Mixture control screws - advice?

I have rejetted my carbs with the dynomax stage 3 kit, and it says to tighten the mixture screws in until they seat, then back them out....

First off, the mixture control screws are on top of the carbs, right?

Secondly, I have 1 of them that won't budge, and the other will only move like 1/4 turn and sticks both ways... I'm using one of those "precision" screwdrivers like you use on cameras and watches and such. Should I put a pair of pliers on the screwdriver and force it? If yes, which way should I turn tighten or loosen?

Please note that I have already sprayed PBblaster into the little holes and let it sit over night. Still won't budge.

Experienced suggestions?

 

Mikuni or Hitachi carbs?

 

Oops. Hitachis on an 82 xj650.

 

This is scarry but easier if the carbs are off the bike. You can heat the area of the carb body containing the screws and the heat will loosen teh gum and varnish that is holding it in. The same is done for carb drain screws. I usually heat them with a cigarette lighter. On the mixture screws I would do this if they are on the bike. Better and safer to use a hairdryer or better yet a heat gun such is used for heat shrink. DONOT crank down on them being that they are made out of brass and can't take the stress.

 

Well, I got one of them loose. The other one is now mostly stripped. I worked and worked at it, but it seems to be crossthreaded or something... it will move righty, but very little lefty
Maybe the PO jacked it up.. Somebody has definitely been into these before.... :?

So I drove to my parents house and picked up my spare set of carbs from the trusty parts bike. I have removed the replacement carb from the "parts" carb rack, and I'm currently cleaning the carb body. I'm just going to pull all the parts (new jet, needle, and the working diaphragm) out of the one that is stripped and put them into the replacement carb, and try to get it on the rack exactly how the old one came off....

Any suggestions?

 

If your parts bike is an XJ 650 then the jets should be the same. If not then the jets should be swapped out too. You only mentioned jet and there are 2 on each carb. The main and pilot jets. If you can keep the same diaphraghms I would. Mainly bcause the slides have seated into the carb body and will be happier in their original place. The stuck mixture screw can still be removed with a reverse drill bit.

 

Update:
I managed to get the mixture screw out after another day of perserverance. I replaced the screw w/one from my parts carbs. She is running now, but not well.

 

I read somewhere that the practice of seating the screw and then backing it out a certain number of times is a very rough technique and shouldn't be used to permenantly set up carbs.

I confirmed this on my Mikuni's. I removed the brass cap from over the pilot mixture screws, and counted the number of turns in to stop on each carb. This varied from 3.5 to 2 turns. So, since the factory set the original correct mixture, I can assume that the correct setting can vary that much with a well broken in carb too.

I am going to be buying a colortune soon, just can't beat that bang for the buck (pun intended). If you have an exhaust gas analyzer, that would be good to set the idle mixture too.

What I did in the meantime is that I pulled my spark plugs after idling for a while, just to make sure it was running rich. If it is running lean at idle, it runs hotter, and it is possible (although unlikely) to do major damage to the engine.

 

That would be correct Richard. I've noticed while browsing info on mikuni carbs that some bike setups come standard with different size jets in the two middle carbs than the outside two.

I can only guess that it is a temp thing as the outside two cylinders have more air surface area and possibly keep cooler than the middle two.

Therefore mixtures may be originally set using these factors as well.

 

BlueMaxim how exactly does the reverse drill technique work? I sheared off the top of one of my mixture screws tonight ( after it finally moved in a turn or so and the bike wont run now of course) and need to remove and replace it.

Can I buy these screws?

 

Yes the screws can be purchased. I believe motorcyclecarbs.com has them. If not then try old bike barn. Here are the links :
http://www.oldbikebarn.com/index.php
http://www.motorcyclecarbs.com/
The reverse drill technique is a reverse cut drill bit that will bite into the top of the screw head. Since it turns counterclockwise it will bite and remove the screw. You can only go so far or you will damage the seat. If yours broke off and has no head left then the threads may be all on the piece that came out. In this case I would try to blow the remaining piece out with compressed air. This will of course require removal of the carbs and a tear down of at least the carb with the broken screw. Teardown being remove the float bowls and diaphraghm slides so that they are not damaged. Take a look at one here:
http://plan9.dnsalias.org:88/xj/photos/ ... C02481.JPG
The mixture screw has a spring and above that are the threads. At the other end of the spring is the oring and washer. So you have some travel with a small diameter drill bit. But if all the threads have broken off then a good shot of compressed air should blow it right out.
Pigmouse was at the NTCC and had some rather hard to turn screws too. I let him turn the last one on #3 so it would be his fault if it broke! Yea..I'm chicken! But we got his bike running! It took all day, and removal of the carbs which we normally don't do at carb clinics but he had such a puppy dog look on his face I couldn't turn him down. We also found out that his parts carbs had a different thread on the mixture screw. I wonder if this is why some are breaking? Some PO forcing the wrong thread into the carb body.

 

Good news. I got the screw out and it is even reuseable. I tried an "easyout" but the brass was too soft anf the easyout just chewed into the brass and spit it out. I then drilled small holes beside the central hole and was able to remove the screw. I will reuse it untilI can find a new one.

Thanks for the links and advice.

 

Good news indeed! Yep, easyouts eat brass for breakfast. However not all the mixture screws are made of brass. I have one in carb #4 that is either steel or stainless. Most likely stainless since it has been uncovered for years and shows no signs of rust. Ain't it scarey to drill on one of your carbs? I hate to do things like that.