GS850-vs-GS750 Parts Swaps/Upgrades

[witttom]

I'm quite confident these questions have been asked before. I tried to find the answers by searching the forum for related threads, but in a sea of information, I just couldn't find my answers. My apologies for asking what I'm sure has already been asked before.

I've owned my 78 GS750 for about 7 years now (I think). Prior to that was a brief ownership of a POS 79 GS850, and prior to that (maybe 10+ years ago) was a POS GS450L. I think that was the first time I joined GSR (I think I've re-joined about 5 or 6 times since). It wasn't until recently though, that I've really taken notice of just how many cross-over components there are between some models. Particularly the 8-valve machines. Over the past few months, I've been parting out a 79 GS750N and more recently an 82 GS850G. I was kinda surprised to see the differences (improvements) between the 79 and my 78 (like folding foot pegs, relocated choke, etc). I immediately saw that as an opportunity for some 'upgrades', which ironically, I never really upgraded anything. In my recently deconstruction of the GS850, I'm seeing the same opportunities. Some of those might really make sense.

I've heard stories of guys building hybrid engines, using the top end of an 850 and putting in on their 750. I'm sure there are a variety of ways to go about this. I really don't want to get into anything that involved. At least not right now. I AM curious about some LESS-involved swaps though. Such as...

Is the electronic ignition from my GS850 able to be swapped into my GS750?

Will the Mikuni BS32SS carbs swap over to replace the VM26SS carbs? Yes, I know they CAN be, but SHOULD they be? The older GS850's had the VM26SS carbs too (right?) with larger jets (#100 vs #102 or so?), but the BS32SS seems like a BIG jump (#115?). I'm not just cramming more fuel into it, but more air, right? I'm using a Jardine exhaust that breaths a little better, so I'm not totally convinced that this would be such a bad thing. But... can I use my stock airbox with such a setup? Am I asking for a tuning nightmare?

Will the 850 cams drop into my 750 head? How much of a difference would there be? I'm thinking this is more than I want to do, but I'm curious.

....and will the oil pan from a 79 GS750 fit on a GS850? I believe it will, but is there a reason NOT to do it (I'm asking for someone else)?

[MisterCinders]

You can hunt down some of these answers through the part no. corss-reference tool on www.alpha-sports.com -- click "Special Tools" and choose "Parts Finder All" from the drop down.

Some of this I have encountered in my current 750/850 project. The 1979 GS850 uses the same VM26 carbs and cylinder head as the 1979 GS750, so parts between those models have a lot of inter-change.

I don't think you'll have much luck swapping VM26 carbs for BS32s. The intake ports are different sizes, so you'll need to address that with an adapter or something. Moreover, the carb spacing may also be different. That poses a lot of rigging to change carbs, then you will have to puzzle out any jetting adjustments, since I don't know that jetting for CV carbs on later GS850s necessarily works for the 1979 GS850.

Couldn't tell you if the later ignition will swap into your 750, but you can get a DynaS for that and be sure.

According to the parts nos., the cams on the 850 appear in fiches for larger bikes (GS 1000) but not the 750s. Same for the oil pan.

[witttom]

Couldn't tell you if the later ignition will swap into your 750, but you can get a DynaS for that and be sure.

The main idea behind that, would be an upgrade that doesn't cost me anything (something that I already have in hand).

[7981GS]

Use the '82 cylinder, pistons, pins, head and carbs.
And keep the kick starter.

BTDT.

Eric

[witttom]

Use the '82 cylinder, pistons, pins, head and carbs.
And keep the kick starter.

BTDT.

But that's a lot of work! Hehe...

[7981GS]

Your 750 will gain a little more than 15 HP too!

Eric

[daturat100r]

Read somewhere on this forum that 850 cams are a bit milder than 750 ones

[7981GS]

Possibly true but all stock GS 2-valve cams are tame.
If you can tell a difference in the cams, you are probably on drugs.

Eric

[postman_pat]

I'm quite confident these questions have been asked before. I tried to find the answers by searching the forum for related threads, but in a sea of information, I just couldn't find my answers. My apologies for asking what I'm sure has already been asked before.

I've owned my 78 GS750 for about 7 years now (I think). Prior to that was a brief ownership of a POS 79 GS850, and prior to that (maybe 10+ years ago) was a POS GS450L. I think that was the first time I joined GSR (I think I've re-joined about 5 or 6 times since). It wasn't until recently though, that I've really taken notice of just how many cross-over components there are between some models. Particularly the 8-valve machines. Over the past few months, I've been parting out a 79 GS750N and more recently an 82 GS850G. I was kinda surprised to see the differences (improvements) between the 79 and my 78 (like folding foot pegs, relocated choke, etc). I immediately saw that as an opportunity for some 'upgrades', which ironically, I never really upgraded anything. In my recently deconstruction of the GS850, I'm seeing the same opportunities. Some of those might really make sense.

I've heard stories of guys building hybrid engines, using the top end of an 850 and putting in on their 750. I'm sure there are a variety of ways to go about this. I really don't want to get into anything that involved. At least not right now. I AM curious about some LESS-involved swaps though. Such as...

Is the electronic ignition from my GS850 able to be swapped into my GS750?

Will the Mikuni BS32SS carbs swap over to replace the VM26SS carbs? Yes, I know they CAN be, but SHOULD they be? The older GS850's had the VM26SS carbs too (right?) with larger jets (#100 vs #102 or so?), but the BS32SS seems like a BIG jump (#115?). I'm not just cramming more fuel into it, but more air, right? I'm using a Jardine exhaust that breaths a little better, so I'm not totally convinced that this would be such a bad thing. But... can I use my stock airbox with such a setup? Am I asking for a tuning nightmare?

Will the 850 cams drop into my 750 head? How much of a difference would there be? I'm thinking this is more than I want to do, but I'm curious.

....and will the oil pan from a 79 GS750 fit on a GS850? I believe it will, but is there a reason NOT to do it (I'm asking for someone else)?

Yes, the electronic CDI ignition from later GS850s (and GS1000s as well) are a straight "drop in" replacement for the GS750. Ignition advances are the same across all three models, so all you need to do is find a CDI box, an advancer, and a sensor plate and you're ready to go. A simple job, will take you half a day (at most!) to fit and you're ready to go!

Mikuni BS32 carbs are not a drop-in replacement on a cylinder head designed for VM round-slide carbs. Ideally, you should source a GS850 1980 or later cylinder head to use the the BS32 carbs. I'm not saying it can't be done, it's just that adapting BS32 carbs to the earlier cylinder heads requires a lot of work matching the intake ports to the carbs and modifying the head to accept the intake pipes. Also, the VM26-style airbox will not work with BS32 carbs and will have to be discarded or replaced with a later model airbox. If you're really handy with a Dremel tool you could I suppose open up the holes on an earlier model airbox and fit suitable intake hoses.

As to your concerns about stepping up to the BS32 carbs - you needn't worry. Yes, BS32s do have a larger bore than a VM26 but it doesn't necessarily follow that they flow more air. BS32 carbs are perfectly suited to cylinder capacities of this size - otherwise Suzuki wouldn't have used them!

GS850 cams will drop in to a GS750 cylinder head, but are not necessary and offer no real performance advantages. GS750 cams actually have the same lift but longer durations. GS850 cam timing is identical to the GS1000, and in fact the GS850 uses a GS1000 exhaust cam.

Hybrid engines (GS850 top end on GS750 bottom end) are a great way to go if you can source the parts. I've built a hybrid 750 myself. I used the cylinder block from a '79 GS750 (overbored, of course), then fitted a 1982 GS850 cylinder head with BS32 carbs (standard GS850 jetting) and a 1982 airbox (the half-plastic design). I also used GS750 cams and standard GS850 pistons. The only reason I didn't use a Wiseco piston kit was for purely budgetary reasons.

A note of caution if you're considering a hybrid engine. Some (not all) GS850 cylinder blocks are NOT a drop down fit into all the GS750 crankcases. Measure carefully before committing yourself, otherwise you will need to remove some material from the GS750 cases to get everything to fit, and that means splitting the crankcases. Ideally, you should use an overbored 750 cylinder block.

And finally, yes, the oil pan from a GS750 will fit a GS850. So will the oil pan from a GS1000, a GS1000G and the GS1100G.

[MisterCinders]

A note of caution if you're considering a hybrid engine. Some (not all) GS850 cylinder blocks are NOT a drop down fit into all the GS750 crankcases. Measure carefully before committing yourself, otherwise you will need to remove some material from the GS750 cases to get everything to fit, and that means splitting the crankcases. Ideally, you should use an overbored 750 cylinder block.

The 1979 GS850 uses the same cylinder head as the 1979 GS750. Although the part nos. differ, the 1978-1979 GS750 heads are all but identical.

The 1979 - 1981 GS850s use the same cylinder block.

So you can use a GS850 block from 1979 - 1981 in a 1978-79 (probably 1977) GS750 and keep the 750 head, carbs, etc.

There is a slight clearance issue between the block and the cases. There are some casting ridges in the cases below the No. 1 and No. 4 cylinders that prevent the block from settling into the last 10mm or so. But you don't need to split the cases to remove that material. Some well-oiled rags to catch the dust and a few minutes with a Dremel will remove the flanges and allow the block to seat. Just make sure to keep the rags in place and take care pulling them out, so nothing drops into the crankcase.

[postman_pat]

The 1979 GS850 uses the same cylinder head as the 1979 GS750. Although the part nos. differ, the 1978-1979 GS750 heads are all but identical.

The 1979 - 1981 GS850s use the same cylinder block.

So you can use a GS850 block from 1979 - 1981 in a 1978-79 (probably 1977) GS750 and keep the 750 head, carbs, etc.

There is a slight clearance issue between the block and the cases. There are some casting ridges in the cases below the No. 1 and No. 4 cylinders that prevent the block from settling into the last 10mm or so. But you don't need to split the cases to remove that material. Some well-oiled rags to catch the dust and a few minutes with a Dremel will remove the flanges and allow the block to seat. Just make sure to keep the rags in place and take care pulling them out, so nothing drops into the crankcase.

You do it your way, I'll do it mine.

Otherwise, you seem to have confirmed exactly what I said.

[MisterCinders]

You do it your way, I'll do it mine.

Otherwise, you seem to have confirmed exactly what I said.

Except that boring out a 750 block to convert to an 850 is far from ideal.

Unless you have a hard-on for CV carbs, you don't need to replace anything but the middle section (block, pistons, gaskets) to convert a 750 to an 850.

The 1979 850 used VM26 carbs with only slight jetting changes.

The 1979 850 used the same head as the 2-valve 750.

The 850's from 1979 to 1981 used the same block.

So converting a 2-valve 750 to an 850 requires no boring (risking thinner sleeves) or replacing the sleeves (expensive), because you can do it with an OEM 79-81 GS850 block and have full sleeves.

Sure, it's a bit scary to grind the flash over the crankcase, but it's not major surgery, and some oiled rags catch the dust. I will do a fresh oil and filter change to capture any stray dust, but the case fitting was not a big deal. If you were doing the conversion in conjunction with a comprehensive rebuild, sure, split the cases. But you don't need to split cases to fit these parts.

I definitely don't "confirm" that the ideal hybrid build uses an overbored 750 block. The hybrid you describe requires modding the one 750 part you should swap out. What's the point of changing out the 750 head for an 82 850, when the original 2-valve 750 head is all you need?

[whiterabbitt]

bigger valves and intake ports, and cv carbs. based on the 850 parts sitting in my shed, compared to the stock pieces on the bike

[MisterCinders]

bigger valves and intake ports, and cv carbs. based on the 850 parts sitting in my shed, compared to the stock pieces on the bike

Must not be a 1979 GS850.

1980+ models had CV carbs, but the first GS850 in 1979 used VM26 carbs and the identical head to the 1979 GS750.

[TeamDar]

Valves are the same size. GS750-GS850, 36mm intake. 30mm exhaust.