SOHC.co.uk Forums > CB350/400

The elusive surge

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Alexrayz:
Hey all, hope you're doing good!

I have made significant progress with my bike the last few weeks I'd like to especially thank Alankelly1 for his most precious help and Nurse Julie for the O-rings and the mounting advice that made the job of replacing the old ones so much easier!.

So: I have stripped the carbs, cleaned and blown every jet screws and passages possible. I have swapped the idle jets I had from an overhaul kit with nice keihin originals, thanks Alan!
The floats have been set to 21mm and checked for holes. The carbs have been bench synced. O rings were replaced and put in place with red rubber grease to avoid shearing. The slides snap back nicely. The mixture screws have been set to 1 1/2 turns out.
After installing the carbs back I have fiddled with the idle stop screw to get to 1200rpm when warm and off the choke. I then set the fast idle screw to 0.2mm clearance.
I get a respectably stable idle at 1200 rpm when warm. Startup isnt too rough.
The problem I have is jsut that the damn thing surges, it can be when operating the throttle or just when left alone for a while. In which case only fiddling with the idle stop screw will make it come down. I then have to fiddle again to bring it back to 1200rpm.
I have removed the throttle cables to make sure they were not messing with anything and indeed no change in behavior. |

Overall it seems like a lean condition problem. But when I turn the mixture screws by a 1/4 turn in to enrich the mix it stutters. My guess would turn to an air leak but doing the "squirt brake cleaner on suspect spots"method to no avail. I tried the inlet boots, fore and aft; the top of the carbs on the gaskets, the depression ports on the boots and the spark plugs just in case and nothing.

Do you guys have any leads or ideas what might cause this?

So close!

Take care

Nurse Julie:
What spark plugs have you fitted Alex?

daz-j:
Hi Alex, apologies for butting in with an extra question but you beat me to it, I was going to ask advice for very similar issues - in my case the engine starts easily but isn't stable when idling - the revs can randomly increase up to ~3-4000, or decrease to the point of stalling, without any throttle input.  Hopefully we can learn together and crack the problems!

I've taken the carbs off a few times now (the joys!) to clean and recheck things - all appears to be ok to the best of my knowledge.  When refitting I've rechecked valve clearances, timing, pilot air screws, vacuum leaks, carb balance - again all seem ok but the issue persists.

So the carbs are off again - I'm slightly suspicious of seals I fitted previously so have just ordered a kit from Julie to replace them.  General advice welcome, but my main question for the forum at this point is what are the worthwhile checks for me to do whilst the carbs are stripped and on the bench please?

McCabe-Thiele (Ted):
FWIW - this a long shot as I do not know at what point the advance & retard mechanism trips in. I would check the mechanism is nice and free plus check the timing dynamically rather than statically.
Likewise as NJ has noted are you on the right plugs & caps &  leads.

K2-K6:
Small observation, have you bench synced the carbs as detailed in "tricks and tips" section of forum ?

There's simple but very influential information in there that sets the carb slides in relation to the throttle lift mechanism to allow correct balanced movement to work without the relationship being compromised.

It's easy to miss this potential conflict if the carbs have been dismantled and rebuilt.

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