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

[Big Jay]

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

[gmansyz]

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.

[Steve]

Now looking for a machine shop in town well versed in motorcycle engines to bore the cylinders out...

If you have not yet found your machine shop, I would suggest that you "go west, young man."

Give Pearson Racing a call at 937-839-9723. He is about an hour and a half west of you, in West Alexandria, oHIo, and is very highly regarded in the racing community. He even has his personal GS sitting in the shop.

.

[Big Jay]

I checked. We have a stack of about 25 copper head gaskets. Can't tell if they are 750 or 1000. Can't remember if that even matters. They are 75 mm.

[TeamDar]

I checked. We have a stack of about 25 copper head gaskets. Can't tell if they are 750 or 1000. Can't remember if that even matters. They are 75 mm.

They will fit GS750/850/1000/1100 8 valve models. Only difference would be the cam chain area o ring access.

[49er]

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.

Well..........the experiment using 0-4 needle jets on the 29's wasn't very successful. The engine wouldn't pull once the needle started taking over. Besides being only .002" less clearance around the needle, it made a huge difference. I refitted the 0-6 needle jets, set the needles at 5DL36-2 and ran 112.5 mains.

The result was a great torque improvement in the mid and upper rev ranges.

I'm not happy with the pilot circuit...........it's either too rich or too lean for some reason. After reading many tuning attempts on these carbs from the archives, it appears they don't perform well in this area anyway, having been designed strictly for high performance applications. I've rechecked the condition of my float needles, the floats and their heights. The heights were set at 23 mm (off the gasket). I will try 25 mm. The needles have light wear rings showing (cant feel a ridge when rubbing a finger nail across them), so that combination may be causing a flooding condition.

After returning from a ride, I left the petcock on prime for 15 minutes while taking a phone call, and found a nice pool of fuel on the garage floor. No oil contamination though, so not all bad. I guess that tells me that the needles really do need replacing.

After fitted the 26's back onto the 894, I wonder why I'm bothering with 29's. Their airflow velocity is obviously superior throughout the whole rev range, giving a strong even torque progression.

Thinking of boring my spare set of 26's out to 28's as an experiment for the 1023 engine.

[Chuck78]

Well, I am paying very careful attention to your tuning trials & errors so that I can get the best tuning possible on my 920cc gs750 when the time comes to swap piston & bore the 850 jugs & rebuild the carbs to match. Thanks a lot for sharing your findings!

My bike, even with dirty carbs & old o-rings, continually impresses me on theway it performs with the wiseco 844 kit. Above 70mph, the acceleration is just "adequate," but up to that speed, the bike's acceleration feels absolutely monstrous! 0-60 is insanely fast, I cant imagine how much torque I'll have going to a 10:1 920cc engine with really dialed in carbs, k&n filter, & HEI ignition!
Looking for wider alloy rims 3.50/3.75/4.25 rear for a 140/70/18 rear tire & 2.50x18 from for a 110/90/18. Got ninja twinpot dual calipers & cbr900rr rotors, ninja adjustable lever & master cylinder, going for sonic or progressive front springs & probably Hagon rear schocks, & gs1100e box aluminum swingarm. Really trying to make this bike a real performer all around.
Having trouble finding a 36 spoke 3.50"+ rear rim in an 18, but I find a lot of 3.50x17 supermoto rims, & some gs1100's apparently had 3.5x17 rims

[tirebiter]

49er,

Any chance you have a chart that shows the needle profile for the 5DL36-2 ? I can't find one anywhere. In fact i can't find a profile chart for any of the older Mikuni needles.

Considering a swap from the stock 5D50 in my '80 GS850LT. Hoping to remedy a slight hessitation just off of idle and have made numerous jet changes already to great effect but still looking for better.

Searching for a needle profile that richens slightly more but just at the top of the needle than the 5D50. Any other suggestion are very welcome.

[Chuck78]

It's on a german language pdf file which is something like a 50 or 100 or 200 page document...big file. I cut and pasted together a jpg of the pertinent ones for my application and loaded them onto one of my threads somewhere on GSR... i don't think I have it on my computer anymore. I will dig for a minute but you can search under my username for jet needle and 5DL36. FYI - the "-2" at the end is not a part of the part number, but rather the slot position that clip goes in to set the height up or down in the slide.

[Chuck78]

http://www.thegsresources.com/_forum/showthread.php?210977-VM26-needle-swap-modified-GS750

[Chuck78]

The needle style that fits the VM26 may be completely different from the 1980+ BS32's, particularly in overall height, so you may need to dig up that entire pdf chart mikuni needle reference listing and search for different needles. They are expensive, typically like $15 each, so you are almost better off just buying a dynojet kit for your particular model bike so you get a performance needle that fits your carb and is designed for your bike's approximate displacement.

I have been searching for the german link on here in all of my old posts with no luck so far.

[tirebiter]

Chuck78,

The link is in post #10 here:

http://www.thegsresources.com/_forum/showthread.php?210977-VM26-needle-swap-modified-GS750/page2

and it still works after registering. Downloaded the file, just now. Thank you MUCHLY !

I hear ya about getting the dynojet kit. If only they published their needle profiles but I imagine that info is probably a well-guarded secret.