SR500 sidecar - setting up the VM36 carb

 Let's just say, setting up a carb is a straight forward process, but it is by no means a quick process.

Only to ultimately end up on the next page and realizing I still have more work to do. 

But *MAYBE* we should track back a little bit and start telling this story from the beginning: As I was a bit unhappy with my TM36 setup, with it being uncomfortably lean in the midrange, which is good for mileage, but terrible for torque and very happy with the top-end performance, I wanted to see if I could fix it. With all attempts failing or leading towards disabling the accelerator pump, I decided to make a bold move and get myself a VM36 and in retrospect that was an excellent decision, I just shouldn't have bought it from the shop I did as I basically had to replicate a lot of steps that would have been available with a pre-configured carb available in other shops. 

So there's a specific SR/XT500 version of this carb available, which mainly differs in jetting (duh), slide-cutout (3mm vs. 2.5mm)

needle jet P-8 instead of Q-5

and needle, which is a 6DH03 instead of a 6FJ06

and obviously a much more sensible 35/240 setup compared to 35/310 in regard to pilot and main jet. 

Sounds good, right? Didn't work at all. You see, I have fallen victim (and for about two weeks at that) of a badly deburred needle-hold-down clip. 

 

So whatever I did, the carb responded lovely at idle AND at the very top. Just everything in between was all but useless. Once I figured that out (the clue was that if raising or lowering the needle has absolutely NO discernible effect on the midrange jetting, there must be something wrong exactly there!), I started to trace my steps back and found out that the P-8 needle jet is too rich and the slightly richer needle wouldn't exactly help either. As luck would have it, I still had a pair of VM34s that I bought for my dad's CX500, but never got round to fitting to the bike and these come (in stock form) with a P-6 needle jet from the same family. 

And this was the break through in all of this mess. Suddenly I could get the midrange lean enough to make sense (it's still too rich, but we'll get to that) and I could start tackling the very top-end again and leaning that out until it would start to pull cleanly all the way.

And this is what this looked like - old battery in the sidecar, AFR displayed tucked in under the tarp, so I could have a look and then drive up and down that hill a stupidizilion of times and check AFR-ratios, take notes, make changes and do it all again. 

So why did I write "still too rich"? Well, because it is. If you look at the second note, you'll see that I erroneously noted down 10.8:1 for midrange (it's actually 11.2:1 after re-checking), but either way this is much too rich for part throttle. Now I don't want the 14:1 climbing towards 15:1 AFR-ratios of the TM36, but I'd like to see it in the 12s, because that would mean maximum torque and with a leaner needle-jet I could also run a bigger main jet again, meaning I would have a bit more thermal safety at wide open throttle as it tends to lean out after a while and once the oil gets REALLY hot (as in around 100 degrees C) I can hear detonation rear it's ugly head... As such the next step is to try out P-4 and even P-2 needle jets to get the midrange lean enough, also in the mean time, I'll probably step it up to a 185 main jet, even though that might cut peak performance a bit, it's better than detonation.