Topic: 650/750 hybrid?

[1981cb650c]

I just blew an '81 650 motor and have been working around the idea of stock rebuild, 650/750 hybrid, or a 750 wedged into my 650 frame
I have been searching this forum and have become fond of the idea of putting 750 liners and 64mm 810 overbore pistons in my '81 650 engine. Lots of info on projects started, (550 cases, 650 uppers, 750 inners) I want to use an entire 650 with 750 guts. has anyone finished one yet? if so:

Parts list please!, exactly which liners, pistons did you use? what years were all the parts out of?
were the pistons pre-notched to accept the valve clearance or was this added?
what kind of compression was achieved? did the piston domes have to be knocked down?
was an external oil line modification required, if so how was it run?
and the big one, WAS IT WORTH IT? what kind of gain was achieved? what is the final displacement?

From what I can gather:

my 650 cylinders bored out with 750 liners, out of a sohc or dohc
750 liners shortened and upper flange machined to fit into 650 cylinders
810 overbore pistons for a sohc 750 at 64mm
existing 650 head, rods, crank, etc
external oil line due to oil gallerys getting machined into. (no more oil leaks!)
custom copper headgasket
cv carbs off a dohc 750 (for increased jetting)
final displacement of 718cc
saw rocker covers off what may be a '79 650 engraved with "650cc sohc" at the wreckers. perfect addition for a 650 sleeper.

It seems that it is not all that much work if I use an entire 650 instead of the 550/650/750 combo. I will most likely need new bearings, pistons, rings, and an overbore to achieve an essentially stock 650. with the exception of the liner swap and related machining expences, for the same $$ in parts, I can achieve a 718cc or am I missing something? Optomisticly I hope to complete the project for next season. I want to put as much info in this or ar another thread. I will also include step by step pictures for all you other 650'ers out there as well as a full detailed report. Lets show the full potential of a 650.

Maybee all 550 / 650 extreme overbore threads could combine there efforts and write a comprehensive overbore page!

[Geeto67]

I have to ask, why do you want to use what many consider to be one of the worst performing carbs ever put on a cb750? If it weren;t so expensive I am almost certain that  all the DOHC cv carbs would have been replaced with CR carbs long ago.

Can't help on anything else except to say that it is probably better to go with the largest overbore available for a stock cb650 and try not to reroute oil lines and all this other craziness. The oil is also the cooling medium in these engines and bypassing it outside the engine  seems to pretty much eliminate that function.

If you really want to go fast, find a way to put a sportbike motor in your stock cb650 instead. I saw an h1 with an fzr600 motor in it once and although I considered it sacrilige to put a 4 stroke in place of a 2 smoke, the bike was faster than any street h1 I have ever seen.

[paulages]

there's no reason you have to stick with 64mm pistons. you'll be lucky to find any at all that don't require some amount of machining to work---mine need 1mm or so removed from the shoulder to clear the head. as long as there is enough meat on a 65mm piston, you should be able to open up the valve pockets for clearance. a second the geet on the carbs also-- i'd use sohc 750 PD series carbs. they're essentially the same as the 650 PD carbs with a larger bore. i guess you would have to source an earlier head to fit the intakes though.

you should be able to get a proper composite head and base gasket from cometic once you know your exact bore size.

geet---not so sure that bypassing the oil lines to the head is worse. i think it may actually aid in cooling, as the lines are out in the open air for that short distance. the only place it is eliminated is where it passes up through the jugs on the way to cool and lubricate the head.

[cb650]

Just keep in mind on 650 the 79/80 head/carbs are diff than the 81/82.   I use the OHC 650 cast rockers from 79's on all mine.   The 79 also had a chrome starter cover.





                          Terry

[paulages]

the pistons i'm using are 3mm over cb750 (sohc) pistons, at 64mm. my machinist has had my cases and cylinders for almost 6 months now, but i'm trying to be patient as he's supposed to be great. the counter person at the shop has back surgery a few months ago, leaving him to work the counter and answer call all day, leaving little time for his own projects. simple overbores, honing, porting, etc. he has people to do, but my project is unusual enough that he's doing it all himself. oh yeah--and the guy has cancer albeit in remission. he
s already bored out the cases and and jugs,  and machined the top of the jugs to fit the 750 sleeve flanges, but the sleeves still have to be shortened and turned down. after this, we'll get to fitting the pistons...