vintage 750 (now 810) racer

^^^ yeah I dont know what the age limits are on your class but the 83-85 gpz is a mono shock and has better cams and better carbs. I have knock a lot of weight off mine. I'm shooting for 350 400 wet. Still electric start though. but there are ways around that too.

If remember correctly, the '82 GPz750 got taller piston crowns and something like squish bands in the combustion chambers, plus some mild porting around the valve stems. '83 - '85 got bigger cams... and bigger valves? I don't recall...

Mine's a 1980 top end with an '83 GPz intake cam + port & polish on an '82 bottom end. BS34s from a KZ1100 with the cutout slides from some GPz750 carbs.

Thanks guys,

Decking is an option indeed. In my CB500 racer I started up with stock pistons and managed to go up to 12:1 by quite simple decking. It worked quite well... third place in championship
The main differnce is that those small sohc hondas have a very shallow chamber, hardly 12c.c so its easy to get good CR by just decking.

Read that the 750 heads are 25cc so initally thought that a very domed piston is needed. not worried about kick starting, pedal will be removed anyway, have an electric roller starter for the races. That said, 800-1k for a set will be out of range for the first tuning round. Injected, I would be interested to hear your recommendations for cams too.

I plan racing in the 750 cc class with a 1976 cut off date so the kz650 is allowed, not the GPZ. But no one will complain if I use a gpz750 block, it's essentially the same motor with just more bore and better heads (I think...)

I'll be up against some really quick 750 Triumph triples, ducs and guzzis, so 80hp might be a bit on the low side, 90hp better

TG

Turboguzzi - you can go slightly negative deck on the barrels and if you plane the head up as close to the valve pockets (remove.04"?) you take away a big chunk of that deep cumbustion chamber.

NOTE - everything must be checked to close tolerances (bondo it and cross-section) and allow for piston/rod expansion... this might be best left to your local motorcycle speed shop or at least a very good engine builder.

Also your mentioning static compression ratios a lot... what your really after is high cranking PSI... static numbers are for stock deck heights and stock cumbustion chamber with a know piston - once you start decking the barrel and the head this mostly goes out the window.

The drag racing guys are up around 300 PSI cranking compression... but for weekend road racing 220 should suffice with cast pistons.

As for cams for the 650/750 head it depends on your port job and valve size... with a full blown pro port job and bigger valves you would be looking for something in the .370-.390" lift range and I think your only option would be WebCam regrinds.

I think .400" lift might be too much... you could ask this guy -

www.teamkz.ca/

Steve has been running KZ650/750 roadracing for maybe 6-7 years and I think he has tried a lot of different combinations.

I believe most running 810cc/cams/big carbs/headwork are making about 80 hp running 180 PSI... you will be very hard pressed to make 90 hp with a stock bore even at an elevated cranking compression - If you can get the weight down to 360 lbs you should be able to race with those other bikes mentioned.

Good luck.

quick calc shows i need to mill 1/16" total to bump from stock 9.5 to 12:1, sounds reasonable, hope the tensioner can take the slack.

Just using CR for now to see how far I need to go, we'll see later if its start detonating

TG

Remeber I said that the 83-85 engine cases will not fit a kz 650/750 frame. The gpz top end will fit the earlier cases obviously. I had read somewhere that the 82 heads had a smaller combustion chamber like 22 cc. You can really cut a lot of weight off these things too. Are you allowed to mod your frame IE mono shock aluminum subframes?

thanks thompson,

I am at the steep end of the Z learning curve... but should have a 650 bottom end available too.

Will be wiser when I start tearing into them motors after I finish bolting together the 500 for this year's first race, that is.

Cheers

TG

T_Dub wrote:

You could always ask around, maybe one of the piston manufacturers will do you a custom set for not too much money.

Where is Burt Munro from the Worlds Fastest Indian when you need him to making custom pistons?

kz650.info has some useful info on the 650-750 combos. might be worth a look to get up to speed on the smaller KZ's.

When you get this going we demand lots of pics, I love the road racers from back in the day.

I met Steve from teamkz at Mosport in 2008.
Nice guy.




A Zephyer he had there:

OK, then, GPZ donor bike is in, its a non runner but motor turns around smoothly, seems to have good comp but is going to be taken apart anyway for my early studies.

soon i'll be wiser....

couldnt resist the urge to strip a little bit and i am pleasently surprised by how good the stock porting is in the GPZ motor!

Intakes are already 29mm dia at the entry so my set of 29 CR's are a straigh fit, even the carb spacing is OK, They slipped right on to the stock rubber manifolds.

the hefty 630 chain will be soon out of the window, but looking at the gearbox sprocket, it has offset. Is anybody aware of a 520 sprocket that'll fit and has that offset too? the rear 520 sprockets will be less of a problem as i can buy them undrilled in any size i need.

would be great to hear what kind of ratios people are running on 650/750/810 racers

Will post pics as soon as i find the charger for my Lumix camera, promise.

Cheers

TG

Sorry for coming late to the party, but I have been off the forum for a few months. I don't know if you have seen these threads/sites before, but if not, they may be useful. Otherwise, please ignore me.

Nice project by the way!

The GPZ Modification FAQ

Ultimate KZ650/KZ750 road race bike

GPz racer is done

And from a previous post of mine...

Re:how to raise max RPM's