Topic: Carb tuning/Idle Issues

[btahrens]

I just got my hands on a 73 CB500 Four and I’ve been trying to get it running well over the past few weeks. I’m in the process of trying to tune the carbs but I’m having a number of issues. Hopefully some of the experts out there can chime in and save me a lot of trouble shooting.

-I have the carbs more or less in sync but I’m getting an intermittent idle surge. Every ten seconds or so it revvs up an extra hundred rpms or so then settles down.

-My clymer manual says to set the idle around 1000 rpm, but the bike doesn’t seem to like that. I’m getting a “slapping” noise at that speed that disappears around 1200 rpm. I read a carb sync guide elsewhere in this forum that says to set the idle at 1200, which seems to solve that issue. Why the discrepancy?

I’m using fresh gas and the carbs and pretty damn clean, so I’m not sure that it’s a fuel/fuel delivery issue.

Any ideas? I’m a motorcycle novice and a half-ass mechanic so please be gentle.

[evinrude7]

have you checked the timing? 

[Cb750 Racer]

I just got my hands on a 73 CB500 Four and I’ve been trying to get it running well over the past few weeks. I’m in the process of trying to tune the carbs but I’m having a number of issues. Hopefully some of the experts out there can chime in and save me a lot of trouble shooting.

-I have the carbs more or less in sync but I’m getting an intermittent idle surge. Every ten seconds or so it revvs up an extra hundred rpms or so then settles down.

-My clymer manual says to set the idle around 1000 rpm, but the bike doesn’t seem to like that. I’m getting a “slapping” noise at that speed that disappears around 1200 rpm. I read a carb sync guide elsewhere in this forum that says to set the idle at 1200, which seems to solve that issue. Why the discrepancy?

I’m using fresh gas and the carbs and pretty damn clean, so I’m not sure that it’s a fuel/fuel delivery issue.

Any ideas? I’m a motorcycle novice and a half-ass mechanic so please be gentle.

If the bike runs fine at 1200rpm then let it have 1200rpm. If you got the carbs properly synched you may be able to hit the 1000rpm mark but if it runs fine at 1200rpm then just leave it there if their are no other issues. you can try adjusting the air idle screws if you haven't already. Adjust your cam chain tenshioner as well that could be your "slapping" noise.