SOHC.co.uk Forums > CB750

Hesitation after @ 4500 rpm

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Laverda Dave:
Have you checked the timing dynamically, maybe the bob weights are sticking or one of the springs has come off and the weights are not being thrown out to advance the timing. It's a free and easy check.
Also check nothing has come adrift under the tank where the coils are, could be a loose connection. Seems odd it started after you filled up.
I was on an IAM observed ride last Friday when we pulled into a petrol station for a feedback chat. When I restarted the bike the indicators didn't work! The fuse was ok as was the wiring and bulbs. I checked the realay and heard it click but still I couldn't find the problem. When I got home I stripped it down and changed out the realay and found it was the culprit. Although I heard it click it had failed inside. A £9 part ended my ride! Strange how things fail when the engine is switched off and then back on again a few minutes later!

Oggers:
Dave

Yes - I too thought about the ATU not working properly so I sprayed it - behind the contact plate -  with WD40, and also oiled the pivot points - although I must admit I haven't checked it properly as the contact plate fixing screws are a swine to remove!  ATU's don't often go wrong though do they?. I also visually checked the connections around the coils/earth/plus leads - nothing obvious. I did though check the condensors. According to Haynes, if points spark on breaking with the ignition on, then possibly one of the condensors is goosed. One did spark, and reading up, what I am seeing when riding the bike could be symptomatic of a failed condensor.  Condensors now on order.....

I agree that all these things are very odd. One minute it is running fine, very well in fact - the next hopeless, for no obvious reason. Britsh bikes are even worse for this!

Thanks for all the info - all useful stuff. Honda RS progressing - slowly. Bottom end done, barrel back on.       

Bryanj:
New condensers, if not Honda mega expensive ones, have a high failure rate and short life. You may be better using an old one from another points plate to check

Oggers:
Bryan

Understood. It will eliminate condensors as the root cause - or not - at least.

Lobo:
Can you ascertain whether it’s all cylinders misbehaving…. or 2… or just the one as this might better point you to the root cause.

If poss can you ride it for ‘an extended period’ in the rogue rpm band, hit the kill switch and bring it to a stop. Then have a look at each plug, plus of course tap each exhaust header to compare temps.

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