Saturday, 17 August 2019

Everyday TR1 - a tale of AFRs at idle

The following blog post might only be interesting to a very limited audience, but as it was quite the eye-opener for me, you just have to endure it now.

It all started with the TR1 suffering from some jetting issues on the way back from the mountains, i.e. it ran pig rich, even though I ran it the same way I had done a few years back, based on my jetting notes.

And guess what it wasn't NOT idleing, because it was running dead rich, but because it was running incredibly lean. The jetting was #20 pilot and #180 mains, air-screw 2 turns out. Blipping the throttle revealed that after being incredibly lean, it instantly went to pig rich, which sort of explained why the bike ran VERY hot. 


So I swapped the pilot jet for a #17.5 and turned the air-screw all the way in. When blipping the throttle everything stay at around 11.3 to 11.5, where it dropped to somewhere like high 9:1s or 10:1 before. With the air-screw all the way in it decided to idle quite acceptably. Yet notably rich(er) than it should with the idle dropping down to around or below 1000rpm.


So I decided to give the air-screw half a turn and bingo, much better. Still a bit rich, but now it started to idle just fine.


So let's give it another quarter turn and whoops, tapers don't increase their flow in a linear fashion. But the AFR seems promising and is pretty much, where I wanted it to end up. But it was already idleing a bit high.


Right then, one full turn and see how it goes. Yup, much too lean now.


So, back to 0.75 turns out and adjust the idle to approx 1200 rpm.


As the title suggests, the engine is running really nice after all the work that has been poured into adjusting the carbs:


Verdict and/or hypothesis: Carbs with very small slide cutouts seem to react very strongly on adjustments to the airscrew and only once you lift the slide ever so slightly that's when actually the pilot comes into play. The other thing I noticed: The air-screw seems to affect the mixture much longer than I anticipated, in reality probably more like somewhere towards mid throttle openings. After going through the various specs and comparing them with the TM38 flatslides I had on there before, I am pretty sure that at least partially the small main-AIR-jet is to blame as the VM38s come with a 0.5 jet and the TM38s have none installed, so that's probably one of the next things I'll play around with... (Little addendum: They have been ordered an unless the shipping company lets me down, I should be in the position to play around with them on Tuesday, when I have a day off!)

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