'77 GS750 - bolt on 80+ 850 head & GSXR slingshot carbs vs MaxiBore VM26's & 77 head?

[Chuck78]

My bike has a big bore kit already in it (we think 2 owners ago put the Wiseco 844cc kit in it) and runs really strong, but I really wanted to build the ultimate classic Japanese bike of my dreams and have a kickstart-only beast, so I am trying to get the most out of the GS750 engine - the largest non-shaft drive GS with a kickstart engine.

I am probably going to watch craigslist for a used GS750 engine since mine runs really strong aside from an oil weep at the front of the cam chain tunnel at the head gasket (common problem area). Nice to have it as a backup in case I really screw up and miss a 9500rpm shift and float the valves or something. I found that MTC Engineering had some old out of production GS750/850 big bore kits left hiding on their shelves (only the turbo versions left now), and I sprung for the last 72mm 920cc 10.5:1 compression piston set that they had. That's the max bore for a set of GS850 sleeves, so I got a 79 850 cylinder block to bore.

So I was considering sending my carbs off to the old school maxi-bore guy to have the VM26's reworked, ported, bored, and flow bench tuned, as this is better than basically any other carb I could bolt to the original head. This service gives you MAJOR power increases, but costs a fortune.

I am forced to use CV carbs when upgrading this 77 GS550B to a CV-only 81-83 GS650 top end, as the 650's head is matched to the 650 pistons and both are a far superior design to the older engines. I learned quickly about the CV advantages, and their quirks. The later flatslide CV's as used on GSXR's and others are a better design, and I am fitting some to her engine with some intake boot mixing and matching.

This led me to wonder how I could get GSXR 34mm flatslides onto my bike... The intake ports on the heads are substantially larger on the CV carbs, so a VM-slide carb type head is not a candidate to retrofit the larger CV carbs. The carb throats are much larger because they have a butterfly throttle that at WOT is still restricting the middle of the carb bore, therefore the openings have to be larger to accommodate.

So I was wondering if I could use a 1980+ GS850 head on the 76-79 GS750/850 engines to allow me the use of the flatslide carbs after finding the right intake boots to fit them????

I say 850 because I think that not only is the GS1000 engine a slightly different design in the cam tunnel area, but also all the other potential candidates that have BS32/34/36 CV style carbs all switched to the TSCC 4 valve per cylinder heads. The Wiseco kits for 76-79 don't fit the 4 valve heads, and the tscc engines can only be fitted with an 810cc wiseco kit for some reason, specific to their heads also. I believe that the GS850's with CV carbs still retained a 2 valve head. Whether or not it was redesigned piston dome and combustion chamber like the 81-83 GS650, I do not know. The 81+ GS650 pistons I am told are not a good idea at all to run with a 76-79 GS550 head due to this.

Soooo... Is this going to be a good potential combination to use pistons designed for a 76-79 GS750/850 head and bolt on a later designed 2 valve head? I think it will bolt up all the same, the 80+ 850 cylinders fit with only slight clearancing at the crankcase. The cam chain tunnel design and more importantly the combustion chamber design is my bigger concern. I will be using I think it is a GS1100 or GS1150 MLS (multi-layer steel) 73mm head gasket that does not need a cam chain tunnel o-ring, if that matters at all.

[Chuck78]

I am tempted to just say screw it and go with the maxibore carb treatment, but the CV carbs' design will definitely give you a smoother and more powerful transition in that 1 second after you rapidly twist open the throttle, no lagging and then sudden power surge like the VM carbs. CV's momentarily keep the needle low in the main jet tube (vacuum diaphragm operates the slide, not the throttle cable like on VM's) as you rapidly dump the throttle wide open (dropping manifold vacuum rapidly for an instant until the engine spins a revolution or three to create more manifold vacuum. Carbs don't work as well with low manifold vacuum in that instant when the throttle is opened rapidly and a large amount, as the jets need engine vacuum to siphon the fuel out of the fuel bowl and atomize it into the air stream. Engine vacuum momentarily dropping rapidly means improper atomization of fuel for that instant if the needle is raised proportionately with the throttle slide... The CV operation keeps the whole slide and needle low as the throttle butterfly is ripped wide open, and allows the slide and the needle to raise up in a metered fashion as the engine turns over a few cycles to increase the manifold vacuum again.

This all happens very rapidly, but leads to the CV carbs reacting much better and more smoothly to a rapid throttle twist. The newer flatslide type CV carbs are better because the slides are skinny and flat and not a larger & heavier round slide, and often phenolic instead of aluminum etc, therefore much lighter and much more responsive to engine vacuum changes. Feel free to correct me if any of this info is off.