Mikuni Mixture Screw fun times

So, just as every other XJ rider knows, the carbs can be more fun than a massage by Edward Scissorhands. Particularly the mixture screws. Three of my screws came out easily with just a small amount of pleading and ginger manipulation of the screwdriver, but as luck would have it, the fourth one would not cooperate.

Long story short, I accidentally stripped the screw head in a way that I could turn the screw in further, but could NOT turn it out at all.

I ordered a replacement set from Chacal and then went to town with a drill and a left handed drill bit. I immediately cleaned it out with some carb cleaner and went to install the new set with delicate care, feeling a great deal of relief....

but it was short lived.

The screw would not go in past a certain point; roughly three full turns in from the top.

It looks like the threads in the carb body are messed up. I don't want to mess up the new screw by getting it stuck and warping the head. What are my options here?

 

Not sure what size but you could grind off a tap and run it in there. I think that Rick O Matic has done that before with success.

 

you could try to do this to one of your old screws
or when someone tells you what size it is buy a plug or bottom tap and clean up the threads
try ace hardware or mcmaster-carr
no starter taps
be careful

 

I sell those taps, but because it's such an odd size, they're darned expensive ($30+)..............better plan might be to find a local machine shop, which probably has that tap as part of a master kit, and throw $15 at them to do it......it takes all of 3-5 minutes to do (but of course, if the threads are destroyed, a tap may not solve your problem).

In which case, a heli-coil might, but again, that's a really odd sized thread.

In which case, I have good used carb bodies available..........

 

what odd size are they?

 

Really odd.....m6 x .50 thread pitch.

 

You're going to have to run a Tap down in that hole.

A Flat-bottomed M6X.5 Bottom Tap.
Very lightly grease within its side channels to capture the aluminum and brass as it repairs the threading.

If the Tap stops. Bring it out. Shoot it clean with Carb Cleaner and start again.
Once you get to the area where the Tap will be cutting its way down through the stripped or clogged area ... you Cut a little and bring it out a bit and "Exercise the Tap left and right" making new threads.

Look for what you need online.
DON'T Break-off a Machine Tool in the Hole.

 

Oh good... I'm glad it's the middle of winter, otherwise I'd be pretty upset about this situation. Time's on my side though. I'll look around for a machine shop in the area. Thanks for the info!

 

Been there and done that!

I went ahead and ordered the tap - cheap insurance if you ask me. When my carbs were all apart last summer, I went ahead and cleaned up the threads on all four. Now I have happy mixture screws that go in and out just like they're supposed to!

 

Before you order the Tap ... Verify that the Screws Thread Pitch.

One Mixture Screw has a more Coarse Pitch than the other.

 

All of the Mikuni mixture screws are those .50 fine threads.

The early 1980-81 Hitachi carbs use a coarser thread pitch on the mixture screws; all the later Hitachi screws are also that very fine .50 thread pitch.