Topic: Engine running unevenly at maintained speed

[Luythen]

Hey

I have a small problem with my 1978 cb 500 four

Yesterday i found myself driving through heavy rain for the first tima on my bike, about 20-40 km of very heavy rain at about 110 km per hour general speed. at one point the engine stalled when i pulled the clutch, later on it did not respond well to throttle in fifth gear. (this all happened during the rain)

Later on i had another problem with the engine, it will not start nearly as well as before, i have to play around with the choke for a while before it runs propertly.

Now the day after the bike drives alright, however when i maintain the speed at ex 90 km per hour it will feel as though it goes ever so lightly up and down in the speed, although the road is straight and i dont turn the handle any.

earlier i have notised that the tachometer fluctuates wildly at high speeds, i cannot feel this in the engine though.

the carbs are all clean, but do i need to sync them?

sincerely
Henrik

[oldbiker]

The fluctuating tacho is possibly down to a dirty drive cable or wear at the cable ends. Take the cable off, remove the inner and clean it well. Check the condition of the square ends of the inner. Oil, NOT grease the inner and replace. If this doesn't cure it then maybe the worm drive off the camshaft is worn or the tacho head itself needs attention.

[SteveD CB500F]

Poor running in the wet (especially heavy rain) is common with these bikes. The water is blasted into the ignition coils under the tank which short out.

[Luythen]

Thx for the answers, i am going to check the tach cable this weekend, and as for the rain thing, it is always nice to know what goes wrong :-)

[SteveD CB500F]

There are some solutions to the water problem.  In the past, I've removed the coils and sealed them around the HT lead with RTV (Room Temperature Vulcanising) rubber compound. I've heard of people reversing the coils so that the joints face backwards, but you may find that the HT leads are then too short to reach the plugs.  There are also posts on this board from people who have fitted CB900 coils with replaceable leads and even VFR coils!

[clarkjh]

sounds like waters getting at the electrical some were.  I got caught in a major downpore last night, heavy rain for over an hour.  I had new tires so I found the bike handling well in the water, but found I had to run 65 ish to keep the rain off my helmet .

I had also just made a new gasket for the points cover, the old one would've let the flood in.

James

[TwoTired]

Yes, the obvious place to check is water invading any cracks in the ignition leads to the spark plugs.  This can bleed off the spark to ground before it reaches the spark plugs.

I had this nagging (poor running in the rain) problem years ago on my 74 CB550.  When it was dry out, the bike ran fine.  So, as a test, I drenched the ignition leads with a water hose one at a time, only to find the bike ran without so much as a hiccup.  Puzzled, I made no changes, only to have the problem reoccur the next rainy driving day.  Fed up, I pulled under an overpass,  removed the tank, and hand dried all the ignition wires and coils with a rag, only to find the problem still there.  So, I stumbled the last few miles home where it finally occured to me to check the points.  Sure, enough, with the points cover removed, I could see water bridging the points.  I dried all that out and the bike ran with it's usual dry day zeal.  In the months to come, I also I also found out that, if there was water in there, the engine's heat would turn it to steam, and redeposit water condensate throughout the compartment and cause odd engine behavior for a few days after the rain. Further, I learned that the stock cork points cover gasket was, at best, good for one installation/compression cycle, and would leak when reused.  It seems that the cork would shrink on the dealers shelf (or was cut wrong during manufacture) so that it wouldn't seal the points cover even when new.  So, I went to the hardware store and found some neoprene sheet of the size and thickness comparable to the cork ones. At home with a #11 Xacto blade, I carefully cut out the neoprene in the shape required to become a points cover gasket, and installed it on the bike.  That ended the engine run problem in the rain.  And, as a bonus, that same gasket is reusable and still on the bike working to this day.  I now make a neoprene gasket for each SOHC4 points cover I own.  I also learned that with reliable power coming from the engine in the wet, it really pointed out the poor front brake preformance.  But, that is another story...
Cheers,

[endobendo]

I've got this same problem on my 76 750 F with Dyna S, Dyna coils and wires.  Whenever it gets real wet it only runs on 1/2 choke.  I guess I could check the sealing at the coils, plugs and point cover a little more closely.  Since it would run on choke I couldn't fathom the problem was electrical.