I had been fiddling around to find good carbs for a reasonable price for my CB 550/650 hybrid engine.
Currently it breathes through early CB 750 K carbs which are not really adjustable for the different running conditions and seem to be a limiting point for a highly tuned engine (ported heads, bigger valves, higher CR, 660cc, hot cam...)
For a long time I had an eye on CR 29ers and Mikuni TMR 32 which are either too big or too expensive.
2 weeks ago I stumbled across some flatside carbs of the first GSXR 750 model, namely Mikuni VM 29 SS carbs and decided to adapt them to my CB 650 head.
For anyone whos interested, here are some photos of the measurements before respacing:
To fit the 650's head carb 2 and 3 need to be about 8 mm closer than the original spacing. The spacing of 1-2 and 3-4 is nearly right and does not need to be changed.
Because I'll stick to velocity stacks, I ordered some with 50mm length and 50mm flange width, which are a straight fit to the carbs.
For the new spacing I made aluminium brackets and shortened the fuel line flanges between 2 and 3 by filing it down 4 mm each.
I also cutted and shortened the shaft and made a titanium linkage to merge both shafts together again.
Another point is to add new bores to the choke shaft to fix the holding screws:
After these measures the carbs fitted the CB650's head, but because the rubber boots are designed to hold a maximum of 38-39 mm and not 41mm carbs, the carbs dont stay in the boots safely. By bending them up and down they become loose.
I hope to get this problem solved by using boots of a Kawasaki GPZ 600, which are also providing a little offset for the outer carbs. Currently I am waiting for them to arrive here.
Finally I shaped the aluminium brackets and rebuild the carbs:
Provided access to the air mixture screws by milling some material off the lower bracket.
That's the current state with the CB 650 carb boots still in place. As I said, I am waiting for the GPZ's ones to arrive and complete the build. I had a try on synching the carbs but the slow jet was much too lean and there was no chance to get the bike running at resonable rpm in idle.
Original jetting is 32.5 slow jet and 97.5 main jet. After a short talk to our Mikuni Import Service I got 42.5 slows and 127.5 mains for a first sync, which were suggested by the importers experience with comparable bikes in displacement combined with the same setup.
Unfortunately I'm scheduled for an internship in Munich at Monday, so the final result may take some weeks...
