Topic: CB650 Engine tune-up for street legal racer

[simon#42]

sorry mike i keep forgetting you are only twenty something !

[SOHC4 Cafe Racer Fan]

The ported head sounds like a plus.  Does your local shop have another spare motor to cannibalize the transmission parts?

[MRieck]

sorry mike i keep forgetting you are only twenty something !

My busa is 16 Simon.

[DresdaHonda]

when the cylinder head is made for 29 mm carburetor, I think it need  larger valves, but what size can be used for street use?  -unfortunately there was only one CB650 engine in the workshop, the rest was CB500 engines.
I have read here in the forum that CB550 and CB650 fit spare parts in the gearbox, so I hope that I can use a shifter fork, from one of my CB550 gearboxes.

[bwaller]

650 intake valves are 31.5mm? Do what you want but I think you could tune 29mm carbs to work. Sorry to be a downer...there may only be  only gear shared between the 550 & 650 gearbox. They are totally different, including tooth pitch with some gears.

[Ricky_Racer]

Since this question is in line with this thread, I don't think I'm hijacking...

Clarification, Please: "(5) Paulages build -- larger overbore pistons in a 650 (requires very skilled machining, since the sleeves will overlap one another)."

CB750 liners in early CB650 cylinders (with either 836 turbo pistons or Yoshi 812's) and a 550 bottom end. Do the liners have to be siamesed to make this work? Thanks!

RR

[SOHC4 Cafe Racer Fan]

I believe so.  That was some finesse machining on Paulages' build.

[Ricky_Racer]

Okay, thanks. I guess I'd better reread his build thread.    RR

[Kemp]

You can also use pistons from the KZ750. Ir has the same wrist pin size and a nice dome and gets you up around 674cc. Soos from the SOHC4 membership made a set for me some years ago. I haven't tried them yet but there is a bit of a downside. When Soos machined the piston crown to fit the shape of the 650 head and have the right deck height, the top ring land is quite close to the piston top at deck height. I was a bit nervous of the ring land collapsing with a theoretical compression of about 11:1 if I remember correctly

[SOHC4 Cafe Racer Fan]

You can also use pistons from the KZ750. Ir has the same wrist pin size and a nice dome and gets you up around 674cc. Soos from the SOHC4 membership made a set for me some years ago. I haven't tried them yet but there is a bit of a downside. When Soos machined the piston crown to fit the shape of the 650 head and have the right deck height, the top ring land is quite close to the piston top at deck height. I was a bit nervous of the ring land collapsing with a theoretical compression of about 11:1 if I remember correctly

Attractive for cost and a slight bump in compression, but I'd still opt for the Dynoman 674 kit with 10.5:1 if I were to do it again.

[turboguzzi]

too many items open at once, so not easy to follow you dude...

as for carbs, those 26mm keihins, actually 27mm, are a great fit for anyhting 500-750 for street use. i raced my cb500/4 with them,  now they found a new home on my kz750 daily rider and they are fantastic. easy to tune, good idle control.

the few mm difference in spacing is easily solved by making some simple inlet adaptors with an angled cut to compensate. look for my racer build thread to see how i done it.