Broken Cam Postmortem Questions

[jgarrelts]

Hello,

I have an '80 GS750ET. 40k miles. It's broken. I'm about ready to put it back together, but the cause of failure is still not definite enough for my comfort.

I was driving on the highway and started hearing an increasingly louder loose valve clearance noise, which turned out to be the left side intake (letter C) cam caps coming apart. Before I could make it to a rest area, the cam broke and the bike shut down. I'd say it went 2-5 miles making noise before the end. Bike ran great up until that point.

I've pulled the head and found no evidence of valve to piston contact that might indicate timing chain slip. Can't say too much about clogged oil passages as there was a lot of debris in the head when I opened it.

I partly suspect bad wrenching by a previous owner. (I've had the bike for just under 1k miles.) The mating surfaces on the head and jug look like they were cleaned with a bad razor blade, leading to some slight oil seepage at the front of the head gasket. Someone who would perform work like this may not torque down a cam properly, either.

The oil level was just at the top of the sight glass. I have normally kept it there due to the oil leak. Possible issue?

My oil pressure light sometimes comes on under deceleration and at low rpm. (It was not on when the engine failed.) Is this abnormal? Reading about it in one of the 4-valve manuals seemed to indicate that the oil pump is a sealed unit and a suspect one should be replaced. Are there clearance specs somewhere?

The bike has had a slight high rpm vibration since I started riding it. I was previously not very concerned with it. Normal? I suppose it could have been a bent cam.

The cam cap bolts were all tight when I removed them. The middles of the caps were pushed up and they came apart in pieces. The cam broke just to the right of the drive sprocket, opposite the carnage. Valves are straight.

I have a lower mileage head to put on, but I'd like to be fairly certain I'm not putting it back together with a problem inside and I'm new to these bikes. Any insight is much appreciated.


-Jeff

 

[Nessism]

That generation of 750 engine had oiling system problems. My guess is the cam oil starved which cause the C cap to seize and subsequently snap the camshaft. Sucks to be sure, but rest assured you are not the only one that has experienced similar issues, and/or spun crank bearings. I've read here that you can/should shim the oil pressure relief spring to boost oil pressure. This mod was common back in the day from what some here have said.

Good luck and sorry for the problems. $hit happens.

 

[GregT]

Yep, classic symptom of low oil pressure, possibly made worse by the PO's poor workmanship.
I'd drop the sump before going any further - if you find crankshaft bearing material on the oil pump pickup screen or lying in the sump, she's toast.
If it's reasonably clear then the oil pressure fix back in the day was 2 X 6mm spring washers above the spring in the pressure relief valve

Good luck

 

[jgarrelts]

When I dropped the oil pan, there was a large quantity of what I had assumed was cam and cam journal that got moved around while the head was getting chewed up. Lots of it by the C caps in the head. The oil filter was quite glittery as well. It all looked like steel and aluminum.

Time to inspect crank bearings? I take it they're steel as well, being rollers?

 

[Nessism]

When I dropped the oil pan, there was a large quantity of what I had assumed was cam and cam journal that got moved around while the head was getting chewed up. Lots of it by the C caps in the head. The oil filter was quite glittery as well. It all looked like steel and aluminum.

Time to inspect crank bearings? I take it they're steel as well, being rollers?

Nope, that engine has plain bearings, not rollers. I'd split the cases and inspect everything. Low oil pressure likely caused the cam problem but there could very well be damaged crank/rod bearings as well.

 

[Guest]

After I blew up my motor there wasn't one piece of that motor I didn't take apart and clean.

 

[koolaid_kid]

Sorry to hear about it. I also recommend taking it all the way down. It is better to do it now while it is not running than get it running again only to see another failure. Best of luck.