Slide Theory, Carb Models, and Jetting

[GMP]

I can tell you now that in mine with the notched slide the NECF was not too bad, and if I could get an NEDE/NECE it would have been even better on the bottom. I may just pop in the NECF with the non-notched slide and see where things fall, but screwing around time is running short and I'll race with what I know. With 250s you spend more time on the pipe so a clean low end is less critical as long as it pulls good out of it. Just shy of an obvious burble is where they like to be when you have to claw up the side of a mountain with no momentum, or pull out of a tight, deep sand berm.

[n_green]

Borrowed from a topic similar to this one over on the Pumpkin forums:

Now back to the notch. I don't fully understand it's purpose. I know what it does but I don't know why the need. I have slides with and without the notch. When you have the idle speed adjusted for an un-notched slide, swapping in a notch slide you will notice a slight speed increase. To get the idle speed back you'll need to tweak the idle speed screw lower just a tad. You will also need to adjust the a/s or swap the pj for a size leaner. I've tested both slides back to back and made the adjustments mentioned and I can't feel any difference in the engine.

The slot alters the flow and possibly the transition time when the pj by-pass outlet reverses and all pj fuel flow reverts to the pj outlet. The only purpose I can think of may be for slightly better launch immediately off idle, but again I can't feel any difference.

[rod]

For those of you wondering about the differences between slides, why doesn't someone just ask keihin (japan) themselves, at least it will be from the horses mouth if they reply.

[v-tec]

Originally Posted by rod

For those of you wondering about the differences between slides, why doesn't someone just ask keihin (japan) themselves, at least it will be from the horses mouth if they reply.

I cant speak japanees LOL

[Jakobi]

Haha good idea Rod. I think you should take the lead role on that one


I don't know if its just the slide. I kind of doubt its entirely the slide thats making so much difference and more likely another difference between the carbs. Today my pilot jet got clogged by some crud. Amazing how much difference such a little hole makes.

[GMP]

No, its not just the slide. Thats why when comparing and recommending jetting here you have to be aware of where people are what they actually have as far as displacement and carb type. Then you can factor in the expected tolerances that cause typical jetting variations.

I've read all the threads on the pumpkin forum and in theory it makes perfect sense, but this is relative to the newer carbs. What does not make sense is how my particular bike with the older carb acts with the notched slide. It is ABSOLUTELY leaner off the bottom with the notched slide, not richer.

Someone with a new carb please measure the distance between the back of the slide and the pilot hole in the spigot, I'd like to see if it is different than the older ASI carb. The new carbs are shorter overall, and jet leaner. If the dimmension I mentioned is different, it might explain something.

[Jakobi]

Hi Glenn,

Most definitely agreeing with what you have said so far without a doubt. Particularly while giving jetting info. Its a huge list of information you have to get before even thinking about making a suggestion.

Besides the slide and notch or non notch, I am finding it very strange that the 36mm carb appears to like richer jetting on the bottom end than the 38mm carb. It goes against logic.

[GMP]

Bike runs like an animal with the JD Blue jetting, non-notched slide and the head mod. Crazy fast on the top end, more than my previous 250s. Raced today on a fast course with some big straights and it has more than I need now. Fuel economy is great as well, used less than 2 gal in 2 hours. It spooges like crazy too.

[twowheels]

Originally Posted by Jakobi

Besides the slide and notch or non notch, I am finding it very strange that the 36mm carb appears to like richer jetting on the bottom end than the 38mm carb. It goes against logic.

It's been theorized that the smaller bore presents a stronger vacuum due to the smaller area and draws more fuel, (somebody could calculate the differences in open area at different throttle openings) I suppose because the engine at a given rpm will pump a set amount of air so the air speed would be higher.

(thinking and typing, perhaps to my peril ...)

[Jakobi]

Originally Posted by twowheels

It's been theorized that the smaller bore presents a stronger vacuum due to the smaller area and draws more fuel, (somebody could calculate the differences in open area at different throttle openings) I suppose because the engine at a given rpm will pump a set amount of air so the air speed would be higher.

(thinking and typing, perhaps to my peril ...)

Affirmative! Logic dictates the higher velocity will draw more fuel for a given jet size, however people running the 36mm carb are finding better results using the JD Blue in the middle clip, 38 pilot, 6.5 notched slide, ASII. My 250 felt too rich with 38mm carb, 40 JD Red middle clip, 7 non notched slide, ASII. Obviously they are running about 170 mains where I'd be running a 180 as per normal theory. The rest has me scratching my head though.