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Carbs/jetting

  • Thread starter Thread starter mike1414
  • Start date Start date
M

mike1414

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Hey all,

So I just recently acquired a new '79 GS 750e (after always having had a '79 550L)!





I purchased it already podded with a 4-1 V&H exhaust; I noticed that the pipes were bluing, but didn't think much of it. Well, I was riding the other day and it was popping more than usual and had lag in the throttle. I figure it's time to re-jet. So, that being said, I don't know what these carbs are. I know that they are Mikuni, thought that a '79 750e ran on VM22's, and I couldn't find what they were in the owner's manual.







That out of the way now, let's talk jets. I understand that each bike is different and have done my reading. Going along with one of the charts that was provided here by another member, I understand that the pilot jets are 15's and the mains (I think, can't remember) are 100's. I was going to start at 117.5 and go with 17.5 pilots although some people have recommended sticking with factory pilots. Any thoughts? Recommendations? I went to Z1 Enterprises to look around at jets, but again, don't know exactly which carbs are on the bike so I need some help before the trial and error of jets.
 
They look like stock 26mm.
Measure the inside diameter on the engine side.

Looking at the stain on the float which could indicate that the carbs have been neglected, i would do a complete carb tear down, clean and o-ring replacement before i started messing with the jets.
 
Makes me feel pretty stupid having to ask that, but hey, never knew exactly how they determined what they were, but I do now. My only issue is - with the top chamber of the carb. The top of the Mikuni VM26 looks almost like it has a screw off top, whereas my carbs seem to have the bolt-down type top. Does this make any different in regards to the jets?





(Top view)


I agree in regards to the quality of the rubber, though, and I am going to do a serious rebuild prior as it looks like the carbs have actually never been opened up (looks like all stock gaskets/o-rings) by the way the rubber is molded. Going to start on that first. Thanks for the input!
 
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What main jets are in it now, which jet needle and where is the clip?
Nice looking bike by the way.
 
The jetting changes are necesary due to the increase in available airflow when the airbox is removed and the exhaust is changed. That much is pretty easy to understand because they can both restrict the airflow. Now think about the jetting needs when the pilot jets are in use. What restricts the airflow? The throttle slides. At idle and low-throttle applications, the throttle slides are restricting a WHOLE LOT more than the airbox or exhaust could ever think of doing. In other words, as far as airflow is concerned, nothing has changed, so why should your jets?

On the other hand, with today's gas with ethanol added, it might not hurt to run just a little richer, but that can usually be done by simply turning out the pilot screw another 1/8 to 1/4 turn, there is no real need to increase the jet size.
 
Quickly off topic, I had to stop when I saw your quote from the movie. My ex-wife was associate producer for the movie and I was an extra and on many of the sets. Also have some original clothing from the movie. Just wanted to say that since it is rare to see anyone quote the great Buckaroo Banzai.
"No matter where you go, there you are"
 
Jetting

Jetting

Hi just a question I read your post about turning pilot screws but I was told by a buddy my bike 1150 gs I have rs38 carbs and Vance Hines sidewinder pipe and my bike is running kinda rough Lean my pipe is turning purple meaning not getting enough gas don't u think I should increase the main jet by at least 1 size maybe 2. To get more fuel. And a good cleaning. The pipe is turning purple
 

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Tkent thanks! I'm loving it, just need to get it running correctly. Since I bought it already podded and piped, and considering the amount of blueing, I can assume that it is not re-jetted and it is the stock mains (which I believe are 100's? and 15 pilots?). I can also assume, as I don't know how to tell forsure, that the clip is in the factory position. Thoughts on figuring out the jet size and clip position?

Steve, in regards to the schematic, I was just posting as to what I thought to be VM26's and the reason why I was confused as to what my carbs are. But John seems to have cleared that up as I didn't know that there was a "SS" version.

Phydeaux, with all of the restrictions that you noted why would it not be necessary to re-jet?

OJ, what a coincidence! Don't run across someone that shares that same passion often, especially not from such a close perspective!

Also, I've been looking around at different sites for rebuild parts - any input? I recall there being a site that someone from this forum put together where you can buy most, or all, of the o-rings, but I can't remember whom. I want to gather the o-rings and gaskets so I can start making moves on the rebuild
 
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oregonjake said:
Quickly off topic, I had to stop when I saw your quote from the movie. My ex-wife was associate producer for the movie and I was an extra and on many of the sets. Also have some original clothing from the movie. Just wanted to say that since it is rare to see anyone quote the great Buckaroo Banzai.
"No matter where you go, there you are"
Click to expand...

Brilliant, clever and funny movie, with bunch of great quotes. Lithgow & Lloyd are supreme. "Laugh while you can monkey boy".
 
Thank, John. Anyone have any input as to determining the current size of my jets and how to identify my float height? Never done anything with the floats/needles before, so if I'm going to start tinkering I'd like some direction at least. The manual didn't seem to be of much help.

Also, I've been searching online - it doesn't seem as though Z1 has the proper jets, anywhere else recommended to get the jets/floats for these
 
Mike, before you start ordering jets tear the carbs down and give them a proper cleaning.
You will then be able to see what size are currently in them. They could already be larger.
The increases popping on deceleration could be from an air leak.
When you say there is a "lag in the throttle", do you mean there is a breakup in the revs when the throttle is applied or something like a lag in the revs returning to idle?
 
Jet size is stamped on the jet - bottom of the main jet, and side of the pilot, take a look, magnifying glass helps. The floats just need a bit gentle cleaning. Float height should 24mm. Go here for new genuine Mikuni jets Jetsrus.
 
To echo Steve, regardless of what you have jet-wise and what you need, you must tear down and clean your carbs thoroughly. Steve gave you the link for the cleaning tutorial. You must follow it exactly and don't take any short-cuts. If you don't do this your bike will not run right and it'll drive you nuts!
 
Steve what I mean by "lag" is that there is a momentary lapse in acceleration when throttle is applied; the bike will catch up, but if I hit the throttle hard, it'll lag. Well, it would. Now it is having a hard time idling. I plan on following the cleaning tutorial through-and-through. I ordered the o-rings and want to order additional float bowl gaskets too, just wondered about the jetting to get an idea of [potential] costs.

John, perfect - thanks for that help. I looked at the jets, but could not see a size, probably simply because a brief spit of carb cleaner didn't do the trick enough. I'm going to hit the cleaning hard (been there many times before with my 550) as I know it often does wonders, I'm just preparing for the worst.
 
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