My replacing the stock carbs

Posted: Thu Jun 02, 2016 6:36 am

reedrunner23 wrote:

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.

not sure what 36 carbs and how you've mounted them - check the line where the float bowl clamps to the main carb body versus a std Honda carb - relative to the angle from horizontal, you may have to fine tune the float hight to compensate if the carb was designed to be sitting at a slight downdraft angle - otherwise it as said, yi got some tiny spec's of dirt in the fuel line causing the float jets to leak, if you see any sign of a wear ring on the jets = their toast

Posted: Thu Jun 02, 2016 5:29 pm

Finally have the carbs tuned up(relatively). Found a massive pilot jet in the 3rd cylinder carb and swapped it out. Currently running 120ish main(drilled) 3rd clip down and 3 turns out on mixture screw. Plugs are white with some brown spots. They dont rev down quickly though which is a bit weird and slightly sluggish at idle. Any ideas?

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

Thanks guys. I rebuilt them fall of last year with all new float valves etc so I hope they wouldn't go bad by then. I have a few extras laying around I will try. The only thing I have changed since then is the fuel line diameter and shorten the line down to connect to the tank easier. Thats why I was thinking the fuel line pressure could be pushing right through the valves and flooding the carbs. Ill throw in the new valves and check the float height again like the picture you attached. If its still leaking Ill switch out the fuel lines and see if it gets any better. Thanks again.

Posted: Fri Jun 03, 2016 10:14 am

just for knowledge - pressure has nothing to do with diameter only height and there is nothing you can do within reality to make any measurable effect with this ( you'd need to have the tank maybe 15ft above the bike and even then
however - what it could be is the material of the hose you used - maybe it doesn't like petrol = the hose is breaking up contaminating the fuel with debris - you see this more often in brake / clutch systems because the brake fluid is more aggressive and hence you need the proper material of hose

another possibility for excessive pressure is a blocked petrol tank vent coupled with a warm temperature causing pressure build up in the tank due to expansion ( occurring when the bike is not running)- this is unlikely as you'd also see fuel starvation occurring ( vacuum as fuel level drops when you ride the bike)

Posted: Fri Jun 03, 2016 10:29 am

Could the float hinges be sticky? Mine were after rebuild and had to take them apart and wiggle them a bit to stop fuel coming out of the overflows.

MB-5 Joined: Jan 05, 2014 Posts: 1 Location: Canada

FYI for anyone looking to use the stock boots in a 1,1,4,4 configuration on BST36ss carb for a dohc cb750, they wont line up and the diameter of the carb boot is to large for the carb even if you did get it somehow to fit. Back to the silicone hose I guess Rolling Eyes

MB-5 Joined: Dec 18, 2014 Posts: 1 Location: Netherlands

Hey Guys,

I am currently running the 31mm carbs on a cb750 with Uni pod filters and a chopped stock exhaust and stock cams. My engine is running extremely hot due to the fact that it probably is way to lean. I got 150 and 147,5 mainjets installed.
Question is to keep experimenting with the 31's and install the dyno kit or buy a set of 36' or 34'mm.. I mean is it possible to get it running correctly with the 31's? It would be a shame if I waist money on a dyno kit.
Thanks!

MB-5 Joined: Mar 14, 2017 Posts: 9 Location: USA

I just got a set so we will de where it goes!

MB-5 Joined: Mar 14, 2017 Posts: 9 Location: USA

I just got a set of the bst31s for a good deal. 128 mains and the dynojet kit installed! Just broke them down and they are soaking. Need to find some new orings and I haven't been able to fish the pilot screw o rings and washer out yet. I'm starting to think they aren't in there!

Posted: Thu Apr 06, 2017 5:24 pm

Hi was wondering if a set of carbs from a GPZ 900 would be compatible with the Honda 900,I know the spacing is probably not the same and the ports are 34mm. Worth a try maybe????

MB-5 Joined: Jan 15, 2016 Posts: 2 Location: Glendale, CA

Hi guys, first off thank you to everyone who has contributed to make this thread a goldmine of info.

I've got a set of these carbs, and for the life of me I can't figure out why cylinders 1 and 4 have 112.5 jets while 2 and 3 have 110 jets. And when I go to change out the jets, should I maintain the offset or should I run all 4 jets of the same size?

DrOlds wrote:

Katana 1100 (GSX1100F)

Type: BST34SS

Main Jet
Cylinders #1 and #4: 112.5
Cylinders #2 and #3: 110

Main Air Jet: 0.6mm

Jet Needle
California: 5DL16
U.S except California: 5DL11
U.K: 5DL7-3

Needle Jet
California and U.K: P-0
All others: P-2

Pilot Jet
California and U.K: 42.5
All others: 32.5

Pilot Air Jet
California: 155
U.S except California: 135
U.K: 150

Pilot Screw setting
U.S: Preset
1988-1990 U.K: Preset (2 7/16 turns out)
1991 on U.K: Preset (2 1/2 turns out)

Starter Jet (choke): 42.5

Float Height: 14.6mm (+/- 1mm)

Posted: Wed Apr 12, 2017 4:14 am

I am not an expert, so guessing here... Could it be that the intake manifold for 1 & 4 are different enough to 2 & 3 to require the different jetting?

Posted: Wed Apr 12, 2017 7:53 am

well spotted - Honda recommend going up on the main jet for 2 & 3 on the CB when you run open velocity stacks - i.e. the opposite of what you've found - each and every set up has its fine tweaks to getting the balance right - i.e. don't take what you've found from a carb for another engine set up and copy it onto a different one with a different airbox / velocity stack / inlet port configuration
ps the GPZ900 carb spacing is 77-85-77 versus CB of 80-100-80, Suzuki GSXR is closer to the Honda at 77-93-77

Posted: Wed Apr 12, 2017 8:19 pm

Hi,
Has anybody long time experience with radiatorhose connectors? I remember that they are not gasoline resistant.
I better ask for that information before i start adapting those gsxr carbs.
Thanks for your input
Cheers
AJ

MB-5 Joined: Jan 15, 2016 Posts: 2 Location: Glendale, CA

Bone wrote:

I am not an expert, so guessing here... Could it be that the intake manifold for 1 & 4 are different enough to 2 & 3 to require the different jetting?

I'm no expert either, so that actually sounds reasonable to me Laughing

The boots in the center are a tad shorter than the boots on the outside. I'll just have to try a few combinations to see what feels best. But given my level of expertise, I doubt I'll be able to tell the different between 110's and 112's.

nlovie wrote:

well spotted - Honda recommend going up on the main jet for 2 & 3 on the CB when you run open velocity stacks - i.e. the opposite of what you've found - each and every set up has its fine tweaks to getting the balance right - i.e. don't take what you've found from a carb for another engine set up and copy it onto a different one with a different airbox / velocity stack / inlet port configuration
ps the GPZ900 carb spacing is 77-85-77 versus CB of 80-100-80, Suzuki GSXR is closer to the Honda at 77-93-77

Thanks for your input. That makes me feel a bit better about experimenting a little.

MB-5 Joined: Mar 14, 2017 Posts: 9 Location: USA

I used 1 1/2" marine fuel line. The kind without the wire reinforcement. It's slightly stiffer than the radiator hose, but I don't have to worry about the fuel.

MB-5 Joined: Apr 14, 2017 Posts: 18 Location: Canada

Can anyone confirm that an 89 katana 750 uses the BST36? And will the carbs work? Found this info sifting through this thread but just want to confirm.

Motorcycle: 88 GSX-R 750
Katana factory carb specifications
Katana 750 (GSX750F)

1988-1996

Type: BST36SS

Main Jet
U.S , cylinder #3: 110
All others: 105

Main Air Jet: 0.5mm

Jet Needle
California: 5EZ53
U.S except California: 5EZ62
U.K: 5EZ61-3

Needle Jet
U.S: Y-1
U.K: Y-2

Pilot Jet
U.S: 32.5
U.K: 37.5

Pilot Air Jet
1989 and 1990 California: 1.25mm
1991 on California: 1.2mm
U.S except California: 1.35mm
U.K: 1.3mm

Pilot Screw setting
U.S: Preset
U.K: Preset (1 5/8 turns out)

Starter Jet (choke): 37.5

Float Height: 14.6mm (+/- 1mm)

Posted: Thu Apr 20, 2017 12:53 am

According to the KatRiders site the '89 Katana 750 does use the BST36.
http://katriders.com/wiki/index.php?title=Carburetor_Settings_and_Specifications

MB-5 Joined: Apr 14, 2017 Posts: 18 Location: Canada

Question: If I was going to run bst36ss carbs on a cb750 with pods, 900 cams and a 4 into 1 exhaust, what jet sizes would I need? Valve clearances are all within 0.05-0.06.

Posted: Thu Oct 12, 2017 3:42 pm

This is just my opinion on this subject of using non-OEM carbs.
All the money that has been spent on this experiment is amazing!
Some have realized that it's a costly and frustrating attempt to improve your bikes performance and others just keep throwing money into it without any success.

All these questions about carb manifold diameter, jetting, float height and so is hilarious.
Save your money for a real set of direct lift carbs.
Ask the guys who use direct lift carbs. FCR, CR Kehnin, and TMR and RS Mikuni carbs. There's a big difference in performance between vacuum lift and direct lift carbs.

Starting with a healthy motor is.also very helpful.
Hopefully Petrat, Genesound., Captain or CBX-tras will speak up regarding this matter.
Sorry if I upset anyone IJMO.

MB-5 Joined: Apr 14, 2017 Posts: 18 Location: Canada

...

Posted: Sat Mar 16, 2024 5:59 pm

I hope someone is still checking this forum, cause I need help with setting up my carbs.
I have a stock Honda CB900F (except for pod filters and 4-1 Sebring exhaust).
I was fed up trying to make stock carbs work for 3 years. And finally bought carbs from GSXR 1100 93-94 in mint condition. I know they are not the most preferred option, but they do mount with stock manifolds if you use a 1-1-4-4 setup.

The bike starts nicely and revs perfectly. Except for rough work below 2500 RPM. It works really RICH. Adjusting the mixture does not help. The main jet is 135. So before I buy the jets/needles I was hoping to ask here if someone has successfully tuned their cb900f with GSXR 1100 carbs. Which jets should I go for?

P.S. I am using iridium spark plugs for stronger spark and for less chance of flooding it while running rich. I've read on another forum that still an ignition conversion to CBR coils is a must for a proper spark. Any opinions on that?

A few photos of how my carbs look when installed with factory manifolds are below. I had to temporarily put some rubber between the carb caps and the fuel tank because the carbs are slightly taller than stock ones. And my tank front mounts were previously lowered by about 10mm jfyi.