Air Striker 2 design question

[mrkartoom]

Anyone know why the new Keihin 38mm AS2 on my 300 does not have the low pressure tower over the needle jet in the venturi?

I know these little half-moon towers are there to create a pressure difference and aid drawing the fuel up from the bowl on 2-stroke engines, but I was sort of surprised to see this model did not have one. I don't know the science of why it is no longer needed, but I do know from Sudco that this is the new design Keihin is delivering. The guy I talked to couldn't explain why. Anyone know?

 

[Bailey28]

Don't know why.... however:

this is what I did on my '10 older Air Stryker II, the one with the round screw on cap. Seemed to help the off idle burbling a little.

 

[mrkartoom]

Yeah there are the towers. Interesting mod. What was the intent, lessen the effect and draw less fuel?

 

[Bailey28]

It is like putting in a larger # slide. Shortening the tower leans the mixture just as the slide starts to raise. Slightly Less signal to the needle jet and the mixture at that point gets a little leaner.

If you wanted to "reverse" this, you could install a lower # slide to richen this part back up. I took 1mm off, I did notice a difference with the off idle burble.

 

[GMP]

This can also be the source of some of the variability in jetting with the older carbs. I've seen some sloppy casting flash on a couple of these. I didn't trim 1mm but did square up the top edge.

 

[elemetal]

There must be some changing theory about how that little shroud works. A friend and I have looked at a variety of the PWK carbs from the earlier screw cap/straight PWK to the 2 screw, PWK AS and there is variation. At first (mid 90's) the shroud was 5mm, then they went to 3 mm, then back to 5mm with a 1mm deep notch in the middle. Not surprisingly the L1 you chose with the needle needed to correspond to the shroud height, lower shroud carb liked a shorter L1. A couple of guys cut their shroud down to 3mm, others notched theirs. No discernible difference aside from an L1 change.

This wasn't done is a testing lab so lots of variables would change guys' results but it did generally point to the fact that the shroud shape could be altered (like needle changes) for tuning but could also be worked with as is (whatever the shape). Interesting that the newest ones (ASII) has no shroud, I'll bet it was to keep the overall carb height down, the new top design makes the carb taller and it has a more shallow bowl to compensate. Also allows the carb to fit more bikes without having to use a spacer block.

My .02 anyways.

 

[mrkartoom]

Good info. More to that little bugger than I knew. I did talk to Carb Parts Warehouse and they said the ones they get have the shroud, so this is a spec from Gas Gas. I didn't realize manufacturers changed anything other than setup.

 

[gasgasman]

My 2011 38mm Air Striker II has the shroud. Looks like the carb on the right in the photo.
It's at RB Designs getting tweeked.

 

[twowheels]

My 2011 38mm Air Striker II has the shroud. Looks like the carb on the right in the photo.
It's at RB Designs getting tweeked.

If it looks like the carb on the right, including the large threads on top for the screw on cap, you hit the jackpot and have an AS I ...

As an aside, are guys pulling out the shroud to work on it (ie, is it replaceable) or is the tuning done in place?

 

[Bailey28]

I bet dollars to donuts that Ron machines the shroud down as well as installs his divider plates.

To work on the shroud, I put the carb in a vice then went to town with a fine flat file. I disassembled the entire thing to get all of the metal dust out afterwards. I know, at least on my setup, that trimming the shroud down reduces the off idle burbling to a minimum.

 

[gasgasman]

If it looks like the carb on the right, including the large threads on top for the screw on cap, you hit the jackpot and have an AS I ...

As an aside, are guys pulling out the shroud to work on it (ie, is it replaceable) or is the tuning done in place?

It is the dreaded bolted on cap.

 

[GMP]

The shroud is part of the nozzle assy. I beleive Ron does modify this as well as other areas.

I'm thinking for a new bike, to avoid all these inconsistancies including carb sizes, it might pay to pick up a used PWK 38 ASI on ebay and send it to Ron for a complete rebuild and modification.

 

[biggstr6]

Yeah, I'm alittle dissapointed in gasgas using 36 & 38 carbs ,you didn't know it till you bought the bike. Not very good business.

Just alot of extra work that wasn't planning on doing. And I'm hoping I dont have to go out and buy a 38 carb to get the 2011 to run like the 08.

 

[gasgasman]

I dont have to go out and buy a 38 carb to get the 2011 to run like the 08.

I'll let you know how the RB Designs carb mods work out.
I have his carb on my 2007 and it is the cat's meow.

 

[Skidad]

I bet dollars to donuts that Ron machines the shroud down as well as installs his divider plates.

.

I sent Ron the carb from my 97 250 KTM and that's just what he did. Lowered the shroud a tad and put a small notch in it as well and of course his divider plate.

Without pulling off the jet block part of his modified carb (to compare to a non modified carb) I have no idea what other mystery mods he makes to the idle/low speed curcuitry besides the shroud mod.