920cc GS750 build - head gasket, deck height, valve sizes, cams, ?'s

[Chuck78]

I'm beginning the planning stages for a late winter or early spring project for my 1977 GS750B. The engine runs awesome as is, and the previous owner said "the p.o. had the engine bored out" but knew nothing more, as it was given to him as repayment of a debt. With as fast as it is currently, and as hard as it pulls from midrange to well above redline, we suspected it may already have a Wiseco 10.25:1 compression 844cc big bore kit.

Wanting to build something bigger and something unique (a GS w/bigger cc's but w/ kickstart engine), I happened upon a loooong out of production MTC Engineering 750/850 big bore kit with 72mm pistons for a clearance price of $250 - last one in stock, hiding on their shelves! (They still have some 69&70mm's but mostly turbo?s w/lower c.r.). I wanted to be able to run 87 Octane still in the summer heat & keep this build very streetable, so MTC shaved .040" off the tops to drop it just under 10:1.
I also got a set of 79 GS850 cylinder jugs (80+ require some case grinding on a 77-9). Now looking for a machine shop in town well versed in motorcycle engines to bore the cylinders out...

I realized reading some other posts that I also would be needing a special head gasket, as the 850 stock was 69mm, 70mm for the largest Suzuki overbore pistons. I was thinking maybe my only option was a custom head gasket from Cometic, but then someone tipped me off that a GS1100 uses 72mm pistons stock. I know a 1000/1100 cylinder block will not work on the 750 mainly due to different cam chain tunnel, but I wondered if the 1100 head gasket would still work? I also got very curious as to the advantage of a copper head gasket, as Cometic custom makes them, and I even see GS1100 copper gaskets available. Will the 1100 work? Should I splurge for a re-useable copper gasket, and are they much stronger than the composite? More questions to come

 

[Chuck78]

Valve size, cams, deck height mock-up

Valve size, cams, deck height mock-up

I will be retaining the original 750 head since it is my only real option (same as 850 head) & probably keep same valve sizes due to money, unless you guys strongly advise going bigger on the valves - any thoughts and ideas on cost of valves and machine work? If I need a valve seat job done, maybe I will consider larger valves. Any strong arguments against a 920cc engine build retaining the stock 750/850 valve sizes? RapidRay?

Probably keeping the original GS750 cams, as they supposedly are more aggressive than the cams that the 850&1000 share, more duration/overlap. No one has tipped me off on valve lift specs between the two as of yet, just duration. I had thought about some Web Cams, but I don't have $500 to drop on cams, as I am trying to keep this bike under ~$2000 invested. The idea of a more aggressive but streetable cam is very enticing, though. I don't want to build an unruly monster that has bad street manners/only likes high revs, but I do want more power at all rpm ranges. Now would be the time to do cams and bigger valves though, but is it worth the cost for the performance enhancement, and only stepping up to a slightly larger "street profile" camshaft for a whopping sum of ~$499?

I was also wondering how to properly measure deck height for head/base gasket thickness selection??? I am familiar with V8 musclecar engines, which is much easier because the cylinders and crankcase are one casting. Is this very critical to measure to get within .035-.050" quench height (piston to head) as with a car engine? With removable cylinder barrels and a base gasket, I wondered if it was now necessary to bolt on a torque plate to simulate gasket crush from the head/studs to measure the piston top to cylinder deck height??? I suppose my buddy could make a torque plate on his CNC for me using a head gasket template, or maybe if I got real lucky, a machine shop in town would have one.

 

[GregT]

This could be a very interesting build. My suggestions for what they're worth are...

Gasket - use a Cometic copper one - with O rings in the corners and i think the 750 uses the big O ring around the camchain tunnel. Pretty sure the 1100 is a bigger block physically than your 750.

Deck height - most times aftermarket bike pistons are correct out of the box. To check just do a trial assembly and nip the barrel down using spacers - this will get you close enough for a street motor IMO. It's not as if you have a wide choice of base gasket thicknesses either....

Cams - yep, use what you've got. Dial them in to whatever lobe centers you prefer. Searching here will give you plenty of opinions and others will be happy to advise here.

Valve sizes - Stick with std at least to start. For a street motor you're after a torque monster which you'll get with std valve sizes.

Put a Dyna on it - S or later, doesn't really matter which either are better than stock.

Have fun

Greg

 

[Chuck78]

After my wife's Dyna-S failed just out of warranty on her GS550, I read several other people posting all over the www about losing fire in two of their cylinders just like hers... I may see if Dyna will sell me just a new replacement 1-4 pickup for the bad unit, but otherwise I will stick with my fresh points setup until they need maintenance. Then I will go with either an 80-81 pickup and custom GM car HEI module setup (search for the "loudgpz" writeup/$40 ignition module build) or else Pete's gsignition.com (I think that's correct) comes highly recommended for a similar cost to Dyna.
I like the idea of the seperate custom made GM HEI module with heat sink, as the delicate electronic components inside the engine case will see very high temperatures A LOT, and I think this is why I saw the Dyna-S fail so young in it's life.

That is very welcome advice on the cams and valve sizes, but I'm still open to hearing more opinions. I'm sure I will be happy with the power upgrades just from the 920cc's and 10:1 compression with K&N filters and rejetted carbs, but still curious how much potential it has with cams and valves upsized. Probably not worth the estimated ~$1000 I'd have to invest, but I'm open to advice. As is with carbs that need torn down (idles for $#^! until it's warmed up 10 minutes), I blow by cars like a rocket, and was keeping up real well with an 1132cc KZ1000 w/CR 36mm slide carbs so much so that the owner couldn't believe I was riding a 750 or maybe 844... I think his bike still needs a lot more tuning!

 

[Chuck78]

I was bummed when my buddy at the local Vintage Japanese Motorcycle Junkyard told me they parted out and scrapped a GS650G that was there forever, but today I found a new arrival totalled GS650G, so I may take my wife's old GS550 engine (bad rings or bores on one cylinder and doesn't like to go into gear sometimes) onto the workbench and open up the case to accept the 650 cylinders, and bolt the shafty 650G's top end on to make the chain driven 6 speed gearbox 8.8:1 compression GS550 into a 9.4:1 674cc screamer! This will then be my practice run rebuilding a motorcycle engine. Since my bike is a real screamer already, I may save the GS750 920cc conversion for the second engine upgrade project for the off-season. Sooner if I finish my house remodel first. I need to make sure the 650 jugs are in good shape first, but I am very excited at the big bump in performance to both of our rides!

I've got a lot of carb needle and jet questions about my 920 conversion still, but would like to hear any other opinions on the above info if anyone cares to chime in still. Thanks!

 

[49er]

Greg has given you some very good advice with one exception, the head gasket.

I would suggest you use an OEM GS1100 Multi Layer Steel head gasket. Suzuki list them as stock replacements for all the 850G, 1000G and 1100G models. They have a bore size of 73 mm, are 5 layered stainless steel (0.043") and won't need any modification for your setup. Don't fit a cam chain tunnel o ring under this gasket, because they are too thick for this later designed gasket.

I have 71 mm 10.5-1 pistons in my '79 850 with stock cams, stock VM26 carbs and a modified stock airbox. The head is stock except for blueprinting and casting cleanup/valve pocket work.

Stock cams, VM carbs and the small port head work extremely well with the extra capacity. The rod ratio of 2.1 on these engines also ensures that the you don't lose much top end kick when keeping most tuning parameters close to stock.

I run a Boyer Bransden MP Digital Ignition which gives great torque from 3000 - 9000 rpm.

Fuel is 90 RON up to 30 deg C, or 90/95 mix above that temperature, to avoid detonation.

I've nearly completed a 1023 cc 850 engine, so I'm looking forward to the performance comparison with my current 900 cc version.

Have fun with your project.

 

[Chuck78]

Well that's more great advice, thanks to both of you guys! What year GS1100 should I order a gasket for, and are all OEM GS1100 gaskets the MLS type?

I almost asked about having Cometic make an MLS gasket, but copper seemed to be highly recommended. I wonder if copper would seal better still, since it is softer?

I have had a lot of experience with old Volkswagen 1.6L Diesels, and the older 11mm head bolt blocks blow lots of head gaskets if not cared for properly. I actually had gasketstogo.com custom make me an MLS gasket for the older VW style, since the new VW TDI MLS gaskets won't fit the older mechanical lifter 1.6's, as everyone raved about how bulletproof the MLS were compared to composite, and my wife drives the caddy pickup so darned hard... MLS are not very tolerant at all of slight head warpage, however!

So I just drop on the 1100 gasket minus the cam chain tunnel o-ring? are the other oil passage o-rings not in the head gasket surface?

 

[Chuck78]

49er, can you give us details of your 1023cc GS850 engine? custom pistons and larger sleeves into the 850 jugs??? Sounds awesome!

Any ideas how much horsepower I can expect out of a 920cc 9.8:1 engine with appropriate jetting, K&N filters, and free flowing 4-into-1? stock 750 @ 8.7:1 or so was between 63hp and 72hp. I think mine has the Wiseco 844cc 10.25:1 kit in it as is, and it's really really fast. Not sure what numbers to expect, other than "whoooaaa, hold on!"

 

[49er]

49er, can you give us details of your 1023cc GS850 engine? custom pistons and larger sleeves into the 850 jugs??? Sounds awesome!

Any ideas how much horsepower I can expect out of a 920cc 9.8:1 engine with appropriate jetting, K&N filters, and free flowing 4-into-1? stock 750 @ 8.7:1 or so was between 63hp and 72hp. I think mine has the Wiseco 844cc 10.25:1 kit in it as is, and it's really really fast. Not sure what numbers to expect, other than "whoooaaa, hold on!"

Chuck, I have a 4-1 too. Dyno was 73 hp (8500 rpm) at the rear wheel. Probably 93 at the flywheel, same as stock GS1000 ET. Mine will just out perform a stock GS1100G.

The 1023 has modded GS1150 sleeves fitted and 76 mm 11-1 JE pistons.

Hard to predict what HP the new one will produce, but I'm expecting a big boost in torque. Will be running VM 29 Smoothbores, but still through a moderatly ported small port head with modded stock airbox. Probably will run an air cooler too, but will trial it without one initially.

The 894 has no overheating tendencies, unless you are caught in stationary traffic for long periods in ambient air temps over 30 deg C.

From my perspective, most people over torque their MLS gaskets, believing that more torque equals better sealing.
I believe that the 5 layer gaskets (some are only 3 layers) are more tolerant to minor high/lows on the mating surfaces and still provide a good seal. However, when they are over torqued, the multi layers compress into a solid mass negating the effect of the multi layer cushion. My 894 is torqued to 30 psi on stock studs. No compression or oil leaks.

 

[Chuck78]

so you used 1150 sleeves in the 850 jugs? I didn't think the 1150 cylinder jugs would work on the 750/850 block due to the different cam chain tunnel.

Are those custom JE pistons, fitted onto stock 750/850 16mm pin rods? That thing will be a monster! wow.

 

[49er]

so you used 1150 sleeves in the 850 jugs? I didn't think the 1150 cylinder jugs would work on the 750/850 block due to the different cam chain tunnel.

Are those custom JE pistons, fitted onto stock 750/850 16mm pin rods? That thing will be a monster! wow.

Yes 1150 sleeves in 850 jugs. Quite a bit of machining. 1150 jugs have different stud pattern and oil galley layout.

JE pistons are made for 16 mm pins. I've lightened pistons to within 12 grams of stock 850 weight.

The 894 has the same weight pistons as stock (198 grams).

 

[Chuck78]

I plan to rebuild/rejet the original VM26 carbs, as they are the same model as the 79 GS850 and 77-79 GS1000. The 1000 actually used smaller main jets, but I suspect that is because the 1000 head has larger valves. One source I read showed the same jetting for the 850 & 750. Not sure on the jet needle, as the taper size/profile will definitely affect the midrange power.
I will be running K&N RC-2222 dual oval filters (like pods but two carbs into one filter) and a very free flowing MAC 4-1 header and homemade glasspack insert. Any ideas on where to start with jetting? Stock was between 97.5 ('79) & 105 ('77) depending on the year (EPA regulations de-tuning them). I figured on a conservative start at 117.5-122.5 (I think they go in 2.5 increments), but with the added cc's, maybe much larger may be in order (130?) with the proper jet needles to keep half throttle from guzzling too much fuel.
That brings up the question of jet needles... Not many people talk about jet needles aside from "raise the needles one notch higher" when they talk about tuning carbs and putting in larger mains. The jet needle inserts into the main jet's needle jet housing, and when the throttle is opened, the tapered needle pulls out of the main/needle jet) and allows progressively more fuel flow as the needle taper inside the jet gets smaller and smaller as the throttle is raised.
Any educated suggestions on what size jet needles to use? A skinnier needle will give more fuel flow to midrange. Maybe looking at the jet needle and needle jet sizes on the GS850 & GS1000 will help. I think they are size designated by letter (letter = machinist's drill size?), which I would have no idea which direction to go still. Maybe just raising them one notch may be easiest, as they are easy to change later without pulling the carbs off the bike.


GS750 VM26 1977-79:
105 main / 5F21-3 jet needle / P-1 needle jet / 22.5 pilot jet
105 main / 5F21-3 jet needle / P-1 needle jet / 15 pilot jet
100 main / 5F21-3 jet needle / O-6 needle jet / 15 pilot jet
97.5 main / 5F21-3 jet needle / O-6 needle jet / 27.5 pilot jet


GS850 VM26 1979
102.5 main / 5DL36-2 jet needle / 0-6 needle jet / 15 pilot jet


GS1000 VM26 1977-79
95 main / 5DL36-3 jet needle / O-2 needle jet / 15 pilot jet
95 main / 5DL36-3 jet needle / O-4 needle jet / 15 pilot jet

EDIT - I had the jet needle and needle jet #'s mixed up, do not reference the quoted text below, above is fixed. Source: http://www.thegsresources.com/_forum/showthread.php?t=200522&page=2


is the “-3” on the needle jet part of the part #, or is this the recommended clip position for the jet needle? This would seem to indicate that the GS1000, sharing the same jet needle as the 850, needs more fuel in the midrange, and therefore the needle sits one clip notch higher to allow more fuel flow???

 

[49er]

I plan to rebuild/rejet the original VM26 carbs, as they are the same model as the 79 GS850 and 77-79 GS1000. The 1000 actually used smaller main jets, but I suspect that is because the 1000 head has larger valves. One source I read showed the same jetting for the 850 & 750. Not sure on the jet needle, as the taper size/profile will definitely affect the midrange power.
I will be running K&N RC-2222 dual oval filters (like pods but two carbs into one filter) and a very free flowing MAC 4-1 header and homemade glasspack insert. Any ideas on where to start with jetting? Stock was between 97.5 ('79) & 105 ('77) depending on the year (EPA regulations de-tuning them). I figured on a conservative start at 117.5-122.5 (I think they go in 2.5 increments), but with the added cc's, maybe much larger may be in order (130?) with the proper jet needles to keep half throttle from guzzling too much fuel.
That brings up the question of jet needles... Not many people talk about jet needles aside from "raise the needles one notch higher" when they talk about tuning carbs and putting in larger mains. The jet needle inserts into the main jet's needle jet housing, and when the throttle is opened, the tapered needle pulls out of the main/needle jet) and allows progressively more fuel flow as the needle taper inside the jet gets smaller and smaller as the throttle is raised.
Any educated suggestions on what size jet needles to use? A skinnier needle will give more fuel flow to midrange. Maybe looking at the jet needle and needle jet sizes on the GS850 & GS1000 will help. I think they are size designated by letter (letter = machinist's drill size?), which I would have no idea which direction to go still. Maybe just raising them one notch may be easiest, as they are easy to change later without pulling the carbs off the bike.


GS750 VM26 1977-79:
105 main / 5F21-3 needle jet / P-1 jet needle / 22.5 pilot jet
105 main / 5F21-3 neede jet / P-1 jet needle / 15 pilot jet
100 main / 5F21-3 needle jet / O-6 jet needle / 15 pilot jet
97.5 main / 5F21-3 neede jet / O-6 jet needle / 27.5 pilot jet


GS850 VM26 1979
102.5 main / 5DL36-2 needle jet / 0-6 jet needle / 15 pilot jet


GS1000 VM26 1977-79
95 main / 5DL36-3 needle jet / O-2 jet needle / 15 pilot jet
95 main / 5DL36-3 needle jet / O-4 jet needle / 15 pilot jet


is the ?-3? on the needle jet part of the part #, or is this the recommended clip position for the jet needle? This would seem to indicate that the GS1000, sharing the same jet needle as the 850, needs more fuel in the midrange, and therefore the needle sits one clip notch higher to allow more fuel flow???

Quick reply before heading out.

There are many suggested starting points from the archives, but as you know there's lots of trial and error involved to getting it right, even when moderate air box mods are factored into the equation.

I have a very free flowing foam filter in my box, so had to play around a bit. I tried the DL31 needles that are run in the 29 smoothbores, but their taper with my needle jets didn't have the midrange punch I was looking for. So I went back to the DL36-3 and and it was slightly fat in midrange and finally settled on DL36-2.5 (shimmed) perfect. I run 105 mains.

 

[Chuck78]

Well that's great info to start out with! Thanks a million for sharing your experience and knowledge. I think yours will be the closest bike to my setup of any on the board here.

Is there any benefit to going bigger than 105 on the mains on your 894cc, probably at the expense of fuel economy, or is that what you found best for overall performance? Only one size up from stock in mains, despite the 44cc increase in displacement and the higher compression??? I guess I should just reference the GS1000 stock jettings to justify it, trying to get a grasp on everyone's tuning experiences so that I don't have to buy all kinds of carb parts and pull them several times until I get them tuned right!

On my 920cc, I may try for a 107.5 mains for starters and the 5DL-36-2 jet needle that you settled on, maybe try to shim it to 2.5 as you did. I realized after reading yours that I mixed up jet needle and needle jet #'s, oops.

49er, Stock 850 needle jet on yours? Pilot jet sizes???
I would like to personally thank you for helping me save a lot of headaches with carb tuning, and the tip on the 1100 MLS head gasket! And thanks to Greg for making me rest assured that the stock 750 cams/valves/head will be more than sufficient for this 920cc buildup!

 

[49er]

Well that's great info to start out with! Thanks a million for sharing your experience and knowledge. I think yours will be the closest bike to my setup of any on the board here.

Is there any benefit to going bigger than 105 on the mains on your 894cc, probably at the expense of fuel economy, or is that what you found best for overall performance? Only one size up from stock in mains, despite the 44cc increase in displacement and the higher compression??? I guess I should just reference the GS1000 stock jettings to justify it, trying to get a grasp on everyone's tuning experiences so that I don't have to buy all kinds of carb parts and pull them several times until I get them tuned right!

On my 920cc, I may try for a 107.5 mains for starters and the 5DL-36-2 jet needle that you settled on, maybe try to shim it to 2.5 as you did. I realized after reading yours that I mixed up jet needle and needle jet #'s, oops.

49er, Stock 850 needle jet on yours? Pilot jet sizes???
I would like to personally thank you for helping me save a lot of headaches with carb tuning, and the tip on the 1100 MLS head gasket! And thanks to Greg for making me rest assured that the stock 750 cams/valves/head will be more than sufficient for this 920cc buildup!

Yes stock 850 needle jets (0-2), but running 17.5 pilots.

I tried 108 mains, but the engine richened to 12.3-1 at 9000 rpm. This also dropped 2 hp off the peak.

Some say I run too lean, but "there's a fine line between pleasure and pain".

I'm tuning the 29 Smoothbores to the 894, and then I'll fatten them up slightly. They will then be close enough to fire up the 1023 and get it bedded in as quickly as possible. I'm a strong believer of the Motorman running in method.

The 29's aren't as sweet in the midrange as the 26's, so I'm fitting a set of 5DL36 needles, 0-4 jets and leaning up the mains for a trial. Hoping to do tests over the next couple of weeks. Plug checks and the butt dyno indicated a slight richness throughout the rev range when using the stock 5DL31 needles and 0-6 jets.

 

[Guest]

I think we have a bunch of big bore 750 copper head gasket. I will check

 

[Chuck78]

Please do, pm me with specs (bore, thickness) & price if you can. Thanks!

 

[Bevo]

I don't think the main jets size will affect fuel economy during NORMAL riding, you are never on the main unless your on a smaller bike.

This is a great explanation on what the carb does at various openings.http://www.iwt.com.au/mikunicarb.htm

 

[Guest]

May I?

May I?

May I say a few things? When I did my 850/1000 conversionb, I put all 1000 G internal parts in the 850. I was geared short like the 850 was but may, it ran well. For the 750+, I think a whole 1100 G top end would bolt right on. I believe the cylinders were the same height. Not the combustion chamber on the 1100 could be larger than the 850 so you would loose some compression. But the 1100 cylinder could be bored to 75mm and could go another 3mm in bore if you replace the sleeves which would be 78mm. Yes, with the small 16mm pin you would need custom pistons made but that shouldn't be too hard if you don't mind spending the money. Then your headgasket would be for the 1100 and the multi layers are the hot ticket.
Oops, now after looking at BikeBandit, I have forgotten that it has a cam chain ilder so that would mean the cylinder block is shorter. Sooo, Maybe, you can get an 850 cylinder, bore it out like crazy, press in aluminum sleeves, weld top and bottom, bore for new iron sleeves and get to the same place? Or got to plated bores instead?
Hope some of this helps.
Laters
Greg

 

[49er]

There's a practical limit to the size of 750/ 850 liners. If you go bigger than 76 mm bores, you loose too much base gasket seating area, especially around the oil galley corner studs. You could redesign that area with weld and re-machining, but addressing the distortion in surrounding areas, wouldn't be worth the time and effort.

Bad base gasket seating on a road machine is a PITA.