Topic: Help needed diagnosing a backfiring issue

[nick5446]

I'm in need of some help getting my bike running. I just bought it as a project, so I don't know a ton about it. It wasn't running when I bought it and I was told it needed "carb work". When I got it home I found that the inline fuel filter was clogged, so I replaced it and it fired up. Didn't run so hot, so I adjusted the valves and the pilot screw to 1.5 turns out on each carb (they were all around 4 turns out). I also confirmed that the accelerator pump works.

When I get it running (not super-easy to start) it runs ok, but it backfires and some fuel squirts out of one of the jets (towards the airbox). Also, it revs quite high when I push the choke knob in.

Any ideas what's going?

[nick5446]

I should add that it's backfiring out of the carb intake, not the exhaust.

[Really?]

I am no carb expert but it sounds like the carbs are dirty, running lean and out of sync.  If the filter was that dirty, what got into the carbs?

[TIM TINGEY, age 55]

Not sure which bike you have, but you might have air leaks at the intake spigots. Though you did a valve adjustment the clearances on SOHC Honda's was .002 and .003 thousands- if any questions arise then set them at .003 and .004. loose is always better it gives the valves more time on the seat. Spitting back through the carb often is a intake valve that won't seal. Also check your ignition timing and point condition. Timing thats too far advanced will ignite the fuel too early-before the intake valve can close.  Lastly, if the idle mixture circuits aren't flowing fuel adequately you wont be able to adjust idle mixture correctly.  One other out-of adjustment area that causes spitting back through the carb is a cam chain that loose-resulting in cam timing thats not controlled. Email me at timmannyatingey@yahoo.com with the model and year. Good luck

[nick5446]

Not sure which bike you have, but you might have air leaks at the intake spigots. Though you did a valve adjustment the clearances on SOHC Honda's was .002 and .003 thousands- if any questions arise then set them at .003 and .004. loose is always better it gives the valves more time on the seat. Spitting back through the carb often is a intake valve that won't seal. Also check your ignition timing and point condition. Timing thats too far advanced will ignite the fuel too early-before the intake valve can close.  Lastly, if the idle mixture circuits aren't flowing fuel adequately you wont be able to adjust idle mixture correctly.  One other out-of adjustment area that causes spitting back through the carb is a cam chain that loose-resulting in cam timing thats not controlled. Email me at timmannyatingey@yahoo.com with the model and year. Good luck

Thanks for the info. Sorry I left out the model info; it's a '79 CB650.

I set the valves at .002 and .004 with the same mindset you mentioned.