Topic: 1971 Honda CB350

[Lil Red]

Hey guys first vintage bike, new to them but not modern sportbikes.

My question is after I ride for about 15 minutes or so, it wants to idle super high - 5k or so. But when I turn it off then back on its fine, something is sticking. Or if I just slow way down in first almost to a sputter without pulling the clutch in yet as I work the rpm's down while slowing down then pull the clutch in when the rpm's go down to about 1,500 it will idle there and then slowly go back up.

Any suggestions what I should start with?

[Gordon]

Welcome!

First things first, we need to find out exactly what bike you have.  The sohc4 350 started in '72.  Is your 350 a twin or four? 

[jamesv220820]

sounds slightly similar to my problem bud i wish i could help ya but i am still diagnosing my problem

james

[Lil Red]

Welcome!

First things first, we need to find out exactly what bike you have.  The sohc4 350 started in '72.  Is your 350 a twin or four? 

Twin

[MickeyX]

Ok, so first off, please move this thread to the "other bikes" area. thanks.  

Clean and set your carbs and synch the throttle cables afterward. I would actually just tune it the whole way from valves on down to carbs so you don't miss something. You could just have an air leak somewhere though. Try spraying some wd40 or carb clean at the boots and see if the idle changes.

There are few of us who have these and are learning as we go. They are simple over all but need to be set precisely or they go all over the place.

[MickeyX]

the diaphragms are hard to come by, new. There does seem to be someone on feebay that has them but they are $$$$$.

[Gordon]

I was able to get a pair of good, used diaphragms for my 360T by buying an ugly set of carbs off ebay for cheap. 

[low-side]

     Here is what I would try:  first, check everything else out to make sure nothing else is contributing to your troubles (valve lash, point gap, timing); next, try spraying silicone spray around your intake manifolds to make sure you're not dealing with a vacuum leak(s); if none of that finds anything, you need to set your idle the old fashioned way.
     Unplug the right hand cylinder and start the bike (if that doesn't work, get the bike running and use insulated pliers to unplug the cylinder while throttling to keep it running).  Adjust your idle screw on the left carb (on the ends of the small arms on the outside of the carbs) until the bike will idle around 800 to 1200 rpm.  Now unplug the left and do the same for the right side.  Now plug them both in (it will idle around 3000 or so) and adjust both sides evenly until the idle is around 800 to 1200.  If you still have a climbing idle, check both exhausts for even output.  If they aren't even, adjust float height and mixture screws as needed until they are even and reset your idle. 
     I hope that's helpful for you.

[ohiocaferacer]

Your mechanical advance behind your points plate is sticking.

Remove the points plate...slide the advance off the cam.....clean...lube and reinstall.

I've dealt with this on the 350 twins alot.

[bobjohnson]

Your mechanical advance behind your points plate is sticking.

Remove the points plate...slide the advance off the cam.....clean...lube and reinstall.

I've dealt with this on the 350 twins alot.

Im having the exact same problem on a CB450, though its not idling quite as high after accelerating (around 2k and falters a bit but generally does not settle back down to a nice idle). Could it be the same thing?