SOHC.co.uk Forums > CB500/550
I need a carb sync - Anyone in North London?
paulbaker1954:
--- Quote from: michtag on September 02, 2020, 03:23:08 PM ---Update:
So I have changed the boot clamps to very nice Jubilee clamps and tightened them carefully. They surely hold the rubber manifolds firmer, but I still have the same erratic behaviour, no noticeable change at all. As soon as the engine is warming up (1 or 2 minutes max) idle starts jumping a bit and then spikes to 3k...then down under 1k for no apparent reason. Throttle is responsive and there is no real hang if I blip it.
At this point I feel pretty confident it is not an air leak, so I will be tinkering with the advance mechanism springs, as an old post by Hondaman suggested weak springs (a common occurrence) could cause a similar behaviour.
At this point I would also be curious to swap my (original) coils with good working ones just to rule out they have anything to do with it, but I don't really want to buy new ones right now without the certainty I actually need them...
BTW I have tested mine with a voltmeter and they seem fine, but I wonder if they may start "acting up" once warm?
--- End quote ---
Having eliminated air leaks then it may we’ll be an issue with the advance mechanism, have you thought of hooking up a strobe timing light I think that would show if the timing was jumping around
michtag:
--- Quote from: paulbaker1954 on September 12, 2020, 02:13:10 PM ---Having eliminated air leaks then it may we’ll be an issue with the advance mechanism, have you thought of hooking up a strobe timing light I think that would show if the timing was jumping around
--- End quote ---
I have thought about it but if the idle revs erratically would that not show the advance mech moving anyway, whatever the reason for the jumping up and down?
One more thing: could the tappet covers o-rings not sealing perfectly cause this? I am asking because I have noticed a little bit of oil just around the rim on 5-6 of them. It's really little, I have to swipe my finger to actually see that there is oil there. Of course, there should not be oil, so I will eventually change them, but i wonder if it could suck in "unmetered air" and if it would end up in the combustion chamber.
Cheers
Nurse Julie:
Nope.
michtag:
--- Quote from: Nurse Julie on September 16, 2020, 02:20:06 PM ---Nope.
--- End quote ---
Thanks for confirming; I thought it would be highly unlikely (I guess I would also be burning all of my oil if there was a straight passage to the cylinder) but it's always good to have expert confirmation.
Cheers
K2-K6:
As julie responded, through the tappet caps is not possible.
"but i wonder if it could suck in "unmetered air" and if it would end up in the combustion chamber." But via worn, damaged, broken piston rings, then potentially yes.
The crankcase is vented on most engines via a breather pipe , and usually on these ( I think) into the airbox or out behind the engine. This is supposed to vent OUT with any oil fumes burnt through the intake to avoid smoke output to environment.
But if the rings are badly compromised it can also suck air in via this route. It would be interesting to investigate this route to see if this is sucking.
You could clamp the pipe to see if the idle speed improves, or place the end of the pipe into a glass of oil to see if it sucks up the oil while running.
Worth having a look at Julie's 400 rebuild thread to appreciate how bad the rings can be with the bike running quite well.
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