Q&A: A not so idle idle
By: Web Editor
Q: My problem is that when I start my 1975 Honda CB550K1 (no: cb550L-1060824) it will not tick over properly. Once warmed-up it will tick over at 1250rpm then without any throttle will fly up to 3000rpm stay there for a few seconds then slowly return to 1250rpm then die.
I have fitted new points and condensers, a Keyster carb kit with 100 and 38 jets, new seals on intake, new air filter and new standard exhausts. I have balanced the carbs on the bench, but it is difficult to use my vacuum gauges with the revs rising and falling like this and I have sprayed Easystart around the joints on the carbs to try and find any leaks. I have even sealed the main jets in with pet seal in case fuel is passing them at tick over. I have spent a fortune restoring this bike and have run out of ideas.
Ian Brown
Chester-le-street
A: Normally, problems with varying idling speeds can be traced to sticking throttles or wrongly adjusted throttle cables, but the fact that the idling speed varies so much without any use of the throttle indicates a fault with the mixture.
Uncertain speeds and, in particular, a reluctance to idle slowly would usually mean a mixture which is too weak, and you have done the right thing in checking for air leaks. One tool which would definitely help would be a Colortune, if you could perhaps borrow one, as this would show you how the mixture varies in each cylinder.
I assume that you have checked the float height and fuel flow in each carb?
1 Response to “Q&A: A not so idle idle”
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YAMAEXUP Says:
May, 24th 2011 at 06:04 pm
The bike in the picture is a CB550F2.The carbs on this & the F1 did not have a ramp on the choke bracket which increased the revs slightly for when on full choke, unlike the carbs on the CB550K3.
I would be checking the inlet manifold rubber 'O' rings. They perish quite easily & are often overlooked when trying to find what looks like a carb fault. The other item to check is that the idle mixture screws are clean. On the F model carbs it is just a tapered screw, the tip of which gets oiled up quite quickly. On the K3 model carbs, the air screw is at the front of the float bowl, & should have a small spring, upon which sits a minute metal washer & an 'O' ring. If any of the parts are missing, this can upset the idle mixture quite badly.