Topic: rapid backfire

[WVCB500F]

Hi all,

I recently had my 73 CB500 serviced and tuned...it was running great and all was well, then the other day I took it to get inspected and stopped at Walmart on my way home (which was my first mistake)...came out approx 15 minutes later, started it up and couldn't even get out of the parking lot.  It backfires rapidly (even saw flames shoot from left exhaust), then bogs and dies when given more throttle.  It will start back up easily, but continues to backfire/spit/pop rapidly and bog down.  I removed the airfilters, sprayed the carbs with some Berryman's carb cleaner (which usually works when carbs start acting up), but no luck.  Since the bike was recently tuned and synched (and in hopes of keeping it this way)...I would like to get a feel for the real culprit before I start pulling carbs and troubleshooting.  I guess I need to know if plugged jets could cause this or am I looking at a problem with plugs/points/condensors? I've read posts about backfiring...but this is more than the occasional backfire.  Sounds more like firecrackers...I think I saw people running back inside Walmart as I was trying to leave!  It was running great prior to this.  Any experience this before or any suggestions?

Thanks,
Brian

[SteveD CB500F]

If it hapened with no warning - sounds like something broke or didn't get tightened properly after the tune-up?

Have you checked the throttle linkage to make sure all four are moving?

[TwoTired]

Jeez, could be anything with that description.  Though we can probably rule out faulty air in the tires. 

Do the easy things first.  Double check the timing and points settings.  You are going to feel pretty foolish overhauling carbs and then finding the point settings slipped.  Did water get in the points compartment?

Also, check for fuel contamination. Could it have been vandalized by the high-class clientelle in the wal mart parking lot?

Check for even head pipe temps.  Cold ones point to the cylinder(s) with the problem.
Pull the plugs and see if the tips show anything odd on the combustion front.

Lookin for clues...

[mcpuffett]

possibly a condensor breaking up?    mick.

[WVCB500F]

Thanks for the responses...I took the points cover off today, started it up and saw excessive sparking coming from behind the left contact breaker.  I found that the contact breaker had slipped back and was sitting up against the points plate.  Not sure how this can happen, but I was able to "pop" it back in place and it's running fine again.  Looks like I'm in for a new set of points (and maybe points plate???) soon.  Is there a spring washer or something I could be missing that holds this in place?  Thanks again!

[TwoTired]

I can only guess from your description that the points are moving around on the pivot pins which do have a retaining spring clip to keep them from wandering.  Everything else is held in place with screws.

Glad you found the problem.

[Harry]

Good you sorted your problem. I would immediately have checked if some joker had switched my plug cables, or pushed a potatoe up my exhaust, but then, hey, I'm a suspicious kind of guy (I have three kids - 95% of the time something goes wrong, they or their friends are behind it ;-)

[WVCB500F]

Yes, the points are moving around on the pivot pin.  I will attempt to replace the spring clip tonight (I have an old set of points and old points plate to use for parts).  I am guessing that I can do this w/out effecting my points gap, since the adjustments are made on the other side of the points (non-moving side)

Harry, never even thought about a potatoe in the exhaust...guess I should keep that in mind.  With two little boys I better start thinking of these things!