Topic: Riding in rain, dead cylinder

[Popwood]

Help me solve my mystery or confirm my suspicion about riding in rain and one or two cylinders die-- for awhile. This happened a few years ago on the first godzilla rally ride. Our group was heading north on two lane blacktop when it started a steady rain. After 20 or 30 minutes my 75 750k developed a dead cylinder. Our group pulled into a gas station for about 20 minutes hoping the rain would let up. It did, and my bike fired up with no issues. Off we went. This happened twice more over the next couple hours and we'd take a break, then bike fired up fine, ran fine. Then the last time it happened I decided to head home. My route was on the interstate and I went for two hours in a heavy, relentless rain and no issues. I've never had the problem again, but I don't ride much in the rain.

My theory -- while riding at lower speed water would get to the plugs and foul. When we'd stop for awhile, engine heat dried all moister out so would run fine, then more rain, dead cylinder, repeat. On the way home, at high speed on interstate, engine hotter so moisture evaporated immediately so the issue disappeared. It could also be that the aerodynamics of slower speed dumped more water on plugs than at high speeds. But should this be a problem at any speed?

Thoughts? Anyone else have a problem like this?

[Deltarider]

Until last year, when all of a sudden the ignition switch failed (soon discovered and after some fiddling I could continue) the only time in 40 years that my bike let me down, was in torrential rain. Some plugcaps got soaked cutting two cylinders out. CB500/550s in particular are vulnerable to this. In European motormagazins including riders reviews by owners this always was a point of criticism and Honda - who closely read what Europe wrote - has tried to solve this with different caps over the years. Might have happened to you. What bike was it?

[calj737]

I would inspect the gasket material behind the points cover. Likely rain getting to them and causing a condenser to misfire.

[ekpent]

 All sound like good suggestions. Running pods in the rain can be a problem also I have found. Do you have the stock air box ?

[rotortiller]

Usually it's the high tension caps, coils or leads shorting out from moisture. If it were a set of points you'd loose two cylinders. Take some DC-4 grease to the wires and caps then try the rain thing again. Sometimes you can get arcing between spark plug wires.

[Popwood]

All stock, including airbox. CB750 K5. I recall checking for moisture behind points cover and it was dry.

On a side note, I had done little to no riding in rain prior. Although the mechanical problem was really annoying, the rain experience gave me great confidence going forward. Must say, though, riding the interstate with big rigs blasting by spraying a fog of mist that made vision limited was very hairy.

[Deltarider]

HT leads and caps should be clean and free of grease. When older they can develop little cracks. Also over time silicates can deposit. Under normal conditions not much of a problem, but in rain they can become conductive, causing parasitic drain.

[andy750]

I had a similar problem in the 90`s with my then CB750F2. Just as you describe - rain would kill the bike. I had a mechanic look at it pour water over it while at idle to replicate the effect (it did not), changed the HT leads/coils/caps/plugs and the points cover gasket - nothing helped. I never did figure out what the problem was and I sold the bike after 7 years ownership to move from the UK to USA. If you do solve it please let us know! Drove me nuts!

Fortunately its only been that CB750 that had the issue -the 2 that I bought since have been in many a rainstorm. 

[Bankerdanny]

My 550 had this issue until I installed new caps. I use a smear of dielectric grease on the insulation to provide additional sealing against water.

[HondaMan]

Help me solve my mystery or confirm my suspicion about riding in rain and one or two cylinders die-- for awhile. This happened a few years ago on the first godzilla rally ride. Our group was heading north on two lane blacktop when it started a steady rain. After 20 or 30 minutes my 75 750k developed a dead cylinder. Our group pulled into a gas station for about 20 minutes hoping the rain would let up. It did, and my bike fired up with no issues. Off we went. This happened twice more over the next couple hours and we'd take a break, then bike fired up fine, ran fine. Then the last time it happened I decided to head home. My route was on the interstate and I went for two hours in a heavy, relentless rain and no issues. I've never had the problem again, but I don't ride much in the rain.

My theory -- while riding at lower speed water would get to the plugs and foul. When we'd stop for awhile, engine heat dried all moister out so would run fine, then more rain, dead cylinder, repeat. On the way home, at high speed on interstate, engine hotter so moisture evaporated immediately so the issue disappeared. It could also be that the aerodynamics of slower speed dumped more water on plugs than at high speeds. But should this be a problem at any speed?

Thoughts? Anyone else have a problem like this?

I've been exactly here, twice. The last time was in 1986 when coming home from Texas, almost midnight, in pouring rain. Each time I stopped to warm up the engine dried out the #1 sparkplug cap and I'd get another 40 miles or so before it soaked in again and I lost #1. After I got it home (and in daylight the next day) I found the top rubber boot of the #1 plug cap had split, and each time it got wet enough it would send the spark around the rubber to the raindrops bridging to the engine. Once in a great while it would also bite my left leg, which I was hugging against the engine for warmth in the dark, wet night. Kept me awake, though!

New plug caps fixed it up for several more decades until 2016 one summer when the 2-3 coil croaked on the way home from work on a 100+ degree afternoon in heavy traffic. I got new coils after that to fix it back up.