SOHC.co.uk Forums > CB750

Hesitation after @ 4500 rpm

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MrDavo:
It was me who had fuel starvation on my Z1-A due to a dodgy pattern fuel tap, but this isn’t similar, your bike ran ok and then it didn’t.

I did have issues with my CB750 K1 running rough, though, after I’d rebuilt the engine,  which turned out to be rock hard ancient HT leads into which the plug caps had been screwed so many times that there were only a few threads of copper left. It all looked ok and the caps screwed on tight, but I had intermittent plug fouling and cylinders cutting out and coming back for no apparent reason. CB750 leads come permanently attached to the coils, you can’t change them without clever surgery, there is a thread on how to do this somewhere. I wanted the bike sorted properly, and was dubious about 50 year old coils anyway, so I made a large contribution to the David Silver retirement fund, and put new genuine coils, with leads, on. The bike ran perfect after that.

This may not be your problem, but worth checking out.

Oggers:
Lobo

Difficult to assess which cylinders are misbehaving. It seems to run OK-ish below 4500 or so, then just runs out of power after that. I can wind the throttle fully open on the road, and then nothing - it remains at @4500 and no more. At a standstill it will rev all the way - which is why I was thinking fuel related. I will try replacing the condensors as one seems to be misbehaving. Ignition on, open points, then sparks across one set of points - which there should not be. Other condensor looks fine.....

Davo

HT leads are pretty new and decent ones. I will check all HT connections though. Simple push fit...Coils could be an issue for sure, but keep that for later....

Laverda Dave:
It may sound stupid but......Are you sure there is nothing blocking the air intake below the seat ( I am assuming the Z650 has the air intake under the seat as most SOHC do and as my Z400J has as well). Theres nothing attached to the underside of the seat pan such as a document wallet that may be flapping open when the seat is closed and blocking the air intake? Likewise, is there anything that may be blocking the air inlet between the air filter and the air box to the carbs? It could well be the condensors though, just a bit odd that it happened directly after filling up but then again this is what happened to my indicator relay last week.

Oggers:
Dave

Pretty sure nothing blocking the air intakes - which is indeed under the seat, but I'll check it out. I've known stranger things to happen! Condensors - hopefully that will be the issue. Perhaps after turning the ignition off/on at the petrol station, one condensor just gave up...who knows. If it is not condensors, then I'm stumped  :'(

Lobo:
Oggers - my previous post was suggesting a method to determine which cylinders are misbehaving. By chopping the engine at problem RPM, rolling to a stop and then pulling the plugs, you may find one / some are wet / cooler than others, which would indicate the problem cylinder(s). Armed with that info will help you massively in determining the cause.

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