Topic: cb350T-- idling problems

[paulages]

i sold my '71 cb350 to a buddy a while back, and wouldn't you know it--it started having every problem in the book as soon as he got it...

i finally got around to supervising a total tuneup (points, condensers, point gaps, timing, valves, cam chain tension, carb clean and rebuild, and carb sync), but it still has a continuing problem. after adjusting the mixture screws and syncing the carbs and cables as per official honda instruction (and as a did countless time on that bike and several other cb or cl350's), it idles at a perfect 1000-1200 rpm.

however, after running around the block it jumps up to about 6000 RPM, and won't come down unless the kill switch shuts the bike down. kick it over again? perfect idle. we have sprayed starting fluid everywhere and have pretty much ruled out vacuum leaks, and two different sets of carbs have produced the same results. the plugs show a nice fat blue spark upon testing.

nevertheless, i can't help but feel like it's some ignition/ electrical issue causing a weaker detonation, that suddenly catches up and makes it run better than when setting everything up at idle.

has anyone else had this problem? poor guy is at his wit's end...

[750goes]

sticky throttle cable/s
sticky slides,
throttle spring getting hung up
cables bining or badly routed - try it with bike running turn bars left and right while revving the motor a bit - if it sticks in any spot - check the cables..
does the throttle snap back or can you push the throttle back down to idle ??

[nickjtc]

Are both carbs CV? What state are the CV diaphragms in?

[paulages]

throttle cables are brand new, and the idle screws are resting on their stops when said condition exists, so it's not a cable problem. diaphragms appear perfect (unless there is some microscopic pinhole i can't see).

[mlinder]

It's gotta be carbs. Hooking up those big mikunis kinda told us for sure, I think.
The big mikunis didn't work because the little 350 wasn't pulling enough to pull up fuel from the bowl. It was just pulling air through the giant throat there.
Anyway, it revved up, and fell all the way back down to dead.

[paulages]

gotta disagree, mark. the only thing that the mikunis told is is that they weren't set up for a 350, nor were they tuned or synced at all. i didn't think it would fly to begin with, but john has been going crazy and you seemed enthused to help, so i figured an attempt couldn't hurt. i don't remember what his vacuum numbers were, but there isn't that big of a bore difference between a 350T piston, and one 750 four piston, nor are the vacuum readings. i'm not ruling out the carbs--in particular something having to do with the diaphragm--but your cycleX mikunis set up for a twin carb cb750 just aren't gonna tell us much. it wouldn't even idle, which rules it out as a useful diagnostic tool if you ask me.

anyway, i'll reinspect the diaphragms tomorrow, but four bad diaphragm seems unsuspect to me. but then again, so does 2 sets of bad coils.. 

[mlinder]

The stroke on a 350t is fully 25% less than a cb 750, as well as having an 12%+ smaller combustion chamber, which is going to make a huge difference in intake velocity, and therefore, draw from the carb.
Anyway, I will come by and see how you are doing today. Interested to see what you turn up. Wish I could have hung out more last night. I figure if we had taped off the intake on those carbs, we could have got it to run on em. At least to tell more clearly where the problem lies.
Oh, yeah, also, I went home to shower before the show last night, and when I came back, the band had already used up the guest list.
Mebbe can get you into our berbatis show next month?

[ohiocaferacer]

Check the mecanical advance behind the points plate.....make sure it isnt binding up. I've had this problem before and its always been the a sticky advance. Pull it off the cam tower and clean it down real good with WD-40......check for again for smooth operation and reinstall.

Greg

[mlinder]

Check the mecanical advance behind the points plate.....make sure it isnt binding up. I've had this problem before and its always been the a sticky advance. Pull it off the cam tower and clean it down real good with WD-40......check for again for smooth operation and reinstall.

Greg

I suggested that yesterday too. He went and checked it and he said the advance was working.

[Cvillechopper]

While working on my friend's 360 T (don't know if you have the same points set up on the cam?) I noticed that that PO had pinched the lead from the left side (cyl 2 I think) between the plate and the head cover.  Had just enough damage that it started shorting out intermittently.  Not sure if that's possibly your problem but we overlooked the one blue pipe, one perfectly shiny pipe detail for the first several hours until his daughter asked why they looked different.   

[paulages]

yeah, greg--i did check the advance and it was working quite smoothly. i was hoping you'd pipe in, as i know you've had several of these guys. maybe we'll pull the advance off of another bike just to be sure.

cville-- i've had the same problem when i was riding a cb or cl350 daily. never really intermittent, but i have grounded the wire to the points before on accident. i'll double check that as well, but it was doing it with the points cover removed altogether.

[mlinder]

Any luck, Paul?

[ohiocaferacer]

Just a shot in the dark......check your spark plug wires where the spark plug caps screw onto the wire. Make sure there is good copper colored wire...if not...trim wire, then rescrew on caps.

Who knows....maybe it could be something simple???

[paulages]

Who knows....maybe it could be something simple???

that i'm sure of...

i'll recheck the plug wires. when testing the plugs against the block, it gets a nice fat blue spark, and i can't find any evidence of arcing from the wires with either set of coils. i still think  an ignition glitch seems the most likely, but i just can't find it. i'll go back through everything tonight.