SOHC.co.uk Forums > CB750

weak spark

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Bryanj:
Check coil voltage with an accurate meter , also at points wires when disconnected it's not unknown for the leads to go high resistance due to length of wire and connections

Lobo:
Hi John,

Whilst you replaced the coils did you actually check / replace with spec coils? Have you actually put a meter across the various wires to confirm good earths, 12v & so on?
You seem convinced its a spark issue... so here from my CB750 K series manual if of any help.
(The manual also includes supplements for subsequent F models.... no changes to the ignition system tat I can see...)

Cheers,
Simon

Lobo:
Further thoughts...

(Morning Bryan. Sorry - doubled up on your suggestion, my posting being typed as yours registered. Only just seen it..)

IF your spark is weak, for whatever reason, it signals a 'lack of electricity' for want of better words.

I'm not familiar with the F1, but suspect the wiring is not far off most SOHCs of that time, but please, do check before trying this.

The battery positive goes to a fuse box, up through the steering head to the ignition, through the handlebars to the kill switch, back through the bars / steering head & finally to the coils. (The wiring likely changes red to black to black / yellow along the way). Anyways, a few loom plugs, switches & a 35yr twisting loom... all hurdles.

What I'm suggesting is a direct jumper wire from the battery positive to the coils +ve. You don't need to disconnect anything else... just run that lead. (leave the kill switch @ OFF throughout this test). Beware now though that the coils are permanently ON, ie don't leave them in this state too long & disconnect the jumper soon after the test. Most importantly DO NOT ride the bike as no way of stopping the engine should you come off ...other than choke perhaps.

Yet more thoughts (don't know the F) is regarding the main beam... is this on with ignition & 'sapping' electricity during cranking? If so remove the head / tail bulbs & try.

The starter motor / battery... are these in tip-top shape... a worn starter may drop the battery voltage right down & really weaken the spark. My K2 was originally wired (po) lights ON @ ignition... it made a significant difference to firing up.

Finally, how familiar are you with the bike? My K2 frustrated the hell outta me on cold starts, despite it having new 'everything' (inc carb set-up / jets). I still don't understand, but I've discovered that to cold start I dry crank it (kickstart) a couple of times, then full choke, wide open throttle (WOT) and press the starter. Amazing, I might have left it 4 months, but always fires up <2 seconds.

Cheers,
Simon.

neat street:
Thanks for your help everyone, I now have plenty of things to check. I will up-date you all on the result.
Many thanks again. John

neat street:
Hi All
Thanks again for all your input, I think I found the problem in the handle bar switch, one screw was a little too long and earthing intermitently. all seems perfect.

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