Topic: CB750 bogs down in the rain

[Tugboat]

This has been an ongoing issue for me. I got an '83 Goldwing about a year ago and haven't really ridden my 750 in the rain until recently, when I was reminded that my bike bogs down when it gets wet.

When I say "bog", I mean that that when cruising at highway speed, and the bike gets wet, it starts to not fire on all cylinders. I can kinda "give it hell" and it will pick up, but will still run kinda wonky. I should note that this has never left me stranded and I've never even had to pull over because of it. It is very annoying though, and I've gotta track this down before I take it out for a long weekend again.

Mine is a '76 CB750 F1 with a '77K motor and, probably, a '77K wiring harness. The '76F has that weird junction box on the frame below the headlight to make up for its smaller headlight bucket. Most of my wiring fits inside the bucket and that junction box, except for one multi-plug that comes from either the LH or RH switch... I can't remember off the top of my head. Anyway, that kinda dangles out in the open. Not sure if this has anything to do with it.
http://imgur.com/a/qMmDu

I thought at one point that my points were just getting wet. Those cork gaskets are not that great, so I've replaced it with a neoprene one from a member here. The issue still persists.

Any ideas what could be happening here? Most likely a short somewhere, right?

[flybox1]

i'll vote for an issue with or bad plug wires/caps    but yes, check all block connectors and wires to/from coils.
something is shorting out.

[Tugboat]

New plugs (last year) & new NGK caps, but I'll check 'em anyway.

[flybox1]

old, cracked wires?

[LesterPiglet]

That connector block exposed to the elements won't help matters.
My bike behaved the same as yours until I replaced the ancient, hardened HT leads.

[ekpent]

Do you have a totally stock and water tight airbox ?,  no mod holes etc.

[Bob Wessner]

What is the condition of your points cover gasket?

[DJ_AX]

What is the condition of your points cover gasket?

+1
Mine used to bog in the rain. Sealed them points up tight, fresh wires & caps... and now I can (and sometimes do) ride in downpours and through deep puddles with no problems at all.

[Bankerdanny]

He said in his original post that he has replaced the points cover gasket.

[750K]

My 77k does the exact same thing, weather it's sitting in the rain or I'm riding and it stars raining. On the throttle it runs on all 4, when I let off the throttle or at idle it sputters. As soon as the rain stops and things dry out it goes away. I live in the PNW so it's pretty frustrating, it rains a lot around here.

So far I have done the following:
New points and condensers, properly timed both static and with a light.
New points cover gasket as well as grommet for the points plate wiring.
New plugs, plug boots, new plug wires with Ngk splice kits.
Air box is in good shape, new filter and rubber boots.
Carbs are rebuilt, new carb rubbers and proper clamps, vent hoses are routed properly.
New fuses, fuse terminals are clean, side covers are on.

These were all things I wanted to do regardless of the wet running issues but none of them seemed to help the problem go away, my next plan is to check the kill switch and ignition switch. If that's not the problem I'm going to get a spray bottle out and fire it up on a dry day and soak things till I find the area. I'm pretty sure it's electrical, I have been wanting to clean all the electrical connections and give them a bit of conductive grease for good measure. The one thing I havent swapped out is the coils, possible that the casing is cracked and they are getting wet. Worse case scenario I'll replace the wiring harness next year, lol.

Keep us posted on a fix if you manage to figure out the issue, I'd love to sort mine out.

[Bob Wessner]

He said in his original post that he has replaced the points cover gasket.

Oops, missed the text below the pics. 

[bjbuchanan]

How does the rubber grommet for the points plate wires look? Definitely a source of condensation/moisture. A dab of silicone is worth it there

I think it is the HT leads like someone mentioned before. Dried and cracked or just old and not meeting up well to the plug caps. Cap screws on snugly but the plug cap itself doesn't fit too great? Try getting some dielectric grease around the cap to HT leads and see what happens

[jtb]

Let me know when you figure it out.

[Tugboat]

So I'm digging back into this, as I've sold my CB1000 and plan on using my 750 on out-of-town trips. Since I first posted I've replaced the coils, points and condensers. The bike has new plug caps and a neoprene points cover gasket. The problem persists! So I'm still looking for answers.

A couple of new details to throw in the mix: some PO replaced the original 3-fuse (glass) setup and has everything running through one blade fuse. The tail light gets juice from a black wire up near where the r/h switch plugs into the harness.. not sure where the headlight is getting power from. Anyway, that's a thing. Also, when I had the motor out last I cleaned up the frame a bit with some paint. I probably neglected to scrape the paint away from where the ground touches the frame; so I've run a 2nd ground straight from the negative side of the battery to one of the bolts that holds the tool tray in place. I scraped the paint off of the frame there, so now I have a 2nd independent ground that I know is good. Not sure if that's doing anything, but it hasn't had any effect on my problem.

So.... anybody have any clues?

[Gene]

Stock airbox? I ask because I ran across a thread yesterday re: pods and rain.

[Tugboat]

Yes, stock air box. But that's what it feels like - like a pod got wet.

[Gene]

Any leaks in the air box boots?  I realize you probably already checked, but just in case.  I had a similar problem with a 360 in the rain, I no longer have the bike but once it dried out it was fine.  Best I can tell it was the plug caps (one side anyway) - but yours are new.  How's the condition of you inner fender?  Is it possible that rain is getting thrown up in there and somehow getting sucked into the air box?  Long shot, I realize - just don't know.

[flybox1]

Hey tug, post up a few shots of your bike from different angles, please.

[scottly]

I'm suspecting the spark plug wires or boots are the issue, rather than the low voltage 12 volt side of the wiring.
I only had a problem riding in the rain once, and that was after riding the bike to work in the rain, leaving the bike in the rain for 8 hours. It was still raining when I left work, the bike started right up, and ran fine for about a mile, when it suddenly had no power pulling away from a stop. It didn't die, and would rev up with the clutch pulled in, but had no power. After letting it run at the side of the road for a few minutes, the heat coming up from the head dried something out, and I rode the rest of the way home with no problems. I suspect that water was flung up off the front tire past my "needle" front fender, drenching the high-voltage spark plug boots.
Make sure the boots are clean, both inside and out, the seals between the wire and boot, and the boot and spark plug are in good condition, and assemble them with dielectric grease to keep the moisture out. Also, make sure the porcelain outside of the spark plugs are clean.     

[DH]

Pretty much covered already, I was gonna add that you can simulate the wet weather conditions with a sqiurt bottle of water
over select components till you hit the affected area/component. Use squirt bottle with adjustable nozzle and carry it in a
coat pocket during test drives.

[750K]

I solved my wet running stumble, while tracking down a charging issue I cleaned and greased all my grounds and wiring harness connections. First ride after the charging issue was resolved was in the rain and no more stumble or stutter.