Topic: '81 CB750K dies on highway

[splnkr]

Hey all, twice now I've gone out on the highway and had my CB750K die on me.

The first time I was able to wait about 10-15 mins and start it back up to ride home. This second time there was no bringing it back and I had to get a tow.

The starter turns the engine over fine, everything sounds right when I try to get it going, but it won't actually start. The thing is that I don't know if this is time-based (e.g. it dies after 20 mins) or speed/RPM-based (e.g. it dies when running at 6k RPM for 5 minutes). Both of them happened around 20 mins in, about half the time of which is spent at around 70MPH, the other half is getting out to the highway. When it dies, it sputters a couple of times and then dies as soon as I pull in the clutch.

Reading on the forums here the consensus seems to be to check fuel first with this issue. I tried cracking the gas cap and flipping to reserve, no change (and there is gas in the tank). Should my next step be to check fuel arriving to the carbs? I am fairly new to working on bikes but am determined to learn. I'll continue reading other threads on the matter but seeing a lot of suggestions from fuel issues to electrical and not sure which avenue to pursue first. Ideally I'd like to progress from least to most invasive checks and eliminate things along the way.

Additional details which may or may not be related: about two weeks ago the starter button stopped working. Determined the solenoid went bad, as I took it out and you no longer hear that click when shorting the starter button inputs. Ordered a new one, in the meantime I've been shorting the battery terminals on the solenoid to get the bike started. All electrical still works fine.

Thank you for your time!

[evinrude7]

81 is a dohc. 

[ekpent]

Spark - fuel- compression are three things needed to run.One way to check for fuel if it dies out again or now is crack open a bowl drain screw on the carbs to see how much comes out. The 750 dohc has an extra device on the carb rack the fuel flows to first before going to the carbs that is vacuum and diaphragm operated that flows fuel when running and stops it when not. That could be bad if its still there.  Lots of people bypass it.
  As for spark have an extra plug around,hook it to a cap and ground it on the head and spin the engine and look for good spark.
  Good luck,no fun getting marooned out on the road.

[calj737]

Could be a failed sidestand switch. Could be a safer switch failing (clutch). Could be the electronic ignition failing once the engine gets hot.

Too little info to diagnose it.

[splnkr]

You're right I don't have enough info.

I'll do as ekpent suggested and check how much fuel comes out of the bowls when I open the drain screw. I was talking to someone else I know today who's familiar with bikes who suggested checking the petcock diaphragm if my bike is new enough to have one. I'm going to check those, and all the fuel filters (one in-line I can see coming out of the tank, and the petcock filter). If still doesn't work, check the filters in the bowls. Will report back when I have more info. Thank you for your help!

[ekpent]

 If you look at this picture you see that to the far right two large hoses are coming out of a device that sits on top of the carb rack. That is the petcock diaphragm that can sometimes go bad. Its vacuum operated as you can see and the one hose not connected goes to the petcock. Look to see if yours has that or not.Not saying that is the problem though just a possibility. Your bike should not have a center stand switch. Be sure to check spark on all 4 cylinders also while troubleshooting since its so easy to do.

[splnkr]

I have good news! I got the seat and tank off and check all 4 spark plugs, all fine. I then bypassed the vacuum powered shutoff and plugged the petcock output from the tank straight to the carb assembly input. Bike started right up! Looks like something may have failed in that shutoff. I'll keep it bypassed until I have a rebuild kit.

Now I'm having the hardest time putting the seat back...is there a clip in the front that's supposed to secure the front? There's a clip but it seems way too far away from the bike to clip in anywhere.