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.

Ultrasonic Cleaning Service

Mcmental ( Chris McNicoll) is a long time member of OSS and he has been ultrasonically cleaning carbs, looms and other engine components for nearly 5 years
Chris operates a 22 litre tank ) and uses an expensive, quality American designed fluid that is used worldwide in the aviation industry.  To give you an idea of what size the tank is, Chris can fit a full GSXR head in the tank as per the photo.
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For a completeGSXR head OSS members will benefit from the bargain price of £45 , including return postage.

For a bank of 4 carbs,  Chris will  strip,clean,rebuild,set float heights for just £45 and that includes insured (£200) return postage. If while stripping and cleaning he finds  anything  that is worn and needs to be replaced e.g. gaskets, o rings etc,  he will source replacements at cost and fit them free of charge.
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Chris  can also clean your complete wiring loom, stripping off all the old sticky black tape and replacing with nice new non sticky loom cloth tape this costs £25 inc return postage.
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If you are interested in using the service you can PM Chris through  his Mcmental trader profile in our trader section. Chris has kindly offered a free loom clean worth £25 to the winner  of our  OSS member competition. Find out more here.

About Ultrasonic Cleaning

Ultrasonic cleaning is a process that uses ultrasound (usually from 20–400 kHz) and an appropriate cleaning solvent (sometimes ordinary tap water) to clean items. The ultrasound can be used with just water, but use of a solvent appropriate for the item to be cleaned along with the soiling enhances the effect. Cleaning normally lasts between three and six minutes, but can also exceed 20 minutes, depending on the object to be cleaned.Ultrasonic cleaners are used to clean many different types of objects, including jewellery, lenses and other optical parts, watches, dental and surgical instruments, tools, coins, fountain pens, golf clubs, fishing reels, window blinds, firearms, musical instruments, industrial parts and electronic equipment. They are used in many jewellery workshops, watchmakers’ establishments, and electronic repair workshops.Ultrasonic cleaning uses cavitation bubbles induced by high frequency pressure (sound) waves to agitate a liquid. The agitation produces high forces on contaminants adhering to substrates like metals, plastics, glass, rubber, and ceramics. This action also penetrates blind holes, cracks, and recesses. The intention is to thoroughly remove all traces of contamination tightly adhering or embedded onto solid surfaces. Water or other solvents can be used, depending on the type of contamination and the workpiece. Contaminants can include dust, dirt, oil, pigments, rust, grease, algae, fungus, bacteria, lime scale, polishing compounds, flux agents, fingerprints, soot wax and mold release agents, biological soil like blood, and so on. Ultrasonic cleaning can be used for a wide range of workpiece shapes, sizes and materials, and may not require the part to be disassembled prior to cleaning.[4] Objects must not be allowed to rest on the bottom of the device during the cleaning process, because that will prevent cavitation from taking place on the part of the object not in contact with solvent

What a difference a week makes!

Just seven days ago we had just a handful of test group members, a few posts and  threads and a lot of work to do. A week later and we have 430 members, 340 topics and nearly 9000 posts. Our busiest section has been members projects with nearly 1000 posts. That statistic alone is the most encouraging of them all because it tells us that we have the emphasis right. We’re grateful for all of those who have given up there valuable time to share what they are building with everyone. That’s what OSS has always been about. If you have a project and you haven’t shared it yet please jump in and do so and remember we like pictures!

Thank you to everyone for their kind words of thanks and support to the Admin team for bringing the site back. Just to see how excited and happy it has made you all is all the reward that we could have hoped for. Thank you also to everyone who has taken the time to give their constructive feedback on the site and it’s function. In the last seven days we have made over 500 coding edits to iron out issues that you have fed back to us . We are really happy with the way it’s all shaping up.

A massive thanks to our Mods who have worked so hard this week too.

If you haven’t yet, please update your profile with your location and get yourself an avatar. There is a useful section on posting pics if you are having any difficulty. If after reading it you are still stuck don’t be afraid to ask a question on the thread.

This week we had our first bike of the month in over 3 years and we are currently trawling the site for our next winner. More details on that later. We are processing our first trader applications so expect some competitions and give-aways to follow too.

On Monday we sent out a press release to all of the UK’s bike press. So we expect some coverage in the coming weeks and months.If you are outside of the UK and you would like us to contact a publication in your country please let us know by contacting admin@oldskoolsuzuki.info

The forum has taken all of our time this last few days and we have barely had time to progress the .info mission on the front page and the vault. We are currently looking for anyone who wishes to produce a technical article or a how to guide for the front page on any subject that they and we  think would be of interest or use to the members. If this is something you would be interested in contact us admin@oldskoolsuzuki.info

If you have any previous experience with word press we are also looking for assistant publishers to mine for old content from the original site and prepare it for publishing. Again please let us know if you are interested.

So all in all a busy but a thoroughly rewarding week. At this rate we may get round to riding and working on  our own bikes sometime next year. Here are some pictures just to prove that we actually we have them.

Steve RENE EFE Minx wm KM Jelle 1100m

So, a brand new site with all of the original OSS values. It would appear that there are still enough of us left to appreciate how good it feels to be home. Let’s go forward from here and continue what we have started.

Thank you all.

OSS is dead, long live OSS

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Bike of the Month July 2015

BOTM 07 2015
After so many years in the wilderness it is great to have our URL live again with a front page and a buzzing and active forum. While the Admin team that built this new site were busy running the Facebook page and plotting the return of OSS one man called Ash took the initiative to start the temporary forum. We and many other OSS junkies owe you a big fat thank you for all the work that you put in to running that forum.
To celebrate the return of the site and as a way of showing our thanks to you for all your work we want to bestow upon you the honour of being the recipient of our very first “Bike of the month”.

Bask in the glory my fiend, you earned it.

With thanks from all of us.

BTW, yes BOTM is back so get building 🙂

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