building ahrma middleweight superbike--need advice

Injected - thanks for the info on the battery size/Dyna-S. I was wondering what would be practical. I believe most of the vintage racing platforms are more or less "sprint" races anyhow.

if its kz650, oversize pistons, cams, 12:1 CR, porting job, easy

26mm are going to be on the small side, they were small even for my honda 500/4 racer....

try finding 28mm VM's or better, CR29's (not low buck...)

second plan, build it around a kz/gpz 550 taken out to 615. lighter, 6-speed, better frame

search for motorcyclist mag gpz550 momba racer story, tell sit all really. took a few middleweight superbike championships...

Turbo - what size pistons did you end up using? With the class limitations I don't think oversized are a possibility in AHRMA unless you drop the cash on a custom set of +1 or possible +2 (I'd have to do the math, but it's pretty tight...) since 685cc is the limit for the 4 cylinders in middleweight. Probably another reason why the 550 might be a better base, but the 650 is what I have. You know, the whole bird in the hand thing.

Edit: Just looked at the Wiseco kit and it's +2 at 700cc. So +1 is really the only possibility unless through increased compression you are able to show that you kept it under 685cc...

DD, mine is an 750/810 with 69mm wisecos, so no much help .

i was thinking exactly that, stock +1mm pistons, though getting high compression will be a challenge so you could weld up a bit the combustion chamber like i did. doable but not for the meek...

plan B, look into high compression CB750 F2 sohc pistons, they are 61mm 15mm and have some dome

plan C look into all sorts of honda monkey and XL perf piston kits

its pretty amazing whats out there on the shelves

Thanks for the heads up. I think I will probably be taking Mark's advice for the first year. I will only be able to attend 1 weekend for certain, 3 at the maximum if things go right anyways.

I will file this info as "to review when you can't make up for your crappy riding ability with existing engine power in the future".

Injected wrote: The guy to ask about road racing with the 650 motor and chassis is Steve Darlington...
I believe he is a member here, if not here is his contact info:

www.teamkz.ca/
teamkz@rogers.com

He was using stock bore with increased compression by milling head and barrel.
The stock 650 head can be opened up pretty good even with stock valve sizes... you want to keep
combustion chamber small and cranking compression between 200-220 psi (you can still kick start at
that level if rules allow)
He mentioned to me that .400" cams may be too much for the intended application with the stock bore - maybe a .360-.370" cam is ideal with 74-78 CFM port work.
It seems that all the 650 cam sprockets I have come across are hard and cannot be slotted with a round file... you need a carbide bit to slot them.
Use stock clutch pack and upgrade to a needle bearing pusher... shim the stock springs with some .030" steel washers.
VM28 carbs are a good bet since they are period correct and are basically a bolt on.
Vent the crank case and cam cover... 3 places is ideal.
Dyna S will run quite a while total loss on a 8 AH battery - check the rules and what local track guys use
He was not running exotic fork/wheels/shocks

I keep running in circles debating what to do, and yet I seem to keep coming back to exactly what this post states. Either I'm starting to lose my memory or I'm just in way over my head. Possibly both.

Does anyone have any flowbench info for a stock 750E head/valve combo?

Larry C should be able to give you some numbers... I believe he has done a few 650/750 heads in the past and has some good data - ask him nicely.

My guy here in Canada did a stage 1 type 650 head for me with stock valve sizes and got 71.4CFM at .350" lift @ 10" test pressure using a cam similar to the stock GPZ750 cam - this was a very conservative port job at my request (street bike)

The VM28's are getting harder to find so maybe look for 26's and bore them - both are heavy but if you bore the 26's they get lighter!

I have been using EBC replacement clutch springs recently - 10% stiffer than stock and only $12.

You should also look at modifying the clutch shock rubbers to transmit all that power more efficiently as the stock ones crumble and end up in the oil pump screen.

One of my bikes has GPZ550 stock cast wheels and brakes - this setup is WAY lighter than the stock 650 stuff... I need to check the weight difference some day.

Thanks, Injected. I have seen both the puck issue and the 550 wheel swap idea mentioned elsewhere. I'll keep my eyes (edit: I do have both eyes still) peeled. I have to keep myself in check between wants and what's required to get on track, as that's the ultimate goal anyways. Always time to do more next year, and the year after, and so on.

You mentioned a 750 head... if I were you I would stick with the 650 head if you stuck using a stock bore size - the 750 while having bigger valves also has a bigger combustion chamber which is great with a 810 kit but not so great with stock pistons.
Your going to give away a lot of precious sealing area using the 750 head on top of the stock pistons.
What you should look at doing is deck the block to get the piston crown as close to the combustion chamber as possible (.035-.040") and use 750 valves sunk onto the stock seats in conjunction with a stage 2 port job which would require at least a .390" cam.
Your gonna want to make a Bondo cast of the piston crown and the combustion chamber at TDC to see what your working with.

Thank you. I'm wondering if the head that was on the 750 engine is actually a 650 head. It has the top idler bolt holes and appears to have the same diameter combustion chamber. I need to get some plex before I can measure the volume, but it looks identical. Valves are definitely larger.

What is the advantage to decking the block rather than the head?

I think for racing you end up decking both anyways... you end up doing both after porting and boring just to make everything square to each other so it happens anyways.
Decking the block intentionally will bring the piston up into the chamber slightly, but you have to be really careful at this point... some builders prefer Zero deck, some -.005" and some +.005".
It depends on that Bondo cast and how closely your piston crown matches the combustion chamber... ideally they should be offset around .040" all over.
The most critical thing is making sure those rods are up to snuff and won't cause any issues at high RPMs.

like injected says, good match of piston crown edge area is the most important thing when you bump up compression.

a bondo casting is a great idea but you could also use plasticine blobs, put them in the fridge and then slice them.

kzrider.com/forum/2-engine/410994-gpz750...=20&start=180#438549

0.8mm is a safe number to go with.

in all of my builds, it all starts form what pistons are available and deciding what CR to use. wouldnt go for more than 11.5 to begin with (on pump gas)