order or ops on carbs

[sipman]

Hi all,

I'm truly glad I found this site. I was planning on getting my bike running and selling it because the fuel economy was so poor. I can't drive 50 miles without having to fill up! If I can get it running right, I'd like to keep it

I have a 81 GS750L. I had carb issues and my local shop replaced the entire bank of carbs. Since then, I've had the poor fuel economy. I'm pretty sure a prior owner drilled out the stock mufflers because they look stock and are pretty loud. I figured I'd need rejetting which the shop promises they did. The bike would take nearly 2 minutes of choking to start. I'd run down the battery all the time! Also, the carb would spill fuel all over the place.

This past week, I bought an O-ring kit and I'm in the process of dipping and cleaning the carbs, and replacing the O-rings. I have borrowed a Carb Sync and I'll be placing an in-line fuel filter on the bike. I'm also considering dumping the fuel and treating or lining the tank to ensure I don't have any loose rust in there.

So my Question is: There seems to be an air bypass screw on each carb. I would use these to adjust the vacuums, right? A friend suggested I need to use the butterflys to adjust vacuum which I feel is too much work!

Second, I should sync the carbs before playing with the jets, right? Also, what am I looking for in terms of identifying if i have the wrong jet in there. I'm guessing I may be over-jetted if I have such horrible consumption? Any input and advice would be much appreciated

 

[Billy Ricks]

Check this thread for jetting specs, http://www.thegsresources.com/_forum/showthread.php?t=73723 . On CV carbs the screw is for the idle circuit. Adjust each one for the smoothest, highest engine speed you can get out of them. Raising the buuterflies is not an option, all you do is increase engine speed. The slides work off of vacuum and opening the throttle plates will just cause the slides to rise and idle to go up. Get the carbs tuned as well as you can before the sync job then check them again after the sync.

 

[Steve]

So my Question is: There seems to be an air bypass screw on each carb. I would use these to adjust the vacuums, right? A friend suggested I need to use the butterflys to adjust vacuum which I feel is too much work!

Second, I should sync the carbs before playing with the jets, right? Also, what am I looking for in terms of identifying if i have the wrong jet in there. I'm guessing I may be over-jetted if I have such horrible consumption? Any input and advice would be much appreciated

Billy Ricks gave you a link to the carb "sticky" at the top of this forum that will get you information on what the stock jetting should be for your bike. Not sure exactly where in that thread the information for your bike is, you may have to scroll through a few pates to find it.

There is no "air bypass screw" on your carbs. And yes, you do use the butterflies to adjust the vacuum, whether you feel it's too much work or not.

Since the only external mixture adjustment on your carbs, I presume that is what you might be thinking is a bypass screw. That is actually the "idle mixture adjustment screw" according to the manuals. The idle mixture is controlled by the selection of pilot air jet (in the carb intake throat) and the pilot fuel jet (in the float bowl, next to the main jet). The adjustment screw only controls how much of that mixture gets into the engine at idle speeds. When you get the carbs back together, turn that screw in until it seats lightly, then back it out between 2 and 3 full turns. That will be adequate to get the bike running well enough to do other tests. When the bike is warmed up, turn each of these screws slowly to find where the engine speed maximizes. Might only rise a few RPM, so listen carefully. If there is no real rise, turn them in slowly until you hear the engine slow down, then back out about 1/4 turn. Do this for each screw. Record this setting for future reference.

To synchronize your carbs, the book calls for warming the engine to normal operating temperature, then setting your idle speed between 1500 and 2000 rpm. Connect the gauges and get your readings. The adjustment screws are on the throttle shafts (the ones that hold the butterflies) between the carbs. #3 carb is the "master" because that is the one the throttle cable pulls on. The manual calls for adjusting #2 to match #3, then setting #1 about .5 cm higher vacuum. Finally, adjust #4 to match #1. To adjust them, loosen the locknut with an 8 mm open-end wrench, turn the adjuster with a long straight screwdriver then snug down the locknut. Very small adjustments make big differences on the gauges, so be gentle. When you are done, set your your idle speed back down to normal speed and go for a ride.

.

 

[sipman]

Much thanks

Much thanks

Hi Steve, Hi Billy,
Thanks for the fast response. Steve, thanks for the step by step. Now assume I can do all that without screwing too much up, I'll then be looking at the plugs to see if I'm rich or lean, right? Then it's trying jets until i think I'm there?

Another Q i have is I'm guessing this will be a difficult job if the tank is still on. If so, is making a 'pony' gas bottle myself an option or should I be buying one? If I go the bottle route, should I be plugging the vacuum line to the petcock so I get an accurate read?

 

[BassCliff]

Hi Mr. sipman,

Yes, when using an external fuel tank, plug the vacuum line from the #2 carb (a golf tee works well). Now let me roll out the welcome mat for you...

Please click here for your mega-welcome, chock full of tips, suggestions, links to vendors, and other information. Then feel free to visit my little BikeCliff website where I've been collecting the wisdom of this generous community. Don't forget, we like pictures! Not you, your bike!

Thanks for joining us. Keep us informed.

Thank you for your indulgence,

BassCliff

 

[almarconi]

Does the bike have the stock airbox on it? I would think that you should be OK with stock jets with your exhaust and a stock airbox. These engines seem more sensitive to intake changes than exhaust. You may want to shim the needles a bit but I wouldn't worry with that until you get the bike running halfway decent. When you go through the carbs see what jets you have in there. Switching the jets without a baseline will lead to lots of frustration.

 

[76FJ55]

When you get the carb torn down record all the jet and needle numbers. and if the needle has multiple clip postions record the position of the clip and any washers present. you may then want to post this data and some of the members may have comments, depending on what is in there. You say the shop was supposed to have rejetted it due to the exhaust mod then may have gone with the wrong jet kit and this will help diagnose potential problems.

 

[sipman]

jets

jets

I'll do that. Currently, I was done the first three when I thought of checking them, so I've only got info on one main jet. I guess it's the most important because it's the main, right?
It's a 112.5.
Now if I go back and get the values, I need to get the numbers from the main jet, the idle jet, and are there two jets elsewhere?
One thing that's confusing, there is a washer on the main jet, does a washer effect the jetting? Do I just use my calipers to measure the thickness and report that back? I think somewhere in my house I have 4 main jets that are just 112. Should i use those instead?

Thanks All

PS. the bike has a stock airbox, at one point I went to foam filters, but I had such a hard time getting the bike to run right that I asked the tech who worked on it. He said the foam pod filters allow the bike to use so much air that's why I have horrible gas mileage. So back to the stock airbox and filter.

 

[sipman]

values??

values??

I found this on www.suzukicyles.org.

[FONT=Verdana, Arial, Helvetica, sans-serif][SIZE=-3][FONT=Verdana, Arial, Helvetica, sans-serif][SIZE=-3][FONT=Verdana, Arial, Helvetica, sans-serif][SIZE=-3][FONT=Verdana, Arial, Helvetica, sans-serif][SIZE=-3][FONT=Verdana, Arial, Helvetica, sans-serif][SIZE=-3][FONT=Verdana, Arial, Helvetica, sans-serif][SIZE=-3]Carburetors: Mikuni VM26SS x 4
Carburetor settings: 1000 rpm. pilot air 1 1/4 T.
Carburetor float height: 0.98?1.1 in.
Main/pilot/needle/jet: 105 /22.5 /P-1 /5F21-3

It was listed under the 750g and 750GL info but in a seperate section saying GS750 Specs. Can i trust they are all the same engines?

Also, This site doesn't show a GS750L. Only GS750G and GL. Do I really have a GL, not an L? It says GS750L on the side panels.
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[BassCliff]

Hi Mr. sipman,

"G" means shaft drive. The 750G was a Japan-only bike. I searched in the carb spec sticky thread at the top of this Tech section and found this. The 1981 should be the same.

GS750
1980 TSCC
Model: L

Idle Speed: 1050 +/- 100 rpm
Carb: Mikuni BS32SS
Id no: 45410
Bore size: 32mm (1.26")
Float height: 22.4mm +/- 1.0 (0.88" +/- 0.04)
Fuel level: 5.0mm +/- 0.5mm (0.20" +/- 0.02)
Main Jet: 112.5
Main air jet: 1.7mm
Jet needle: 5C32-3rd
Needle Jet: Y-5
Pilot Jet: #42.5
By pass: 0.8mm
Pilot outlet: 0.7mm
Valve seat: 2.0
Starter jet: #50
Pilot Screw: 3 and 1/2 turns back
Pilot air jet: #150
Throttle cable play: 0.5-1.0mm (0.03-0.04in)

Edit:
After taking a second look, I found another set of numbers. Basically the same except for the needle jet/jet needle.

GS 750
1980- 81
LT/LX/EX Model

Carb ???????. BS32SS (4 separate carbs)
bore size ?????. 32mm (1.26)
id no ???????. 45400
idle r/min???.... 1050 +/- 100 r/min
fuel level ?????. 5mm +/- 0.5 (0.20 +/- 0.02)
float height ???? 22.4mm +/- 1.0 (0.88 +/- 0.04)
Main jet??????. 112.5
Main Air jet????? 1.7
jet needle?????.. 5C28
Needle Jet?????? Y-4
Pilot Jet??..????. 42.5
By-pass?.?????? 0.8, 0.8, 0.8
Pilot Outlet?????? .7
Valve seat?????? 2.0
Starter jet?????? 50
Pilot Screw............. Preset
throttle cable play ?. .5-1.0 (0.02-0.04)

There are also a couple of 750 manuals on my website. Please download them and read them.

Thank you for your indulgence,

BassCliff

 

[almarconi]

It's a 112.5

That's the stock main for your bike.

there is a washer on the main jet, does a washer effect the jetting

The washer is just that a washer. It does not effect the jetting. Just install it under the main.

Underneath the pilot plugs are the pilot jets. Remove these and clean them, record the number thats on them. Stock are 42.5 for your bike.

I figured I'd need rejetting which the shop promises they did. The bike would take nearly 2 minutes of choking to start. I'd run down the battery all the time! Also, the carb would spill fuel all over the place.

It's a shame that a shop would mislead someone like this. Run..Run away and work on the bike yourself. Get a copy of the service manual and do the work yourself. Your carbs are probably in need of a good cleaning and adjustment. Properly set up the bike will start pretty easily and come off choke within a few minutes.

 

[Steve]

Properly set up the bike will start pretty easily and come off choke within a few minutes.

I guess my wife's bike is not properly set up.

If you already have helmet and other gear already on, you can start the bike and pull out the driveway within 30 seconds.
By the time you are in second gear, you can take the choke OFF. :dancing:

None of that "few minutes" stuff for us.

 

[almarconi]

None of that "few minutes" stuff for us.

Consider yourself fortunate. I guess alot depends on the temperature outside.

 

[sipman]

clarification

clarification

when I said the bike would take 2 minutes of choke, I may have mislead you. I'm totally unsure of how long the bike ran on choke, but it took about 2 minutes of pressing the starter button, taking it off, pressing it, taking it off, pressing it, etc before the bike would fire. Off choke, it wasn't an option without having jumpers from my car there.
It hasn't been fun. But, I'm really encouraged by this forum because I think I may actually have a bike to ride that gets decent mileage if I actually listen to the advice you all have been giving me. I live in Philly and my folks live in princeton. It's a decent drive, but one I've never been able to make on this bike without having to fill it up along the way. From what I'm hoping it is a trip there and back without a fill up. It's about 100 miles round trip. My last bike was injected and there is much to be said for that simplicity, but I'll need a new emoticon to express the width of my smile if I can get the bike running right!

 

[sipman]

specs

specs

So here's what I know as far as jets:

Main jet is 112.5
Jet needle has no markings I can see
Needle Jet is y-4
Pilot jet has no markings
pilot air jet is 760

From what I can tell, it seems accurate.

 

[Steve]

when I said the bike would take 2 minutes of choke, I may have mislead you. I'm totally unsure of how long the bike ran on choke, but it took about 2 minutes of pressing the starter button, taking it off, pressing it, taking it off, pressing it, etc before the bike would fire.

WOW, you really need to do some work on your bike.

Starting drill here (with her bike) is:
Set 'choke' to about half
Verify kill switch is set to RUN
Turn key ON
Verify neutral light
Push starter button for about 2 seconds
Adjust engine speed to stay under 2,000 rpm by using 'choke'
Put on helmet and gloves
RIDE
After getting into second gear, turn off 'choke'
Continue ride.


(My bike is virtually the same, except I keep the 'choked' idle speed to about 1200. Warm idle is a whopping 700 rpm.)
.

I live in Philly and my folks live in princeton. It's a decent drive, but one I've never been able to make on this bike without having to fill it up along the way. From what I'm hoping it is a trip there and back without a fill up. It's about 100 miles round trip.

No reason your bike should not make it on one tank. True, the '81 L-series bikes had smaller tanks (about 3.5 gallons, and about 1 of those gallons is in reserve), but you should be getting at least 40 mpg, so the 2.5 gallons in the main tank should take you the 100 miles.

My wife's '82 850L has a 4.5 gallon tank, and we typically go about 140 miles before even putting it on reserve. Unfortunately, I don't think a later tank will fit on your bike without some modifications.

.

 

[sipman]

UPdate

UPdate

So this evening, I started up my bike. I have the freshly cleaned carbs, with what i think are stock jets in on it. I tried to adjust the bypass screws to get to a higher RPM, but none made any effect. So I guessed my balance may be so out of whack. I figured I'd try and come back to it.

The bike started up well which was encouraging, but balancing the carbs was nearly impossible. I tried to balance #2 to #3 but the idle went up and down and the balance would never hold steady, never mind even. (I did plug the vacuum line to the petcock. I had attached a 16 oz pony bottle to the bike and it positively chucked it. I needed to refill it 4 times!!! Now I'm no expert, but that seems like a ton of refills for a bike that's idling at 1500rpm, right?
So after failing on #2 and #3, I decided to sync # 2 to #1 which worked. Then #1 to #4 , which also worked. I had the bike idling at 1K. So I though I was set. A quick blip of the throttle, and it's once again too high idle, then another blip, it's too low. SO I can't even imagine what makes this bike vary it's idle like this.
Then I decided, maybe I gotta just ride it for a few and open it up, so I installed the tank. I've replaced all the black gas lines with vinyl so I can see everthing. Once i connected the #2 Vacuum line on, I could see both it and the main fuel line had fuel in it. Does this mean my Petcock has failed? Also, after riding a little, the bike was smoking out the tailpipes like crazy. I'm assuming it's running rich from #2 sucking gas from the petcock, but would it make sense for all 4 pipes to blow out smoke? I'm thinking it's black smoke, not white but it was late so I was only seeing it when cars had their lights on behind me at red lights.

So in recap, my bike is still guzzling gas, I think my petcock has failed, and I;m not sure if the idle adjust screws do anything on the carbs. Any idea if theses symptoms mean one problem?

 

[Suzuki_Don]

The varying idle sounds like an air leak to me. Have you turned the mixture screws in till they bottom out, and then back them out by 2 1/2 turns. Try richening the mixture screws by another 1/2 turn and see if this makes things settle down. It definitely sounds like you need a new petcock. Don't try and rebuild as it rarely works.

 

[sipman]

mixture screws

mixture screws

Hi,

I bottomed out the mixture screws and it seemed to do nothing. So I set them all at 1 1/2 turns out. I'll try and additional turn. How do I chase down a leak? Do i spray carb cleaner all over? wait for a rev