Right cylinder, pops, crackle,s and backfires???

[Nine_Iron]

Hey guys, new to the forum here but lots of info here....great site!

Anyway, I acquired a 1975 xs650b a few months ago and have been working on it all summer. The guy I received it from had it for 20 years and then let it sit for 6 years. I've finally got it somewhat running but am having a bit of a problem. I can start it almost immediately but it idles rough and sometimes not at all. I can tell the right cylinder is not hitting, I can hold the right side exhaust and can't even touch the left side. When I hit the throttle the right side will start hitting but it is very hit and miss and it pops, crackles, and backfires horribly. I can hold the throttle and it continues to run but the backfires never stop yet the left side purrs like a kitten. I have noticed a black oily deposit on the spark plugs after running for a few minutes like this.

So far I have replaced (with brand new parts) the coils, points, condensers, and plugs. The tank had a ton of rust which I removed and I completely 100% cleaned both petcocks and carbs. I left the carbs at the exact factory settings and they are completely stock. I feel 100% confident that the carbs and fuel are free flowing with nothing plugged up. And I've been through the timing 5 times....checked with an ohm meter and both points just start to open at the F mark.

All four valves seem to be traveling fine with the covers off so I decided to check compression. I was getting a reading of 120 psi on both cylinders but I did however try filling a little 3&1 oil in both cylinders and both cylinders gave me a 150 psi reading.

Any advice?? I have racked my brain on all this. Bad cylinder rings? Fuel mixture to rich on the right side? I am nonstop changing from fuel problems, back to ignition problems, back to engine problems, and all over again.

 

[gggGary]

You don't mention that you have synched the carbs? The 75 has two throttle cables so it can be a bit tricky. Are both throttle shafts free moving and return quickly to idle? How about the diaphragms, do they both drop very slowly when you block the vent with your finger?

 

[gggGary]

Also have you tried spraying carb cleaner around the throttle shafts and intakes while it's running to see if it shows up an air leak?

 

[5twins]

You say you've cleaned the carbs but you may not have cleaned them good enough. It sounds like the pilot circuit is still plugged on that right carb.

 

[dragwagon454]

I have a 1975 xs650b that has the exact same problem as you. Did you ever get it resolved? If so what did you find? Thanks

 

[gentlemanjim]

I'm thinking slide diaphrams

 

[wesleyonoel]

Quick check of idle circuit on right carburetor might answer some questions. A simple shot of ether (starting fluid) directed on the intake side of the carburetor (filter works) will tell you whether fuel is making it through the idle circuit freely. If the right side "picks up" after a shot of ether, there is a problem in the idle circuit. There are wee-tiny orifices in the venturi which are easily plugged leading to idle trouble. Try this quick and easy method to either eliminate or identify the trouble. Best wishes...........Wesley

 

[Tech7]

Given that the bike backfires/misses throught the entire RPM range, (according to the original post), I'd be more inclined to think an ignition problem, perhaps a bad plug wire. An inline spark tester would help with the diag.