Sunday 2 September 2018

Everyday TR1 - new TM38 carbs

And one day everything is working fine and going smooth. And you get bored. And you strife for improvement. Now even though I have a set of perfectly good Mikuni VM38-9s, I have had a set of TM38-85 sitting on the shelf for way too long.

But first start with a problem aka. oil-leak. Admittedly I had already bought some aluminium roundstock to repair the lower valve-cover thread. Turns out, those heads leak substantially less, when tightened up properly.

The stock jetting with #22.5 pilots and #230 mains is "a tad" on the rich side, a more conservative #20, #190 was chosen for a first start.

Interestingly the rear of the carb is tapped for a air-jet, but the port is run open. As the parts out of the VM38s will fit, I might give those a shot (6DP4-needle and 0.5 air-jet instead of the stock 6FJ4).

The slide assembly is relatively straight forward, the paper gasket got greased up so it won't stick to the cover or carb body.

The throttle cable holder is vastly different and looks a bit like an afterthought. Those M3x10 screws will have to be swapped out for allen bolts as I foresee them to seize in the aluminium slide in the long run. With that being said, a bit of copper-slip should prevent that for now.

The 51-6506 2in1 throttle cables (available from various XS650 shops), are a bit too long, so shortening by about 10-15mm is due in the long(-er) run or you end up with lots of slop on the throttle-grip. A bit of decent allen-bolt hardware should make maintenance a lot easier.

In order to be able to oil the cable from the top, a bit of shrinkwrap seals the adjuster against the cable.

The cable is too long by about 10 to 15mm, so at the very least the bit from the junction box to the throttle grip has to be shortened. Once I get to sync'ing the carbs, I'll probably have to shorten those as well.

The benefit of the TM38-86 is their 47mm outer diameter on the engine side, which makes them a straight fit in the stock inlet rubbers.

Fuel line routing is dead simple, but squeezing a filter in there might present some challenges as there's really not that much space.

Bit of hose-clamp-magic as I seem to have run out of good quality neoprene fuel hose and the normal stuff gets mushy within days, with the fuel we have available overhere.

So what's the first verdict: Jetting will take some effort. Overall the length is very similar to the old VM38s due to the longer inlet rubbers. The sync plugs on the stock rubbers are a heaven's sent for getting the carbs dialed in and there's once more a lot to learn. First step will be to shorten the cables to reasonable lengths and then play around with various air-jets as the mid-range is running very lean. Mains are too big, which is weird as it's the same size that works with the VM38s, but maybe this is due to the synchronization not being what it is on the other carbs.

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