Make a slow bike fast - KZ750 Twin

doc_rot

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After talking to a guy in Canada about the KZ750 twin he races I have decided to build an all out track/race version of my own. I have been slowly accumulating parts for the engine and recently picked up a complete chassis for the build. Its the roughest bike I have ever bought. Literally everything, except the frame and the engine cases, will be sold or junked.For $100 with a title I don't think I can go wrong.

I have never done any road racing. I've done a couple track days and loved it but it is expensive and hard to get out there. Right now the plan is to build a track bike that will see the street once in a blue moon. In reality this project will mostly be an excuse for me to make cool shit and make poor financial decisions. That being said; most of the modifications I plan on doing should be legal for WERA Vintage 5/6 should I choose to race down the road. Goal is 80-85HP to the wheel and around 375-390 lbs.

This thread will proceed rather slowly at first, but I'm starting it now so I can document things as they happen rather than trying to dig up photos and info down the road. Stay tuned for vigorous turd polishing.

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canyoncarver

'hacking is learning'
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Right on !. You weren't kidding. That thing is/was rough. I'm in to learn the build info you've found.
 

pidjones

Over 1,000 Posts
I call them "fence row rescues", because that's where I found them. A little bit, maybe just find a bolt, each day and you will surprise yourself with the progress.

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doc_rot

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Because knew I was junking so much I literally just cut off the whole sub frame without dissembling. The black stuff on the ground in the picture of the frame all came out of the subframe, full of rust. lol. luckily I dont think the structure has been compromised by rust.
 

Tanshanomi

New Member
Subscribed! I saw Fred French's KZ750B race in the big twins class at Brainerd in the late '80s, and it definitely left an impression.
 

doc_rot

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thats awesome. ive seen this youtube video of a guy that bought that old race bike but never found any details on it.

 
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sav0r

Coast to Coast
As I scrolled down I legit thought that seat was potting soil until I got to where I could see the rest of the motorcycle.
 

pacomotorstuff

Coast to Coast
Chinwagging with some of the old racing relics I know regarding the KZ750B, they remembered some guys here in Canada tried to dirt track them back in the '70's. The biggest problem seemed to be keeping the balance shaft chain from flying apart at racing rpm's, aside from the motor needing all of the normal upgrades from street bike mild motor to monster racing motor (there wasn't anything stock inside the motor by the time it was ready to race).
Having said that, in vintage road racing a number of years ago, there was one doing pretty good in its class, but I wouldn't want to add up all the time (and money) that went into making it that fast. I'd heard that there was a big bore kit for these bikes at one time, but couldn't find any info to share with you as regards displacement, mfg., etc etc.
If I find any other info on the B's, I'll post it for you.
Pat
 

biker_reject

Over 1,000 Posts
Why on earth would you build a track bike out of a 40 yr old commuter bike?


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doc_rot

Oh the usual... I bowl, I drive around...
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Why on earth would you build a track bike out of a 40 yr old commuter bike?


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I spell it out in the first post.
"In reality this project will mostly be an excuse for me to make cool shit and make poor financial decisions."

Also I can't believe I'm explaining this to someone who put USD forks on a virago. What would you prefer? That I go buy a 2010 R6?
 
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doc_rot

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I bored the new set of cases this weekend. Im now prepping the cases for paint. I was thinking black for the cases but part of me wants to paint them gold like the '83 Wayne Rainey GPZ. I got a bunch of colors from Gun Kote and sprayed them out. I'm not in love with any of them - still on the fence.
 

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Rider52

Over 1,000 Posts
Cool project! I had one in the 80s. Got it on trade for another bike. Extended forks, 18" apehangers etc. I pulled off all choppery stuff and the non essentials, did some Joe Minton mods and rode it around Cleveland for a couple of years. It was a lot of fun and we ran it a Nelson Ledges a couple of times.
 

doc_rot

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The crank balancer on these engines is not terribly well engineered. Its heavy and complicated, and probably one of the reasons why the bikes were never really raced.

I was alerted to a problem with the crank balancer sprockets. These sprockets are broached and the broached slot holds springs and pins that act as a dampner for shocks coming through the balancer chain. The problem is that these broached slots leave the sprocket quite thin and brittle in this area and it is possible to shatter the gear on a hard shift. The solution is to machine a ring with an angle and press it on the sprocket boss. Attention must be paid to provide adequate clearance for the balancer chain. I used 4140 as it is incredible strong and is typically used in applications like these. I shot for a slight interference fit (0.0005"-0.001") and then also tacked the ring on with silicon bronze. I lapped the face on a slab of precision granite to make sure the welds were not proud of the sprocket boss.

The other issue is with the balance weights. The slots broached here leave a very sharp edge. The material is iron so it is brittle and this edge is prone to chipping. Every 750 twin engine i have torn down has had chipping here. I used a dremel with a grinding stone to put a very slight chamfer on the edge to prevent further chipping.

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doc_rot

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The engine build is what started this project, I was initially building this for my other 750 twin, but decided an engine such as this need a chassis that was built to the same standard.

Prior to this project I had never gone that deep into 4 stroke engine theory. This project had me reading several books on tuning 4 stroke engines, lots of late nights reading on the interwebs going into calculations and theory and then trying to apply that to my fairly conservative engine build. Looking back knowing what I know now there are some things I would have done differently but you gotta start somewhere. Below is what I've settled on, its not the end all be all, but its the culmination of my research and how much money I was willing to sink into this project.

Transmission - 1983 twin, under cut by APE. The 83 twin has a lower 5th gear ratio than the 76-82 models resulting in a higher top speed.

Clutch - Basket from 83 twin - this clutch basket is aluminum saving over a pound in rotational mass, it also actuates from the right side of the engine allowing a much larger output shaft sprocket. Stock clutch disks with 20% stiffer springs from a Ninja 650.

Balancer - additional rings pressed on sprockets

Case - Bored for big sleeves, oil gallery tapped with 1/4" NPT for oil cooler, APE HD cylinder studs, grade 12 main studs.

Crank - Balanced and mirco-polished by Revco Precision

Connecting Rods - Custom rods from Carillo. uses King bearings for a ZX -14. the ZX14 uses bearings with the exact same dimensions as the KZ750 the only difference is the location of the bearing tang. these things are pure art. its a shame they are hidden in the engine.

Pistons - Custom JE pistons with custom iron liners - 86 mm. 10.25/1 in this application

Cylinder head - custom valve train and porting from Schumann Motorworks. 1mm oversize titanium intake valve - stock size titanium exhaust valve. Custom bronze valve guides to neck the valves down from stock 8mm stem to 7mm stem. beehive springs and titanium keepers.

Cams - Web Cam custom grind. IN: .406" lift @230 degrees of duration. EX: .418" lift @ 261 degrees duration

Exhaust - 1.5" 18GA 13"L primary, step up to 1.625" 18GA 13"L primary, 2" 18GA collector 16-20"L. I've always wanted a set of high pipes despite being impractical on a race bike. Ive ordered the bends from Burns. I want to build something crazy.

Carbs - TBD but likely a set of FCR 39 carbs. back up will be RS-38 neither of these carbs would be technically legal in WERA but i love pumper carbs. easy enough to switch out later if i need to.

Ignition/coils - Optical ignition and coils from Power Arc.
 

doc_rot

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Bottom End assembly is complete.

Cases were very corroded. They were too big to fit in my crappy HF blaster, so 2 layers of duct tape and i got them blasted for $60. The blasting revealed how pitted the cases were so I chickened out and went black on the case paint. cleaned manaicaly like 4 times before painting with KG- Gun Kote satin black and baked them in the oven.

Whoever tore apart these engine cases did a number on the cylinder mating surface. There were some deep gouges from them prying off the cylinder. I wanted to make sure there were no high spots so I very lightly lapped this surface with some 220 grit on a surface plate. this revealed a low spot and a little more lapping removed it.

I removed the sludge plugs on the crank and after a thorough cleaning I reinstalled the plugs and tacked them in with some silicon bronze.

The Carillo rod bolts are tightened to the amount of stretch with these CARR bolts the stretch is .0065"-.0075". this was about 45 ft lbs. I clamped a drill vice down to hold the crank during the process so i could get accurate measurements with a micrometer.

I found some grade 12 socket bolts to replace the main crank case bolts. fitted with a thick hardened washer.

Sadly this project is being put on ice for the moment. and this engine is getting boxed up and stored for probably at least a year until i can get a shop setup again. I am moving out of state and will be dealing with all of that.





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