Motorcycle Repair: Timing a 1982 Yamaha 650 Maxim, yamaha 650 maxim, yamaha maxim 650

Question
Hi Chris,

I have a 1982 Yamaha Maxim 650 with only 38,000 KLMS and I think my problem may be the timing. It sat idle for a couple of years, then I took off the carbs and gave them to a friend to be cleaned and it ran great for a couple of days and then it started to back fire, I checked the coils & they are alright, changed a couple of the spark plug caps & the plugs still backfires and will not start...

Any suggestions...

Thanks,
Dwight

Answer
Hi Dwight.

Backfiring will often occur due to one or more of the following factors;

1) Valve timing and gap.

2) Bad fuel.

3) Carbs being out of synch. This will also cause other proformance issues as well.

4) Cylinders not firing, causing heat combustion in the cylinder when the engine warms up.

Let's approach each possibility one at a time.

1) Have you checked the valve timing and gap?  If not, then do so.  The repair manual will have the specs for you.

2) You said that the bike sat for a while and you had the carbs cleaned.
 Did you clean, or preferably, reseal the tank and replace the fuel line?
 Have you been using the fuel that was left in the tank?  If you did not get rid of any residue, from the bad fuel, that was left in the tank then you have recontaminated the carbs and they need to be recleaned.

3) Since the bike ran okay immediately after the carbs were reinstalled, then the synch is probably not an issue.  Double check it if you wish to be certain.

4) You say that you have replaced the spark plugs and caps, but did you check the coils?  spark plugs and caps rarely, if ever, go bad.  Follow the test proceedures in the repair manual for the coils if you are not sure that you are getting spark.

Start with that and see what you find.
Good luck and ride safe.
FALCON