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carbs 1 and 2 rich...out of ideas

  • Thread starter Thread starter gearheadE30
  • Start date Start date
G

gearheadE30

Guest
Background: the bike is a 1979 GS750, 22k miles, doesn't burn oil. Carbs have been dipped and rebuilt with new orings. Stock pilot jet, 122.5 mains, needle in the 4th position with no shimming. K&N pods, Kerker exhaust. The exhaust has the end cap but no baffle. No idea how it ended up like that, but it is what it is and it isn't terribly loud.

The problem is, at most throttle positions, cylinders 1 and 2 are incredibly rich, to the point that the bike surges some. The plugs are black after riding. Cylinders 3 and 4 look pretty much perfect. Possibly beside the point, but I might drop the needles a half position and put the 125s back in, because I lost some power up to after going to the 122.5s.

I'm at a loss as to why 2 cylinders are so far off. I did check float height with a micrometer; if I remember right, I set them at 24mm. I don't know if there is a good way to actually check fuel height in the bowls, but if there is, I would absolutely love to do it. Also, could a bad coil or something do it?
 
If it's rich, going up in main jet size will make it richer.

Are the plugs black after a chop? Or just after riding a bit? Without a plug chop, you cannot tell where the richness is coming in.

Your VM carbs tune from the pilot up. What are your pilot screws at now? Work on getting the pilot settings right, then move up to the needle.

To check fuel height in the bowl, Z1 has a float tester that screws into the drain plug hole and allows you see where the actual fuel level is in the bowl. It costs about $10 IIRC.
 
did you clear the air jets? the little holes opposite the air screw, looking into the airbox/intake side of the carbs
 
If your bike surges then that is a sign of a weak mixture. You're going to have to do plug chops to sort out where you're at.
 
I realize going up in size goes richer-this is just relative to the two carbs that are working correctly. I will hold off on messing with that until I can get all carbs to do the same thing.

Plugs are black after chops at anything 25% throttle and above, though it's especially bad around half throttle.

I would have to check pilot screws, but the fuel screws are all set to 1.5 turns and the airs are at high idle. It idles very well, with no bogging or hesitation.

Air jets are clear, though I could blow them out again.

This is more like cutting out on the 2 really rich cylinders temporarily and coming back in. I have a lot of experience tuning automotive ECUs on fuel injected cars, and it feels dead on a rich cut rather than a lean surge. I should have been more clear on that. On a side note, going from 125s to 122.5s did improve the cutout condition, so that backs up the theory that it's too rich.

Also, the carbs are sync'd.
 
It's not a bad coil as 1 & 4 and 2 & 3 are the ignition pairs.

Fuel screws are a bit too open - about 3/4 is usually about right (but that's probably not your main problem).

You can check fuel level in the bowls by removing the float bowl drain plug and replacing with a threaded nipple attached to clear tube - the manual shows you how.

How does she run flat out with WOT?
 
I would get a new baffle for that pipe and recheck what you did in the carbs, needle clip position, jet size etc. Something does not add up. Start your fuel needles at one turn out, and adjust them in depending on plug color after driving several miles at under 25MPH after the motor is at operating temp. Shut the bike off like you are doing plug chops. They should end up somewhere between one and three quarters of a turn out from lightly seated. Air screws will be roughly twice what the fuel screws are set at. Your fuel bowl level should end up between the bottom and top of the bowl rim. Measurements are done with the gaskets out of the carbs. If you are consistent, a fuel level sight tube is available to show where the exact level is. I would check it as the bowl fills to make sure the needle shuts it off at the proper level and then with the motor running.
 
I did recheck the carbs and they are all good. I'll try the fuel screws again, though I started them at 3/4 the first time I put the carbs on the bike and that was significantly lean. I also double checked the float heights when I was checking everything else, and I did set them with the gasket out. I'll try the clear hose sight and see what that gets me. Good call on checking as it fills; that should be interesting.
 
I have had similar experiences because the pods were actually blocking of the air pilot openings in the end of the carb, but with K&N i would imagine they are ok. The ones I used had a huge step right there that completely interfered with the pilot holes???? but worth a look?
 
Thanks for the tip, but that doesn't seem to be the issue here. I'll have to try out the other ideas this coming week and see what happens.
 
basics

basics

Gearhead,

Fellow Hoosier

I have rebuilt several GS carbs and can think of few problems other than something basic wrong in the carbs. Hate to be simplistic, ur certain all carbs are rebuilt correctly with exactly same parts? Is it poss u have fuel leak running down the line onto the brace and wicking into older intake boots or loose synch screws? Could u have a diaphram tear? You said u had o-rings changed have u double checkd the o-ring on the main jet tower? Just the most random ideas I could muster as u have covered the basics. Also i'm curious are u running a jet kit?
 
light bulb

light bulb

It just hit me. On the big mouth side there are brass air jets that if not tapered (made smaller) will force fuel out of the carbs, when running pods and a free flowing exhaust. Hence my question if your running a jet-kit, mine provided a reducer. I also think u would see better results with some type of baffled exhaust. Just a suggestion. gl
 
He has VM 26 carbs on the '79. I agree that something is inconsistent and doesn't add up though. If the carbs have been done right, maybe it is something strange like the previous owner parked the bike in the exact same place all the time and the sun dried out and ruined the plug caps on 1&2. The old caps definitely go bad in time. One thing I didn't think about is that with jetting for pipes and pods the vent hoses on either side of the vacuum port should be removed, but is not part of this problem.
 
OldVet66 said:
He has VM 26 carbs on the '79. I agree that something is inconsistent and doesn't add up though. If the carbs have been done right, maybe it is something strange like the previous owner parked the bike in the exact same place all the time and the sun dried out and ruined the plug caps on 1&2. The old caps definitely go bad in time. One thing I didn't think about is that with jetting for pipes and pods the vent hoses on either side of the vacuum port should be removed, but is not part of this problem.
Click to expand...

That is a great random idea!
 
when I asked if the air jets have been cleared I was asking if they have been poked out. I had an issue that was similar and it turned out that the air jets were slightly clogged; air would pass, but not enough.
 
Well, on a whim I pulled the carb vent hoses because they were the only thing I could think of that linked the carbs, and BAM it works correctly now. Any ideas why that would be? The hoses are clear and unkinked...
 
quick update-because of the pods, the carb vent lines were no longer in stable air behind the airbox. I had just zip tied them to the frame rail, as I didn't realize how big their impact was. The fix was to run the lines into the battery box area up under the cover that holds the tool kit.
 
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