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GS750 850 upgrade

  • Thread starter Thread starter dlanes
  • Start date Start date
D

dlanes

Guest
I read about this and need a little guidance. What is all needed? I'm sure the carbs will have to be rejetted, but what size should I be looking for, or will I need another set of carbs? What about the bottom end? From what I read 79' gs850 is ideal. This is going to be a winter project for me.
 
dlanes said:
I read about this and need a little guidance. What is all needed? I'm sure the carbs will have to be rejetted, but what size should I be looking for, or will I need another set of carbs? What about the bottom end? From what I read 79' gs850 is ideal. This is going to be a winter project for me.
Click to expand...

You need the cylinders and pistons and rings only. Jetting, heads, everything else is identical.
I have them if you want a set.
 
tkent02,

I assume you have done this swap...did it make a big difference in power and torque? I have read that a GS750 makes about 63 HP and a 850 about 78 HP so that is substantial. Is there any issues with balance using the larger 850 pistons?

Sounds like a pretty simple and cheap upgrade.

Brian
 
Youll also need a gasket kit. From a 79 850.And a ring kit. Id suggest getting the wrist pins from the 850 pistons as well, but thats just me. I dont like mixing wear parts like that if it can be helped. Its not as cheap as you might think, you'll likely have a couple hundred into it depending on what you'll pay for parts, but its cheaper than the 750 bore kit, and you only lose .25cc per cylinder in comparison. They will not give you a compression ratio change like the bore kit will either.
 
I've read on the forum that some grinding is required to make the 850 barrels fit into the cases, as I'm interested in this upgrade too.
 
Coady said:
I've read on the forum that some grinding is required to make the 850 barrels fit into the cases, as I'm interested in this upgrade too.
Click to expand...

Grinding is required only if you use the 80-81 jugs. They were a little different because of the change in the head in 80 for the emmissions required CV type carbs. If you find a set of jugs from a 79 850 they will drop right on to the 750 cases with no grinding required.
 
TheCafeKid said:
Grinding is required only if you use the 80-81 jugs. They were a little different because of the change in the head in 80 for the emmissions required CV type carbs. If you find a set of jugs from a 79 850 they will drop right on to the 750 cases with no grinding required.
Click to expand...


Thanks for the info. I hadn't found a definitive answer on that.

My 750 has some vertical scratches in the cylinders - goes great (as long as I use hot plugs) but burns about a litre of oil per 1000kms and smokes a little when ridden hard and when hot.

I've been looking at these 750 cylinders and pistons on ebay as it seems a better/easier option than reboring: http://cgi.ebay.com/ebaymotors/ws/e...0420498843&viewitem=&sspagename=STRK:MEWAX:IT

But is it worth holding out for 850 parts?

I'm not in a hurry to pull down the top end again but reckon I'll be keeping this bike for a while so want to get it sorted.

Sorry for hijacking your thread dlanes.... :)
 
Coady said:
But is it worth holding out for 850 parts?

I'm not in a hurry to pull down the top end again but reckon I'll be keeping this bike for a while so want to get it sorted.

Sorry for hijacking your thread dlanes.... :)
Click to expand...

No holding out required, I have two sets of pistons/cylinders, and one set of new Suzuki piston rings and gaskets.
Let me know if you want them.
 
what about a cam and springs while its apart?, and atelast lapp the valves
 
cyclefvr2 said:
what about a cam and springs while its apart?, and atelast lapp the valves
Click to expand...

Changed the valve stem seals and measured the springs a couple of months ago, when I replaced the head gasket. That's when I saw the scored cylinders. Put it all back together and, as I said, it goes great (compression 135 - 145psi), but the oil usage is bugging me and it's impossible to read the plugs and fine tune - although I reckon I've got it pretty close.
 
Great thread..having my 750 going back together i've been on the lookout for a set of 850 cylinders/pistons..maybe an upgrade late springtime if i cant find any sooner...good info here tho thanks!


DDM
 
I've been wondering just how big you could bore the 750 cylinders. If you machined them out larger could you use the sleeves from an 850 or even a 1000? I guess what I'm asking is if anyone knows the max you could bore one out to. I was thinking the other night about the crazy idea of taking that relatively light 750 engine and boring it all the way up to something in the 900's by using stock 1000 pistons. Probably not feasible... but then again maybe it is.
 
doctorgonzo said:
I've been wondering just how big you could bore the 750 cylinders. If you machined them out larger could you use the sleeves from an 850 or even a 1000? I guess what I'm asking is if anyone knows the max you could bore one out to. I was thinking the other night about the crazy idea of taking that relatively light 750 engine and boring it all the way up to something in the 900's by using stock 1000 pistons. Probably not feasible... but then again maybe it is.
Click to expand...

Just so you know, the GS1000 engine is lighter than the 750 engine so I'm not sure it's worth the hassle.
 
Nessism said:
Just so you know, the GS1000 engine is lighter than the 750 engine so I'm not sure it's worth the hassle.
Click to expand...

Yup, not worth the hassle. Plus, about 3mil is probably as large as you can go on the stock sleeves before they're too thin. Maybe if you used 850 jugs and punched em out etc etc. But by that point I'm just gonna find a 1000cc motor and mod the mounts on the 750 rather than mess with all that
 
Nessism said:
Just so you know, the GS1000 engine is lighter than the 750 engine so I'm not sure it's worth the hassle.
Click to expand...

HA!
Well, that trashes that idea. I knew the 850 was heavier, but didn't think the 750 was. That's what I get for thinking.

I'm rolling the idea of a cafe or GP build around in my head, so trying to come up with ideas for getting max horsepower out of one of the smaller engines. Obviously the 750 is a deadend. Also looking for a use for my set of stock 1000 pistoins.
 
doctorgonzo said:
HA!
Well, that trashes that idea. I knew the 850 was heavier, but didn't think the 750 was. That's what I get for thinking.

I'm rolling the idea of a cafe or GP build around in my head, so trying to come up with ideas for getting max horsepower out of one of the smaller engines. Obviously the 750 is a deadend. Also looking for a use for my set of stock 1000 pistoins.
Click to expand...

If you're making a cafe the 750 isn't a dead end. But I would think the only use your going to find for your stock 1000 pistons is gonna be in a stock 1000 motor. Maybe punch out an 850....but why do that when you won't gain the CR increase...
 
TheCafeKid said:
If you're making a cafe the 750 isn't a dead end. But I would think the only use your going to find for your stock 1000 pistons is gonna be in a stock 1000 motor. Maybe punch out an 850....but why do that when you won't gain the CR increase...
Click to expand...

Yeah, no real reason to punch out an 850 when it's a heavier engine than the 1000. As you said, I'd just do the mods to make a 1000 engine fit.
 
If you're looking for a performance increase then use the tried and true methods.

1) Exhaust and pod setup - cheapest and easiest, make sure you tune carbies well though or you will end up with less horsepower.

2) Carbies - get some smoothbores to suit. Once they're set up they'll decimate all.

3) Head shave - Bump that compression ratio up a tad. 20 thou off is a safe bet.

4) Mild cam - often referred to as a 'Stage 1' cam, these will give an increase in performance while retaining most of the stock usability and reliability.

If you do all of that you'll be the envy of many bike owners, not to mention have a well-sorted machine that'll have quite a few more ponies than stock. Nothing to challenge 'modern' bikes, mind you, but definitely enough to surprise people. I would agree, however, that the best option is probably just going to a 1000 engine, especially if your bores are scored. All the work mentioned above, plus the boring/pistons/rings you'll need for your current unit, would probably run in excess of a 1000 engine and almost certainly not give you as much horsepower.

Summary? If you're after power, go a larger engine. If you want to get the most out of the engine you have, do numbers one to four and write back to us.

Cheers mate - boingk
 
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I have a fairly built 1000 engine in my G. I just see a lot of 750s come around cheap, so seeing what the limitations of that engine were as far as using stock parts from a larger engine. I had no idea it was so heavy (the engine), which basically makes a 750 build (for me) need to look in the direction of a full engine replace. 750 is a little big for what I want to do anyway, how heavy is the 550 engine.. anybody?
 
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