SOHC.co.uk Forums > CB750

First ride misery.

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Graeme77:
I think its cahrging properly, but ive had the battery fully charged since this issue started and its still the same.

As far as checking the charging system, i think maybe i should explain what ive checked already.

I checked it out of curiosity a few weeks back. Voltages rose with revs but not quite high enough. I tweeked the voltage screw to get them up a little. I also re-did the core gap.

When i checked it today it was showing 15.5v at 3000rpm, so i tweeked it back down a little.

Im wondering if this voltage was high enough to kill the ignition unit. I guess i have no way of knowing if it may have gone even higher.

If the regulator was bad, what symptoms would show?

Battery is back on charge now, but it reads over 12.5v at the moment anyway.

Im sure the ignition box is holding the idle up, but nothing seems wrong, so i think its broken.
It did once idle at 1500, it then gradualy rose up to 2500, stayed there for a while, then died.
Im sure the idle stop on the carbs is right, the box is making it idle high, then when i try and tweek the idle down im just shutting the carbs off.

Graeme77:
I recharged the battery and tried again.
No different.

It was idling at 2500-3000rpm. I got a timing light on it was half way between F and the full advance marks.
However, it doesnt change with revs.
I got the engine to briefly run at 1500rpm but the advance was the same.

The box is stuck somehow and not advancing/retarding.

This took 5-10 mins, at which point i noticed the headers for 2/4 were glowing cherry red!
I shut it down and walked away :(

Graeme77:
Could i take the whole bikes electrical system out of the equation by running the electronic ignition from a seperate, fully charged battery?

See how it runs then.

That would remove all possibility of any problems with connections etc.

BigAl (Alan):
I would look at replacing the main jets No 105 as previous posts, and check for leaks in the inlet rubbers.
I think the early CB 750 No 120 jets have been fitted

Good luck

Alan

K2-K6:
If the advance was not going back down to base level you would get those sort of effects i.e.fast idle and too much heat (from advance with no increase in fuel flow, making it leaner than ideal).

You should be able to rig up a power supply from a good car battery maybe just to the ignition pack to take that variable out of the equation if the pickups are not earthed back to the engine, I'm unfamiliar with the circuit used on these.

If no advance function, can you swing the timimg plate back to retard it and see what response this has at tickover?

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