Increasing Slow/Pilot Jets from #45 to #55
 

I've just run into a brick wall and need some help with my STG1 build. My dyno guy doesn't have any #55 slow jets, so I called my local dealer and the highest they can order from Honda are #52s.

Can someone point me in the direction of some quality #55 slow jets?

try these guys - http://www.jetsrus.com/

they seem to have the 424-21 (as I recall) size 55 slow jet.

http://cgi.ebay.com/ebaymotors/ws/eB...tchlink:top:en

I just found this on Ebay and they look like they'll work. Can anybody comment? Are our carbs made by Keihin?

Will do!
:woo:

yes our carbs are Keihin carbs.

Jetsrus are showing 23.5 mm long slow jets and they may work, but mine measure 28mm so I don't think I'll order those just to be safe...

it says over all length under 424-21 as 28mm????

Slow Jet http://www.jetsrus.com/photo_jet_slow_keihin_k26.JPG
OVERALL LENGTH = 28mm

Cool, I musta been lookin at the wrong one! I'll go get them & Thanks!

Sounds like a bona-fide tuning nightmare!!

I'm not sure how you can change everything that much and hope to see a flat torque curve, without new needle profiles and a whole lot of dyno time. Even then, only maybe.

If I understand the problem correctly, the designer starts with a carburetor, sized for the mass airflow. Then designs an airbox, through lots of modeling and testing to make the carburetor work with the heads and exhaust system.

But I must admit I'm just a home builder, bewildered by the simplest jetting job...

LOL!

Just grab a beer and stay tuned my fine feathered friend:popcorn:

I went in under the VTR1000 Superhawk and it was showing me the wrong part# that's why it was too short!

You got a part# handy for mains too?

Perhaps I am sticking my nose where it doesn`t belong but isn`t the jump from a 45 to a 55 pilot a HUGE jump? Are you certain you need such a large jet?

i have no idea on the mains. I went with a Factory kit as it gives you a variety, even though you probably will never use them!!!

Skoshere, You gots a #195 & #200 left in that kit?

I cut the top off the air box:eek:

gotcha!

The way I figure it, he is trying to dump so much fuel into the bike that the mere force of the gas being pushed out the exhaust will squirt the bike forward. ;)

All this for a "stage 1" seems like a lot of mods.. What's a "stage 3" like? I shutter to think.

I am staying tuned for sure though! I hope it works well and is a real screamer!

I use these guys for carb parts:

http://www.carbparts.com

In over 20 years, they have never told me, "Yeah, we can order that jet for you."


Rex

Ok, so let me ask you this. I don't know the answer, but it would be a good one to know in ths case.

A stock airbox is designed with a snorkel, so that the only inlet to the airbox is between the cylinders, in a calm area of air. This region is supplied with cool, ambient air through the inlet ducts in the front of the fairing. You also get a side benefit of part of this ducted air flowing over your knees and displacing the hot radiator air outward, to keep you cool. But for the engine, there is a constant, mildly pressurized source of air for the intake to the airbox. The main goal is to minimize pressure fluctuations in the supply air and maximize the supply pressure. There may be other design issues, like cutting intake noise. It is not designed t be a ram-air system, per se.

So now, you cut off the top of the airbox. The supply air now depends on the air pressure on the underside of the gas tank, which is open to the sides to the outside world, which is whizzing by at 100 MPH. We know from our high-school physics class, where they describe how an airplane wing works, that high velocity creates low pressure.

So my question is this:

Is the air pressure under the gas tank higher than the air pressure between the cylinders?

If you want better airflow to your engine, it better be.

RCVTR: you do have a good point there, he should seal off the sides and back of the interface between the tank and the airbox if he wants to improve airflow at speed.

Otherwise the low pressure created by forward velocity will try to suck air out of his carbs.

Or he could build a Ram Air duct like the Moriwaki bikes had:
https://www.superhawkforum.com/forum...ad.php?t=21875

That's exactly how a carburetor works...it's called "venturi principle". Air flow speeds up as it passes through the venturi, a narrowed area in the throat of the carb. This creates a lower pressure at the main nozzles. Higher atmospheric pressure coming in through the carb vent pushes the fuel out of the float bowl through the main jets and out the main nozzles, which dump fuel into the venturi section of the carb throat.

Plus I'd think you'd create a ton of turbulence by cutting off the top of the air box.

A step in the right direction might be to install a Flo Commander to at least have a common air supply to the pilot jets.

http://www.ablett.jp/bikes/vtr/air_box_lid.htm

http://www.ablett.jp/bikes/vtr/air_box2.htm

http://www.ablett.jp/bikes/vtr/air_box3.htm

Best of luck, but I've never heard or read of this working well.

I have spent a couple of hours trying to get that to work... Both on a stock engine and now on my modified engine... Simple answer... It makes more power, more torque and nicer curves with easier setup with an unmodified air box than anything else... I have had limited success with some mods to improve airflow in the snorkle (not remove it) but that's about it...

And I think I have said this before... I have unlimited access to a dyno... So when I say spent hours, well... You get the picture...

sorry no - the biggest jet in there is 188

Honda part number 99103-MT2-0450 is for #45 slow jet
Honda part number 99103-MT2-0500 is for #50 slow jet

I tried to see if they have a 52 or 55, they do not.

Thanks everyone for your input!

This bike will either be the 1st street bike to go topless or yet another proof that the topless airbox just wont work. Either way I gotta try it just in case I can figure it out...

Also, I ordered the 195 & 200 mains at jetsrus under part# N424-36.

I can almost guarantee that it will idle like shit and foul plugs with 55's in there. The motor simply cannot get enough air to burn that much fuel at idle. Think about it, even with stock airbox and exhaust there is not that much restriction at idle speed. Fluid friction (of air) increases as velocity goes up, and velocity is very low at idle. Very few guys are getting the bike to run correctly with #50's even with engine mods and full exhaust systems (The TBR full system is the only one I'm aware of that will run with this idle jet).

Thanks Greg, that's good to know. It may need #50s or possibly even lower, but a well known SH tuner from across the pond said to start with #55, so I'm gonna give'm the ole college try:brownnoser: