Well, Condition 1 seems pretty common with these units. Mine does it quite a bit, but usually only if I dont let it come back up to a full idle before shutting it off.
Condition 2 I have not had, but I would guess that the crank just happens to be in the right position to fire the plugs when you turn the key to run. Since both plugs fire at the same time on this engine, if there are unburnt fuel vapors in the muffler, the spark will detonate them. Just a guess, though. Only thing I can think of.  ???
All I can suggest is, before you shut the engine off after a cut, let the rpm's come all the way back up to full idle speed.

GlennG

Thanks for the reply.  The mill operator often shuts the mill off for a minute to converse and then cranks it up again before the fumes dissapate from the exhaust.  I will explain this to him . 

Glenn

Randy

Quote from: GlennG on October 17, 2006, 12:38:56 AM
   I need some help cause I`m stumped. My buddies LT40 with an Onan 24 hp engine will backfire( violently ) on a semi frequent basis. It backfires only on 2 conditions .

Condition 1 is when the mill is warm , the engine is running and the ignition is swithced to off.

Condition 2 is when the mill is warm and the engine is off but the ignition switch has been selected to run just prior to engaging the starter.

Condition 2 is going to give someone a heart attack. Compete silence then ...BOOM!

The mill runs great, both plugs are tan , I see no evidence of exhaust leaks. The engine idles , accelerates and runs strong.

Anyone ever had a problem like this? I`m baffled

Thanks in advance
Glenn



Mine does the same thing when I turn it off-------To stop it from happening, I just let the engine return to Idle and run for 15/20 seconds to give the muffler time to cool down--------If not------It will give people a scare. What causes it is the muffler is hot and will cause the gas that doesn't burn after the ignition is turned off----to explode. Never had the 2nd problem. Randy

ladylake

Glen  You could slow the idle speed down a little if it's fast and adjust the idle jet on the carb a little leaner.   Steve
Timberking B20  18000  hours +  Case75xt grapple + forks+8" snow bucket + dirt bucket   770 Oliver   Lots(too many) of chainsaws, Like the Echo saws and the Stihl and Husky     W5  Case loader   1  trailers  Wright sharpener     Suffolk  setter Volvo MCT125c skid loader

VA-Sawyer

I was hoping somebody would have a sure fire ( no pun intended ) fix for this problem. Every once in a while, mine will backfire after shutdown. About  1 or maybe 2 seconds after the engine comes to a stop...KaBoom! Once in a blue moon it will backfire on startup. The starter grinds through about two revs of the crankshaft and then, KaBoom! and it's running. 
I think more than one customer almost had a heart attack over it.
VA-Sawyer

Polly

8) i to had the backfiring problem on my lt40 mill my john deere gator and my yazoo o turn  lawn mower guess what i got to looking and all three had same muffler type i believe a new muffler of this type cost approx 169.00  i then thought epa and pollution regulations  the gator and onan had two manifold pypes going into the muffler so i took hack saw and cut muffler off and took i inch black pipe nipple and welded to the manifold pipe no problem doing this the almost tellescope inside one another  then i went and purchased two mufflers the old screw round type about 6.00 each  and connected to my i inch black pipe backingfiring stopped  fuel usage decreased  so i got dual exhaust on my onan and my jd gator and no more backfiring  i took one of old mufflers apart  and they are designed to burn exhaust gas before it can get in air hence backfiring the farmers pulling tractors use to put spark plug in exhaust manifold to make tractor backfire on purpose old jd sounded real good this way  so to get rid of backfiring go back to old type mufflersl

jpgreen

I've got a simular backfire problem.  Excess gas is the culprit for sure, from refried beans mostly...  :D

I've also done what Polly suggested with Onan mufflers.  I had one rig that would crack the flange mounts on those double mufflers.

One thing that should be done with air cooled engines is NEVER shut them down hot.  Allways warm up, and cool down idle for at least 3-5 minutes. You'll double the life of one by doing that..  8)
-95 Wood-Mizer LT40HD 27 Hp Kawasaki water cooled engine-

Polly

 8) 8) i forgot to add a couple of other things when talking about using two old type mufflers instead of stock muffler on the onan most importantly the exhaust backpressure will decrease which will not only increase the power of the engine but most important let the engine run cooler  and add a lot of time to the life of the engine also my jd gator has kaloseka engine i am from ky so excuse spelling  i modified exhaust on it also and it made a big noticable difference in it also  by way white beans are better then brown less gas

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