Honda cb350 CV carb issues

L

lhemrick

Active Member
I have a 1971 Honda CB350K3 I'm trying to bring back to life and am having some carb issues. I have replaced the diaphragms, cleaned and put new kits in the carbs. I reinstalled them and have a problem I'm unsure about. Understand I have never owned a set of CV carbs before so I need some help. From time to time the engine will take off at idle while sitting still to around 5 grand. The only way to stop it is to turn the engine off. They are fine on the refire until you throttle it up and they take off again. Can anyone help with this problem?
 
How did you replace the diaphragms? They are NLA. I assumed you got good used ones?
Anything sticking or binding in the carbs?

Check for air leaks around the manifolds.
 
I purchased some new ones from www.jbmindustries.com and put them on my slides. I have checked for air leaks and found one on the left carb boot, fixed it but it didn't stop the problem.
 
From all accounts I've heard, the JBM Industries diaphragms are a very good replacement. My first thought was an air leak around the carb boot, probably worth checking again just to be sure. Also, have you sync'd the carbs since the changes?
 
your problem stems from the throttle cable adjusters between the carbs....one or both is screwed out too far simulating a slightly twisted throttle......if one is out further than the other the engine will rev creating a vacuum that will ultimately pull the other carb into play and increase the rpms. mine does this all the time upon re-assembling the carbs to the throttle cables(unless i'm lucky and get it right by accident).....what i do is lift the tank up so there's room to get my hands in that stupidly small space under the frame and between the carbs and use the choke to slow the engine speed while trying to spin those adjusters. i heard of another method that requires you assembling everything and sticking your fingers into the carb to feel exactly when the butterflies move when you turn the throttle. i don't quite understand how that's possible so i'm going to ask him again next time i see him how he does it.
good luck
greg

screwing the throttle adjuster into the little arm on the carb should reduce the tension in the cable.....un-screwing the adjuster would then put more tension on the cable.....if my memory serves me correct, if i'm backward sorry, just adjust until the problem corrects
 
does it build to 5k rpm or is always sitting there?

sounds like a points/advance problem... just getting stuck
 
CV carbs are one of the simplest designs there are. Are you positive the problem is carb. That is always the 1st thing a rooky starts tweaking as soon as the bike acts weird. If you pipes and filters are stock. And is set to speck, than its not the carbs. If they're not, and you didnt address 1 mod at a time. You have created a monster that only 1 with experience can kill. I cant give you the yrs of experience in a message.
 
for your symptoms, vaccum leak, cable hanging up, or poorly routed, timing advanced to far. etc. the list goes on.... And yes carbs are lean, if thats the problem....? try bringing your idle screw out a full turn, if it gets better, you'll know thats the problem and your going in the right direction. if not, could be a number of things.
 
No this bike is completely stock (pipes may be aftermarket but are full length with baffles) and I replaced all the jets with the same sizes that were in it. Using the stack air cleaners too. The bike will crank fine and idle at 1100 and you can throttle slightly but if you take it too far it will start toward 5000 on its own (even if you let go of the throttle) and sit there till you kill the engine and recrank it. You can even ride the bike if you take it easy but get on it and it takes off on it own! I will also admit I've never dealt with a cable carb setup like this either. So I am kinda at a loss as to what is going on, course if I knew I wouldn't be asking for help. Thanks for everyone input so far and I'll take it to "WORK" tomorrow and see if I can remedy the issue with the advice.

THANKS!
 
I'll bet your cable is either sticking ( has too radical of bends in it ) or simply needs lubed. Or you have a vaccum leak some where. when it's idling, spritz some water around the boots, butterfly shasf in the carb bodies etc. if you find the leak the idle will "drop" from the water being induced.
 
hmm...the "idle at 1100" make me question my fix a little......does the throttle grip snap back home with ease,lube? while putting the tank back on maybe twisted the split cable shortening one side and lengthening the other ......if throttle grip snaps back easily, check and see if the idle screw is floating above where it rests on the carb. if it is floating, squeeze it to its seat with your fingers and see if it slows the engine that way. this, of course is if you've got too much tension on your cables and it's one way to find out if your butterflies are not seating completely with the throttle grip sitting untouched at idle. this problem does allow you to ride around because you are under load until you throw it into neutral or pull in the clutch and the engine loses that load. check it out, it's a cheap easy way to at least eliminate one possible problem before messing with anything else.

good luck
greg
 
Its a brand new cable which doesn't mean its not sticking but it reduces the chances a lot. By the way what is the proper setting for the air mixture screws to begin adjusting the carbs? Neither manual tells me where to start.
 
sorry, but you're on your own there....even tho mine hadn't been run in 17 years, i knew it ran when parked so i never touched that screw....it worked before, it'll work again and for once that minimalist approach worked out for me. you don't need to listen to me, but i'd say one thing at a time and change as little as possible until you get a better feel for your particular beast.
 
Do you have a manual?
The carbs are not clean if all the jets and needles were. It removed and checked.
Really it's one of two things
-Air leak, you have been given a few ways to check that
-the physical throttle plates or slides are not working as they shoul.
 
It takes a lot to get me down and I'm aware of the issues of getting old bikes running again after all I own seven others but as I said before this is the first twin and surely the first CV type carb setup I have attempted. I'll keep trying and It will all work out sooner or later. Another question; how much top end noise are the little engines supposed to have? It runs a little louder than any other I have and am just wondering if its a trait or is something wrong with the reassembly. It runs good and strong just a little noisy in the valves.

"You will never find the answer if you don't ask a question"
 
at 50 mph. mine sounds like your dragging a knight in shining armor down the street by his nuts! but seriously, there is a lot of valve noise in these guys and i've adjusted all over the place even pushing the limit of the required clearances to see what would happen and still not a lot of change in the "loose sewing machine" or "can of bolts" sound that thing pumps out.....so i put it back to spec and accept that people can tell it's me coming long before i get there.
 
I had nearly identical symptoms when I first fired up my 73 350, and resolved by firmly re-attaching the rubber boots from carb to engine. IF, like mine, yours are original they are probably very stiff, and its very easy to get an air leak when fitting the carbs back on to the engine.

Or it could be any of a dozen other issues as the guys here have already mentioned.
 
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