QuestionI have been working on my 1971 Honda sl. Great bike. I replaced the points, plugs, plug wire caps, battery, adjusted valve tappet clearance, put new carb kits in both carbs and set timing per the manual. the bike starts easily and runs/idles very well. It also runs quite well at rpms 3K and above. However, on lower RPMs it runs rough and seems like it backfires when you back off on the throttle. I adjusted the carbs per the manual but I can not seem to get it to run smoothly at low rpms. Do you have an idea how I can get it to run w/o the backfire or miss fire at lower rpms.
Thanks much for your help.
Bill
AnswerBill.... Start with the basics... you've adjusted the valves, so check the compression. Should be in the 170 psi range. Once you know you have good compression, then you can move forward into troubleshooting.
There are stampings on the carburetor, denoting the calibration series.
Series "A" carbs require 25mm float level, 120 main jets, #40 idle jets, needle clip at 4th groove from the top and one turn out on the idle mixture screw.
Series "561B" carbs carry a #125 main jet and all else is the same as above.
You didn't tell me if you have stock air filters and exhaust system components. If so, then the carb calibrations should be good to go. If you have put on pod filters and changed the mufflers, then all bets are off. Start with at least one main jet size up. When the bike starts to stumble, try adding some choke a little at a time to see if it improves or gets worse. If it gets better, then you need to increase jetting again.
Straight through mufflers are going to backfire no matter what you do...
You didn't mention checking the mechanical spark advancer to see if the weights and point cam are working properly. Pull the points plate and check the advance/retard function. Common area that is overlooked and causes problems.
Finally, if you used the KEYSTER brand carb kits, they may have calibration issues, so use OEM jets and needles whenever possible. I'll assume that you put the carb slides in the correct sides. They are left/right specific, with the cutaways facing the air filters.
Bill Silver
PS///// There is a misfire condition that can be setup when the ignition points are both closed momentarily, during the firing cycles. This occurs when the point gaps fall below .012" in most cases. The points need to be fed separately while the coil windings develop their magnetic field. With small point gaps, there is the possibility that both points can be closed, thus both coils are being energized with insufficient current available to fulfill that task. Always be certain that the points are gapped out about .014" maximum gap, then set to open at the F and LF marks respectively. This is a somewhat subtle effect that isn't always apparent to troubleshooters faced with ignition system malfunctions.
