SOHC.co.uk Forums > CB500/550

Fuel Height

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K2-K6:

--- Quote from: Steris on May 06, 2021, 08:34:28 AM ---Would low compression cause slow running issues? I'm struggling to think of anything else that's causing my problem. It runs lovely over 3000rpm sits at 60 70 mph great, it also ticks over at 900rpm when cold, but once hot either dies or revs itself up to 3500 rpm. 🤔🤔🤔

--- End quote ---

Have you any compression readings for it?

Runaway idle speed is always excess air however it gets the supply to the combustion chamber.

murdock:
That the air screw rotates 3 turns and there are no changes in idle is not the right thing to do, I think you have the problem in the slow jets, I think you should reduce, which jets do you have at 38, 40?

JezzaPeach:
I had the same with a hanging idle and found one or two carbs needed balancing.

Steris:
Hi Merdoc
Everything in the carbs are original  so 40 pilot jets. I'm going to  remove the carbs again this weekend and drill out all the brass plugs in the pilot passage ways and make sure they are 100% clear..

deltarider:
I wouldn't drill out anything. If yours is a CB500, realise that a common mistake is trying to achieve a highest possible idle rpm by turning out the airscrews. No doubt the engine will idle nicely and pur like a kitten, but it comes at a price: driveability is gone and you'll experience a poor acceleration. Once the engine is hot, rpm will not drop. Realise these are quite simple carbs that lack acceleration jets. Therefore airscrews need to be set as is specified in the manual, even when you need to adjust the big idle knob to arrive at an acceptable idle rpm. Mixture will be quite rich and I wouldn't be surprised an exhaust gas meter would show 5% CO. I recommend an idle rpm of about 1100-1150. Also realise that after a cold start, you will need to maintain a raised idle to prevent stalling. Some do it by keeping the throttle grip twisted a bit (as described in the Owner's Manual), others use the big idle adjuster knob, which they will reset at a traffic light, once the engine has warmed up sufficiently.
Verify fuel supply is constant without hesitations. Abstain from 'extra' inline fuel filters; I had nothing but trouble with them. If float valves and needles are somewhat sticky, a fuel additive like Redex may help.
Realise that todays gas is not the same as in the 70s.

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