Topic: 1973 Cb 750 stalls out after some riding, usually when shifting

[Jroth]

I just inherited a 1973 cb 750. It had been a project of my fathers a couple years ago and ran excellent. It has been sitting in a garage for a couple years and I’m currently working some kinks out. New spark plugs, Pulled the carbs, cleaned, set to Hondaman specs, float heights set before re installing and clear tube test after.. Set clutch to Honda man specs, still releases late and  has a grinding noise but not sure if the oil hasn’t circulated there yet. The real issue comes after I ride it for a little while. Clutch will grind when disengaged but seems ok in gear. After a little bit and usually during a shift up or down, the bike will sputter and stall out. I can get it started if I wait for a little while. There’s plenty of oil and does not appear to be clogged. After maybe 1/2 hour, the bike will start right up and run sounding normal except for clutch noise. I thought maybe a warped clutch plate? Any thoughts?

[calj737]

Sounds more like a fuel vent issue. Have you looked under the fuel cap and cleaned it from any rust, and insured the vent hole is open?

Try a test ride with the cap not locked and see if it solves the issue. And you also want to make sure there are no funky loops in the fuel line causing an air bubble.

[Jroth]

I did clear the vent in the gas cap but could check again. I haven’t tried a test ride with the lock undone. Will try that out! I think the gas lines are good but I’ll give that a look see. It just completely stalls out when I pull the clutch. Is that just coincidence?

[calj737]

The clutch has nothing to do with the engine “running” in that way unless your bike has a Clutch Safety Switch that has somehow gone bad and is causing an electrical short? That’s the only notion I can come up with. 

[ekpent]

 Does it have stock ignition points and condensers or an aftermarket set-up ?

[Jroth]

It has the transistorized ignition. I tried the open cap test and it’s still happening. I can hear a light scrape noise/squeak while the engine idles as if it’s struggling to turn over. I thought maybe I wasn’t getting enough oil circulation but my oil light isn’t on. All four exhaust pipes are hot too

[Patrick]

Are you, by any chance, running inlines fuel filters?

[calj737]

I can hear a light scrape noise/squeak while the engine idles as if it’s struggling to turn over.

Does this sound continue with revving the engine? Does the sound change tempo with revs? If you pull the clutch lever in, does it go away?

As much explanation as you can mange here please...

[Jroth]

No fuel filters. I’m not sure if the sound goes away when reving, it might. I just checked tappets, cam chain, points gap but not timing. I need a timing set up. It makes a similar sound to what the starter sounds like before the engine starts. Except it begins making the “starter sound” while running. The noise persists even when clutch is pulled in. I pulled my spark plugs and they were wet and Carbon fouled so I changed them. I thought it was just because the bike was sitting. The new set I put in came out looking the same after only a week of trouble shooting.

[Spanner 1]

Your start button on the r/h control is 'sticky' and will connect randomly while riding in hot weather engaging the starter.  ( have personal experience ). Take the r/h control apart and free it up. WD is your friend here .

[Jroth]

Could that be fouling spark plugs? Or is that a different issue? I will try the starter today.

I did change handle bars. I  threaded thel wires through a stock set of handle bars. Nothing shorted but I had trouble fitting the starter back on. There’s a small metal tab inside that I assume gets pushed over the wire once it’s in place? I wasn’t sure but that seemed right. Maybe I just need to tighten up the slack and grease up that button. Could timing be screwing this up? Worth buying a timing light?

[evanphi]

Def worth buying a timing light.

I had a funky starter button when I first bought the bike because I had aftermarket bars and clamping too hard on them caused the starter to run! Go over all your wiring in that RH switch and make sure there's no shorts.

[calj737]

Def worth buying a timing light.

I had a funky starter button when I first bought the bike because I had aftermarket bars and clamping too hard on them caused the starter to run! Go over all your wiring in that RH switch and make sure there's no shorts.

Especially insure that you have adequate wire length from lock-to-lock. It could be you're RH wires are too short, just slightly, and when you turn right, the wires are pulling the START contact onto the bars and shorting you out.

The fact that you indicate a noise similar to the starter motor while riding is concerning. It means that possibly the START is activating.

[Johnie]

Start should not activate unless the clutch is pulled in or the bike is in neutral unless those features are disabled.

[Jroth]

Ok so after inspection I don’t think it’s the start button. It appears fully sprung back and doesn’t stick. Doesn’t pull in when turned either direction. I checked the timing on 1-4 and that was pretty much dead on. I tried to check 2-3 but the timing light wasn’t lighting up. Am I missing something in this process? I pulled spark plug wires and checked for power, they are all getting power. All exhausts are hot. I listened to a couple videos of problematic Honda’s and someone had a video up that sounded similar to mine. He said the problem ended up being a float needle one notch out of sync with the rest. Maybe it’s that? Should I pull the carbs to check that, or can I leave them in? Would that cause the engine to stall out? Or is it more likely something internal engine-wise?

[calj737]

You’re missing the point. A shorted wire won’t effect the button position on your bars. To diagnose, open the RH switch pod, start the bike, and turn lock-to-lock. See if you can reproduce the problem. If not, reinstall and repeat. Does the problem return?