After having (sort of) cracked the nut that used to be the TM38 flatslides, I set my eyes on finding out, if I could sort out the niggles that those VM38-9 still had. A little disclaimer before we set off: At this point we're working on dialing in the last few percent of a carb's performance and this is definitely not universally applicable or in more common terms: This works for my setup: XV(s)1100 engine, sucking through the frame with a semi-open airbox and an equal-length 1.75" primary, 2in1in2 exhaust.
There's quite a bit of backstory to why I chose the VM38s in the first place, especially as the guys in the German TR1 Forum are huge fans of Dell'orto 36 and 40mm carbs. After living with a 40mm pumper Dell'orto on my KTM LC4 for a few years, I am definitely not so much a fan and thus chose what is essentially the Japanese copy of the same carb. It lacks an accelerator pump, but sports slightly higher standard jets and needles and an a lot tighter fit than the usual Italian counter part. It also has to be noted that this specific version of the VM38-9 was meant as an upgrade for Ironhead Harley Sportsters and thus I originally had rather high hopes that the expensive bits like the needle and needle jet and the slide cutout would be "about right" for a V-twin. This part turned out to be true.
So originally I restarted this quest with the settings I left these carbs the last time before I switched to the flatslides: 17.5 pilot /185 main /3rd notch on the needle /0.25turns on the airscrew. Result: a good starter, but some lean-pinging once the slide lifted and some rather solid performance of about 170kph topspeed.
As I had some tremendous results on the XS-Triple-Sidecar with needle-shims, (yes that's actually a thing and you can buy phenolic washers with standardized thickness) I gave it a shot on the old girl. That's where something interesting happened: The bike got leaner instead of richer, which led me to the theory that the initial taper is very shallow together with the comparatively HUGE slide-cutout on one of these. Also starting the bike became an absolute mess. (This is also where the post about swapping starter-clutches comes in.)
Therefore I chose to lift the needle a full notch and things started to behave a bit more predictably. Bike suddenly was overly rich on slide liftoff, but then steadied out at around 11.8 to 12:1, when lifting the slide, which promised a very, very beefy midrange. As can be seen below, I adjusted the air-screw and got it to a healthy 12.2 to 12.4:1 AFR at idle. But still, starting was an absolute mess. It would spit back and not want to fire up, but when it ran... oh well.
So after a lot of going up and down with the air-screw and regardless of the readout on the AFR, I decided to throw in some #20 pilots and next to no effect on the AFR*, but it fired right up again and ran like it always should have and was quite a bit beefier at low RPM. This begs the question: Due to the slide-cutout did I always obscure a lean-condition on the carbs?
The final jetting I arrived at: 22.5/175/4th/0.5 turns (pilot/main/needle-position/airscrew)
It starts perfectly, even at close to 0 degrees Celsius, it pulls strong all through the rev-range and it's thermally stable. Idle goes down 100-150rpm, when very hot, indicating that it might be a bit on the rich side. Fuel consumption has settled down to around the 6.0 litre mark, which sounds hefty, but bear in mind that the bike is fun to ride and I live close to the German Autobahn, so there might be some frequent trips to evaluate the top speed of the bike, still not breaking the elusive 200kph mark, but we're getting close, thanks for asking though, to be figured in into the numbers.
*Addendum August 2023: by now I have moved it up one notch and went with 22.5 pilots as I also had to swap ignition boxes. Other than that this setup has been absolutely mint.
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