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backfire #780757 08/06/19 9:54 pm
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RGSROB Offline OP
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trying to coax my 51 mac back to running properly after a long rebuild.
Started right up but as the motor warmed up started to backfire as the throttle closed.
Went through the 276 carb , quite thoroughly-took jet block out etc-carb cleaner through all of pilot system.
Problem still persists.
Is there any chance that a faulty mag could cause this?

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Re: backfire [Re: RGSROB] #780789 08/07/19 6:51 am
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kommando Online Content
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Did you drill out the pilot jet with a 16 thou or number 78 drill, carb cleaner and blowing air does not work on such a small hole.

Re: backfire [Re: kommando] #780796 08/07/19 10:59 am
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RGSROB Offline OP
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Used a wire that passed through pilot and pilot bypass -
Initially bike fires right up and then as it warms the bottom end running becomes erratic and backfires on throttle closing.
took the plug out and was surprised to find it very sooty.
No sign of float bowl flooding.

Re: backfire [Re: RGSROB] #780809 08/07/19 1:35 pm
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Offhand it sounds like the carb, but... . Is this an automatic advance or a manual? Those symptoms could indicate that the timing is retarded. Could the timing have slipped?

Ed from NJ

Re: backfire [Re: RGSROB] #780899 08/08/19 9:27 am
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kommando Online Content
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The classic symptoms of a mag failing is for it not to create a spark when hot, normally its the capacitor failing and cooling down gets you back the spark. You can eliminate the mag by warming the motor up until you get symptoms and then quickly removing the plug and earthing it to the head and see if kicking over gives you a good spark. It does sound less like a mag failure and more carb related.

Re: backfire [Re: RGSROB] #781638 08/16/19 11:46 am
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after trying two carbs I decided to change the magneto, still the same symptoms of weakness.
All that was left as likely culprit was carb or exhaust system.
Managed to get new jet body gasket for the 276 carb and new pilot. The gasket was much thicker than the paper item that had been fitted.
The bike did run better but still popping and banging on the over run.
decided to revisit the new Armours silencer which is supplied with a removable baffle identical to a venom.
However this silencer does not have the fixed baffle that the venom does .
The fixed baffle provides back pressure which may effect carburation.
So temporarily I have bonded a large washer into the end of the removable baffle.
The bike ran much better with no popping or banging.
hopefully that is the end of this saga


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