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Cross threaded mixture screw

  • Thread starter Thread starter BoulderBiker
  • Start date Start date
B

BoulderBiker

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I've got a set of carbs that overall are in very good condition minus the fact that the previous owner had cross threaded the mixture screw on one of the carb bodies.

Any of you folks ever come across this scenario and have some suggestion on how to salvage that carb body? Hate to junk a good set of carbs over something so seemingly simple.

Thanks everyone.

BTW bout the possessed carbs on my running bike, they seem to be doing it much less and don't rev as high on the occasion they do, so whatever's stickin from time to time it seems to be going away, *shrug* go figure.
 
You're in luck, I just successfully dealt with this problem a few weeks ago! Get a 6x0.50 metric tap bit and ream it out while upside-down so the shavings don't fall in too far and clog the passages. Make sure you clean it carefully (while still upside-down) and get all the metal bits out before putting it back together (WD-40 on a q-tip works great, the metal shavings will stick to it). The new threads might not be the best, but this isn't a load-bearing screw, so as long as it screws in and holds at all, you'll be fine.

You can get the (*extremely* hard size to find) tap bit for just a few bucks at http://www.jlindustrial.com/ and a new mixture screw (if needed) at www.mikesxs.com. The new mixture screw from mikesxs might be longer but it'll work fine.

Happy fixing!
 
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It is still in the carb body, I have not tried extracting it. Kinda wanted to get people's thoughts before possibly making it worse.

Did you find that you had any problems with getting a good seal after retapping the hole or are your carbs synching fine and not having any problems?

Thanks a ton!
 
Oh, didn't realize you've still got it stuck in there. You're on your own for that part! ;-) Plenty of advice in the archives here about removing stuck fasteners. Hopefully if it's still got a good slot on the head you can just back it out with a little muscle (like I was able to do).

The carb I repaired in this manner is working great. You'd never know I did it. The screw action is smooth just like the other three and no troubles with the mixture at all. Just did a vacuum sync on the carbs and the bike is running great. AFAIK the seal is by way of the o-ring at the base and does not depend on the threads. Like I said, if your new tapped threads work at all, you'll be fine. My threads turned out great, though.

After having done that repair once, if I ever encountered that problem again, it wouldn't even bother me. It took a while to figure out what to do, what size tap I needed, and to actually find somebody selling that size of tap and replacement mixture screws, but the actual repair was a piece of cake and I have total confidence in that carb now.

Of course if the screw is sufficiently stuck, that's a whole different story. Stuck fasteners are THE WORST.
 
Awesome, I'll give it a go in the next week or two!! Its probably not stuck, I just haven't touched it since I got them from the previous owner. I'll post back here with my results.
 
Since its a mixture screw, I think your assumption that is likely isnt stuck is correct. I would try a battery drill running in reverse and about a 1/8" drill bit.
I would run the drill bit into concrete to take any edge off of it and roughen the tip. I would also either wrap the drill bit with some electrical tape so it would not further mess up the inside threads of the carb or run the bit inside a plastic soda straw or something so as to not risk destroying what is left of the threads.
With low speed and light pressure, I think you may be able to back it out.

Also, the carb body is aluminum and the screw is brass. If you put the bare carb body in an oven at about deg for 10 mins, it should expand slightly more than the brass screw. You may be able to turn it out with a small screwdriver if it is a bit tight.

Earl
 
earlfor said:
I would try a battery drill running in reverse and about a 1/8" drill bit.
I would run the drill bit into concrete to take any edge off of it and roughen the tip.

Earl
Click to expand...

[-X [-X [-X

The advice about being careful is good but dulling the bit and running in reverse is an invitation for the bit to walk causing all kinds of trouble. Better to drill super slow, with some grease on the bit to catch the chips.
 
The bit cant walk if you have it sleeved and you want to back the screw out, not drill into it.

Earl

Nessism said:
[-X [-X [-X

The advice about being careful is good but dulling the bit and running in reverse is an invitation for the bit to walk causing all kinds of trouble. Better to drill super slow, with some grease on the bit to catch the chips.
Click to expand...
 
earlfor said:
The bit cant walk if you have it sleeved and you want to back the screw out, not drill into it.

Earl
Click to expand...


:oops: Good points. I stand corrected although it's hard to visualize what good dulling the tip will do.
 
It should take only a tiny amount of torque to turn a mixture screw. Dulling the tip of the bit would increase friction.

Earl

Nessism said:
:oops: Good points. I stand corrected although it's hard to visualize what good dulling the tip will do.
Click to expand...
 
Earl I had never thought about using a straw to protect threads but that is a good idea.

One other tip might be to get a set of reverse or left hand drill bits that cut while in reverse. I use these when taking out the plug of the carb air screw covers.

It is interesting to dull the tip of drill bit. When I was installing helicoils in some mangled bolt holes, my brother in law, a machinist, told me use an old dull bit. His reasoning was that a new sharp bit would have too much bite in the soft aluminum and that it would want to screw itself into the metal before cutting.
 
Your brother in law is right. A sharp bit will have too much bite for drilling soft materials such as aluminum. A large bit probably wouldnt snap off when it grabs, but if a small bit grabs, it will snap. For drilling plastics such as plexiglass which will crack if the bit grabs, I always run the bit into concrete to dull it before drilling.

Earl

 
WHat I do is to use a drill that just fits into the hole using it on slow speed Drill just enough to get a pilot mark on the mix needle (instead of trying to center punch it) Then drill it with left hand drill bit 5/32 & #3 easyout
Sometime if the drill catches they will unscrew on their own with just the drill BE CAREFULL not to drill too far
 
So you're saying the screw is basically down in its hole about as far as the others approximately? I can't see how anyone could cross thread it that deeply. I can see cross threading a few turns in but not to within 1 or 2 turns (if set similar to factory) of bottoming. I think you have the typical stuck mixture screw.
If the screw head is in good shape to accept a good fitting tool, I have good luck by turning the screw in 'til lightly seated first, then I pick out any crud from the now exposed threads with a straight pin and add a drop of lube. Then I start backing it out.
If damaged or it won't turn at all...I don't know if you plan to do a complete carb cleaning/rebuild or not but if you look at how recessed the screw is and how its position compares to the others, the screw MAY be set very similar to the others and it may be best to leave it alone, pending testing. If recessed similar, you can turn in one of the other screws and maybe that will give you an idea of how far out the stuck one is from seated and if it's in its normal "range" of adjustment. I only mention this as a "lazy" option that may stop you from getting in too deep. If you want to clean and rebuild, you won't be able to do a typical cleaning job with the screw in there or replace any worn o-ring. You can still shoot cleaner up the pilot passage. Rinse immediately with fuel so the mixture screw o-ring isn't attacked.
Still best to get it out. But if you think the screw is adjusted close to the others and removing it may ruin the carb body, it may be best to leave it, test, and if testing says the screw or its adjustment is causing a problem, then try to remove it.
 
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