Mikuni BST36SS, O Ring X, not shown in parts fitch

[buffalobill]

Ok, after much frustration searching for a vacuum leak causing a high idle at 3000 RPM, I found this O ring not shown in parts diagrams and may not be included in carb rebuild kits.
The plastic part it seals is not shown in parts diagrams.
I had to order this O ring from: The O Ring Store LLC
Actual item as listed: 3/8"ID X 1/2"OD X 1/16"CS AS568A-012 Chemical Resistant and High Heat Viton O-Rings: Fluorocarbons (FKM)
Not pricey, only 6˘, minimum of 9, plus shipping.

Unscrew main jet:


Take out slider and needle assembly, you can see top of plastic part:


Pull out plastic part:


See O ring X, as I call it:


Replace with new 3/8 ID x 1/2 OD, 1/16 CS, Viton O ring.

[Kreylyn]

It's included in the K&L rebuild kits. Or at least, has been prior. I've always had one to replace it with anyways.

Krey

[buffalobill]

It's included in the K&L rebuild kits. Or at least, has been prior. I've always had one to replace it with anyways.

Krey

Good to know. I wasn't sure because they don't show the actual kit for all carbs, just a generic photo.
Anyway the last O ring kit I bought was from a small output supplier, and it did not include this ring. Probably because it's not listed or shown in the parts diagrams.

[Lachie]

The slide guides are not available from Suzuki
If needed can be got from Moto Lab Derek is very helpfull

https://store.moto-lab.com/products/655/throttle-slide-guide/876/suzuki/gsx750f/1989

I replaced mine due to wear and theres a rightup on Moto Lab site about it

Here's mine that I replaced as you can see the slide face is quite worn and were the slide hits the bottom is also worn down.



That grey powder on the guide is the plastic wearing when I opened up mine it was all through the diaphragm and everywhere

[buffalobill]

That's very interesting!
What effect do worn guides have on a running engine?

[Lachie]

Main problem is it accelerates needle and needle jet wear so will run out of tune get rich etc

You likely will checked these for wear anyway when rebuilding the carb

Mine were quite bad but that was after the bike had done 80,000m with definite banding around the needle and a lot of wear.

My take is that if your experiencing wear with the jet needles then check wear of the slide guide and replace if you think there is too much. Can also contribute to wear in the slide itself. Possibly, also upset the air balance to the diaphragm

result is that it will just determine how often you need to check and rebuild the carb.

From my knowledge only the BST36 from the pre had these slide guides they went to a different design with the post 36's

[katanarider]

How sure are you that those are actually Viton ? The reason I ask is because I just went through this same issue. Do they have the same texture & flexibility as the intake boot o-rings ? Let me explain...
I found the o-rings and slides were not available for my Kat 1100 either.
Originally, I replaced mine with Nitrile Buna-N (black, soft & smooth), but then I found that the carb cleaner I used to test for vacuum leaks was eating through the intake boot o-rings. Not only that but I also replace my diaphragm slide o-rings with the wrong size because I was getting bubbles near the needles. Now, as of last week to be exact, I ordered Viton o-rings for my diaphragm slides and have since obtained a nice set of digital calipers to insure proper fitment ( Kat1100 carbs have two different size o-rings !) Anyway, these Viton o-rings are brown ,stiff and sort of rough feeling (not exactly silky smooth). So, what else I've come to think about is, the replacement o-rings for the intake boots (from Suzuki) seem to be the Buna-N material because they have the same texture & flexibility and the carb cleaner had negative effects. The next vacuum leak test I do will be with a small squirt bottle of gasoline.

[92xjunker]

Just some O-ring info

http://www.manuf-rubber.com/pdf/Material-Guide.pdf

[katanarider]

Notice on the Nitrile do not use with list, you'll see Acetone.
Spray carb cleaner is Acetone based !
You'd think something labelled as "carb cleaner" would be Nitrile safe, don't be fooled.
I took another members advice and used engine starter spray, but that ate the Nitrile too ! Brake Cleaner spray may work because its supposed to be most friendly to rubbers for obvious reasons, but I don't know if I'd trust even that at this point. Some mention propane, but I couldn't get my RPMs to increase with that. Even injecting the propane directly into the air filter didn't make the RPMs move.
Bill, on a side note, I think I remember you saying your engine idles sort of high anyway. While researching my Kat 1100 carbs a bit further I found this article explaining the jet needle and pilot jet combination. Scroll down to the bottom.

http://www.geocities.ws/brian_sandrasmith/Katcarbs

[buffalobill]

Finding useful info on how to tune Mikuni carbs is really scarce!!!
I find diagrams about how they work, and how to clean them.
But nothing actually detailed. Plenty of misleading anecdotal suggestions that conflict.
One huge problem is just turning the idle jet screw while the engine is running. There's no clearance for a hand to reach up with a screwdriver. I have a special angled screwdriver tool but it's got no feel, I can't tell if it's engaged with the screw head.
As it is, seems a person has to become a multi subject expert, material engineer, historical researcher with many hours of tuning frustration/experience, just to get their one bike on the road and working well!!!! LOL!!!

Notice on the Nitrile do not use with list, you'll see Acetone.
Spray carb cleaner is Acetone based !
You'd think something labelled as "carb cleaner" would be Nitrile safe, don't be fooled.
I took another members advice and used engine starter spray, but that ate the Nitrile too ! Brake Cleaner spray may work because its supposed to be most friendly to rubbers for obvious reasons, but I don't know if I'd trust even that at this point. Some mention propane, but I couldn't get my RPMs to increase with that. Even injecting the propane directly into the air filter didn't make the RPMs move.
Bill, on a side note, I think I remember you saying your engine idles sort of high anyway. While researching my Kat 1100 carbs a bit further I found this article explaining the jet needle and pilot jet combination. Scroll down to the bottom.
http://www.geocities.ws/brian_sandrasmith/Katcarbs

The high idle was eliminated by this O ring replacement.
But not ideal yet, wandering/fluctuating below 2000 rpm. While riding below 3000rpm it's rough and chugging. The weekend I will try adjusting idle jet screws to eliminate this roughness.
It's entirely the idle circuit I'm having problems with.

[Lunatic]

If you're rejetting, in a nutshell:

1) The main jet is used for top end; 7,500 RPM to red line.
2) Needle clip position is for mid-range; 5,000-7,500 RPM.
3) Low end, float height; 2,000-3,000 RPM.
4) Idle and low RPM cruise is the fuel screw.

You have to start at the top and work your way down. Pick the best main jet first, then select the best needle clip position. Followed by float height and fuel mixture screws.

Note: There are several companies that make longer fuel screws. They hang down past the casting allowing you to get your hand in there.

[Lachie]

http://www.factorypro.com/tech_tuning_procedures/tuning_carbtune,CV,high_rpm_engines.html

A good source of info for tuning

And this writeup is quite good for CV carb explanation

http://advrider.com/index.php?threads/the-bst-40-bible.347184/

[buffalobill]

Thanks for these postings. I've read these, but forgot it all because it's been a few years since I had to re-tune some carbs.

FYI; these are on a 92 GSXR1127 engine I put in a 1990 750 Katana. The BST36 rack came off the 750 engine.
As of today (11-3-18) I got my Mikuni BST36 rack tuned quite well. Steady idle, only slightly rough between 2000-3000 rpm under load. Pulls really hard all the way to 7000 rpm in 2nd, but I can't really test it in 3rd gear because redline would be 130 MPH. However, I did redline it in 3rd on a race track last June. The engine felt good too. Could not shift to 4th because the next corner was coming up quick.
Took all of Sept-Oct weekends to get this tuned right.
I also experimented with bigger main jets for top end, but those were too rich.