My replacing the stock carbs

Posted: Sat Sep 05, 2015 4:47 am

If it just simply runs out of rpm with no miss then probably too much carb, not enough motor to keep the slides all the way open.

That sounds like 4th gear or even 3rd also, way too much rpm for the speed you give there. At 8000 in 5th you should be up around 170 or so.

MB-5 Joined: Jan 23, 2012 Posts: 6 Location: fort collins

So I just tore apart my set of first gen and I thought I was being careful,but when I was using a hammer impact I knocked loose a check ball, a small o ring and little brass washer and a tiny spring. I have no idea where they came. I need a little help please. I can show pics if needed.

Posted: Tue Sep 29, 2015 5:15 pm

Only a maniac or a butcher uses any sort of hammer impact on a carb.

Posted: Tue Sep 29, 2015 11:45 pm

generalyen wrote:

So I just tore apart my set of first gen and I thought I was being careful,but when I was using a hammer impact I knocked loose a check ball, a small o ring and little brass washer and a tiny spring. I have no idea where they came. I need a little help please. I can show pics if needed.

There are very few places that a check ball and spring can come from. A photo will solve the mystery. But please take the trouble to make it well-lit and focused.

BTW, I've used a Milwaukee cordless impact driver on 700 carb sets and counting. Most indispensable tool I own. Seriously. I also sell hammer-style impact drivers for JIS screws such as you'll find, well, everywhere on any Jap bike.

Don't be discouraged. Most of us are here to help with the occasional goof because we've made so many of our own.

OSC
www.oldschoolcarbs.com

MB-5 Joined: Jan 23, 2012 Posts: 6 Location: fort collins

So I found out where the check ball went, it was for the choke detent. The o ring and washer was for the pilot screw, but I have 3 small springs that I'm stumped on. I know what the larger go to but I have no idea where these bastards go.

MB-5 Joined: Jan 23, 2012 Posts: 6 Location: fort collins

Nevermind I figured it out they are for the pilot screws. When I took the bike out I was only able to get to 1/4 throttle, anymore than that and it would cut out. I tried pulling the choke and it didn't effect anything but have a higher idle. I have 130 mains, needles on 2nd clip from top. Idle screws at 2.5. And my elevation is 5280. Stock 750 engine but I will be installing 900 cams this week. I have not checked clearance but I obviously will after installing the new cams. And I have 1/4 fuel hose. Anyone know if these settings are conducive to my elevation?

Posted: Mon Oct 05, 2015 9:56 pm

I have an impact too but it gets nowhere near any carbs I work on. Even when I was paid to do it I never used them. But then I don't break parts either. Ever.

OP, you are terribly lost if the engine is totally stock. CV carbs self adjust to elevation largely but you are way out of shape with that main jet, it is huge. The needle appears to be dropped to make up for that error. Or, ???? Best thing one could say would be go back to stock settings and start over and make smaller changes. Adjustable needle says non-stock needle as well. The slides may be drilled if you have those needles. So probably lost there too, you have no valid proven need for any of that.

Valve clearance issues are what largely set people onto the carbs to screw them up on these, the carbs largely not being the issue other than they clog the idles up. Meaning set valves first to a known good condition THEN go to carbs. But just try telling the average American male that.............can't do it, you just have to take the bike or car, fix it, then choke them with it along with the whopping repair bill. Then before you know it they have it messed up again. Or the 800 pound gorilla, male testosterone. I have met the enemy and he is ME.

All because they miss the fact that you absolutely cannot carb reliably for an engine that is not rock solid in running ability. The valves being off mimic that, it's just as bad as trying to rejet an engine with one cylinder that has no compression. It ain't happening, OH, you may THINK it will, and so much the worse then for you. Hard to see straight with 800 pounds on your back.

A major reason I gave up on hi-perf work some time back. I was spending more time pissing the customers off over their misapplied concepts of what and how to do things than I was doing true hi-perf. Having to dog people when they start to screw up is very frustrating. My drive to get things right was doing that to them and me, I found it is better to sit back and watch the show. And keep a big reserve of popcorn on hand at all times. 'Cause boy does it get good. There's comedy, irony and sadness at every turn, and the movie is aptly named 'The Brick Wall'........................

Doing all the 'trick stuff' to carbs only puts you in the doghouse if you are running a stock setup that would work otherwise PERFECTLY without all those 'tricks'. So many lose sight of that though in misplaced enthusiasm.

You jet DOWN for elevation if anything, not up. Less air higher up means even stock settings could be rich.

And putting 900 cams in an engine that won't run 1/4 throttle now???? How quick can you say screwing up there? Why add one more major variable when it is so screwed up it won't run now???

ONE THING AT A TIME.

All I can say is don't expect things to get better without some major changes. Not trying to be mean at all but it is what it is. The rules of physics will always prevail and they don't care squat about enthusiasm or how many trick parts you have. .

Posted: Mon Oct 05, 2015 10:45 pm

generalyen, please post a photo of your carbs. You make reference to adjustable needle clips, which don't exist in stock VB carbs of any sub-model. Carb make/model makes all the difference. Plus, there's no scale to the spring photos, like a dime alongside them, and the springs themselves aren't pilot needle springs for VB either. if anything they look like balancer screw springs but without scale we're shooting blanks.

FYI, we jet leaner at higher elevation. If you ride from 5000' down, you can keep stock jets and make a small adjustment, if at all, to the pilot needles. If you go uphill consistently, then we need to make a permanent jetting or needle change. It's also relative to environment. 7 thou in Nevada high desert is a far cry from 7 thou in woodsy Northern California.

OSC

MB-5 Joined: Jan 23, 2012 Posts: 6 Location: fort collins

I thought 130 sounded really big, those are the mains that came in the rack when I bought them. I know that with less air means less fuel needed. I just didn't know what would be a good base point for my altitude. Just checked the lobes on the 900 cams and it looks like they are beyond service limits. So I guess I will need to be returning those cams. Anyways I also checked clearance and they are all within .004 - .006. I am also re routing my fuel line because it appeared to be getting slightly kinked. I don't have anything done to the engine because it only has 7k on it.

Posted: Tue Oct 06, 2015 3:32 pm

Any time we use non-stock carbs it's an adventure. Different cams add to this exponentially, and you're further complicated by elevation.

We find a setting that at least gets them running and then make stepwise changes one at a time beginning at idle and from there upward.

OSC

Posted: Tue Oct 06, 2015 4:25 pm

Are those Mikuni carbs from a Kawi?

Posted: Tue Oct 06, 2015 7:19 pm

Changing up carbs on a stock engine=no man's land for many.

It can be done but it also separates the doers from the wannabe doers as well, that don't leave very many standing at the last.

MB-5 Joined: Jan 23, 2012 Posts: 6 Location: fort collins

They are off a 86 gsxr 750 or so I was told by the seller on ebay. What would be a good main jet to start with at my Elevation?

Posted: Wed Oct 07, 2015 2:40 pm

As has been mentioned, if you don't have compression and valve clearance numbers you really shouldn't even be considering anything else.

But even if you're in good fettle there we have a ton of variables to contend with. For one thing you can't use the stock airbox so it's either pods or stacks--either of which will increase by many orders of magnitude the difficulty of getting them in a good tune.

Your elevation will in turn greatly exacerbate above.

I hate to be fatalistic but there is no set prescription for your setup. Experiment and be guided by what you observe.

All in all your project is crying out for stock.

OSC

MB-5 Joined: Jan 23, 2012 Posts: 6 Location: fort collins

I just ran them after I ran new fuel line, and I think that was the culprit for the 1/4 throttle condition. I had to bend stainless tubing to make the tight bends without kinking. Will rev all the way to red line but is running rich like you guys had anticipated. I ordered 122 mains and will drop needle down one clip and will try again. Will post results.

MB-5 Joined: Feb 24, 2014 Posts: 5 Location: Racine, WI

I have some 36s and fuel leaks out the right side 2 carbs and it comes out of the intake pilot holes. Any ideas? Not sure if thats a float issue or not.

Posted: Wed Nov 11, 2015 8:38 pm

reedrunner23 wrote:

I have some 36s and fuel leaks out the right side 2 carbs and it comes out of the intake pilot holes. Any ideas? Not sure if thats a float issue or not.

sounds very much like it and its one of the list of basic carb set up must's
there is great knowledge on here ref: carbs - maybe sometimes we also need the basics - after valve clearance and no air leaks
1) absolutely no float bowl chamber leaks
2) are the slides closing against the stop plate - never hanging on the cable
3) is there a good flow to the carbs
4) can you blow air into / out of the pilot screw air and fuel jets - i.e. not blocked
5) get the bike running on the idle screw only - no point in moving fwd until you achieve this and a workable idle ( rough - maybe but running)
6) mark your throttle in quarters or thirds ( tipex etc..)
7) familiarize yourself with each quarter ( or third) with the associated circuit - third's = idle / needle position / main jet
Cool

test adjust whilst limiting throttle to whatever segment your wanting to adjust / feel the need
Now this is NOT perfect / correct or the whole story - but if your starting with a blank sheet - its a good starting point, it is also more related to direct lift versus CV - but it can work to some degree with CV - the gsxr 36mm carbs do respond to this - nice carb

if you don't see response to change - jet size / needle position etc.. = your out to lunch on the basic range setting's of whats in the carb and need to make much more detailed changes - like needle / spray tube size, pilot air jet changes etc.. = time to get an expert

Posted: Mon Nov 16, 2015 2:21 pm

Hi would a set of mikuni 32mm of a FZR be worth playing around with for a 900

Posted: Mon Nov 16, 2015 5:15 pm

FZR carbs? NO they will not work. FZR carbs are downdraft type. You need carbs that are horizontal type.

Posted: Mon Nov 16, 2015 7:13 pm

Ah well thanks for the info

Posted: Tue Dec 22, 2015 3:56 am

Anyone able to verify these are bst36's? I was told they are from a 87 gsxr 750 but the guy doesn't really know much bout em. Thanks!

Posted: Tue Dec 22, 2015 8:08 pm

Peteroo, those are 36mm. From a GSXR1100 or CA model GSxR750 or Katana 750

Posted: Tue Dec 22, 2015 8:47 pm

sounds like they shall work and at sub 100$ quite nicely.

Posted: Wed Jan 13, 2016 7:26 pm

Melchiro,
I am starting another build and have picked up a set of 34mm GSXR carbs that are exactly like the carbs in your first post of this thread. I have a stock 750 engine, V&H header, D&D can, Dyna 2000 ignition. I see that you had your carbs set up for your 750 with 122.5 mains, dynojet needles 2nd clip from top, idle screws 3.5 turns. What size pilots did you have? The carbs I have came with 110 mains, 32.5 pilots, idle screw 3 turns out and the needles have clip heights that are different all around.

Posted: Thu Jan 14, 2016 3:10 am

Lucasmer50 wrote:

Melchiro,
I am starting another build and have picked up a set of 34mm GSXR carbs that are exactly like the carbs in your first post of this thread. I have a stock 750 engine, V&H header, D&D can, Dyna 2000 ignition. I see that you had your carbs set up for your 750 with 122.5 mains, dynojet needles 2nd clip from top, idle screws 3.5 turns. What size pilots did you have? The carbs I have came with 110 mains, 32.5 pilots, idle screw 3 turns out and the needles have clip heights that are different all around.

My pilots are stock, size 32.5, just like yours. According to the gixxer forum, 32.5 were the pilot size for all U.S. Suzuki bikes that had the BST34.

I would start with a good cleaning, verify the float height at 14.6mm, then go with the existing jetting, but with the clips in the middle slot on all four needles.

Posted: Thu May 19, 2016 4:00 pm

Slapped the carbs on (36's) and did a WOT plug test and this is results. Granted they are not new plugs by any means but im thinking pretty lean with 124 mains. Anyone have a suggested jump in jet size to tan these up? Only noticeable sputtering/bogging is around 1/2-3/4 throttle which i could lower the needle clip but looks like if i change the mains it would likely fix the sputtering and get better plug reading at WOT. Pictures are poor, but i think they show enough. the plugs are out of 1 and 4 cylinder btw

Posted: Sun May 22, 2016 11:06 pm

Do these carbs run big head mukuni main jets usually? I measured the jets and they are the size of small head mukuni... possibly dyno jets as they aren't stamped

Posted: Fri May 27, 2016 2:31 pm

interesting link comparing dynojet/kehin/mikuni jets, only wish i found it before i bought some jets. http://www.jetsrus.com/FAQs/FAQ_mikuni_vs_dynojet_vs_keihin_sizes.htm

Drilled the DJ jets to equivalent of mikuni 120 and no hesitation at WOT so time to synch and retest.

MB-5 Joined: Feb 24, 2014 Posts: 5 Location: Racine, WI

Anyone have any issues with the line pressure being too great for the float valves? I am still have issues with gas leaking out of all four pilot air jets (front side) and have no clue what is wrong. I have checked float level, cleaned, changed every rubber piece in the carbs all with no luck. Not sure if the line to the carb would be a factor in this. I am using a smaller diameter fuel line. Short piece off the tank, fuel filter, leading into a "T" and equal length lines going to each side. Curious if anyone else has had any issues or have any thoughts on it.

Posted: Thu Jun 02, 2016 6:24 am

Fuel flowing into the carb bowl is not shutting off properly by the float needle..

Because maybe the floats are cracked and they no longer float, or,

The needle tip/seats are compromised, maybe the wrong needle tip for the seat, maybe the needle tip is cheap rubber so the ethanol laced fuel is compromising the tip, so fuel is leaking past, or,

Maybe you are checking the float height level wrong...

With a carb bowl removed, try visually seeing if the fuel stops coming out by raising and lowering the float with your finger, one carb at a time..

The below pic, is the float height check.. BST34 is 14.6mm, BST36 is 14.6mm