My replacing the stock carbs

Posted: Wed Jul 25, 2012 1:30 am

JJam wrote:

Is this when you are riding or just in the garage? If there is no load on the bike, it makes a difference. What do your plugs look like?
Tuning carbs is different on every bike and location.

On a different note. I got these BST's in the mail from Davelepka. I will be making the adapter hoses with inner tubes and will have them on hand ready to bolt your BST's onto your bike.

If someone doesn't have the time too make them? Give me a call & I will hook you up.

Thanks, JJAM

These are the exact same carbs I have, but I can't find any pictures as to what needs to be plugged off. Mel's seem to be slightly different in the pics he provided. The stock carbs on the new 750 I picked up are not running right, so I would like to try these carbs instead. Stock 750 BTW with proper valve clearance, and other than the carb issues runs really strong.

Twinstar Joined: May 21, 2009 Posts: 54 Location: Michigan

Thought Id trow this out there if anyone is interested Please message me. I have given up on these carbs and my bike......Id like to go back to stock stock carbs and airbox. Id like to trade my carbs for stock ones and an air box. My GSXR carbs are from a late 80's early 90's. I also have Dyno Jet needles, extra jets and new slide springs. I Just thought I'd throw this up if I dont hear anything I'll put it on ebay. I can email pics.

***Edit- The carbs look just like the pic above.

Posted: Sat Jul 28, 2012 12:47 am

Bryon, are you giving up because of jettings issues, or the adapters? I would believe that if you have tried many jet combinations and the rest of your motor is in order? Then the problem is that your BST slide glides are worn out. If you pull the slides out and look in where they came out of, you will probably see that the glides (part where the slides, slide) is worn and the slides will move back and forth when installed.
This lets air pass around the slides and causes all kinds of problems. There is a fix for this which has been mentioned, in another BST thread. The fix costs $160.00 plus you will most likely need new emulsion tubes and new needles. You already have the needles, assuming that they are correct for the CB-F needs.

Otherwise start over with stock parts, like you are wanting, or better yet, save your dollars, sell your BST's on ebay and get some CR-S carbs or the like, that are not worn out and fit your motor.

It's never easy, thats just the facts. Sad but true.

Peace, JJAM

Posted: Sun Jul 29, 2012 5:08 pm

PM sent- I have two sets of stock carbs and an airbox- I know how frustrating it is to get these set up- thankfully I live 5 miles from Melchiro...........

Twinstar Joined: May 21, 2009 Posts: 54 Location: Michigan

JJam wrote:

Bryon, are you giving up because of jettings issues, or the adapters? I would believe that if you have tried many jet combinations and the rest of your motor is in order? Then the problem is that your BST slide glides are worn out. If you pull the slides out and look in where they came out of, you will probably see that the glides (part where the slides, slide) is worn and the slides will move back and forth when installed.
This lets air pass around the slides and causes all kinds of problems. There is a fix for this which has been mentioned, in another BST thread. The fix costs $160.00 plus you will most likely need new emulsion tubes and new needles. You already have the needles, assuming that they are correct for the CB-F needs.

Otherwise start over with stock parts, like you are wanting, or better yet, save your dollars, sell your BST's on ebay and get some CR-S carbs or the like, that are not worn out and fit your motor.

It's never easy, thats just the facts. Sad but true.

Peace, JJAM

The slides aren't worn at all, engine is in great shape 140psi in all cylinders. I think my issues is the stock cams, Like someone a few pages back said these will work on the 750 stock, but better if you have 900 cams installed. I could get it running with them, but couldn't get them tuned correctly. I havent riden in 2 seasons now, so I just wanna go back to stock setup and go from there.

Posted: Thu Aug 02, 2012 8:54 pm

Bryan81 wrote:

JJam wrote:

Bryon, are you giving up because of jettings issues, or the adapters? I would believe that if you have tried many jet combinations and the rest of your motor is in order? Then the problem is that your BST slide glides are worn out. If you pull the slides out and look in where they came out of, you will probably see that the glides (part where the slides, slide) is worn and the slides will move back and forth when installed.
This lets air pass around the slides and causes all kinds of problems. There is a fix for this which has been mentioned, in another BST thread. The fix costs $160.00 plus you will most likely need new emulsion tubes and new needles. You already have the needles, assuming that they are correct for the CB-F needs.

Otherwise start over with stock parts, like you are wanting, or better yet, save your dollars, sell your BST's on ebay and get some CR-S carbs or the like, that are not worn out and fit your motor.

It's never easy, thats just the facts. Sad but true.

Peace, JJAM

The slides aren't worn at all, engine is in great shape 140psi in all cylinders. I think my issues is the stock cams, Like someone a few pages back said these will work on the 750 stock, but better if you have 900 cams installed. I could get it running with them, but couldn't get them tuned correctly. I havent riden in 2 seasons now, so I just wanna go back to stock setup and go from there.

Yep....put 900 cams in it and it is a game changer...

Jebby

Posted: Fri Aug 03, 2012 5:11 am

Honda in the past did not give two shits for performance. Same way with the clutch,nice and calm,if it slips no problem.

The 750 cams are so mild they are not worth keeping in a motor. They are good for core to have real cams hard welded and ground. Since no one rides these now and damn few 30 years ago the cost to make cams is about 1/3 the new price and now closer to total of what they are worth.

The 900 cams are decent the lift is a lot more and lift basically helps power everywhere. 1100 cams better yet but trying to find "non worn out" cams the job.

Now carbs wise get something aftermarket. Then no EPA and all tuning parts are available.

No matter what the carbs came from if it is an EPA bike there are no tuning parts. The jet kits are basically to do the bike the carbs came from and not that good anyway.

I only said this dozens of times. These bikes and basically all Hondas in this era just do not respond to Mickey Mouse work. Ride the bike dead stock,you will not do much to a 750 anyway.

They work fairly well stock carbs,airbox,and stock jetting. Maintaining the valve clearance is more the needed thing to "keep" the performance vs trying to get more.

Twinstar Joined: Feb 03, 2013 Posts: 59 Location: Sweden

Hi all,

I am a bit confuset. should i go with the 34 or 36mm carbs?

Stock engine Dohc cb750f 1982
4-1 exhaust
Individual pods

Posted: Sun May 05, 2013 12:30 am

Just a note to say, I finally got my installed BST31ss's to make my little cb scary fast for this old, Goldwing riding cripple...lol...I had trouble with the visibly spotless carbs having the air pilot jets being clogged. I have since acquired another set of these as parts to complete a set of BST34ss's and will give them a try just to see which ones I like best. Right now she screams to 5k, has a bit of a gurgle just before she is an arm stretcher/shoulder wrecker to redline! I am so glad I learned how to read! {lol} Thank you all for the insight, the experience and for sharing it all! beerchug

Posted: Sun May 05, 2013 3:13 am

I just checked ebay. Those carbs have gotten pricey. However I would love a set. I have no problems with my stockers at all but would like to run pods.

Posted: Fri May 10, 2013 1:38 pm

If anyone is looking for a set of these carbs I have a set to sell. I purchased these: http://alturl.com/c5qc9
Rebuilt by "Partly Chris's Motorcycles".

All rebuilt the same as described, and I will be putting them up on eBay but thought I would offer here first. I paid $395 + $45 shipping but will sell for $300 plus whatever shipping is.
These are Mikuni BST36SS carbs, 36mm. Jetting is OEM.

I ended up selling the bike, I didn't even get a chance to mount them, so I don't need these sitting around.

Posted: Fri Jun 07, 2013 5:49 pm

Point of interest after spending a couple hours on the GIXXER forum....
The floats on these carbs have multiple recommended heights...the reason
being as it affects the mixture!
That...and the air bleeds are the real keys to these carbs...

Jebby

Posted: Mon Jun 10, 2013 11:06 pm

A lot of great information on carburetors

MB-5 Joined: Jun 01, 2013 Posts: 6 Location: Madison, WI

I after reading this thread I purchase and installed a set of 36mm slingshots from a 1988 GSXR 750. All is working fine just working on getting the right jets for my stock 1979 CB750K. Right now with Dynojet installed I'm running the 120 Mikuni jets.

But one thing I don't think was not mentioned and maybe I'm too green but - how do you deal with the extra slack in the throttle cable? Is there a cable that would work better then the stock cable that came with my K?

Posted: Fri Jun 28, 2013 3:46 am

I left mine alone....doesn't bother me....ctluba made his own.....some have them custom made.

Jebby

MB-5 Joined: Jun 01, 2013 Posts: 6 Location: Madison, WI

Jebbysan wrote:

I left mine alone....doesn't bother me....ctluba made his own.....some have them custom made.

Jebby

I was hoping that maybe a CB550 or something smaller might fit it. If it was only an inch to an inch and a half longer I'm getting use to the little bit of slack but I am liking how the carbs work and how the bike runs with these carbs, from the little that I have drive it since the conversion. I'll get some more play time next week.

Posted: Sat Jun 29, 2013 3:49 am

Jebbysan talked me into making a set of BST adapters for another member. Plus if interested I can make some velocity stacks that fit the oval inlet spigot on BST's.

The stacks are hand formed the old school way.

Keep it in a wheelie, Jim

MB-5 Joined: Jun 01, 2013 Posts: 6 Location: Madison, WI

mespock wrote:

I after reading this thread I purchase and installed a set of 36mm slingshots from a 1988 GSXR 750. All is working fine just working on getting the right jets for my stock 1979 CB750K. Right now with Dynojet installed I'm running the 120 Mikuni jets.

But one thing I don't think was not mentioned and maybe I'm too green but - how do you deal with the extra slack in the throttle cable? Is there a cable that would work better then the stock cable that came with my K?

I found that I am running slightly hot with the 120 Mikuni jets. As of now I was told to use the 140 dynojet jets that came with my kit. After my first run with the 140 DJs I am seeing my plugs starting to fade from white.

Also what are you guys using for your Crankcase Breather ports?

Red CB1100F Joined: Jul 19, 2010 Posts: 6420 Location: Ct

For the crank case breathers I used some cheap little air filters from the auto parts store. Some guys run hoses from the breather to back under the tail and then put the little filters on the end of the hose. This set up works fine for me. The filters get a little gummed up after a while. They are cheap to replace.

For the throttle cable, I shortened my cable up. There is a great video on youtube, a "how to" to shorten cables. It worked great, three years and going strong. I also had a cable made by "motion pro" (I think that was the company) because I planned to use my home made cable just as a temp. As I said, the home made cable has held up fine and I never installed the custom made one.

MB-5 Joined: Jun 01, 2013 Posts: 6 Location: Madison, WI

ctluba thanks this helps ..

Here is my 79 750K ...

MB-5 Joined: Jun 01, 2013 Posts: 6 Location: Madison, WI

My Crank Case Breather filter

Posted: Thu Jul 18, 2013 1:33 pm

Well, I'll jump into the fray...

1982 CB750K with 900 cams, 4 into 1 exhaust, Uni foam filters, Accel coils.

I started off with the stock carbs, and was offered a set of 93 GSXR 600 Mikuni's for cheap. They were in outstanding shape, and very clean inside and out, so I bought them.

I've read through this thread, and am currently trying to get the radiator hose mod going. First try was a piss poor attempt on my part. I blame the heat index of over 100. Very Happy

I could really use some advice on the jetting and the carb boot situation. It's not my first rodeo, I've built other bikes before, and am fully capable, and have the correct tools for all portions of my build. But I have to say, getting this set of gixxer carbs on my Honda has been quite a task.

Perhaps some of the learned members of this thread could offer up some advice?

What type of hose is being used, and are inserts necessary to make a smooth transition from the carb to the head?

Is the stock 32.5 pilot jet too small for my combination of motor and carbs?

Also, do these gixxer carbs use small or large head Mikuni mains jets? (since the carbs are used, I'm checking every possible angle, thus my question. I believe they should use the small heads, but the inlet sure looks like it's designed for the big heads.)

Hopefully you can offer up some wisdom. I'm fresh out. Must be this heatwave.

Thank you for your time and patience.

Loudest143

Posted: Thu Jul 18, 2013 2:04 pm

First off welcome to the site!
Second....where are you?

The large head jets are what is in them.
Jetting is impossible to tell you for many factors...but let's start with the fact that there are about ten versions of these carbs.
Some have rubber well plugs in them...some do not.
On the primary...the air bleed is more critical for tuning than the jet itself...as it is an emulsion type jet.
Yes the brass tube are critical for transtion and keeping them taught to the head....also to keep hose from collapsing...
The dynojet kits have adjustable needles which is nice...but I do not recommend drilling the slides.
Post a pic and model number of what you have.

Peace,
Jebby

Posted: Thu Jul 18, 2013 3:06 pm

I will have a lot more info to give on the jetting subject when I finally refurb my own and ultrasonic clean them.

Jebby

Posted: Thu Jul 18, 2013 6:56 pm

Quote:

First off welcome to the site!
Second....where are you?

The large head jets are what is in them.
Jetting is impossible to tell you for many factors...but let's start with the fact that there are about ten versions of these carbs.
Some have rubber well plugs in them...some do not.
On the primary...the air bleed is more critical for tuning than the jet itself...as it is an emulsion type jet.
Yes the brass tube are critical for transtion and keeping them taught to the head....also to keep hose from collapsing...
The dynojet kits have adjustable needles which is nice...but I do not recommend drilling the slides.
Post a pic and model number of what you have.

Peace,
Jebby

Thanks, Jebby. I'm in Columbus, Ohio. The carbs are the 18E GSXR 600 carbs. In fact, earlier in this thread there is a link to my exact carbs from an ebay ad. I bought them from my good friends at The Rice Paddy. 93 gsxr carbs

Oddly enough, they came to me, anyways, with small head main jets in them. It just didn't seem right from the get-go, since the mounting surface for the main jet is perfectly sized for the large head jets. Also, they did not come with any rubber well plugs. (stock CB750 carbs had them.) As you know, buying used means doing some detective work, and I'm not afraid to do some research, and ask questions.

If I read your reply correctly, you recommend brass tubing/pipe for the internal strengthening of the radiator hose mod. Previously I had read someone using PVC pipe. I can see the benefit of the brass, so I will source some of the correct size at my local Lowes or Menards. Easy enough to get ahold of, I imagine.

Regarding this mystical air bleed that you speak of... I'm familiar with the front inlets on the carbs. I guess I need to do some more reading about these air bleed circuits, and educate myself on them. I feel a bit foolish that I've overlooked that entire topic!

This set of carbs is listed as having stock jets of 32.5 pilot, and 105 main. I know that a dynojet kit was installed, but I will recheck the slides to see if the PO drilled them. Frankly, they looked untouched. I'll look a little closer, and double check that and confirm. I will source some standard Mikuni large head jets here locally, and get started into some tuning. But that will come AFTER I solve this radiator hose mod complication.

Thanks for your reply. Look forward to more advice!

Loudest143

Posted: Thu Jul 18, 2013 7:33 pm

This was in this thread but hard to find...
http://www.gixxer.com/forums/showthread.php?t=159718

Read it.....one of the most compelling pieces of info is that the float level can affect the mixture...as the float
and pickup assembly is one piece.

You will need to start with at least a 120 jet in these....try to find some quick change float bowls that came on the
early to mid GSXR 750/1100.
I finished a set of Katana 750 BST36SS for a member here James Knouse....he is going to install them with the
custom boots courtesy of JJam (Jim Sawtell) and tell me the outcome. They have the Dynojet needle three from the
bottom and 124 mains......BUT...these carbs had the rubber well plugs.....this will affect overall jetting. Some had them, some
did not...I am still researching as to why.

These are great carbs.....light, simple, no choke to F@#k with and they plain WORK.....but you must make sure your bike is
in perfect tune first...advance,valve lash, ignition....before you tackle them.

Jebby

Posted: Fri Jul 19, 2013 2:39 pm

I did find a full set of the correct Mikuni mains in both 115 and 120 in my 'jet box'. The little parts box that you keep tiny little things like jets, needle spacer washers, and other such things. I'm not fully convinced yet that the stock boots won't work. They're SO CLOSE to being right on the money, that it's worth another shot tonight with the 120 mains. It's got to be really close with that setup. If I can confirm that the stock boots are an issue, I'll eliminate those, and fabricate the radiator hose mod. *SIGH* I'd rather stick with the stock boots, but I guess it's possible that they are causing a rough entry to the head. It's all part of the process.

Obviously, I'll keep updating my findings.

*EDIT*

Oh yeah, I'm satisfied that my advance, valve lash and ignition are working properly. The stock carbs and I spent some hours together, and everything was working well. The top end of the motor was rebuilt by the PO. Port and polish, 900 cams, and proper shims. (I checked and measured)

Posted: Sat Jul 20, 2013 3:28 pm

Two steps forward, one step back. It's always that way with carbs, isn't it? Very Happy

I used the 1/1/4/4 boot combination of the stock boots, and the gsxr carbs fit perfectly. Radiator hose? I don't need no stinkin' radiator hose! I did use standard hose clamps at the carb side, to insure a better seal..

Started with 122 mains. Mixture screw at 3 turns out. Stock pilot jet, DJ needle, 2 positions showing above the clip, no extra shims.

It wouldn't start with choke, only with starter fluid. WhiskeyTangoFoxtrot! My carbs are showing perfect balance on the Motion Pro manometer at idle and when the throttle is turned. So at least the carbs are balanced with each other, that's good news.

Went all the way down to my lowest jet size-- 115. Mixture screw 2 turns out. No change to needle.

It started with choke. (positive development, IMHO) Idled rough, but far better than the 122. Has a small amount of throttle response, but just bogs at about 1/4 throttle. Again, it's just not any where near a proper idle. Adjusted the the idle screw up to around 3K and there was no noticeable change, positive or negative. Again, the carbs are still balanced.

So here's where I'm my brain is on this... Float level, and vacuum could be causing me issues.

Float level is set at 14.6mm. That was what I found to be the stock setting for the 18EO GSXR600 carbs. I'll look into that a bit further to confirm the setting. My carbs have the metal tubes on the side of the carb from the base of the carb above the bowl to the top of the slide. Now the vacuum issue.... I have the two large spigots in between 1 and 2, and 3 and 4. Similar in size to the fuel inlet between the same carbs. Of course there are the vacuum plugs on top of the diaphragm that I have plugged.(manometer attaches to these ports, I believe) Then the odd ones... They face horizontally out of the top of the carb body, just below the slide lid. I have not found any information on what these are for exactly. Perhaps someone can advise?

Well, back to the moto shed to pull the carbs out and continue the process. Let me know if you have any thoughts.

THANKS!!

Loudest143

Posted: Sat Jul 20, 2013 3:35 pm

They are not clean in the circuits...the choke is really an enricher....if it will not start...there is no fuel
from that circuit.
Carbs can be very clean looking and not be worth a shit internally.
Spray out all of the circuits and air bleed with Berryman carb spray with the red nozzle tip tapered down
at the end with sandpaper.

Jebby

Posted: Sat Jul 20, 2013 5:09 pm

Thanks for all your help, jebby. I'll give them a thorough cleaning. What do you think about the float height? 14.6mm is what I have them set at.