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Mikuni Carbs

  • Thread starter Thread starter Anonymous
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Anonymous

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The last post that I placed on this forum was not too helpful to me. So I am trying again. I have searched the other forums, but need some basic information about my Mikuni carbs. The bike is a 1978 Suzuki GS750E, the engine number is GS750-67340.

#1- How do I find out which carbs I have, 29 mm , 30mm or whatever was on these bikes. I do not see any numbers on the carbs themselves.

#2- What are the stock jet sizes, one book I look at says main Jet 100 and Pilot Jet 15, but another says Main Jet 102.5 and Pilot Jet O-4. I am confused. I would also like to know what jets I need to change to.

The bike has foam element filter pods and a Vance and Hines 4 to 1 exhaust.

The bike runs and idles great, until it is warmed up then it will die when you,let off the throttle. I have had the idle adjustment knob turned in all of the way, but it will still die.

The bike is in need of a synchronization also, that I know. I really need to pull the carbs apart and clean them completely. If I do take them apart, what settings should I use to take it to the shop for the synchro.

Thanks for any help in advance.
 
Re: Mikuni Carbs

mpinkorea said:
The bike is a 1978 Suzuki GS750E, the engine number is GS750-67340.

#1- How do I find out which carbs I have, 29 mm , 30mm or whatever was on these bikes. I do not see any numbers on the carbs themselves.

#2- What are the stock jet sizes, one book I look at says main Jet 100 and Pilot Jet 15, but another says Main Jet 102.5 and Pilot Jet O-4. I am confused. I would also like to know what jets I need to change to.

The bike has foam element filter pods and a Vance and Hines 4 to 1 exhaust.

The bike runs and idles great, until it is warmed up then it will die when you,let off the throttle. I have had the idle adjustment knob turned in all of the way, but it will still die.

The bike is in need of a synchronization also, that I know. I really need to pull the carbs apart and clean them completely. If I do take them apart, what settings should I use to take it to the shop for the synchro.
Click to expand...
Your carbs, if stock, are Mikuni VM 26mm.
Stock main jets are 102.5
Stock pilot jets are 15
The reference you saw about "pilot jet 0-4" is incorrect/an error. "0-4" refers to the needle jet which shouldn't need to be changed.
As for what jets to use with foam pods/V&H pipe, I can only suggest a starting point: try 122.5 mains. Raise the jet needle from its factory position (3rd) by placing the jet needle e-clip in the bottom (5th) position. OR, a little leaner would be to place the e-clip in the bottom position but also install a jetting spacer (about .022" thick) directly on top the e-clip. This would be position "4 1/2". Be sure to re-install all stock plastic needle spacers in same order. Your stock jet needle will only work in one of these two positions in my opinion. If still too lean, you need a jet kit.
Last, try turning out the pilot fuel screws (underneath) an additional 1/2 turn out as a starting point. If not rich enough by 3 turns out, try a 17.5 pilot jet and return pilot fuel screws to original settings. Always adjust the side air screws by the highest rpm method.
As you said, you must have clean carbs with good o-rings to start with. The valves clearances and ignition timing must be correct before the vacuum synch too. You MUST vacuum synch as part of jetting and re-synch every time you disturb/move the VM jet needles. If all is done correctly, the stalling should go away and the bike will run well.
 
Keith thank-you very much. I will take this information and run with it. I don't know if I am going to go with the foam pods now that I may have to put a jet kit in. I will most likely stick with the factory airbox. This should leave me with enough stock tunability except for a step larger jets. Thanks again.
 
Keith thank-you very much. I will take this information and run with it. I don't know if I am going to go with the foam pods now that I may have to put a jet kit in. I will most likely stick with the factory airbox. This should leave me with enough stock tunability except for a step larger jets. Thanks again.
 
You're welcome.
Just some added info if you just have the pipe and run the stock airbox WITH THE LID still on.
Again, jetting is trial and error, but I'll give my best guess.
Try at least 107.5 mains, but you may need up to 112.5? I'd probably start right in between... 110's.
You may not have to raise the jet needles, but I suggest taking a plug read after a solid 1/3 throttle position test after a couple of miles, then chop off and read. Do what the plugs tell you.
For the pilot circuit, test first. Then make a small 1/4 turn out adjustment on the pilot fuel screws for starters if you're a little lean or cold blooded. I test the pilot circuit in 4th gear, level or uphill ground, at about 35 mph, steady couple of miles. Plugs should be clean or been burning correctly to begin test. The bike should be completely warmed up and this will also help eliminate any start up choking effect on the plug colors. Do this pilot circuit test AFTER the 1/3 and full throttle main jet tests are completed and you've determined the needle and main circuits are good.
And as I said earlier, synch the carbs before bothering with performance/plug tests.
And to add a little various info that will help you:
-Adjust the side air screws for highest rpm, but be sure the bike is idling at 1,000/1,100 rpm's to start this adjustment...
-Check your manual, I believe your carbs have a "master carb", which means you synch the other 3 slides/vacuum levels to match it. I believe your master carb is #3. If you have a master carb, it's easy to tell because there will be no adjusting screw/holder nut above the slide. If all your carbs have adjusting screws, start at any carb, but it's generally easier to adjust the higher vacuum levels down to the lower levels that you first see at start up.
-Besides the cleaning and check/replacement of all inside o-rings, I suggest new manifold o-rings and check the manifolds too for good condition. Replace the stock Phillips manifold screws with Allens. Apply a coat of hi-temp grease to the new manifold o-rings and torque to 6 ft/lb. Robert Barr, a member here, sells o-ring kits for your carbs at a very good price. You'll have to order the manifold o-rings from a dealer/Bike bandit or your local shop.
-And if you do decide to run the foam pods, REMOVE the two float bowl vent lines and leave the ports open to breath. This will avoid fuel starvation. Remove the vent lines only if you run pods. Hope this helps.
 
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