Engine back fires when I throttle off.

[Rustrob]

Hi all you boffins out there. Hope you can help me. I went for about a 160 km. ( 100 mile) ride today and about half way stopped at a mates place for about an hour. When I continued my journey I noticed every time I throttled off for a corner or whatever, the bike back fired, only on one cylinder I think. Also with the throttle at cruise it appeared to have a bit of a miss. Any ideas?

 

[DaveO]

If it was running OK before I would look for leaks where the exhaust joins the cylynder head, checking that the nuts haven't backed off. I would also check for leaks around the carburettors.

 

[DaveO]

If it is genuine backfire it could be that you have a firing problem on the side of the backfire. A plug breaking down, or HT lead needing replacing, a coil problem or points needing cleaning, adjusting or replacing. The lack of a strong spark on one cycle leads to excess fuel being ignited on the next.

It could be poorly adjusted valves (too tight) or valves burning out but, if it were me, would check out the simple stuff first.

 

[Rustrob]

Thanks for those thoughts. I decided to have a good look at the problem and it's amazing what you find when you look properly. Checked the ignition department, where I really thought the problem was but every thing appeared fine there. Then I pulled off the carbs, straight away I see a problem. One of the throttle slides has turned a quarter turn, weird. Stripped the carbs and found the slide was loose on the diaphragm. That was on one carb but it didn't stop there, the float in the other carb was half full of petrol, obviously a hole some where. All fixed now, soldered the float and put a few drops of a good glue on the diaphragm so hopefully it will be all go tomorrow. Strange tho as the bike was going reasonably well. Hope this helps some one with similar problems.