Topic: '78 cb750k Dies After Heating Up

[specialone70]

Had to replace my head gasket so I tore apart my whole bike and while I was in there I replaced my piston rings, honed the cylinders, and replaced the cylinder base gasket and valve cover gasket. Started it up and it ran just as well as before. Then I rebuilt and bench tuned the carbs since the previous owner had them in awful condition. I got everything back together and took it out for a spin and everything was running fine until I was nearly home (after riding about 10 miles) when I put in the clutch at a stop and the engine died. I started it again and had to give it some gas to get it going. I went about 100 feet, put in the clutch to shift and it died again. I tried restarting it again, but the electric starter was struggling to turn over the engine, and after a few turns the motor just sounded too weak to turn the engine at all. I started walking it home and after 2 miles of pushing and 45 minutes later I tried starting it again and it started up with a little throttle and I rode it 1 more mile home. I checked the oil tank and there was smoke but I figured it was from blow-by from the new rings. The next morning I started it again and it seemed fine. I took it for another 5 mile spin and a similar thing happens. The way it died made me fear there was an oil issue, and after a reddit post a few other people confirmed my fear that the oil wasn't circulating and then they suggested I make an account and ask the geniuses on this forum. Do you guys agree with the oil not circulating? Why wouldn't it be? What can I do about it? Is my engine trashed for riding it that far with no moving oil? Could it be an electrical issue? I've been at college since this whole deal and the bike has been here at home so I haven't torn into it at all yet. Thanks for the help. This whole thing has been a huge stress.

[70CB750]

I would think the engine would be locked by now if there was an oil issue.

Pull valve tappet covers and see if there is oil on tappets and around.  You can also spin it using the starter - kill switch off - and watch the oil delivered.

Is it possible for you to rig an oil pressure gauge to the galley plug?  It would give you more info on oil than anything else.

How hard is to turn the engine over by the kickstart?  It should be easy enough to do by hand.

Just few points to get you started, good luck!

[specialone70]

The fact that the electric start struggled so much to turn the engine made me think the engine was starting to lock, but didn't get hot enough to expand and seize the piston to the cylinder. I had the kick start off since I didn't have the clutch cover on yet, so I'll throw it on and give it a shot and see if there's anything flowing around the tappets and post again in a bit. I don't have a pressure gauge I can put on there right now but I might try that later. Thanks for your help!

Update: I opened a tappet access cover and everything was bone dry, which didn't surprise me since it's been sitting for a couple months. I gave it a couple turns with the kick start, but everything was still dry after that. The kick start also didn't feel right in my hand. It sort of felt like metal rubbing on metal, but I'm not 100% sure. It sounds like it's definitely not circulating oil so I guess my next question is how much damage did I do by riding it 15 miles without any oil? And what would cause it not to flow? I didn't mess with the oil pump. I'm pretty sure I hooked up all the oil hoses correctly. Someone on reddit suspected some silicone gasket sealer might have clogged the system, but I can't imagine that would stop all flow. Are there any common mistakes people make that would cause this?

[70CB750]

Kickstart is not enough to create pressure to provide oil for the head.

Silicone gasket sealers are known to kill these engines, by clogging oil nuzzles (under cam towers) that supply oil to the top.

You will probably need to pull the head and see what the damage is.

[Kickstart]

Do the test Prokop (70CB750) suggested: 
With a couple of tappets covers off (one from each side of the bike) use the electric starter for a minute and see if you get oil splashing around up there.

What does "bone dry" mean?  I screwed up a head gasket change many years ago and oil starved the top of my engine... the side that was starved was charred black.

You might just have a bad battery or an electrical problem.

Where did you use silicone gasket?

[specialone70]

I checked it with the tappet covers off and held the electric start for about 5 seconds at a time, twice. Still nothing. I guess the next time I'm home (in a couple weeks) I'll have to try holding it longer. Nothing was charred, so that's a little reassuring. As far as dryness, when I touched a tappet it there was oil on my finger, but not enough to form a drop. Just a little light layer. I just used like 8 small dabs of gasket sealer on one side of the head gasket, so I don't think there was enough to clog anything. I also recently found out that the piston rings could have been installed incorrectly, so keep that in mind.

[fdbrat]

Did you gap your rings?

Also how many AH is your battery. These bikes like a fresh battery.

Also with my experience with mopeds, this sounds like a bad condenser. Bike runs great cold but when it warms up it dies and or no start till its cool.

I hope you get it figured out.

[Kickstart]

If you didn't get the silicone near the oil passages (especially the two supply studs in the rear) then I don't think there would be an issue with silicone particles getting clogged where they shouldn't be.

I don't see how the rings would cause an oil delivery problem... charge up that battery and do the test again for about a minute.

The initial problem you described could be an electrical problem. In fact someone here had a similar issue and it turned out his ground wires weren't making a good connection to the frame and/or engine.

[specialone70]

fdbrat: I don't think the battery's AH's are a factor, since it started right back up after things cooled down. As far as the condenser: is there a way to check if it's good or do I just have to replace it? And my dad seemed to think it was an electrical issue, so I'll cross my fingers that the condenser is the only problem, or a loose connection somewhere like Kickstart suggested. And I know the gaps were separated around the pistons before we put them in the block.
P.S. I'm a new member to this forum and I'm really impressed. You guys rule!

[fdbrat]

Be sure to wear gloves if you are unsure of this test.
Read this link and have at it.

Also next time it dies, check to see if you have spark on all for plugs.

The Ah states how many times you can crank the starter before its dead.

There is lots of things to check. Lets just take this one step at a time, and verify what's good.

http://mgaguru.com/mgtech/ignition/ig129.htm

[brewsky]

Started it up and it ran just as well as before. Then I rebuilt and bench tuned the carbs.............

That might be a clue right there.....

Did you pull the push in slow jets and clean them?

Was it "running fine" when cold with choke on?

[DH]

FWIW, and this may be the long way around, Go with the general consensus...Hook up a volt meter and check the battery voltage. I'd say it ought be at least 12.2-12.4, otherwise re charge it. Is the battery good to begin with? Will it hold a charge?
 If you're concerned about oil pressure/flow
you can spin the engine with the spark plugs removed and the kill switch activated. That'll make it crank faster and easier. Watch for the oil light to go out. Does it? It may take a bit, if you're going in short bursts with the starter. See if it turns over smoothly wi the kicker. If that checks out, put the plugs back in. Start it, With the kick starter, and note how much effort is required on the kickstart lever. run it for 30 secs. or so after the light goes out, then shut it down. Remove the tappet covers and check for oil around the valve gear.. Or run it for a few seconds with the tappet covers off,  just long enough and check for oil splash (messy) It should be wet on and around all the valves. If it isn't, there's an issue. (find out why) You did reinstall the oil jets back in the head during engine reassembly, eh? If it's ok, start it and, with a volt meter across the battery, note the charging voltage with the engine revved above idle.(quick check) If it doesn't increase when revved, there's an issue. Find out why.. If it's discharging, It
can indeed cause the symptoms you describe. If it does charge,
run in short cycles, (shut it down if, at any time, it makes any unusual noises,) restarting it each time with the kicker, and
note if it requires more effort each time you restart. If it gets
progressively worse, there may be an internal engine issue,
which could of course, mean POSSIBLY, but not limited to, a ring issue, (correct size, end gap, ring land clearance) I've heard that rings that are installed wrong can cause excessive oil consumption. maybe
someone with  expertise in that area will have more to add
concerning piston rings. They may need a break in period to loosen things up.
That's how I'd check if it were mine again, FWIW. Check/verify (one way or another)/ repair. Only run the engine long enough to verify and no more. If the above checks to be what you think is
ok, could be a problem elsewhere as has been suggested.