Pilot Mixture Screw thread size?

I'm working on my XJ650 4k0 project and I have my Hitachi carbs apart for cleaning and refurbishing and during re-assembly I am finding the pilot mixture screws feel uncomfortably tight when I preliminarily fit them minus o-ring washer and spring. As a precaution I'd like to run a tap down the thread. Please can anybody confirm the thread size. I am measuring 6mm x .5 pitch with my thread pitch gauges but I would feel more confidant if this was confirmed.

 

Well, as no-one has been able to help me on this one I've brought a pilot mixture srew into work and put it in the Shadowgraph. It projects an image onto a calibrated graticule at about 30x magnification. I was then able to accurately measure the pitch of the thread. It's M6 x .5 pitch. The tap and die are now on order and should be delivered by the time I get back from a 2 day trip to Glasgow to see Fleetwood Mac. Happy days!

 

M6X.5 "Bottom-Tap" (Flat-bottomed Tap)

A Pointed "Through-Tap" will ruin the lower orifice.

 

before you run that tap in there you should try a fine wire brush oh the screws and a small bottle brush on the carbs. what's the "H" number of the tap? it could leave you with permanently loose screws and fine threads like that are easy to cross thread with a tight tap

 

Cheers Rick, I've ordered a plug tap, oh and thank you very much for the massive contribution of technical help you have provided to this site. It has been a really huge help to me.

Thanks for the input polock, I'll do a test hole in a scrap piece of aluminium and check the fit before I attack the carb.

Vacuum gauges and colortune just arrived yesterday so I'm looking forward to fun and games very soon.

 

Cheers Rick, I've ordered a plug tap, oh and thank you very much for the massive contribution of technical help you have provided to this site. It has been a really huge help to me.

Thanks for the input polock, I'll do a test hole in a scrap piece of aluminium and check the fit before I attack the carb.

Vacuum gauges and colortune just arrived yesterday so I'm looking forward to fun and games very soon.

 

Sounds like a good Saturday night project. :twisted:

 

If your idle mixture screws are already tight from day one even without O-rings and springs, then I bet someone has already cross-threaded your screws for you.

I'd be very very careful with a tap down there. First off, if the screws have been cross-threaded, then you want to make sure you chase the first "original" thread instead of the newly created bastard thread from the improper insertion.

And then as mentioned by others, the tiny hole at the bottom of the screw hole is very important. And not only that, but the smooth surface higher than that is important for the O-ring seal. You must insure that you do not run the tap in deep enough to mess with any of that stuff.

 

after some investigation, metric taps don't use a "H" number, they use a "D"
number.
http://www.rntap.com/tap-technical-info ... imits.aspx
we really have no way of knowing what size your screws are so, try this.
put some rubbing compound or tooth paste on the screws and run them in and out a few times, clean them off and see how they feel then.
i'd use that tap only as a last resort

 

Problem overcome! Thanks for all of the help and suggestions. I only had to screw the pilot screws through the die, clean them off and hey presto, they all screwed beautifully into the carbs. I then removed them and installed the O-rings, washers, springs and finally the pilot mixture screws themselves, with a tiny smear of silicone grease. All felt nice and smooth all the way down and back out by 2 1/2 turns as a starting point for tuning. So much better than before. No need to run the tap down after all. Oh, and Fleetwood Mac were fantastic.

 

Good. I was worried about the whole tap thing...

 

I see you backed out the Pilot Mixture Screws 2 1/2 turns. Did you keep them there or was more adjusting needed? How do you know at what point to leave them alone?

 

Final tuning is done with a colortune but only after the carbs are vacuum synced and all other essential checks and adjustments are made. I think I ended up somewhere between 3 and 4 turns out. I suggest you search for carb tuning posts by the great RickCoMatic for more detailed information.

 

I want to throw my $0.02 in here as a warning to anyone looking to chase the threads on the pilot screws.

In the U.S. models the threads changed between '81 and '82. Yours may not be the same as his.

 

That explains why the el cheapo Taiwanese repair kit I bought on ebay had mixture screws with a course thread. Only parts I ended up using were the gaskets, lesson learned.