QuestionI am building a 305 scrambler, which I am able to get to run, but it spits and occasionally backfires out the left carb only. I have double checked the valve lash, point gap and static timing and they seem to be ok. The carbs are pretty well synchronized and I still get it. Could it be that the left half of the cam is off a notch and throwing the valve timing off? That's what it seems like but I don't know how that would be possible to be off on just one side. Also the engine decellerates very slowly after the throttle is closed. Ideas?
AnswerDave, I have written several restoration guides covering problems such as this.
First, the cams really can't get "a tooth off" as they are splined to the camsprocket. IT is possible that the retainer nut can work loose, causing the cams to shift back and forth in the sprocket, but there is a lot of racket noticed and a timing light will show a lot of jumping around with this problem and/or loose rivets in the camsprocket assembly. Often the spark advancer springs get stretched or break, so the ignition weights stay out too long, causing a delayed slowdown back to idle speeds.
I imagine that running ignition timing will be a good bit off of the static settings due to excess sideplay in the point cam/camshaft interface. When this happens, static timing can be okay, but fully advanced timing is not matching on both sides above 3,000 rpm. Setting the timing correctly at full advance, which is critical to long engine life, will lead to one side being 3-8 degrees behind the other side at idle. We developed an electronic ignition to smooth out all the ignition woes, which really can't be fixed any other way, unless you replace the cam and point cam shaft assy. Be sure that there are no air leaks at the cylinder head/exhaust pipe joint and that the rest of the system is air tight. IF you have non-stock exhaust or intake system, you will have to keep making adjustments to the needle height and main jet until you get it right again.
Generally backfiring through the carbs is a sign of a lean condition: bad o-rings in the carb/insulator junction, warped carb body flange, ignition timing incorrect, idle mixtures not set properly, float levels incorrect, blocked idle jets and/or emulsion tubes to name a few items. Be sure that the points are clean, gapped correctly and then watch for excessive arcing across the point faces. This would indicate a loose condenser connection or failing condenser unit.
The best way to check carb synch is to drop the bowls and main jet holders, then watch the needles as they rise when you open the throttle. Adjust as necessary.
Bill Silver
www.vintagehonda.com
