750/850 Swap Noob

[MisterCinders]

Checking over the parts list for this and am puzzled by one entry. The o-ring at No. 6 in the fiche (sorry for the huge pic, it won't let me resize).

When I did the top end last year, I recall an oblong ring in that area, but the fiche shows a simple round ring as well as the oblong space on the gasket. Where does that round ring go? Why is there no oblong ring?

[rwcfrank]

I just did the 750/850 deal. Quite honestly I probably could have torqued down the head and the cylinder would have "self clearanced" It's mostly casting flash that has to be removed so maybe its a case of which bottom end you have. year to year or just how much flas was left over after the casting was cleaned. I was worried that if I torqued down the head and the flashing to cylinder clerance was insufficient it may either break the iron liner or leak because it was not down all the way. FWIW I have a 78 750 bottom end with an 82 850 cylinder.

[OLDENUFF]

The rectangle o ring is deleted by the new design MLS head gasket. The 2 egg shaped Orings go around the locating dowels on the base.

[MisterCinders]

The rectangle o ring is deleted by the new design MLS head gasket. The 2 egg shaped Orings go around the locating dowels on the base.

That's why I am confused. The fiche does not show an egg shaped o-ring for that spot, but it does show a round o-ring at No. 6.

The 750 base gasket had the egg-shaped rings, so I remember generally what goes in the different locations. This fiche has no egg-ring, even though the base gasket still has that egg shaped cutout.

Where does that round o-ring in the fiche actually go?

[MisterCinders]

I just did the 750/850 deal. Quite honestly I probably could have torqued down the head and the cylinder would have "self clearanced" It's mostly casting flash that has to be removed so maybe its a case of which bottom end you have. year to year or just how much flas was left over after the casting was cleaned. I was worried that if I torqued down the head and the flashing to cylinder clerance was insufficient it may either break the iron liner or leak because it was not down all the way. FWIW I have a 78 750 bottom end with an 82 850 cylinder.

Yeah, I plan to dry fit the jugs to see what clearance I need in the bottom end.

[OLDENUFF]

I would guess that #6 is the egg shaped oring. You are using 850 gaskets right?

[Spyder]

If you are still having troubles with the old gasket on that jug, soak it in acetone. It will eat any gasket off of any metal without hurting the metal. Old trimmers carb dip. Works great and you can buy it anywhere they sell paint.

[MisterCinders]

I would guess that #6 is the egg shaped oring. You are using 850 gaskets right?

Yes, I am using gaskets for the 1979 GS 850.

[MisterCinders]

After a little grinding in the cases, the swap went silky smooth.

Test ride this evening went well.

Gotta get some hard riding in to break in the rings. Not a hard task on that 850 chain drive.

[7981GS]

What and where did you have to grind?
Any pictures?

Eric

[MisterCinders]

What and where did you have to grind?
Any pictures?

Eric

No pics of the grinding, but the jugs would not seat by about 10mm because of some casting flange in the 1 and 4 openings in the bottom. There was very little metal to remove, so with some oily rags to catch the metal dust and little Dremel work on those ridges, I was able to dry fit the jugs. The ridges stick out about 1mm or so and catch on the sleeves, so it was not to bad trimming them back.

Another GSR member, Nerobro, came by to help.


On Saturday, I tore down the old 750 top end and stripped and dipped my carbs. I also had fit the new pistons and fresh rings. On Sunday, Nerobro arrived on his gleaming white unicorn to save me from the disaster in my garage.


Here is a close up of the pistons with one of the old ones for comparison



At that point, it was time to install the block.

First we got the 2 and 3 pistons in place.



Then the 1 and 4 pistons got on board.



Now that everyone was comfortable, it was time for the head.



Now time for cams and timing.





With the cam timing sewn up, I did a quick valve adjustment check. All were in spec, but the No. 3 exhaust valve was at the tighter end of spec. So I swapped in a thinner shim. Then we buttoned up the top.



After that, I rebuilt my carbs to sort out some jetting on the needle and main, and account for the new pull of the 850. So far, she is digging the new guts.

[rwcfrank]

How would you rate the power to before with the 750 jugs? I am waiting for my frame to come back from the powdercoaters so I can start assembly on my 750/850.....

[MisterCinders]

How would you rate the power to before with the 750 jugs? I am waiting for my frame to come back from the powdercoaters so I can start assembly on my 750/850.....

On paper, it should be about 15 HP. Only had a short rIde in town, so it's too soon to tell. Feels healthy though.

[MisterCinders]

Did some more test riding to break in those rings. This 750/850 tears up the road. The increased power and torque, the fresh top end and newly cleaned carbs (they were clean, but I stripped and dipped them while doing the new top end because . . . of course I did) are like a perfect storm of engine running.

Did a quick compression test and the results are around 122 psi cold, and dead even across all four cylinders.

Thus, I am chalking this one up in the "win" category.

There are some lingering jetting issues, as I adapt to the new displacement. The stock VM26 carb jetting for the 1978 GS750 and the 1979 GS850 is almost the same, so I made only minor adjustments to the needle and main. Those changes were needed with the old engine, since it was running slightly rich in the upper circuits.

I also continue to have a weird starting issue, but that's for another thread.

[MadCapsule]

Mister C, if you need an extra set of hands with this project let me know.
Edit: Never mind. Didn't read through the whole thread.