Bike of the Month April 2020

Yes, really; you’re reading a BOTM article on the very first day of the month. The world is in shambles, and here I am actually getting some work done; I don’t know either, it’s not even a joke…

Ask the question; “What is a Streetfighter?” in a group of people that we have been surrounded with over the years, and chances are you kickstart a full-on brawl. Whatever your opinion, there’s no denying the fact how it started out.

Dig out your oldest SF mags and see for yourself; if it’s just Slabbies and Slingshots with 916-seats; you need to go further back, because it didn’t start out that way. Most were chopped up GS’s, GSX’s and some built out of other inferior brands we won’t mention, hardtailed or not but near all of them  were very hopped-up Jap 4’s in a modified factory frame.

Actually, they were all pretty close to the ET you see in the 2 pictures above, albeit faster. The bike you see here is owned and build by our friend EFEchop (obviously), Karl, for friends. Bought in 2011 as a mildly modified 750ET, built in spirit of the very first Streetfighters as mentioned earlier, Karl took his time to build it over the years to what it has become.

A bike not just reminiscent of the “proper” ones in the old days, but a bike that can show up the best bikes from way back when, and now, not only for the fact that it actually gets used on the road. Over the course of 9 years of ownership, Karl used a lot of tried and tested ingredients to get the bike to this level, but it takes more than just the right bits to end up with a decent bike. You need to know how to actually put the whole lot together too.

The weedy 750-lump was rightfully changed to a TSCC1135, complete running gear replaced with obligatory Slingshot parts, frontend mated to the headstock with thug yokes, just like it was done all those years ago. Swingarm on the opposite end balanced out with fancy shocks, exhaust routed thought the hole where the monoshock used to live; this bike has everything and makes it work.

Engine fed with a bank of fresh RS36’s, Dyna ignition and coils for a better bang and every other thing you can think of (bar a turbo); it’s been done to this bike. And then, there’s the paint..

Most bikes you see built now have very abstract paintjobs or even wrapped. These days it’s rare to see a bike airbrushed to within an inch of it’s life; it’s almost a forgotten art. More bikes need airbrushing; there I’ve said it. If there was ever a better example to stake my claim, this bike is it.

Btw; he has a few more nice bikes too; priorities and all that..

Anyway, without further ado; congratulations Karl, your Pork Chop is this month’s Bike of the Month.

Discuss here

Buildthread here

Bike of the Month March 2020

As February has come and gone, you may have noticed a apparent lack of BOTM that month. They made that month too short; that’s my story and I’m sticking to it. It didn’t help that the weather was utterly miserable and riding bikes in the summer sun was a long distant memory. Cue March and we’re inching ever closer to spring. Yesterday was the first decent day of 2020 here and I even managed to get the bike out.

I don’t ride on the road very often anymore (not here anyway) but if there’s one thing I get the most gratification out off, it’s showing up modern machinery with our older bikes. Having the powerrangers scratch their Rossi-rep lids in disbelief how they just got left for dead by a bike older than themselves, usually ridden buy a guy in jeans and trainers. But, enough about me…

The above is best done on a bike that is very understated and one that, in the eyes of the unknowing, just looks “old” My pink-neon wheeled EFE doesn’t fit this category but the 1100M Oilyspanner built that you see above is one of the best I’ve ever seen.

Even if you do know what you’re looking at, you’d have to look twice to see all that has been done to the seemingly stock-looking bike. Starting off fairly standard a few years back when it replaced a (much) later model GSXR, all was done to have the older bike get in the realms of modern sportsbikes.

Weight was shed anywhere and everywhere possible; roughly 40kgs (!) saved over stock and with a modern frontend swapped with the endlessly outdated (and questionably sprung) original Slingy USD’s, the rear was balanced out with a very trick raceshock from Nitron.

The buildthread of this bike reads as though a proper hands-on journalist is using it as a longtermer, with a wealth of information on chassis and especially carb-setup. Jetting all done on the basis of experience, “feel” and the use of a private road (officer), the bike has become what it should’ve been in 1991, had our friends in Hamamatsu had the technology of today.

Still very much a project but you can’t help but tip your hat to the work already gone into this bike to make it what it is now.

Congratulations Oilyspanner, your bike is this months Bike of the Month

I’d have left the purple wheels though…

Discuss here

Buidthread here

Journey to the center of Mikuni’s BST38SS carbs

Journey to the center of Mikuni’s BST38SS carbs.

When I was studying my new 38mm slingshot carbs my eyes fell on the small rubber hose which runs along the outside of the carbs from the float chamber to somewhere above the intake. I disconnected the hose and started tracing the circuit inside the carb.I did this by reconnecting the hose to one of the fittings and bowing into it. So by hearing where the air escapes you know the routing of the circuit.bst38ss-1

The top fitting connects to the uppermost hole in the bellmouth, but when I blew into the fitting of the float chamber I seemed to have hit a dead end because there wasn’t any air escaping. I noticed a small plug which looked like a jet inside the float chamber. I removed it and now I could blow trough it. First I thought the jet had been clogged but after closer inspection it really was a plug instead of a jet. So there was a hole in the bellmouth that connected to the float chamber, but the hole was plugged. I had some sleepless nights trying to figure out what the function of this would be.bst38ss-2

Then I decided to do some investigation on the web. I didn’t expect to find much info on Mikuni carbs on the web, but suddenly I found this article deeply hidden inside Factory Pro’s website…


Power Jet Circuit, GSXR750, as installed on air cooled gsxr750 w/ 38mm Mikuni carbs, 90-92

Power jet carbs – Mikuni’s great addition to a carb used in a high rpm application.

The power jet adjusts high rpm mixture, in the gsxr750 – from 10 to redline, in 1/3rd the step of a main jet change. Changing a main jet, in the 38mm carb, as installed on the gsxr750, adds or subtracts up to 2% CO per main jet change – when the CO% needs to be adjusted in in .2%-.4% for best power attainment.
Changing the power jet allowed much finer increments of change and, just as critically, happened to change the fuel delivery curve to what was optimum for the gsxr750 – something that would have required main air jet changes and other modifications to attain, but would still leave the main jet fuel delivery steps too coarse.
Strange. This Powerjet circuit works wonderfully when tuned on the stock airboxed gsxr750 (and it’s pretty straightforward to tune on our EC997 Low Inertia Eddy Current dynamometers unlike simple inertia dynos.

The method of operation is as follows.
At full throttle, as the rpm increases, at exactly 10k, there is enough of a pressure differential between the float bowl and the airbox interior to draw fuel up the black hose on the LH side of the carb and exiting through the hole at the top of the bellmouth of the carb.
The fuel is metered by a jet that is located in the bottom of the float bowl. The jets are sized in increments of 2.5 or .025mm. Usual size for a gsxr750 with a stock airbox and air filter might be between #58 to #67.5.
The power jet circuit, when properly tuned, adds the equivalent of 2-3- main jet sizes “on top” of the main jet, so, if you were not using the power jet circuit, i.e. had a “0” or blanked jet installed with a #125 main jet, you would use a #117.5 with a #62.5 power jet installed.

Since this particular circuit works on the pressure difference between the float bowl and the airbox interior, it is absolutely affected by any change in the pressure differential. If the air filter is changed to less restrictive unit or the airbox inlet is modified, creating less restriction – the power jet area (size) should have to be increased above the usual size, though, a BMC or K&N, as installed for stock replacement, may only require 1-2 sizes increase in the power jet (in addition to +2-+3 on the main jet circuit).

If the airbox is removed, there is no longer a sufficient pressure differential to pull the fuel up the ~2.5″ vertical rise from the float bowl to the outlet in the bellmouth and the circuit is no longer effective.

Why is the Powerjet circuit difficult to tune on a simple inertia dyno and easy on our EC997 Low Inertia dynamometer? According to the former owner of Dynojet, the powerjet circuit simply doesn’t work because there is a lag in fuel delivery at 9.5k rpm – creating a flat spot there. It turns out that the reason that he saw that is that the dynojet dyno has insufficient load to simulate the Real World Loading ™ that is present on the bike in 4th and higher gears on the road or track. There is a slight delay in the onset of Powerjet fuel delivery, but it’s only vaguely present in second gear in the real world, and not present in higher gears due to the slower acceleration rate that occurs when you are actually riding. If you were racing, as Yoshimura USA and other non sponsored, large US Suzuki sponsored teams (we lent them carbs for the Finals) verified, the kit outperformed anything dynojet had to offer.

How to tune:
1. Install the main jet that produces the best power at full throttle / 8k-9k.
2. Install the powerjet set that produces the best power at full throttle / 10k to redline.
3. Raise or lower fuel level to get best power at full throttle / 3k.
4. Recheck main jet and needle height if you needed to lower the fuel appreciably.
5. Adjust fuel screws for best idle.
Note – this is the “short” tuning list!

Benefits:
The size of the main jet DOES affect the low and midrange. Excess leanness isn’t usually the problem on these carbs. Using a #117.5 vs. a #122.5 main jet (PJ equipped vs. using a #0 PJ ) leans and crispens the lowend and midrange for better off idle and corner exit performance.

There other applications on other motorcycles that use circuits that are called “power jet” circuits that work on different principles – some are electronically controlled and work in the midrange like RGV250, the RS250 for upper topend, where they activate and deactivate through different ranges and still others work for different reasons and by different principles.
“Power Jet” is a catchy sounding name and it gets used every few years or so…

Why did Suzuki specify that US and UK models, for example would have a blank or “0” jet installed, disabling the circuit and other countries, like Canada, got the activated power jet circuit (though with pretty odd settings)?
Emissions? I don’t think so. With the basic fuel level and needle settings virtually the same on both applications, using the larger main jet, as required with the circuit blanked, would only increase hydrocarbon emissions under measured conditions.

At any rate, the circuit works extremely well in dealing with the coarse main jet metering steps of the older style gsxr750 carbs – 1st through 5th place at the 1990 WERA Grand National Finals used our Factory Pro #CRB-S06-1.0 Carb Recalibration Kit. Pervasive kit use followed for the next couple of years -until 1992, the last year of the power jet.


 

Says it al really, but what I can’t figure out is why mine have size 0 jets fitted as my carbs came from the UK and so should have a functional circuit according to the article.
But anyway, as I am using separate K&N’s the powerjet circuit won’t be able to function properly so I removed the tubes and plugged the outlets inside the bellmouths.
This way you won’t have to disconnect the tube every time you want to change the main jets which can save you a lot of dyno time and therefore money. Now you only have two screws for the top cap and two for the float chamber which makes them very service friendly.

Thanks to Factory Pro for restoring my good night sleep!

Now that we are talking carburation technology I would like to point out two other things that are important.

When I remove the airbox and fitted separate K&N’s there were a few hose fittings that I didn’t know what to do with.bst38ss-4 In the middle of the bank of carbs there’s a 14mm big hole which acts as a breather for the float chambers. You need to connect a hose to this which is about 30 centimeters long to

A.) prevent dirt from entering the float chambers, maybe you’d even fit a small filter to the other and of the hose. A good and cheap trick is to nick some of your girlfriend’s nylons, put a piece of it at the end of the hose and keep it in place with a tie-rap.
B.) create a kind of buffer for the air pressure below the diaphragms. This is very important for the same reason you need to add tubes to the fittings of the float chamber breathers.

You need to connect a tube about 20 centimeters long to the fittings bst38ss-3of the float chamber breathers which are located between carbs 1&2 and 3&4. If you don’t do that the air pressure inside the float chambers will become very perceptive to pressure changes outside the carb like when you get some sudden sidewind or pass a big lorry.
I didn’t believe this at first until a dyno operator did a run before- and after fitting the hoses. The hoses made the powercurve much smoother and therefore made it easier to choose the right jetting.

Marc Salvisberg from Factory Pro Tuning says;

In the US, with a stock airbox, we didn’t have ANY problems with crosswinds, even 40-50mph gusting crosswinds at full lean at 100mph boogie. Actually, there is one problem – getting broadsided with a 50mph gust WILL push you off the track! Willow Springs in southern California. I thing that the biggest problem was the carb tuning as rides with our carburetion setups could: run with or without float bowl tubes, tuck their knee in of out, draft to the inside or outside of another rider while in a strong crosswind! It’s been a few years, but I definitely do remember the lack of problems with crosswinds. Urban myths started by someone in the States! Do the hoses affect the carburetion? Perhaps, to a very small effect. Less than running the bike again and increasing the crankcase temp 10F!

The only thing I can say is that we did a run with- and without the tubes installed and the effect was very clearly visible on the dyno graph. So when you fit separate K&N filters be sure to fit those hoses for the horses!

Thanks to Sandro Serafini, creator of Evo2 for the delicious carbs.