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Jammed Air Mixture Screw. Tips?

P

patrickhimself

Forum Apprentice
'79 GS750L is the bike.

I was cleaning my carbs in an attempt to get ready for spring. Getting everything put back together on the last carburetor and my mixture screw gets jammed like it is cross threaded. After a few tries to get it seated properly I'm stuck with a bit of a mangled mixture screw (just the head) and (somehow) a tore up o-ring.

Soooo...what do I do here? I don't see anything jammed in there. Can I retap the hole? Or am i going to need to buy a new carb for this thing?
 
Pictures are worth a thousand words on this one. Are the threads in the carb body obliterated or are there still some good threads? Are the threads shot on the needle or just partially? It all depends on how bad you buggered up the threads.

If you only messed up the first couple threads in the carb body, you might be able to clean them up enough with a small dental pick to get the needle threaded properly. Harbor Freight taps run to the large size for fit and leave you with loose threads more often than not, so I would use a good bottom tap if you can find one in the proper metric size, with lots of oil.

I am sure that was clear as mud.
V
 
Good luck finding a tap for that extra fine thread. I haven't been able to. Maybe someone else will chime in with a good source.

If the mixture screw is still in the hole, and the slot is damaged to the point of no usable slot, then you may have to use a dremal tool and cut a new slot in it by cutting straight down the top of the tower. You may try to use an EZ-Out too, but that could leave you with a bigger problem if you break off the EZ-Out while trying to extract the screw. If you have the screw out, then I'd just pick up a new mixture screw. I think that these tend to jam up because the thread pitch is so fine and just about anything can jam them up. Clean out threads with soap and water and use a Q-tip swab. That may remove the small particles that could be jamming things up. Inspect the threads and needle for any signs of crud. I think the factory used some thread lock on the screws. It's the main reason I found that HEAT is your best friend. If the O-ring is already shot, then don't worry about using too much heat. Unless you're using a welder, you wont do any damage by applying heat to that area. Just make sure the rubber diaphragm is removed before you apply the heat.
 
I'll try and take some pictures when I get home tonight.

The mixture screw isn't stuck in the carb. It only goes about half way in and sticks. I'll try and see if I can get at the threads in the carb body with a pick or something. They don't look too bad, so hopefully it is just dirty...

JTGS850GL, what do you use the heat for? Burning off residue?
 
Amazon sells taps for these. I think they're only $10. It's a fine thread on the tap. I would only use a tap to correct the threads. I'd recommend you use oil when chasing the threads too.

I had the same problem. The review of the tap I bought from amazon was even from someone fixing an old Mikuni carb.

Apologies, but I can't remember the size tap I bought. Could dig it out later if your desperate.

Ollie.
 
The heat would be to expand the carb body to extract the screw. Since the screw is not stuck you do not need it. Use a magnifying glass to check the threads that you are having trouble with. If the jet does not screw in easily, then something is wrong somewhere.
V
 
Oliverstrother said:
Amazon sells taps for these. I think they're only $10. It's a fine thread on the tap. I would only use a tap to correct the threads. I'd recommend you use oil when chasing the threads too.

I had the same problem. The review of the tap I bought from amazon was even from someone fixing an old Mikuni carb.

Apologies, but I can't remember the size tap I bought. Could dig it out later if your desperate.

Ollie.
Click to expand...

Thanks Ollie. I'll take a look.
 
tkent02 said:
I wouldn't give up on it yet. Try the tap.

If not, I have several extra VM 26 carb bodies.
Click to expand...

I tried the tap, it slipped right past the threads. I could try a size larger, but I'm afraid the screw won't fit properly then.

Let me know what you'd want for one of those carb bodies. It's the right most carb on the bike.
 
might be the washer or o ring they are hard to see

might be the washer or o ring they are hard to see

That might be to obvious but can you "see" light at the end of the tunnel?
 
No, the o-ring is out of there. If you look at the first image, you can see that the carb threads are ripped out of there at the top of the hole and flattened out deeper in (vs, the picture of the clean threads). I have no idea how this happened. The screw came out just fine...
 
Got a new carb body from tkent2 (thanks again, Tom!). Got it installed last night and had just enough time to check that the bike started up and ran. Will synch tonight and—hopefully—ride tomorrow.
 
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