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78 gs750 carb problems

  • Thread starter Thread starter Rama
  • Start date Start date
R

Rama

Guest
First off, this is my first post here and I tried to find an appropriate thread to make a proper intro but didn't find one, so hello from Georgia.

I recently picked up a 78 gs750 in semi running condition to have as a project and am having some strange problems with getting it to run right. Before making this post I did a pretty good bit of searching for info both here and other online sources to try and get an understanding of how to tune these carbs (VM26SS). Right now the bike will idle fine but when I try to ride it, it picks up very poorly and stumbles a lot. Someone has put pod filters on it, and it has 130 main jets, 15 pilot jets, and some strange jet needle that is not adjustable. The only marking on the jet needle is 53H and I can't seem to track these things down and believe them to be some type of aftermarket deal. Has anyone ever seen or heard of this type of needle?

Anyways, I set the carbs at 1.5 turns out on the pilot fuel screws and 1 turn out on the air screws and then fiddled with things from there a little bit. After idling around my neighborhood I did a plug check and the right two cylinders and very lean while the left two cylinders are very rich. I certainly wouldn't claim to be an expert but I have never seen this big of difference between two cylinders that were set alike before. Two plugs on one half of the motor were white and two on the other half were black. I've seen subtle differences but never polar opposites like this. It might be possible to tune it out but still...I've never come across a bike that would have that big of a difference in fuel screw settings in order to run properly. Maybe an eighth of a turn or even a quarter difference between carbs but I would be willing to bet that it would take far more than that to tune this out. If anyone has any insight on this or knows of anything that could be causing this I'd sure like to know what is going on here.

Thanks in advance and if there is any other info that would make this easier to diagnose, I will do my best to provide it.

Oh yeah, I forgot to mention that it has a 4-1 header with a Rinehart can which strikes me as odd.
 
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chuck hahn said:
A few questions.
Did you FULLY disassemble them and replace the orings on all the jets and fuel tubes?
Have you done the valve adjustments?
Heres a link to the factory service manual..select the 8 valve manual.



http://members.dslextreme.com/users/bikecliff/
Click to expand...

The first time around I did not fully disassemble, however, I just got done pulling everything apart and going over things with a fine tooth comb, and I did see some orings that may be questionable. After I got it apart I quit for the night, but I'll be back on it tomorrow night.

No, I haven't checked the valves yet, but it's priority has been escalated on the official list of things to do. I'll probably do it before I put the carbs back on as well as rough bench sync using the drill bit method.

Thanks for the link to the manual. That will give me something to do at lunch tomorrow.

I don't know if I'll have time to get everything I need to do done tomorrow night, but I'll post back with my findings.
 
Heres the thing...everything needs to be done because ALL work in symphony with each other..one off..all suck. So, replace ALL the orings at the same time with new ones so your 100% sure that not an issue. Set the bottom ( pilot ) screw at 1 turn out from lightly seated at the bottom of the threads. Side ( mixture ) screw at 2 turns out. Get the valves done. I think the 15 pilot is too small for your setup..I would go to at least a 17.5 On the needle there should be 5 slots. the clip goes on the center clip ( as your starting point ). Once you have the carbs setup and the valves done, then get it started and warm it up GOOD with a fan blowing across the engine to keep it from overheating..a fan on each side is also a good idea. Now set the big idle knob so the bike is idling around 2,000 RPM..sync the carbs. As you adjust each carb the idle will change so youll be adjusting that as you go along too..keep it around 2,000 as much as possible. Once you get all 4 carbs as close to the same vacuum youll then be ready to do any fine tuning. I think youll find this will also help eliminate a great deal of that response stumble.
 
OHH and if you didnt notice..that long brass tube that sticks down from the carb bodies...thats the bleeder tube. There are 2 sets of holes right where that tube goes into the carbs. Take a bread tie wire and poke out the holes and also down the small hole at the end of the tube. That tubes purpose is to belled off any excessive fuel from the pilot curcuit..lots of folks dont realise that if its clogged it wont dump extra fuel back to the bowls and the cylinder sucks it up and then your running richer than you need to be...
 
OHH #2...also check the timing (and points / condensors if it has the stock ignition).
 
All of the above, and 130 mains are way way too big even with pod filters. It's drowning on gas.
 
Hey Josh...The jets I am sending tomorrow have those 112.5 mains...what do you think??? Send them to him instead of you??
 
chuck hahn said:
Hey Josh...The jets I am sending tomorrow have those 112.5 mains...what do you think??? Send them to him instead of you??
Click to expand...

Nope. Too small. He needs somewhere between 115 and 120 depending on what his exhaust set up is..
 
chuck hahn said:
Original post at the bottom says 4 into 1 and hes running pods.
Click to expand...

Oh well the he needs probably 117.5 or 120. Maybe even 122.5 but I doubt it. Honestly I think 117.5 is probably workable but you might have to futz with the needle quite a bit so a lot of times it's easier to go with a step larger main and only move the needle down one or maybe one and a half slots toward the point. It might be a little wet at WOT but not many of us spend a whole heck of a lot of time there, at least on the street.
 
Rama, I'm running K&N's with a V&H 4-1 ('78 GS750E)

15 on the fuel jet (2 turns out) and 117.5 mains. E clip on position 4.

Runs like top, although I haven't finished my plug chops I'm damned close.
 
Thanks for all of the suggestions and advice!!! I'm about to go out and work on the carbs some more. The only real problem that I can see right now that I am having trouble finding a solution for is the fact that my needles do not have any slots for adjustment or an eclip. From what I can find it looks like it's stock according to all of the parts break downs I can find. If I get a minute I'll post a picture of the needles so you all can see what I'm talking about. Aside from that, I'll be heading over to my local shop to pick up some jets tomorrow at lunch.\

Edit - Here's a picture of the needles. If it becomes necessary, I have some .005 shims that I can use raise the needle if I end up being lean. If I am rich.......I'm screwed.

photo.jpg
 
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I got a set of donor carbs and am going to the storage unit in the morning..I will take them out and see if they are the stock or not. If they are then you can have them. Hang loose a few more hrs and I may save your bacon and some cash too boot. Stock SHOULD be a 5F21-3...clip in the center groove.
 
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chuck hahn said:
I got a set of donor carbs and am going to the storage unit in the morning..I will take them out and see if they are the stock or not. If they are then you can have them. Hang loose a few more hrs and I may save your bacon and some cash too boot.
Click to expand...

What I wanna know is WHERE those needles came from? I've never seen a set like that. They look home made turned down on a lathe.
 
those arent mine Josh. Those are what he has in there now..and your needle jets are in the maill due for delivery Saturday.
 
They look OEM from something cuz they are stamped..self made ones i dont think anyone would bother to stamp them???
 
chuck hahn said:
They look OEM from something cuz they are stamped..self made ones i dont think anyone would bother to stamp them???
Click to expand...

Yeah I see that stamping in there now...looks like "...3H" no idea...
Odd.
Thanks for the needle jets chuck! Oh hey whenever you think about it, would you mind snapping a pic of that seat pan you were telling me about? Does it have hinges on it??
 
You know, I wonder if those were out of a early early 77 "B" model? Those had slightly different carbs. Mainly in the needle. It wasn't spring loaded. I had a set of 77 "B" carbs but it's been so long I can't recall if they had slots or not...
It wouldn't surprise me...
 
Rama said:
Thanks for all of the suggestions and advice!!! I'm about to go out and work on the carbs some more. The only real problem that I can see right now that I am having trouble finding a solution for is the fact that my needles do not have any slots for adjustment or an eclip. From what I can find it looks like it's stock according to all of the parts break downs I can find. If I get a minute I'll post a picture of the needles so you all can see what I'm talking about. Aside from that, I'll be heading over to my local shop to pick up some jets tomorrow at lunch.\

Edit - Here's a picture of the needles. If it becomes necessary, I have some .005 shims that I can use raise the needle if I end up being lean. If I am rich.......I'm screwed.

photo.jpg
Click to expand...

Guys,

All those fuel screws have the tips snapped off - are the tips still in the carbs?

I've never seen a Mikuni with needles like that
 
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