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Quick Diagnosis 1980 GS750L

posplayr

posplayr

Forum LongTimer
Past Site Supporter
TGSR Superstar
Hi,
I'm new here as I just bought a running and pretty well preserved 1981 GS750EX. However I ran across and picked up a mystery 1980 GS750L parts bike ($250). The parts bike had a full fairing and about 30K miles on the odmeter. The number two plug was black and the others almost seemed lean (see attached). The bike has not been registered since 1986 so it has sat for many years, so I decide to do a compression test prior to screwing with the carbs to try and get this thing to run. My GS750EX is running fine so I'm just trying to assess if I have a basket case here are is it a plausible runner or just a rebuild candidate.

I put a starer in and put it on to a charger. The starter will spin the motor with plugs out. I pulled the carbs completely off the motor.

Dead cold I got

With light oil #1 60 psi and with motor oil 90 psi
" #2 30 psi " 60 "
" #3 60 psi " 90 "
" #4 60 psi " 90 "

Having read the forum I know I'm supposed to do this hot, but what can I say. It was encouraging to get real consistent numbers between #1,#3 and #4. Initally I though maybe #2 had a burnt valve from mis adjustment. I also pulled the cam cover and cranked the motor till all valves were closed on #2 and made sure I could raddle the cam followers. I did not botehr to use a feeler guage but it felt about right considering I used to have to adjust vales on an old 1969 Datsun 2000 roadster (I also have some experence adjusting multiple carbs which will come in handy with this gange of CV carbs.

So my question is, well yes this bad boy is supposed to have "only" 30K miles and yet it appears to have bad rings on the #2 cylinder and the others are none to high even if they are even. Is this a 130K mile bike although that might have been a challenge given it was only ridden 6-7 years.

The story was from the guy I bought it from is his dad picked it up in trade for some work and eventually go it to run but had problems with one of the cylinders (guess which one). It has been a garage queen and stored and transported for the last many years.

One last item I did notice is the cam lobes on the intake side are very good, but there are some minor grooves on the full exhaust side (all the way across). If this were a car and it had 30K miles I would think that exhaust cam was in pretty bad shape.

I'm wordering if this thing was likley mistreated, misadjusted , just a flucky broken compression ring?

At this point I'll likely just button it up and keep it around for parts. Now I'm not sure the motor is worth much of anything. Beside this the bike is fairly clean and complete. The most notable which I'm going to swap over are tail light fixtures, engine covers, exhaust pipes and break clutch levers. Other odds and ends.

Appreciate any possible insight.

Posplayr :cool:
 
Hrmm..get this.. I picked up a 80 750E off a fellow board member, knowing full well it had problems. Guess which cam is scored up bad? Exhaust. He said he was riding it, it started running crap. he shut it off, waited a bit, fired it back up and was fine for five or so miles, then it acted up again. His thoughts, and i concur, is that it was sucking something up and blocking oil flow. Perhaps thats what's gone on with you. Parts are pretty available for the 16v motors from what ive seen on ebay. Chances are you could have that thing torn down and slapped back together pretty quick and pretty enexpensively. OR option two, since you obviously have cam damage, and possibly piston, ring and or cylinder damage...BORE IT OUT!! Get some cams off ebay, or if you feel froggy, get some high lift cams and what not. I have a leftover 2v 750 motor that i am planning this for myself.
 
I dont know what to make of it

I dont know what to make of it

but I forgot to mention that the crank case was full of oil over the clear window full mark, It also smelled a little like gas. Given how old it is not sure if it is relevent but seems somebody put too much oil in. Oil viscosity also seemed a bit thin but relatively clean all considered.

How might the engine be sucking up stuff and loosing lubrication to the exhaust cam?

Cam chain seemed a little slack (1/4")

Well it is alway nice to know you have a spare engine. Generally it looks clean inside and I'm pretty sure the transmission is still good.

Thanks for the comments.

Posplayr
 
AHH well that for sure makes a difference. My guess then, is that the motor was running well rich. Fuel was pouring into your cylinders, washing your cylinder walls down, diluting your oil and thereby ruining lubrication. You could always try changing the oil and filter, making sure your carbs are properly jetted and mixed, and firing her up to see what happens. Chances are that she may run halfway decent. I was told that if the scoring isnt HORRIBLE, you can run the motor with the same parts inside since they've worn the same. Maybe get some miles out of it while you hunt for a new top end. Just a thought.
 
I wouldn't trust those numbers with fuel in the oil and it sitting for so long. Considering all this those numbers are quite good.
You should have two bikes when you're done. Check out all the extra info on the left side of the home web page.
 
Hey howdy hey!

Hey howdy hey!

Mr. posplayr,

Let it be known that on this day you are cordially and formally welcomed to the GSR Forum as a Junior Member in good standing with all the rights and privileges thereof. Further let it be known that good standing can be improved with pictures (not you, your bike)! :-D

Thanks for joining us and good luck with your project.

Thank you for your indulgence,

BassCliff
(The unofficial GSR greeter)
walmart_greeter2.jpg
 
posplayr said:
but I forgot to mention that the crank case was full of oil over the clear window full mark, It also smelled a little like gas. Given how old it is not sure if it is relevent but seems somebody put too much oil in. Oil viscosity also seemed a bit thin but relatively clean all considered.

How might the engine be sucking up stuff and loosing lubrication to the exhaust cam?

Cam chain seemed a little slack (1/4")

Well it is alway nice to know you have a spare engine. Generally it looks clean inside and I'm pretty sure the transmission is still good.

Thanks for the comments.

If the engine is making oil and it smells of fuel the petcock is faulty. Those spark plugs you show are good for #1 & #4 and getting on the rich side for #2 and #3 is really rich. If the petcock vacuum line runs off #3 that is why it looks so black/sooty.
You need to change the oil and replace the petcock. Since the engine has sat for so long, the carbs will need a through clean! Check the "In the Garage" forum for cleaning details.
After cleaning the carbs, checking the valve clearances, fitting a new petcock and charging the battery, you should be able to get it to run.
Then it's time to assess whether it's viable to finely tune as a runner!!
Click to expand...
 
49er

49er

>>>If the engine is making oil and it smells of fuel the petcock is faulty. Those spark plugs you show are good for #1 & #4 and getting on the rich side for #2 and #3 is really rich. If the petcock vacuum line runs off #3 that is why it looks so black/sooty.

Just so happens the lever on the petcock was broken off. I don't have the key for the locking cap so I will have to swap over the tank on my other bike. I can clearly see I came to the right place. I just dropped the exhaust system so I will have to put that back on. The carbs will take a while longer to do. Sound like the engine has 1/2 a chance to run.

Thanks very much.

Posplayr \\:D/
 
CafeKid

CafeKid

I may have overstated the wear/damage to the exhaust cam. In the attached picture you can see that the two left exhaust lobs are more wore than the right intake lobes. There is enough wear on the exhaust to just catch a finger nail. maybe .005".

I'm starting to think that this is really in pretty good shape.

Thanks again.

Posplayr 8)
 
Hi!
With the ex.cam looking like that, I would take it completely out and check the bearing surfaces before attempting to run the motor. This might be a sign that it has starved for oil.

Just my .02

-blo
 
60 psi dry compression readings are very low, even if the engine was cold. Not sure how much they would increase when warm but a fresh engine will typically run in the 150 psi range so keep this in mind.

If it were me I'd pull the top end off and measure the bores and check the valves before putting the engine in service.

Just my $0.02.
 
After sitting for twenty years, it's almost assured that the rings are rusted/stuck. Try squirting some penetrating oil in the spark plug holes and hand rotating a few times over several days and see if your compression numbers don't perk up.

I think that exhaust cam wear on the four valve 750 engines is fairly common. My '81 shows a bit of exhaust cam wear after 14,000 miles, but the intake cam is perfect. Another 750 I've looked at (this one an '82) showed the same pattern as yours with about 20,000 miles on it.

If you can actually feel any wear on your cam surfaces, it's gone. If you're patient, you can generally find one for the '80-'81 750s pretty cheaply on eBay. Make sure you get good, close up pics before buying though.
 
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