GS750 to 850

[KW]

I've managed to get hold of some GS850 barrels and pistons which I intend to fit to my GS750. Will I need to do anything with the carburation or should it still be ok with the standard 750 settings?

[Anonymous]

mate, bigger motor more fuel you will have to set up carbs after you fit 850 kit for sure ozman

[SqDancerLynn1]

Check block skirt for interference with the 750 crank, check piston pin diameter,check the bolt pattern, It may not work??? Just putting the 850 pistons in doesn't make yours a 850. the crankshaft stroke is also different. Let us know if you make it work

[KW]

As far as I was aware the 850 barrels/pistons are a direct replacement for those on the 750. From all the information I can find the strole is exactly the same at 56.4mm, just the piston diameter is different. Have I got it wrong???

[Anonymous]

Hi KW,

I've done this mod to my GS750.
Bought a GS850 block and pistons from a wrecker's yard with the later model cylinder head which accepted the CV carbs.
As it turned out, I didn't use the heads (too much trouble to find a mount for the choke cable, wanted something quick and easy), so I used my existing GS750's head.
I had to do some minor modifications to open out the crankcase openings, but apart from that it was a simple "drop in" modification.
I used the GS750 carburettors (26mm Mikunis in standard configuration) and was quite happy with the power increase - never had any carburetion issues.
Have now done 60,000+ kms with this mod...no problems to speak of. Noticed slightly higher fuel consumption, so it could probably go a bit leaner, but it's not worth the effort to correct. Seems to be running just great.

[Anonymous]

By the way KW,

Your research is correct, the GS850 and GS750 shared EXACTLY the same stroke dimensions at 56.4mm, the GS850 had 69mm pistons however as distinct from the 750's 65mm items.

It was, for all intents and purposes, an overbored GS750 with shaft drive.

Regards,

Pat

[Anonymous]

mate, bigger motor more fuel you will have to set up carbs after you fit 850 kit for sure ozman

Not true. Increasing the bore size, while retaining the same carbs, valves etc. will simply have the effect of increasing the velocity of air through the carbs. (Each time the pistons go down the cylinder, they pull a larger volume of mixture through the same size holes, therefore velocity needs to rise.)
The increased velocity through the carb venturis will automatically draw more fuel from the float bowls, without the need to increase jet sizes.

[Anonymous]

If i wanted to do this to my '79 750E, all i would need is the pistons, rings and sleaves from the same year 850 right? Then press in the new sleaves and reattach everything right? No drilling or machining? or will i need to open up some room for the new sleaves at a shop?

[Anonymous]

If i wanted to do this to my '79 750E, all i would need is the pistons, rings and sleaves from the same year 850 right? Then press in the new sleaves and reattach everything right? No drilling or machining? or will i need to open up some room for the new sleaves at a shop?

It's probably not going to be that simple. Some sort of modification is going to be needed somewhere. For me, it was a case of enlarging the crankcase openings so the GS850 block would fit onto the 750's crankcase. It was very minor but it needed to be done.

I'm assuming (!) that you have (or will have) access to a GS850 cylinder block in order to extract the new sleeves? Do a trial fitment with that first and you'll see what I'm talking about. Then you can decide which way to go.

Maybe the sleeves themselves can be machined to get the necessary clearances?

Pat