Japan Meets Britain
Correct guys.
In simple terms colder denser air usually needs larger main jets to work with the higher oxygen content of the denser air. For the same volume of air drawn in, there is more mass of oxygen so it needs more fuel to maintain the correct A:F ratio. Conversely, warm days need smaller main jets. That's somewhat of an oversimplification but it's close enough for government work as they say.
So if I have a choice for all-round riding conditions, and don't want to be constantly changing jets back and forth when the temperature drops or rises 20 degrees, which way should I err? I mean, if I'm on a road trip, and I start riding in the morning when it's 40F, and by the afternoon it's 77F, what jets should you use? Would the 240's cause it to be too lean in the morning? But if I have the 250's in there, would I be fouling the plugs in the afternoon?
Last week when we did the road testing in the 70+ degree weather, I found the the throttle didn't make much difference from about 3/4 to full throttle with the 250's, but with the 240's it was a very smooth transition from 3/4 to full throttle. On the other hand, the plugs started to get good colour with the 250's on the 45 degree testing day. Of course, these were relatively short rides (only about 20-30 minutes in length max), so maybe I need to go on a one hour ride or longer to really see.
Hi Vince,
Like I said, it ain't no two -stroke. If the setup is that sensitive to the weather, something isn't optimal. I never change main jets because of the weather, winter or summer. I do that in my KR1-s stroker with flat sides (very sensitive bike and carbs), but not for the bomber and CB72. Jensen assembly of Japanese motorcycles requires great peace of mind (Pirsig)
I've never had to change jets either due to weather (and I've ridden the bike in everything from 30F to almost 100F with no issues), so maybe what I really need to do is take the bike for a good ride and see how it does. I was just wondering if there was something that I wasn't doing in terms of jetting adjustments that I should be doing.
And I suppose if you only had a choice between having a bike run lean or rich, rich would probably be the preferable of the two, so perhaps the 250's might go back in there (I'll let John have the final word on that though), but I can't imagine only one jet size (240 vs 250) would make that much of a difference. I think more time on the bike is really needed to make the best judgment. It is safer to run the richer jets than leaner, but how did you determine that the 250 was right on that cold morning? You mentioned color. Mixture at top end leaves the tip white and only a small mixture ring deep down inside the plug.
That plug chop must be made at peak load conditions though. The reason is that plug temperatures change dramatically from idle to WOT. What we need is the right jetting at WOT when the motor is nice and hot. Anything less and the plug color is not telling you much at all. Most riding on the street is not WOT at 9,000 rpm though. It's at around 1/4 to 1/2 throttle at say 5-7k revs. At that point the main jet is influencing things but the needle and needle jet are more important. Performance will change form cold mornings to hot afternoons, and the "best" jetting is one that's slightly safe for hard riding on a hot day. And if that loses a little on a cold morning, that's just a fact of life that we all live with. The difference is that most of us are only vaguely aware of the concept and you have tasted it. The only real fix is EFI........
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||