Learning more about B4 jetting

[andrewsw]

I had opportunity this weekend run quite a lot in conditions I'm not used to. See page 5 of: http://forums.banditalley.net/index.php?topic=10769

We were running from between 1k and 3.5k feet above my typical altitude and had air temps an easy 15-20 degrees F above typical for this time of year. That translates into super rich, but by how much? Anyone have a clue about converting that into jet sizes?

I'm currently running 97.5 mains, no snorkel, k&n, full yosh system, 32mm carbs, '93 US spec. I have always had a top end issue... running out of freight train at about 95mph and only creeping up above there. On this trip, I did *not* have this problem. In fact she was pulling like a champ and at 100 mph at some of the higher, but still flat, runs she was stilling pulling a bit. I'm assuming I can move up to 99 or maybe even a touch more on the main. I don't actually have any sizes between 97.5 and 102.5 to test with, hence my question on converting 2k feet and 15 degrees into a jet size...


Meanwhile, I"ve made an interesting discovery on the pilot circuit.

I have had trouble with the classic lean idle symptom -- hanging revs. But it doesn't do it when I blip the throttle, so much. Mostly it happens when I come down from speed and pull the clutch. The revs hag at 18-2000 and sit there for quite a while before pulling back down. A quick blip (or two or three, the firmer, and more chop to it the better) seems to correct it. I've got the a/f screws at 3.25 turns out and have had them as far 3.75 or more turns out with some of that symptom still there.  I can induce the symptom with a throttle blip by bringing the revs up to about 3 or 4k and then slowly closing the throttle. Then it will hang for a while. A quick blip doesn't do it.

Also, I've experienced a little lean surge at low revs, part throttle cruising. In a low gear, 2nd or 3rd, running steady throttle between 2k and 4k revs it will oscillate just a little bit. I don't *think* it's chain bind as the frequency is wrong. It makes a sort of (here goes...) wah-ooh-wah-ooh-waa-ooh sound with a frequency of about one waa-ooh per second or so.

When we were up in the mountains in the ungodly stupid heat, I did *not* have hanging revs. I did get some rich-idle rev drop on a throttle blip, but that was very minor. I did suffer from low revs after shutting down for 20-30 minutes and restarting, but it was so beastly hot, that I can't help but think that was just from the damn gas boiling in the carbs while they sat over the engine and cooked.

So I'm thinking that it's a case of being lean on the pilot jet and trying to compensate with the a/f screw. I'm thinking of moving up one size in the pilot, bringing the a/f screw in an appropriate amount (what about two turns or so?) and see what that does. I'm thinking that it's hanging on the pilot jet when it gets the chance... coming down from speed with a closed throttle, but the blip has enough of a chop to it that it just doesn't get the opportunity to hang.  I know it doesn't really work that way, but it seems like it is.

any thoughts?

[Banditmax]

Stick the 102.5's in is my reckoning on the matter. And what can you lose by nocking the pilot jet up a bit?

[andrewsw]

nah, the 102.5's were too much. They caused it to crap out at 85-90 again. And the fuel mileage was shot to hell too... which is only a minor annoyance, but still.

I really need to get some stupid dyno time. I just don' t have the knack of really dialing it in. It's all so much a guess on my part. I think I've go the wife prepped for a dyno expense in the spring.