Author Topic: Carb backfire then seizure !  (Read 721 times)

Offline a10 bobber

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Carb backfire then seizure !
« on: 19.08. 2014 12:38 »
Hi, I had suffered a backfire so cleaned the float bowl out.it had what looked like timescale in it.
 All back together and it ran beautifully for about 50miles, then the backfire through the carb started again.I headed home but the misfire worsened and got so bad it blew the gauze off of the carb bell! It seemed less worse at high revs so I tried to get home in a lower gear, then a mile from home it seized instantly!
 The day after it was still seized solid.although I feel a complete idiot for trying to get it home, I had no idea it would seize.
 Any ideas what could have happened? Earlier I had notice that the end cover to the magneto with the kill button, had lost a bolt and was loose so I fitted a new bolt and tightened it all up.
 After it seized I noticed a bronze ring had popped out around the primary out drive (i have a newbie open clutch and belt conversion) and the carb bellmouth was oily.
 Regards Paul.

Online Topdad

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Re: Carb backfire then seizure !
« Reply #1 on: 19.08. 2014 12:58 »
Hi Paul, no point beating yourself up about it ,whats done is done. Did you check the jets in the carb for deposits blocking them . An lean mixture would create backfires and I think increased engine heat as would slipped timing. Taking it apart will bring to light more details around the head/combustion area .Hopefully you'll be able to salvage some parts but check everything carefully . Feel sure some more input will be arriving shortly ,best of luck BobH
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Online RichardL

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Re: Carb backfire then seizure !
« Reply #2 on: 19.08. 2014 13:58 »
Paul,

Amongst my own recent discoveries, I learned that the magneto end cover on an ATD-equipped machine holds the magneto cam ring in place and aligned with the stop pin. I don't really know if this is the same for manual advance (and don't know which system you have). If your end cover was loose it does seem possible that the cam ring moved, which could explain timing that caused heavy backfire through the carb.

For your sake, I'm hoping for no serious damage or, worst case, damage limited to the top end, being much easier to deal with than a complete disassemble.

Richard L.
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Offline muskrat

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Re: Carb backfire then seizure !
« Reply #3 on: 19.08. 2014 21:13 »
G'day Paul,  *sad*.
Most heat related seizures only damage the pistons and bores. The heat would have been produced by one or both of the problems mentioned above. Was the end cap fastened before the 50 mile ride? I can't see the cap going on fully if the cam ring was out of position but due to wear it may have moved a tad. Auto or manual advance? Then there's the scale in the carb. If you see it in the bowl it IS elsewhere in the carb too. Then add a mist of oil being drawn in (should provide extra lube to top end) blocking the pilot air passage, leaning out the mix.
Take photo's of the postmortem.
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Offline a10 bobber

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Re: Carb backfire then seizure !
« Reply #4 on: 23.08. 2014 18:23 »
Thanks for the reply guy's.engines out and off to brother in law for dissection!
 Will keep you posted.