Modern carb on wombat

[bcruder]

This may serve as an intro but I do have multiple re-animation projects on the same bike and will share them later.

Bob

I have a wombat with a combat top end and the GEM reed valve that Hodaka sold in the early 70's. Its studs had the same 48-50 mm spacing as for the stock manifold and that worked fine with the stock 24mm flange mount carb.

Back around 1980, I replaced the 24mm with a 26mm with the same flange dimensions and using the same tuning parts. I only had to shorten and widen the molded rubber air box connector. That worked fine in Missouri at 1000 foot elevation with leaded non-ethanol gas.

Forty years later, I am retired in Colorado at 6500 foot elevation, lower temperatures, and unleaded gas with ethanol. I could not get the 26mm jetted near as well as before, even using the same Full-Bore oil at the same 20:1 (had a full case). I did not want to buy another set of jetting parts for a 40 year-old carb and expected modern carbs to be easier to jet accurately. Should be simple?

The now-common spigot-mounted flat-slide carbs are generally easier to work with and offer a wider range of needle sizes and tapers. The Keihin had somewhat more modern passages and offered more jet and especially needle options than Mikuni.

I chose a Chinese-made 30mm keihin PWK model with power jet. Those things are beautiful, far better machined than the old Mikuni carbs and at least as nice as the Japanese Keihin carbs (which are also made in China). I did not expect to pull enough air for any fuel to pass the power jet. For those who seek cleaner off-idle performance, there will be 28mm - 32mm sizes without the power jet but with the air striker bell mouth.

There were several surprises.

I looked for spigot-mount adapters and found that the intake manifold flange must have been wider on the Combat and Super Combat. The adapters either had a wider hole spacing or had the same 48mm-50mm spacing but had their hole center line offset from the carb center line. Either way, the holes had to be elongated to fit the studs.

The stock wombat rubber airbox tube connector was too small for the 50mm carb mouth and the materials that I remembered from 1980 were no longer available. Eventually, I found a radiator hose made for large diesels that had the right 50mm id and fused several inches of that to the stock airbox connector. The result was beautiful but that molded hose cost more than I had paid for the carburetor.

The as-delivered carb jetting may have been right for sea-level but was far too rich for 6500 ft. The supplied spare needle, main jets and pilot jets did prove about right. That was trial and mostly error because none of those tuning parts were labeled for size.

Trying to convince wife to move to our other place in really-really-rural West Texas. It has enough room for its own track at about 3200 feet and is always 20-30 F warmer than Colorado