SOHC.co.uk Forums > CB500/550

MISFIRE ON NO4 CYLINDER

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deltarider:

--- Quote from: K2-K6 on May 14, 2025, 10:44:29 AM ---If there's nothing fallen out in the carb (main jet, emulsion tube etc) then it's worthwhile with very close examination of idle circuit to be absolutely certain there's no impairment.

If the air passage into the carburettor for that circuit is blocked, then effectively it'll run full bleed on that idle jet as soon as you start it. Start will be OK as it's "choked" by this effect and won't be seen at initial running.

You could try turning the idle screw adjustment on that #4 to assess if it's doing anything. No response indicates it's blocked for air supply.

If so, you can remove the screw itself, spray carb cleaner down into there, then blow out with compressed air to see if it'll clear without dismantling.

--- End quote ---
This ^ is a simple test to verify the idle circuit is doing anything at all.
Turn the airscrew of carb #4 3 turns out and wait some 30 seconds and then fully in and wait another 30 seconds. If you notice within that minute some change in rpm - this does not have to be much! - the idle circuit still works, at least... to some degree (!). The change in rpm is often not much, but... when there's no change at all, it indicates the idle circuit of that carb is disfunctional and needs to be inspected. The good news is: you don't have to pull the rack as yours is a CB500 and with the rack in situ, the #4 is the most accessible of all four carbs. So, if you are any familiar with what's inside, you may just remove the floatbowl. All parts are accessible for inspection. You can unscrew the slow jet and spray carb cleaner in the orifice where it sat and hope the best of it. The main jet is easy to pull, so you can inspect its O-ring. BTW, you pull (and press in) the main jet! Do not twist the main jet! You may damage the tiny O-ring.
BTW, those who have no better filtering than pods for the air intake, run the risk of getting dust in the small orifice in the carb that is for air to the idle circuit (pilot system air jet in the pic below). Another good reason to abstain from pods IMO.

blade918:
hi all sorry for delay holidays double checked compresion all ok all elecs ok so now going to remove carbs swap all inards from carb 4 to carb 1 and vice versa will let you know how this goes

blade918:
update removed carbs 1 2 3 inlets on head lovely and clean no4 black soput hand over inlet kicked her over and you can feel air being sucked in but also blowing back so im guessing inlet valve is a problem head off next to inspect

Sesman:
Are you sure the valve clearances are correct on that cylinder eg not too tight?

K2-K6:
I'm mildy puzzled that a compression test would give good reading but have significant loss back out through a closed valve.

Are you certain of the compression reading  ?  No difference at all from the others  ?

We had, on forum some time ago, something similar in that a recalcitrant cylinder was defying logic of all combined thinking chucked at it here, ultimately to be found in a "strangled" internal pipe within the header section of one exhaust pipe .... believe that was a 500 too.

Worthwhile adding that inspection to the pile along with valve clearance to check, as mentioned above.

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