CB360 lost power today

J

Joker_Da_Man

New Member
1976 CB360 with 5000 miles that I picked up two months ago. I have ridden it about 200 miles. PO put new carbs, points, spark plugs, and fuel filter on it earlier this year. It has no mufflers as the PO cut them off after he was rear-ended this spring. It has the header pipes to right below the footpegs, then nothing.

I was cruising down the interstate at 7k rpms, sixth gear, when I backfired a few times and lost a lot of power. I pulled over and nursed it to the next exit about a mile away. It seemed that it could hold 40mph but not anything more. Immediately I jumped to thoughts of lost ignition in one of the cylinders, but both pipes were still hot and still puffing warm air out as I let it idle (though it died after about a minute of idling--not normal). I thought 3-4 minutes of riding would be enough to let the dead cylinder's pipe cool down quite a bit, but do I need to double check the lost ignition theory?

After stopping and checking that, I tried to nurse it a little farther towards home. I made it about 2 miles and then it started backfiring more so I decided not to go further as I was still about 8 miles from home. I noticed that there was smoke puffing out of the drainage hose that attaches to the very top of the engine (valve breather, right?). Not billowing smoke, just maybe 2 tablespoons worth of smoke with each fire of the engine. Is this normal? It got me thinking maybe it's valve trouble.

I'm not sure how hot these air-cooled engines are supposed to run. It was only 40F today so I didn't think it would be as hot as it was. I could manage to unscrew the dipstick but it was too hot to lift out with my bare hands.

Any ideas? I suppose it could be a fuel delivery problem but that smoke has me thinking it's something else. I've got the bike back in my garage; what should I check on it now?
 
could be running lean due to a number of problems, causing it to overheat. Make sure its cold and check your valve clearances, double check timing and check your float bowl height. Could be starving the engine of gas. My bike ran great for hundreds of miles until i really started to get on it for longer periods of time, then just cut out like you said. Turned out to be 2mm off on one of the float bowls, thats all it takes.

bob.
 
Hmm I just read on another site that the airbox covers are very important to make sure the airflow is restricted correctly. The PO didn't have them on and only had one in the parts box that he gave me. Also the air filters look pretty dirty so I might splurge on some new ones for peace of mind.

So I think I will get my exhaust and intake sorted out. And once I get the exhaust pipes on I can tune it up in my garage without causing excessive noise for the neighbors. 8) Also the air filters look pretty dirty so I might splurge on some new ones for peace of mind.

Of course this assumes your theory that I was running lean and overheating is correct. It seems plausible to me. If I get a nice enough day (North Dakota) I will have to start it up and see if it runs normal again for a bit to confirm that it was a temporary problem.
 
Check the oil, in fact it would be a good idea to drain oil and see if it has metal in it.
Running without silencers or air box covers will probably be lean enough for a partial seizure, particularly if it wasn't re-jetted.
If your really unlucky, it may be cam bearings, remove points cover and see if there is oil in there
 
I drained the oil and was only able to find 2 pieces that were maybe metal. So small I couldn't be sure. Also there was maybe 20 specs of some sort of grime. Seems OK to me. I suppose there is a filter too that might be worthwhile to cut apart.

There was no oil inside the points cover.

I plan to pop the top off to check the valves as soon as I get my Clymer book in the mail. It wasn't immediately obvious to me which bolts I needed to take out to get in there and I expect that will be in the book. If everything looks fine in there I will go on my vacation in Mexico (!!!!) with no worries about it, and then order exhaust pipes and airfilters when I get back home.

Edit: Does a "partial seizure" typically result in any damage?
 
Yep.
Rings may be stuck, pistons could be 'smeared'
Only way to find out is take it apart
The oil filter is on the end of crank, clutch cover has to be removed.
It's a centrifugal filter which works pretty good at removing any particles, finer than most cartridge type filters can manage
 
sounds like you may have done top end damage from a seizure as pj said....ck compression.
good luck,
joe@ vcycle
 
I got delayed on this (got sick, broke my 12mm socket, busy, etc.) but finally got down to business tonight. Is it possible to get the rocker cover off without removing the engine from the chassis? I got all the bolts out but it looks like there are sleeves that require the cover to be lifted straight off rather than slid out.
 
Yes you can pull the engine damn near down in frame. Just takes some wiggling and you may have to remove the bottom engine mounts to let it settle a bit to get the barrels off. First time I had mine down, I had it to the pistons in the frame.
 
I finally opened my eyes and saw that I could take off the triangular bracket which the top engine bolt attaches too, so the disassembly went pretty easy from there. I took some pictures of the cylinders and one of the spark plugs for you to take a gander at. You should be able to view them all here: http://mine.axvigs.com/cb360/

The cylinders seem to have a pretty nice shiny finish to them, and no vertical lines (stuck ring would have that symptom, right?). I suppose I should have tested for compression before I tore the whole thing down. After I had the cam shaft taken off I was turning the engine a little using the kick starter and could hear some weezing of the air leaking. Is that normal? All the valves should have been closed but do they actually seal airtight? I suppose it could have been the sound of air getting sucked in.

Anyways, looking forward to what you guys think. Hopefully I can polish up all dirty bits, throw it back together, tune it up, and go for 100.
 
All looks pretty good really
Cylinder was a bit rusty in places but has obviously been running fine.
Have you pulled cylinder off yet?
It's the only way to check piston/rings and big ends.
Pic 42, around 4 o'clock position is it alloy showing or oil on cylinder head making it shine?
 
Valves should be liquid tight. Test them with the head stood on its side on one set of ports and pour acetone or gasoline into the other ports. None should leak out from behind the valves into the combustion chamber. Let it sit a few minutes, should really take a few seconds. Then pour out the stuff, flip the head over and test the other side.

When I got my bike it had two bent exhaust valves and shitty seals on the intakes. I got newer exhaust valves and lapped them all in until they held fluid. No leaks and way more compression.
 
Couldn't figure out how to get gas to stay on top of the valves and not just get the whole combustion chamber wet, but when I turned it over it held gas in the chamber for several minutes.

Piston and rings pics are here: http://mine.axvigs.com/cb360/pistons/

Is it normal for the top part of the piston wall to be that dirty? It is sort of rough if I run my fingernail across it, but not deep scoring.
 
Instructions are in post above.
If head is turned to 'sit' on intakes, fill exhausts.
Check for leaks, empty fluid into something and sit on exhaust side to check intake valves.
It may help to do just one at a time, plus, put some gas in a paper cup (or 'hard' plastic)
turn head 'backwards (towards valve springs) to empty ports
Sitting inverted and filling combustion chamber is a waste of time
 
Commence the vigorous head-to-desk motions. I just realized that I need to pour the gas in the intake or exhaust ports, not on top by the springs. That should work A LOT better...
 
Valves did not leak when properly tested tonight. PJ, that shine in pic 42 is the metal glaring. It's not shiny elsewhere because it's covered in carbon, but the carbon seems to have been knocked of sometime right there.

Do those pistons look OK?
 
Pistons look OK.
Next, Pics of cam bearings and rocker arm pads.
None of the valve adjuster screw lock nuts are loose?
The only other thing would be timing 'slipped'
 
Pictures of cam "bearings" (don't see any bearings in there) and what-not: http://mine.axvigs.com/cb360/camshaft/

Valve adjuster screw lock nuts are tight.

Timing slipping would be the cam chain slipping?
 
Looks in real good shape, pit in one rocker pad isn't going to be a problem bearings are all good (machined directly into head, no balls/shells, etc)
Is that a mark on cam nose in pic #6?
Are all the cam base circles dark grey/black (polishing all the way round cam lobe/base circle shows tappets were too tight)
Nothing much left to check at top end, if all the screws holding points/timing plate on were tight.
Have you got clutch cover off to check clutch hasn't rubbed inside cover and oil pump pick up is clean?
It's probably OK as the top end is good
 
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