KLXSTER,
You could reduce the size of the main jet air jet. That will richen up the high RPM range. The .5 mm air hole is correct for the stock air box, lid and snorkel. It works for me.

 

Well, the second chart is 136/3N.. The only chart using 4N is the third one, and that is with the 132..

But yes, there are questions that need answers between the DGR AFR's on chart 1 and chart 2.. I watched and participated in all dyno sessions. I know the operator operates the throttle in a consistent manner. Therefore, the variances should be due to adding 5N and 3.5 out( 3.5 turns out on the fuel screw) to the #136 main jet..

Since this run, I have reset the float level from 15mm to 14mm.. I am going to try to get a Dyno run today with the stock header + PC4 ..

One point - I am convinced the FMF continues to make bigger HP well past its' chart cutoff point of about 8400 rpm. This is from ride tests where it just pulls like hell all the way to ignition cutoff (10.5K)..

 

I haven't dyno'd a bike (or car) before - but why do the charts basically start at 4500 RPM and cut off at 8500/8700 RPM? I'd be interested to see from down low, all the way to redline - is it just the dyno operator's preference?

 

IceBikeDave, I am unfamiliar with the mod you describe and don't understand the underlying theory behind it.. If you have data on before and after and details on the mod, I'd be more than happy to try it.

 

I thought I was the only one here that believed that, glad to see I'm not. At least in gear 1-3 I can hit the rev limiter without even thinking about it and there is power there. I can be cruising at 8 or 9k rpm and open the throttle and get to the rev limiter almost instantly, again, in gears 1-3. It's very useful for riding in heavy traffic as I do daily and shifting would be slower in cases where you just need a quick pull to get ahead of someone. I don't have the full FMF as you do so you probably much better top end with the full system than I do.

 

rgmr250, the dyno operator controls the rpm range that is charted.. I have asked him to start at 3.5-4k and go to 9.5k.. But that is not what happens.. He pushes a handheld trigger button to start and stop the charting while the engine is rev'ing.. I was sure he was hitting it at 4k and going to 9.5k... So I don't know..

 

IceBikeDave, Spent an hour online researching the Main Air Jet.. Looks like the FMF system could need a reduction in my main air jet orifice size to "fine tune" the upper end. Problem is, I cannot find data to help me determine what size I should mod to.. For instance, making the orifice smaller will richen the upper end - but how much smaller ? I need a formula or ratio for AFR change % per Main Air Jet change % ...etc etc

Go too far with restricting the Main Air Jet and atomization is lost - or suffers - and performance is lost. Apparently..

 

Klxster,

Sorry for the delay. I had my left hand thumb-base joint rebuilt yesterday. I am typing one finger with my left hand.

So, how dose the main jet air jet affect the fuel flow?

First some facts:
The area above the fuel in the float bowl is at atmospheric pressure. Practically always.
The vacuum at the top of the “needle jet holder” (emulsion tube, thing you screw the main jet into, or whatever it is referred to) varies. Not much at idle and much more at full throttle at high RPM’s.
The air jet supplies air to the chamber that is just outside that emulsion tube.
As the vacuum above the “NJ holder” increases the liquid in the nj tube rises and is sucked into the engine.
There are a series of holes in that nj holder. When the suction becomes high enough air will be sucked through the top holes and when there is more vacuum, then, the lower holes will bleed air. So, the main jet dose not have gas being “pulled” through it until just below max power output . The liquid gas just wells up into the mj holder cavity until just below max power output.

If the mj air jet was removed, and just a big hole supplied air to the chamber that is just outside that emulsion tube, the mj would likely never see any suction. Alternately, if you plugged the air jet with a small tapered rubber stopper, the mj would see suction at or near the beginning of the slide starting to lift. You can try this rubber stopper experiment.

The correctly sized air jet will prevent an overly rich mixture as the engine speeds up under a load. Or provide it if you wish.

The stock snorkel creates a good amount of vacuum in the air box at high RPM’s, we hate that. With the lid removed, less vacuum and more air to the engine, we like that.

That stock air jet works great with the stock snork. Open box top, lean running, not so much at 1500 rpm, really bad at 7500.
You can adjust that high rpm lean condition with the air jet. And probably in combination with the MJ also.

How to install different main air jets:

What we will do: Install a threaded bushing in the Air Jet cavity to accept threaded Jets from Jets r us.
We will be drilling and bonding in the completely disassembled carb body.

What we need:
(1) Locktite 680 retaining compound. 1 small drop (McMaster Carr)
(2) M3.5 x .6 tap (McMaster Carr)
(3) Forster #16 screw driver (McMaster or forsterproducts.com)
Screwdriver not needed BUT it is a fine tool.
(4) Jets: Description: GENUINE Keihin main jet size 40 – 45 - 50 (SKU- 1001-108 ) Jetsrus.com . The 50 is for going back to stock.
(5) Some Sintered bronze bushings. QBCbearings.com BSNPLNMP030506 PLAIN SLEEVE BEARINGS Each $1.43
I bought 7 so I could evaluate bonding strength.

Procedure:
Make sure the bushings slide into the existing jet cavity, On mine the bushing to housing fit was just a little less than a light press fit. The bushing slid in with finger pressure and the final positioning with a small screwdriver.

Disassemble and clean the carb. Q tip and carb cleaner for the air jet cavity

De oil the bronze bushings so the locktite will bond to it.
De oiling procedure. Soak the bushings in carb cleaner for an hour or so.
Place them on a white tissue to dry and bleed oil into the paper. Or ultra sonic clean it (them). Dry completely.

Drill the original jet. Drill the existing jet to.125”. I placed the carb body on the drill press table spigot end down. Just go through the brass. (don’t drill through the camber into the pilot circuit)

Bond the bushing into the carb. Spread a small amount of locktite in the carb air jet cavity, use a small wire and smear it around, try to get it halfway down the hole. Don’t glop it in, this stuff gets hard and will cause problems if it gets into a small passage way. Apply a small amount to the bushing side. Slide the bushing in so the exposed part of the bushing is about 1/8 “ pact the curved portion of the inlet bell. Set the carb on the diaphragm flange face and let it bond. I left mine overnight to cure.



Tap the bushing. In the same drill press I chucked up the tap oiled it well, disconnected the drive belt, had the wife hold the carb, I used one hand to turn the pulley and the other to move the tap down. Tap about 6 to 8 turns total. It takes very little effort. The tap drill size for a M3.5 x .6 tap is 3.1mm, the bushing id is about 3mm, I tapped the 3mm hole. Sintered bronze taps like butter.

That’s it. Put the 50 jet in just to prove to yourself that you didn’t screw things up. If I was trying this on a no lid situation I would next try the 40 jet.
Your main jet might need to be smaller now.

Somewhat related. Why do some(most) 2 strokes not use air bleed in the mj circuit.

 

Outstanding !! With this mod - if someone will "dial it in" for us - You could expand the power producing capability of the DJ kit ... For instance , 132/5N/FS3.5 and .4mm main air jet , could produce better AFR's and more power than I have now with 136/5n/FS3.5 and .5mm main air jet..(near sea level and lid off of course)

 

Your description of the processes involved are easy to understand and jive completely with what I found during research.. As I understand it, the emulsion of the fuel as it travels up the "well" is important for providing good atomization once it gets to the top of the "well" and hits the air stream... The main jet provides the metering of the raw fuel into the emulsion tube - the main air jet provides the metering of the air into the emulsion tube... A balance occurs or is created between the MJ and MAJ - the result of that balance is what your spark plug has to ignite at all engine speeds where the needle is unseated..