Rain Man!

[Pete]

Hi Everyone,
I joined this forum expecting to find people asking about the problem I have, having been told that it is a common problem. I haven't opened every message, but at first glance I don't see anything similar.
The problem? - My CB750Four K7 (1978), runs beautifully- until it starts to rain! When it rains, or even if there is a lot of spray from a wet road, the bike starts to run badly, backfires, intermittently runs Ok, then bad again, and will conk out completely. It will then start up again, but continue to run badly. I need to use the bike daily, and here in the UK, a bike that doesn't work in the rain is frequently useless.
I've had it looked at, some parts replaced (supressor caps), but has not cured it. Any suggestions?

[SteveD CB500K0]

Hi Pete.

I'm afraid that it is indeed a common problem.

Once you've looked at the plug caps (make sure that they seal against the HT lead), you will need to look at the ignition coils.

The problem stems from the fact that they are mounted beneath the tank with the HT leads exiting forwards. The leads are not detachable and feed into the body of the coil.  In this position they collect all the wet and crap sprayed up off the road and with something this old (even when they were new!) the damp gets in and shorts out the coil output.

Three possible fixes:
1. Don't ride in the wet  (OK, OK...)
2. Fit some DOHC coils (look on www.sohc4.net/forums for people who have done this and posted instructions - one member even used VFR coils...)
3. Do what we did in the 70's:
Take off the coils and put them somewhere warm to completely dry out.
Clean and rough-up the body of the coil around the HT lead exit point with something sharp.
Apply RTV (Room Temp Vulcanising) gasket goo over the area to seal the coil/HT lead joints
Allow to \"vulcanise\"
Refit coils and voila!

[SteveD CB500K0]

PS.

If you are in SE England (within 50 miles of Reading?) give me a call and I'll come over and help.

Steve
07802-307696

[JonnyB]

I agree with everything Steve said, I would just like to add, check the condensor cover, Mine fits non-to well, and when it really ?isses down, it gets wet in there too.

[Pete]

Thanks Steve,
I will try out what you suggest, and let you know of any results. I will be away for a week, and then again for a fortnight over Easter, but I will have a go when I get the chance.
I'm in Newport in South Wales, and working in Bristol.
I'll upload some pics of the bike too.

Pete

[jimshea]

I had a 76 F1 that had the same problem. Has your bike got a black plastic box at the front just above the engine where all the wires are terminated? If so try and seal this box from the elements with some kind of rubber sealant.