vintage 750 (now 810) racer

Hello to all,

I have been vintage racing quite successfully a Honda CB500 four in the Italian classic bike championship for the last three seasons.
Being quite tall, i am strongly considering going up a class so my size & weight are less of an issue......

Been considering running a KZ650/750 in the 750 class, and will most likely purchase a title-less bike soon.

I have self tuned my current racer to a pretty high level (61RWHP out of 500 cc) but still would like to hear ideas from members who have race tuned a 650/750.

My main question is where to get high comp pistons. Most of the high performance work I've seen in the forum includes the 810 piston kit. regretfully, I am limited to 750cc, 66mm bore so cant go that way.

If anybody knows of high compression 66mm pistons (11:1 - 12:1) I will be glad to hear. will post the same question in the engine forum.

Thanks and see ya.

TG

I dont know of any high compression pistons but the 83-85 gpz750 cams will improve performance. You could also deck the head to increase the compression ratio. A dyna ignition would also help. You could also port and polish the head depending on what rules you have. I'm sure someone else will pipe in. I'd like to see some pics when you get started.

Welcome TG.

Paul (pz1morton) races a KZ650 with a GPz750 motor in the VRRA here in Ontario.
He may know some of the info you reguire.




"Hey Rob glad you had a good time at the show.thanks for taking pic's of my bike as i too forgot to bring my camera and i worked the booth all three day's to boot,plus worked all weekend too.when you get a chance email me your phone number.thanks again.also did you happen to notice the power plant.82 GPZ750 race prepped engine.lots of goodies for this year's racing.
Regards Paul
VRRA #757"

thanks guys, will pm paul

we dont have too many rules other than the 750 cc displacement limit and no flat slides carbs. Other than that its pretty open, so definitely considering using a GPZ engine as my starting point, should be legal.

It's pretty competitive here, so stuff like heavy porting, Keihin CR's and electronic ignition are on my list.

from the little homework i've made, stock sized (or nearby...) hi compression piston's seem to be the issue.

got a copy of Kevin Cameron's interview of Muzzy back when he built the first 750 superbikes and there's a picture of a very yummy KHI works HI COMP piston there. guess its unobtainable stuff today.

will scan the article.

TG

I believe the 82 head has a smaller combustion chamber than the 83-85 heads. the 83-85 cams are bigger though. the 83-85 engine cases will not fit in the earlier kz's so your best is just mod the kz engine with gpz or aftermarket stuff

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

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.

yes, that's an option of course, but first i would like to see if there's something on a shelf somewhere.

my april 1983 issue of cycle magazine shows 66.4mm special khi pistons with high domes and 11:1 CR used by muzzy for Rainey's GPZ750. would be really nice to track these...

scanner refuses to work, will post the pictures soon

TG

i have a real rank set of .520 lift regrind cams for a 750 .

timebomb33 wrote:

i have a real rank set of .520 lift regrind cams for a 750 .

.520 lift? WOW, I think I'll pass..... its a road racer not a dragster...

TG

here's the piston I am after

I had a custom set of pistons made up by JE in Cali about 5 years ago and they were on the order of $800 all in. My local distributor recently said that those pistons now would be at least $1200 - things have gone up that much in recent years.

Since this sounds like a weekend racer and not a world championship level bike you might be better served sticking with the stock pistons... while cast they are still plenty tough and avaliable used.

Get some bondo out and do some squish tests... you will see there is a lot of room to play with on those combustion chambers by decking both the barrel and the head. Put some extra squeeze in the motor and raise the cranking compression up to 220-240 psi, you can still kick start it at those levels and you will see the same power increase you would get from those unobtainable high dome pistons.

Sorry but I doubt there is anything sitting on a shelf somewhere... it all got torn up on the track... I guess you could give Muzzy a shout, but that was another lifetime ago.

Make the bike as light as possible and with 80 hp it will be fast enough for stuff to happen.

Turbo, A couple thoughts, 1st, Kz650's are numerous, & pretty cheap w/o titles. Wisco offered 720 kits which would keep you in class, But, A GPZ would start you out with better suspension/brakes and carberation. why re-invent the wheel? I loved vintage racing because it allowed a dirt poor privateer to have fun.luck G