High revs won’t drop back down

[Craizeehair]

I picked up a new project bike last week, a CB350 K2 as I fancied something different to play with.

I knew it had a few things to sort, one of which is when the bike is running it ticks over fine and will rev fine and go back to tick over but it will at some point hit 4-5k revs and just stay there, it won’t drop back down, you can’t twist the throttle forward to bring it back down. If you kill the engine and start it again it will tick over fine but will stick at high revs again when revved up.   

I haven’t had it long enough to really do much and was hoping I could get some pointers before I go tearing in and set off down the wrong path. I have had a quick look in the carbs and everything seems ok at first glance, diaphragm, jets etc all seem ok. 

Any pointers as to where I might start?

[Athame57]

Any pointers as to where I might start?

I would be getting new throttle cables myself. Also I would check the movement of the throttle arm, maybe the shaft that is holding it and the 'butterfly' et al is sticking and slow to close again because it needs cleaning and greasing.

[Craizeehair]

Thanks, I will check all that but everything seems to snap back closed but the high revs maintain, I haven’t left it too long but it will stay there for a good 30 seconds to a minute before I kill the engine.

I haven’t done enough testing to be sure but it may be the new it warms up rather then from cold. My initial thoughts are an air leak, carb issue or spark advance, just wondering if they are reasonable assumptions and places to start?

[McCabe-Thiele (Ted)]

This reads like typical so called "Hanging throttle" problems.
If you use the site search facility this subject has come up frequently.

Ths is one of many posts on the subject.https://www.sohc.co.uk/index.php/topic,31510.0.html

[SteveW]

Air leak on inlet rubbers?

[DomP]

I agree with air leak at the inlet manifold, I was in denial for a while because I'd fitted new orings, turned out they were undersized cross section, correct size fitted solved what had been a long term issue.

[Craizeehair]

Thanks for the suggestions, looks like I need to find some time to get the basics checked and go from there. I don’t think it’s the intake rubbers as I replaced them already as the others did look pretty ropey.

[Rozabikes Tim]

Various utube videos I have watched used a spay of brake cleaner at tick over on the rubber manifolds. If that changes revs, there is a leak!

Never tried it myself but makes sense....

[DomP]

Yes I'd changed mine too which is why I continually overlooked them as the problem.  My new orings are significantly thicker

[Craizeehair]

I’ll try spraying around them and see what happens, I’m pretty sure it only happens once it’s warmed up?

[Toko_Jo]

Always worth checking carb rubber diaphragms on those CV carbs for degradation/splits.

[Craizeehair]

What I’ve found out so far, the diaphragms weren’t great, one was split and one was wearing through, both have now been replaced for NOS from DS.

It still had the same issue after changing those, I did squirt some easy start around the new intake manifolds and the left hand one did cause the revs to drop slightly.

I went back to basics and adjusted the cam chain, checked the valve clearances, no issue there.

When I checked the compression it was rubbish, around 80! Squirted a new drops of oil down there from the oil can and it improved to around 120.

I’m going to do a top end rebuild at the least and may go deeper, so the hanging idle will be addressed if needed after that. I’m going to take my time and hopefully have everything in place to start it when the nights start drawing in and in the meantime, concentrate on riding the two bikes that are ready, willing and able through the summer months.

Thanks for all the responses, very much appreciated.

[K2-K6]

From that description, it sounds like it could be a bit "windy" somewhere in the engine.

Before stripping it, you could try putting the crankcase breather into a glass of oil when you get conditions that cause it to rev. If it sucks the oil up into crankcase then that would suggest something amiss with rings in allowing it to draw air past piston on downstroke to supply even with throttle closed.

Could be worthwhile checking like this to give more indication before dismantling. 

It may have broken or badly worn rings/bore/piston in there.  Or, possibly very stuck rings from gumming/corrosion if sat for extended period.

[Craizeehair]

That’s interesting, another thing I didn’t know. I knew I shouldn’t have started stripping it before updating here…

I’ve taken off the points, spark advancer, air filters and carbs already!

[K2-K6]

Aah, ha.

It doesn't matter too much as any of those conditions I've mentioned above are going to be very apparent on inspection. Ultimately you'd have to do the same strip down if it ducked up oil, just that you'd have a little more evidence for "conviction" if that was present 

Hopefully you'll be able to pinpoint much in the way of untoward conditions easily.

It'll be interesting to see what you've got in there once apart.