Saturday 25 May 2024

The SR500 sidecar - fitting a VM36 carb

 Before we get into the topic at hand, nope the blog ain't dead, but lately I had to decide on how to use the available time and decided that making some actual progress versus documenting said process was the better choice, mainly because the Suzuki-project has somewhat spiraled out of control and other commitments kept me well busy lately. 


Enough excuses, time to get on with "it" and "it" is fitting a VM36 roundslide instead of the TM36 that used to live in there before. In essence the TM36 runs pretty good, but in order to make it work with the accelerator pump, midrange has to be jetted very, very lean. This on the one hand has some rather negative effects on the torque curve (it feels rather weak unless you crack open the throttle and ride it like a little racebike) and, and this is the bigger issue for me, especially in warmer weather I have huge detonation/pinging issues, when using the throttle a bit more sparingly. With the VM38s on the TR1 running like they do, it seemed like a bit of a no-brainer to go with a VM36 for the SR500 sidecar. In theory this will most likely have some negative effects on the fuel economy, but this will get me some much needed increase in torque.

 The internals are as simple as always, a main jet and a pilot jet and a little dish to make sure the mainjet can always get some fuel in the bowl. Mine is the 2-stroke version (something I was not so happy about admittedly as there is a specific SR/XT500 version, which is jetted slightly differently, but Mikuni being Mikuni, you can find the jetting online and then get near this setup, with just a jet swap. I am currently running 30/240 with a very open airbox and free flowing exhaust and what am I supposed to say, the old girl is instantly as fast as with the TM36 and starts first kick. She just hangs a bit at 2/3 throttle, but once you get past that you'd better be careful as she revs to self-destruction in an instant. 

 During assembly I swapped the floatbowl bolts for M5x20 stainless allen head bolts with spring washers, so now I can take off the float chamber without removing the carb from the bike. 

Talking of which, if odered from KEDO, there's an adapter kit for SR500s running the later 48T airbox to connect the carb to it. (And as that's a legal requirement overhere it comes in pretty handy.) It consists of a press-fit plastic reducer and some instructions to shorten the stock airbox hose by 30mm. Which is all nice and good, but there's so little room that you have to fit the inlet rubber onto the carb as well and then essentially slide it in from the side. Doable, but very, very tight.

And with the carb in place it was only a matter of firing the old girl up and hooking up the wideband lambda sensor and see what happens. At which point I found out: stupidly rich (not surprising, I had overlooked the fact that it came with a #35 pilot jet downlow) and very good at the top (cause I had already changed that one.

When your AFR sensor comes out of the exhaust looking like that, you know you're onto a winner.

So what's next: I am seriously contemplating buying the leaner needle and needle jet of the VM36-4 specification as that might solve the hiccup at 2/3 throttle instantly. It is VERY driveable already and feels a lot more relaxed, some naysayers might say a lot more boring, but then again it now feels like a small sidecar with an oldskool thumper engine, which quite frankly is what I want from the sidecar.