750 twin engine build

Some years ago, a fellow that raced a 750 twin in WERA racing told me about all he had done to get 90+ rear wheel hp. He had spent five years and lots of money getting to that point. It's possible to get near that point without spending so much money, and I'm going to give you some ideas. I had planned to do this myself, and have a lot of the required parts, but it's not in the cards.

In no particular order.

Yamaha 350 Warrior pistons, 3.346 bore (max oversize) JE part number 222085. That translates to 885cc
Heat the 750 cylinder block in your oven to ~350 degrees so the liners can be pushed out. With a TIG welder fill in the top groove where the liner lip resided, then use a boring bar to level the weld to the cylinder walls. Cut .100 off the top and bottom of the block to set the piston deck height level with the deck. If the compression is to high then you can lower it by opening up the combustion chamber. Send the cylinder block to one of the companies that replate late model blocks, like Millennium Technologies and have the bare aluminum bore plated. Send the pistons (at least one) with the block so they can hone it to size. No more liners.

www.mt-llc.com/

If you're really hardcore and want to remove the balancers, don't, just remove the balancer drive chain, there are oil holes on the balancer journals.

Put the biggest valves that you can squeeze into the chambers, bore the valve spring pockets to 36mm and get the 36mm buckets from a kz1000 based drag bike, or buy them from APE. You will have to figure out what springs to use and what shim under bucket hardware. Talk with a cam regrinder to get a usable cam profile, just keep the valve lift at less than .500" or you'll be breaking cam chains.

Bottom end is just a standard rebuild. If you do away with the starter don't remove the starter driven gear (the one on the crank), there is an oil hole under it that will bleed off your oil pressure.

Electronic ignition off a 82 -84 KZ750 twin, or off a KZ440 (second choice).

Hi Lou! And anyone else that's still around from the halcyon days of yesteryear

wow thanks for the info, saving this for a later date!

DoctoRot wrote: wow thanks for the info, saving this for a later date!

Same here. Later in my dreams. lol.

Great info! I was looking to build a big bore 750 twin but ended up selling the bike before i got far with it. I know of a guy here in CPH, Denmark who have a 900cc KZ twin with 40mm flatslide pumper Mikuni's. A beast of a bike! But he told me to keep it under 900cc, he is having a lot of trouble starting the bike. He bent the kickstart shaft (!) in first try. I guess i had too high compression but he said it was "only" about 10,5:1. I have seen him start the bike on the electric starter after he lowered the comp but the starter motor and clutch is really fighting him. The 886cc pistons would be the 85mm right I was planning on using 83mm for a 864cc.
Wiseco Yamaha YFM 350 10:1CR 83-85mm pistons.

Just some specs to keep the discussion going.

Standard valve sizes on a 1978 KZ750B Twin;
42mm intake
35mm exhaust.

Intake runner 38mm
Exhaust port 35mm

The exhaust camshaft have 280' duration and intake cam have 260'. So a exhaust cam in the intake would move the peak power up a bit.

Love the idea of lining the bore instead of having new liners made and boring the block.

is this the bike you are talking about?

I believe that is the first big bore bike Per from Wrenchmonkees built, 970cc.
The bike I was talking about is his own bike which have a similar 970cc engine in it.

very cool. thanks for sharing