Motorcycle Repair: 1980 cm400e Engine pinging sound, octane boost, fuel octane

Question
QUESTION: Hi I just finish rebuilding carbs on my 1980 cm400e
the bike has 2000 original miles I rode it for about an hour  it was misfiring when reving but nothing too bad.
but then when I parked it and reved the engine a bit it started pinging, any suggestions?

ANSWER: Was it pinging with no load on the engine?  If so, you have a severe ignition timing problem or oil getting into the cylinder lowering the fuel octane.

Regards
Rich

---------- FOLLOW-UP ----------

QUESTION: good evening
the engine is pinging with no load even on idle
I replaced the spark plugs (the old ones had no signs of any problems or deposits or oil )no blue smoke either.  and the backfire is minimal only on very high raving..I also got some octane boost thinking maybe the gas was causing the knocking, but pinging is still there.. could a bad valve cause this on a 2000 mile bike?
is there a way to pinpoint the problem before dishing out hundreds on a mechanic?

Thanks
Eric

ANSWER: We may be talking about two different things here.   The terms "pinging", is usually used to describe pre-ignition in the cylinder.  It sounds a lot like someone rattling a can with ball bearings in it.  

We're now talking about backfiring.  That's a popping out the exhaust.  That is typically caused by an ignition problem or a lean fuel mixture.   Raw fuel is getting into the exhaust and igniting.  It can be a plug not firing correctly or the mixture too lean for the plug to ignite the mixture.

You will need to check the ignition timing and if that's good, try applying a little choke when it's missing out and backfiring.  If the problem improves, it's a lean mixture and you'll need to probably clean the carbs.

Regards
Rich

---------- FOLLOW-UP ----------

QUESTION: Thanks Rich, I opened the valve cover and found one of the exhaust valves adjustment screw and nut came loose and bounced  and that 's what caused the pinging. I set them back and adjusted the valve to the required spec. in regards to the backfire you maybe right, however I just rebuilt both carbs as well as installed an external gas filter do you think the carbs need cleaning? or maybe I should try and adjust the jet pilot screws? originally I set them to the manual's spec
I'll try opening the choke

Thanks

Eric

Answer
Adjust the idle air mixture to the highest idle speed.  If you adjust the screws in you, will hear the speed slow down.  If you back the screws out, you will also hear it slow down.  Put the adjustment half way between those two points.

Regards
Rich