GS 850G ...Head gasket, valve or ring job ?

[49er]

Hi Guys & Gals
I have just joined your very informative and interesting group.
You have a great resource which I hope to call on and be able to contribute to over the years.
I own a '79 GS 850G with 78K's in reasonable nick and have owned it for 4 years. As far as I know it is standard, except for a 4-1 exhaust and performs quite well. It starts and runs well if used daily. When allowed to sit for 2 weeks or more it requires short bursts of cranking for up to a minute on average.
Because of this reluctance to start, I decided to do a compression test. The range when hot with the throttle wide open was 115 - 125psi. When the engine cooled the figures dropped by 10psi.
The engine has a moderate oil leak somewhere between #2 and #3 cylinders which could be coming from the head or barrel ends.
I am planning to take the head off to check the condition of the gasket,valves and pistons and check out ways of improving performance without loosing reliability/flexability.
What is the standard valve to piston clearance on these engines?
Can I machine metal off the top of the barrels, or must it come off the head? Is there a difference in the wrist pin height/diameter between a GS850 and GS1000 piston?
Does anybody know the factory endfloat for the camshafts?
Hope I can pick some brains here.
Cheers

 

[humhead]

GS 1000 piston pins are 2mm lager in diameter. Milling the head or block will mean you'll have to use adjustable cam sprockets & degree your cams. (this in addition to possible deck height &/or piston to valve clearance issues) You might want to try a thinner head gasket. This way, if you run into clearance issues, it's easy to go back to stock! Compression is listed as 128 - 187 psi with a limit of 100psi for a 1000. Assuming that an 850 is similar, you're on the low side of normal. Camshaft deflection is listed as .03 with a limit of 0.1.
1mm over stock pistons & rings are still available in the US.
If you're looking to improve the performance of your 850, you might want to consider installing a 1000 or 1100 engine into your 850 frame. Search the archives for info - I remember reading about it on this site. The modifications needed were minor. I hope this helps.

Terry

 

[49er]

Thanks for the info Terry. I was told that dropping either a GS1000 or GS1100 engine in requires slight mounting changes and fitting the matching drive shafts and diffs.
I would prefer to keep the 850 in as original condition and appearance as possible.
You mentioned some figures for camshaft deflection, I was wanting to know the factory endfloat tolerances. Like most of these engines, mine has a top end knock at idle when warm.
Thanks for your help.
Cheers
Ian

 

[49er]

Back again. I've finally got around to stripping the top end down. Found minor wear on the valve stems,guides and seats. Just a re-lapping job reguired. The head gasket is a multi- layered steel one. It has been reused and was coated with silver paint, probably to help resealing. I think the oil leak was caused by a cheap sealing method used around the timing chain housing.
All pistons checked out fine. Minimal wear of the rings and grooves.
The bore is the problem. Rings set in the stroke area give gap readings from 0.022in - 0.026in. Standard should be 0.004-0.014in.
I guess others have gone through this exercise.
Is it cheaper to rebore and fit oversize pistons or find/make new standard barrels and fit new standard rings? Are softer, non chromed top rings available?
Thanks Ian.