850 barrels DO in fact fit in the 750 cases.....

[tkent02]

Actually this engine has been rebuilt, fairly recently. O rings around the cylinders and all the rubber seals are still pliable, gaskets were not stuck, no leaks at all anywhere. Cylinders, pistons, head, all of it looks way better inside than the 850 stuff I am putting on.
It just didn't work.
WTF?

[49er]

Actually this engine has been rebuilt, fairly recently. O rings around the cylinders and all the rubber seals are still pliable, gaskets were not stuck, no leaks at all anywhere. Cylinders, pistons, head, all of it looks way better inside than the 850 stuff I am putting on.
It just didn't work.
WTF?

Sounds like the only posssible cause left for your low compressions is poorly seating rings. Have they been fitted up correctly? Do the bores look like they were honed recently?
Could the PO have reused the old rings and not fitted them to the original pistons without honing the bores? You should have seen some blowback/fuming through the breather and had a smoking exhaust as a result of that.

I would look elsewhere for clues around your poor acceleration though! Probably retarded ignition timing. If you are still running points, check that the centifugal advance mechanism is still fitted and working. If electronic, check that it's timed correctly, and that the black and white stator wires are orientated to the correct coil. Even check that the coils are matched to the electronic ignition. If they are rated incorrectly, that will cause poor performance.

[hoyterb]

"Actually this engine has been rebuilt, fairly recently"
Cam timing ? It affects power and compression.

[bwringer]

"Actually this engine has been rebuilt, fairly recently"
Cam timing ? It affects power and compression.

Exactly what I was thinking. Bet this was off by a tooth or two.

[tkent02]

Exactly what I was thinking. Bet this was off by a tooth or two.

When I got it, cam timing was one tooth off, can't remember which way.
Fixed it, not much difference in compression. Not much power before or after.
Adjusting the valves made more difference, but not much.
Ignition timing was spot-on, set it there myself. Stock points ignition, all functioning perfectly.

Now I'm thinking I may have fallen for the "clogged exhaust pipes trick" again.
That's the only reason I can think of to have no power at all.
This has bitten me on numerous engines.
Do stock pipes tend to clog up after a while running rich?(stock jetting at 5,500 ft.) They are black inside but not thickly so.
I can put these pipes on my 1000 to see if it kills it....
Would not explain the low compression though.

[Nessism]

Did you do the oil in the cylinders trick after your first compression test? I'm guessing not or you would have mentioned as much. At any rate, the oil helps determine if the rings or the valves are at fault.

As a side note, based on your seemingly prolific rotation of GS bikes, those decent 750 parts are likely to see reuse again (with new rings of course).

[tkent02]

Did you do the oil in the cylinders trick after your first compression test? I'm guessing not or you would have mentioned as much. At any rate, the oil helps determine if the rings or the valves are at fault.

Yes, I did. Numbers came up just a tiny bit, only 1-3 PSI per cylinder indicating valves are at fault.

As a side note, based on your seemingly prolific rotation of GS bikes, those decent 750 parts are likely to see reuse again (with new rings of course).

Yes, and I have been wanting to try the 850 conversion anyway, so no worries. I guess if it works well I will be looking for 850 pistons from now on. :-)
It's just puzzling. I tried the compression on one cylinder of the 1000 just now, then again with the exhaust completely blocked, it went down only about three PSI. That might explain the low power but not the low comp readings.

Oh, well.

[Big-Jim]

The cylinders that come with the later CV carb 850s are different, the sleeves are too fat to fit.
The later head goes on the earlier cylinders just fine, identical except for the intakes. Still deciding which carb/head setup I want to use. Would prefer the CV style but my CV head will take more work and money to get in good shape. Can't find anyone local who works on these heads, might just take the best head, lap it a while and call it good for now.

This is the set up that I am running on my gs 750/840. I use an ported 850 head with 34mm flat slides. better mid range and top end punch but the CVs are alot smoother too
Jim

[tkent02]

This is the set up that I am running on my gs 750/840. I use an ported 850 head with 34mm flat slides. better mid range and top end punch but the CVs are alot smoother too
Jim

I have also a later 850 head with CVs, not ported at all.
I would rather use this due to the better compensation at different elevations with CVs.
I live at 5,500 ft, my recreational riding is all up higher than that, up to 14,000ft, around town stuff is all at 5200-5500.
SO, do you think the CVs are a good idea in this case?
Are they more powerful than VMs or is your power due to other mods?
I would have thought not from my experience with 750s, my old 1978 750 was faster than the '80 and the same as the '83 I had last year.
I think my CV head needs more work than the VM head, but I can do it if it's worthwhile.
And, does your engine vibrate much more than it did as a 750? Did you do anything to alter the weight of the 850 pistons?
Thanks,
Tom

[SqDancerLynn1]

Their was mention in a previous post about the need to reuse the 750's cam gears & to eliminate the idler gear that the 850 uses. Unless you split the casas and change the timing chain

[tkent02]

Their was mention in a previous post about the need to reuse the 750's cam gears & to eliminate the idler gear that the 850 uses. Unless you split the casas and change the timing chain

750 and early 850 heads are identical, same part numbers on all the bits and pieces except the cams themselves, idler gears are the same, tensioner parts, sprockets are all the same. The 850 uses 1000 cams for a broader powerband...
I might try both cams to see which I prefer.