93 Honda CB750 Nighthawk By solymar (Read 1,700 times)

[solymar]

So I have a 93 Honda CB750 that has just over 10k mile on it & have owned it for the last 2 years. It's been running just fine until today. I went to the store, got there just fine & about half way back it started running terrible. It was like either it was nearly out of gas (nearly full actually) or it was possibly running on 3 of 4 cylinders. It kinda chugs if ya can understand what I mean by that. It will eventually get up to speed, and it doesn't die at lights.
I've checked & cleaned the petcock & replaced the fuel filter, checked my oil & air filter (no problems there). I pulled my #1 spark plug & it was wet around the outside edge but the rest of it looked normal. I haven't pulled the other spark plugs as it got too dark for me to see (no garage) & the mosquitos were feasting on me!
Anyone have any ideas as to what could be wrong &/how it went bad mid flight? Any insight would be appreciated & thanks in advance.

[cyborg]

A few things come to mind,,,,my first guess is a coil is starting to fail,,,it'll run fine cold then when it heats up from operation it shorts inside,,,had a couple of them go bad myself,,,,or one or more of the plugs is going bad,,,as a side note there's basically no miles on the machine,,10.000 is nothing on a honda,,,I bumped into a guy about a month ago that had 320,000 on one,,,never had the motor apart,,,altho it did smoke a bit not too bad considering the miles,,,so my bet goes to electrical,,,also do you have a big ball of keys ?? I've seen ignition switches be affected by big key rings that "turn the bike off intermittently " when jiggled around by bumps

[solymar]

Wow, 320k is crazy. I've read about people putting 200k on them, but I thought they were the rare cases. If I could get 100k out of mine , I'd be ecstatic!
Thanks for your insight, I was thinking about the coils as well. Being that the bike is nearly 25 years old, I wouldn't be surprised if it was one or both of them. I'm about to pull them this morning and test them with an ohmmeter. The other thought I had are the carbs, if one of the jets got plugged by some dirt. I'm really hoping that it's not them as getting them off is no big issue, but getting them back on is a royal pain.
As for the keys, I only have the one key on the ring along with a small plastic key tag, so I don't think that would be the issue. Though, I have to admit that I probably would have never thought that would be an issue even if I had a bunch of keys on the ring.
Thanks again, I'll keep you updated.

[cyborg]

I'd bet one or both of the coils going away,,,if it runs fine till it gets warm,,, that's a major indicator

[cyborg]

They may test fine cold,,,,

[solymar]

I decided to just replace both coils, wires & spark plugs. I wasn't able to get full readings on the coils with my multimeter, the leads aren't long enough to reach inside the coils for one of the tests.
I also pulled my carbs so I can clean them out too, tho I'm not looking forward to having to put them back! Honda leaves you next to no room to reinstall them. Since I pulled the carbs, I decided to go ahead and relace my throttle, choke & clutch cables since their all oe parts. Technically their is nothing wrong with the cables, but since their nearly 25 years old, it's probably time to change them out.
It'll be a couple of weeks before I can order the parts, but as soon as I get them, I'll let ya know what happens.
Thanks again!

[cyborg]

Just lube up the original cables and roll them up in a big ziploc bag that way you'll have known good spares

[solymar]

Makes sense. Always a good idea to have backups. Thanks for the suggestion.

[cyborg]

Update us when the parts get in,,,like to know if it gets cured

[cyborg]

What's happening with the 750?

[cyborg]

Get the bike sorted out?

[solymar]

Not yet, kept having leak problems with my carbs. First with the float bowl gaskets & now from the fuel joint in between the #2 & #3 carbs. Luckily the new fuel joint & o-rings are supposed to arrive Monday. I’ve already replaced the coils, wires & plugs.
Once I have the carbs back on & holding fuel without leaking, it should start back up again. I figure at minimum it should be back to where it was when this all started, but I’m really hoping it’ll be running like it should be.
I’ll update you soon.

[cyborg]

Great news!!! Good luck,,,those are excellent machines

[solymar]

Got everything back together with no leaks, yay!

Unfortunately when I try to start it, it turns over a few times and then backfires thru the exhaust pipe. Give it full choke & some throttle and it really backfires!

I checked the spark plugs and they seemed dry but do smell like gas.

I’m not exactly sure what the problem is atm, so I’m hoping for some suggestions.

[solymar]

Fyi, I’m using a aux gas tank with the gas I use for my mower. Since I just used that gas yesterday for the mower & the mower started right up, I don’t think the gas is a problem.

Things I did to my carbs...I took off the diaphragm covers & the float bowls, cleaned everything the best I could without separating the carbs completely. I replaced the main jets, slow jets & float needles. My floats are not adjustable and after inspecting them I didn’t see any issues & no need to replace any. Everything seemed to operate smoothly before installing it, but I’m sure that I’m not infallible lol.

Any thoughts you might have would be appreciated.