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Rapping noise after backfire

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Alan Chalou

09-02-2007 03:13:32




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I have an MTA that I've owened for a number of years and I work it on light duty jobs. The other day when I started it, it backfired so bad, that now it has a rapping noise on the top of the engine. Could the timeing have been changed? Help..!!




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Dave BN

09-02-2007 10:04:24




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 Re: Rapping noise after backfire in reply to Alan Chalou, 09-02-2007 03:13:32  
Without being able to hear it I'd guess you have a bent and sticking valve or a bent push rod or valve lever. It's not too hard to take the cover off the top and have a look. You do need to figure out why it backfired so badly. Was the engine hot when this happened? Dave.



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Janicholson

09-02-2007 08:40:01




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 Re: Rapping noise after backfire in reply to Alan Chalou, 09-02-2007 03:13:32  
Carbon buildup can be blasted loose by a backfire and be caught between the piston and head causing the same noise as the screw (in the other post) Sea Foam, a light spray of water from a mister type sprayer into the intake, (just enough to cause it to almost die from just above idle), (or pulling the head off and checking it out, are your possibl answers.
The worst case is a broken ring or cracked piston.
Best of luck, JimN

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John-Paul

09-02-2007 08:09:25




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 Re: Rapping noise after backfire in reply to Alan Chalou, 09-02-2007 03:13:32  
Knock could be spark knock from whats causing the backfiring. Id check to make sure there is no air leak at manifold and carb. Backfiring can be from a really lean engine, and a lean engine runs hot, which in turn "can" cause spark knock(engine contiunes to fire after intended since plug too hot). Does it smooth out some when you put choke on. Ive seen them so bad, you can put hand over intake of carb, and the engine not only still run (very bad sign) it runs better. Heh.

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Alan Chalou

09-03-2007 04:27:56




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 Re: Rapping noise after backfire in reply to John-Paul, 09-02-2007 08:09:25  
Thanks guys for all your excellent info...I found the problem last night. What I thought was a rapping noise was actually an exhaust leak, I'm assuming that with the hood on and being parked beside the barn the echo sounded just like a rapping noise. The exhaust manifold had a weak spot and from the backfire it blew a hole through about the size of a nickle..... .....whew...!! The next thing is, why the backfire. Maybe water seaped through the weak area. We just had an inch of rain the day before. The engine starts good runs great, good power and no spark knock. Thanks again

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Allan In NE

09-03-2007 09:46:33




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 Re: Rapping noise after backfire in reply to Alan Chalou, 09-03-2007 04:27:56  
Alan,

Backfiring is almost always caused by something wrong in the ignition allowing the charge to explode in the exhaust system instead of in the cylinder where it belongs.

When the fuel/air mixture isn't lit off in the cylinder as it should be at the proper time, it is then naturally pushed on out into the exhaust. The "late" occurring ignition pulse then couples with the "hot" charge in the muffler when the exhaust valve is open and you get that "bang" in the wrong area.

It's nothing to mess with. I've seen 'me rip the exhaust right off of the engine and open that muffler up like a tin can.

Look for bad/frayed wiring in the primary, wrong dwell setting, loose distributor shaft bushings, tracking cap, broken rotor and/or bad/corroded secondary wires. Rusted/bad connections at the coil tower.

Allan

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Ken L.

09-02-2007 06:57:24




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 Re: Rapping noise after backfire in reply to Alan Chalou, 09-02-2007 03:13:32  
Had this happen to a freind of mine years back. He called and said that his M backfired and there was an upper end knock after that. Checked above the head and everything seemed good. I pulled the head and found a screw head embedded in the #2 piston head which, would hit the cyl. head each time the piston was up. Pulled the carb and found the head of a screw from the choke butterfly missing and the other screw stretched and bent. If you've checked what the other guys said to check and have found nothing, pull the carb and check the four butterfly screws.

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Allan In NE

09-02-2007 04:09:39




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 Re: Rapping noise after backfire in reply to Alan Chalou, 09-02-2007 03:13:32  
Yank the muffler off and see if the noise disappears.

Allan



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El Toro

09-02-2007 04:08:06




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 Re: Rapping noise after backfire in reply to Alan Chalou, 09-02-2007 03:13:32  
I would take a look at the pushrods on the valves
to see if one may be bent. Hal



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