Need help with '80 CB750K possible misfiring

[HondaTurbo]

Hi gang,

I was searching for information on my '80 CB750K and came across this forum! I'm looking for any input as I am somewhat new to the repair of motorcycles. I bought this bike a few months ago from someone who was using it as a daily driver. It sat over the winter, stored properly as far as I know. It was riding fine for its first three rides, although I knew it had issues when I got it because during idle it makes a clunkity-clunk noise.

During this last ride the exceleration felt very rough and jerky, like it just wasn't picking up like it should. It started to backfire like crazy when up to speed. It eventually stalled out at a stop sign. We had to push it to start it as the battery was dead.

I'm thinking there is multiple issues with the bike, but it was bought as a project. I appriciate any input and please feel free to ask me questions about it so you can give clearer answers! Anything is appriciated. I was hoping to ride a few more times before summer ends. I was planning to take everything apart for a rebuild over the winter--but perhaps will have to do this sooner than later?

-Tara

[TOOLS1]

Welcome. The clunity clunk noise is most likely because the carbs are out of sync. Which they probably are, since most of the other problems you mention point to the carbs being dirty. I would start with a through cleaning of the carbs.
TOOLS

[HondaTurbo]

Excellent, thank you! I will hopefully have time to start going through the carbs tonight.

The clunking noise sounds more like it is coming from the engine itself, but I am hoping all of these issues are linked to one thing. If it is the carbs, it should be easy.

While riding the "oil" light occasionally comes on and shuts off as well.

[sgtslag]

SeaFoam is your friend...  Get a can of it, and add according to directions, to both the fuel tank, and the crank case.

For the fuel system, it will help dissolve varnish deposits within the entire fuel system, but it won't dissolve everything, unfortuntely; it is the easiest thing to try, though.  Run that tank of fuel down as low as you can, before filling up, to get the most out of it.  It can really help if the varnish is not too much, or clogging the jets.  It's easy, and inexpensive (~$9 per 16 oz. can).

For the oil, add the correct amount to the crank case, start the engine, and run it for five minutes, on the center stand; with the rear wheel spinning freely, let the engine idle, and shift through all five gears, to thoroughly flush the transmission, as well as the entire engine, of deposits.  It will dissolve varnish, carbon, and other crud, within the engine, and the transmission.  The crud will be dissolved and float in the oil, being captured by the filter, as well as being suspended within the oil.  After five minutes idling, the oil will reach 170-180 F, which is when it suspends deposits within itself.  Then shut it down, and change the oil, and the filter -- if it wasn't black when you started, it will be at this point, with all of the crud it loosened from the engine, and the transmission.

If you have to let the bike sit for more than a couple of weeks, without being ridden, treat the gas with SeaFoam, to stabilize it, and to prevent formation of deposits within the fuel system (up to 12 months only -- after that, the gas will turn to crud...).  Periodic treatments in the gas tank, will help ensure a clean fuel system, so I recommend adding it at least once per month, during riding season, as a preventative measure.  Cheers!

[HondaTurbo]

Thank you! I will try the SeaFoam as well. Funny my fiance just told me he ran it through his bike--but once my bike has issues he has no suggestions. Time to take it into my own hands!

[rrgunslinger]

This may be a stupid question but does it have oil in it? Sounds like it seized.

On Tue, Jul 17, 2012 at 12:57 PM, HondaTurbo [via Honda CB750'S] <[hidden email]> wrote:

Hi gang,

I was searching for information on my '80 CB750K and came across this forum! I'm looking for any input as I am somewhat new to the repair of motorcycles. I bought this bike a few months ago from someone who was using it as a daily driver. It sat over the winter, stored properly as far as I know. It was riding fine for its first three rides, although I knew it had issues when I got it because during idle it makes a clunkity-clunk noise.

During this last ride the exceleration felt very rough and jerky, like it just wasn't picking up like it should. It started to backfire like crazy when up to speed. It eventually stalled out at a stop sign. We had to push it to start it as the battery was dead.

I'm thinking there is multiple issues with the bike, but it was bought as a project. I appriciate any input and please feel free to ask me questions about it so you can give clearer answers! Anything is appriciated. I was hoping to ride a few more times before summer ends. I was planning to take everything apart for a rebuild over the winter--but perhaps will have to do this sooner than later?

-Tara

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NAML

[HondaTurbo]

Yep it has oil in it, and the oil levels are just fine. I had just done an oil/filter change on it right after purchase.

[shinyribs]

HondaTurbo wrote

Thank you! I will try the SeaFoam as well. Funny my fiance just told me he ran it through his bike--but once my bike has issues he has no suggestions. Time to take it into my own hands!

LOL! I about laughed myself silly. Welcome to the forum,and good luck with your bike. It sounds like you have a can-do attitude,which is all you really need. These bikes are so easy to work even i can sort some of the stuff out.  Good luck and have fun.

[Lucky 1]

Take a DC inductive timing light and clip it onto each spark plug wire.
You will see if each cylinder is firing or not. no wrenches needed. LUCKY

[HondaTurbo]

Took the carbs off last night and will begin cleaning tonight.

Noticed the intake Carb boots are looking rough. They are very hard around the edge and some minor cracks around the edges only (before the clip). From other's experience is it worth soaking them and trying to salvage them or just buy new ones? I noticed for a rubber part they aren't exactly cheap. Ones I found are almost $100 for the full set. And the expense officially begins! :)

[HondaTurbo]

I answered my own question and found a intake boot set for $64 shipped, so I will be replacing those. :) I want to do this the right way, not the cheap way...

[sgtslag]

Don't mean to insult you, but be careful how you clean those carbs:  carb cleaner will destroy rubber, and possibly plastic, bits!  There are plenty of detailed carb how-to articles, and videos, on the Internet.  Cheers!

[HondaTurbo]

Thanks, I'm not exactly new to cleaning/rebuilding carbs. I'm just new to Honda's.

[James]

i tried the seafoam following sgt instructions and my bike runs better now. i will still clean the carbs this winter but at least now i'm riding around. good luck