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

tkent02

tkent02

Forum LongTimer
Past Site Supporter
No grinding required. Fit them on the engine today. This with the early style 850 barrels on a 1978 750.
Now I just need to put in some pistons, and a bunch of other little parts and go for a ride.
 
Good info Tkento2 for people performing upgrades. I'm curious to learn how you judge engine smoothness. 850 seems to be very smooth, not sure about the 750 in it's native state.
 
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Nessism said:
Good info Tkento2 for people performing upgrades. I'm curious to learn how you judge engine smoothness. 850 seems to be very smooth, not sure about the 750 in it's native state.
Click to expand...

A 750 is very smooth indeed if it's all working right...

Just noticed the 750 pistons are lighter than the 850s...almost 8 grams difference each, about as much weight as the ring set. 156.7g for the 750, 164.3 for the 850.
Not sure what effect this will have on smoothness. As long as all four are matched it shouldn't be a problem I hope? I can lighten them some with a Dremmel but that seems to be a lot.
Experts?
Ideas?
Lighter wrist pins?
 
Shouldn't matter - 4-cylinder inline engines are static balance. As long as the pistons & rods all match you're fine.
 
Caution is indicated here. I gotta say, I had to grind the cases where the 850 cylinders went into the block on mine. This was on a 78 gs750 block & an 82 or 3 850 cylinders, pistons, & head. I sold it before I ever fired it up but there was no way it fit til I did the grinding. Maybe the earlier 850 cylinders fit but you need to do lots of measuring first.
I used the later 850 parts so I could use the cv carbs.

Paul
 
nastyjones said:
Caution is indicated here. I gotta say, I had to grind the cases where the 850 cylinders went into the block on mine. This was on a 78 gs750 block & an 82 or 3 850 cylinders, pistons, & head. I sold it before I ever fired it up but there was no way it fit til I did the grinding. Maybe the earlier 850 cylinders fit but you need to do lots of measuring first.
I used the later 850 parts so I could use the cv carbs.

Paul
Click to expand...

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.
 
Billyboy said:
Very cool!
Let us know how she runs!
Click to expand...

May be a while, I'm stuck in two places. Still can't dig the top ring out of one of the pistons, and looking for someone who works on these heads around here.
If I have to ship it to a machine shop somewhere anyway, who's the best?
 
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Something is wrong here, the old 750 had low compression, around 60 to 90psi. That's the reason for going 850. It was really a dog.
The pistons, rings and cylinders I removed all look new, fit tightly.
The intake valves look very good.
The exhaust valves look tight, except the sealing surface was black with carbon, they had been adjusted too tight before I got the bike. The head held liquid, did not leak down after an hour or so...
The exhaust valve guides are just barely a little bit loose, intakes are tight...
So where did my compression go?
I expected to find badly pitted or burnt valves.
I have taken apart engines with lots of visible wear everywhere that had higher compression than this.
 
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frozen rings?

tkent02 said:
Something is wrong here, the old 750 had low compression, around 60 to 90psi. That's the reason for going 850. It was really a dog.
The pistons, rings and cylinders I removed all look new, fit tightly.
The intake valves look very good.
The exhaust valves look tight, except the sealing surface was black with carbon, they had been adjusted too tight before I got the bike. The head held liquid, did not leak down after an hour or so...
The exhaust valve guides are just barely a little bit loose, intakes are tight...
So where did my compression go?
I expected to find badly pitted or burnt valves.
I have taken apart engines with lots of visible wear everywhere that had higher compression than this.
Click to expand...
 
tkent02 said:
No, all free, all grooves spotless, no wear, dirt, carbon, or rust visible anywhere in that ring area. Looks new.
Click to expand...

Weird. Maybe the loose valve adjustment?

Does your compression tester work? Did you have the throttle open??
 
robertob said:
Weird. Maybe the loose valve adjustment?
Click to expand...
They were tight, set them all correct. Tests before and after were a little changed, not much.

Cams were working fine, valves going up and down, it was all good.

robertob said:
Does your compression tester work? Did you have the throttle open??
Click to expand...

I borrowed the tester that I used at the time, and recently test my 1000 with the borrowed one and my known good one. The results were fairly similar, close enough. And yes, the throttle was open.


The 60-90 number is not that low, this is at 5,500 ft and there is less air here to compress.
Every engine reads lower here than at sea level, every vehicle is slower.
The difference between cylinders is way high, though, and the underpowered anemic way the engine ran led me to believe it was wasted inside.
Very slow, couldn't evade attacks of mindless minivans by pulling away from them, it was too slow. Old women in Hyundais got me off the light. It started to pull a little at about 6,000 or 7,000 RPM, but not much. Maybe a 25 horsepower machine, at best.
Even pulling the kickstarter through by hand it had little soft thumps instead of big hard Whomp Whomp Whomp you would expect to feel.
 
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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?
 
tkent02 said:
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?
Click to expand...

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.
 
"Actually this engine has been rebuilt, fairly recently"
Cam timing ? It affects power and compression.
 
hoyterb said:
"Actually this engine has been rebuilt, fairly recently"
Cam timing ? It affects power and compression.
Click to expand...

Exactly what I was thinking. Bet this was off by a tooth or two.
 
bwringer said:
Exactly what I was thinking. Bet this was off by a tooth or two.
Click to expand...

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.
 
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