Topic: CB750 A crankshaft in CB750K motor

[caferacer77]

So i am still doing reaserch on building a performace 750 motor and I read in a few places about using a CB750A crankshaft in the K model engine in order to use a hy-vo primary chain. I guess this is supposed to cure the noisy transmission and wear of the stock primary chain, but is there other improvements there? My other question, what is a hy-vo primary chain? Also what are the similarities and differences between the two crankshafts, will the stock K model rods fit on the A model? how easy or hard is it to make this conversion and what about the tensioner? has anyone done this before?
thanks,
fabrizio

[dusterdude]

if im correct,a hy-vo chain is like a timing chain in a american v8 engine,whereas the normal chain is a double roller like the rear drivechain.

[JLeather]

The hi-vo chanin is supposed to be stronger.  Putting a 'matic crank into a K bottom end, however, is a HUGE pain in the ass.  You need a different clutch-basket sprocket as well, and a different tensioner.  You'll have to make your own set of hi-vo primary chains.  It's not a drop-in and go kinda thing.  IMO, there are people building some 130hp+ engines running normal primary chains.  Just buy a good, new set of primary chains and a good, new cam chain and use those.

[Terry in Australia]

Yeah, the hi-vo chains are a lot stronger than roller chains, but as JL mentions, there's a fair bit of work (and a lot of money) involved, too.

Personally, if you're building a sub-100 BHP engine, I'd say stick with new OEM primary chains and a Tsubaki cam chain, with an M3 racing camchain tensioner assembly and your engine will be as reliable, as it is fast.

If you're building a 1000+ cc engine with over 100 BHP, I'd use the CB900 or CB1100F crank with Hi-vo (primary and cam) chains. If I was building a drag bike, I'd use a much modified CB750A engine, with that 2 speed auto. "Horses for courses". Cheers, Terry.

[JLeather]

For bracket racing nothing beats a 750A.  Yeah, you'll only run about a 13 second 1/4 (and that's with some engine work) but whatever light you cut is your exact time +/- .003.  They seriously run that consistent.  If you can cut a .500 light they'll run and exact ET every time.

[Terry in Australia]

One of the older members here, (Bodi?) ran a CB750A based drag bike with lots of mods, and from memory it was running around 10 second quarters, or maybe less? Apparently those old beats are very strong, he compared them to a GM "Powerglide" transmission? 

[Bodi]

Not me
I've read details of the hyvo swap. It ain't easy for sure.
And I have no clue about racing the "A". For one thing the tranny isn't made to shift - it's a two range thingy with "city" and "highway" gear ratios... you can have acceleration in town or low highway revs but as far as I know you're not supposed to shift on the fly... but I'm regularly wrong. I did ride a 400A once but not a 750A.

[JLeather]

There's a dude on the dragbike forum who races a 750A all the time.  He said he's got something like 1,000 passes on a stock bottom end, and every pass he shifts it 1-2 at full throttle just like a manual-valve-body auto tranny.  I was under the impression that 1st was to get you going and then you were supposed to let off the throttle to shift to 2nd in a stock 750A.  I've got an A motor I keep meaning to build something fun outta.

[dusterdude]

yea,thats my boy chad,and he wears out all those faster bike too.his bike is deadly.

[Dave Wyatt]

I shift my 750A all the time.  It's the only way you can make the thing perform.  As someone mentioned, Chad shifts his all the time on the strip, he finally wore out/broke a tranny a while back, after years of full throttle abuse.  They will take a beating and  just keep on running.