SOHC.co.uk Forums > CB750
Hesitation after @ 4500 rpm
Oggers:
Lobo
If new condensors don't sort it, I'll give it a try....
Cheers
philward:
I have had condensors go on me on 2 separate occasions and in both cases, the symtoms where the same. The engine misfired as I accelerated - surging then holding back as I rolled the throttle on, This was across the whole rev range and not just at higher revs.
Lobo:
Oggers, have reread your initial post, and see you have an additional in-line filter.
If no joy with other routes, consider taking this out for a short trial. I too used one in a small Yamaha, which would loose power at WOT (on hotter days), but run fine at small throttle settings. The in-line filter was hard to see as buried in the crankcase along side the carb, but upon inspection it appeared only half full, and somewhat airated. Removing it solved the issue.
I’m guessing it was fuel vaporisation - a fairly well documented phenomenon. Typically it rears it’s head when an engine has been switched off for a while, allowing heat to soak into the fuel lines. This fits with your petrol station fill up…
Worth a try I’d guess if al, else fails.
Oddjob:
Have you got one of those bottles that hold fuel while you sync carbs? Just thinking, try fitting that if you have and go for a quick run to see if it's a fault inside the petrol tap.
Oggers:
Chaps
I am very pleased to report I have just ridden the bike very quickly up my very convenient, quiet test hill just outside the village, and I can report all seems well. In fact far better than before! It pulls like a train. The problem did indeed appear to be condensors, and in all probability, one of them was on the way out creating less than adequate sparking before it finally gave up.
Couple of points - pun intended....
Intially I must have inserted the fork connectors of the condensors to the small bolts incorrectly. I do realize there are insulating washers and that the contact fork terminal sits in there also. Initially the bike would not start after replacing the condensors. I then fiddled about further, and inserted both fork terminals under the washer which sits under the head of the bolt. The insulating washer follows this, which would seem to insulate the bolts/forks from a bracket and thus the contact breaker plate? When it would not start, I feel the fork connector(s) may have been in contact with bracket/contact breaker plate. I am no sparkie so lesson learned perhaps....which is carefully note how things are connected/attached before removing them!
Everything pointed to fuel as the problem. I guess the condensor died when I turned the ignition back on at the petrol station. One for the brain's hard drive - not to be wiped. Thinking about it, the bike now performs so well that it may be worth replacing condensors on other bikes as a matter of course. I have no idea how these things behave and if condensors degrade over the years, but for £10 from Fleabay, it may be a worthwhile thing to do on the 400/4 and the GS1000.
Very many thanks for all your suggestions and help. I find these forums are a truly beacon of kindness in this increasingly disagreeable world.
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