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29mm mikuni air screws

  • Thread starter Thread starter kahuna
  • Start date Start date
K

kahuna

Guest
I had a leak in plugs that are in the vacuum lines to carbs 2.Once I replugged the lines I had more power but she was popin and fartin, so I tried to adjust the air screws and totally got lost.It is running but she has a flat spot at initial excelleration then she gets it,then she starts popin.Am I heading for the shop and vaccuum gauges or is there an old school way to get a starting point on all 4 screws then maybe work by carb # to # @ certain rpm?any tips welcome.1983 gs 750 thanks,kahuna
 
Somewhere around 2 turns out is a good starting point maybe 2.5 if you have a pipe for stock carbs
29mm Do you have smooth bores???
 
reply to sqdancerlynn1

reply to sqdancerlynn1

hey,We've talked before couple years back,good to see your still gs.yes on the smoothbores.Just to be certain on starting point, is it still 2 turns?,and screws out equals more air, as screws in equals more gas.I will also read my plugs but I always worry about runnin to lean as its not to healthy for rings.
 
Keith made this post a while back re 26mm VM's tuning the 29's would be similar
Keith’s post
Stock carbs (U.S.) are VM26SS. I don't know if Canada bikes got 26mm or possibly 28mm? I do know 33mm are not stock anywhere. Sounds like your bike is starving for fuel above 6,000 rpm's. The jet-needles need to be raised and the main jets increased and probably the pilot jets increased too as I posted.
Your jet-needles are inside your slides.
As for the screws...the PILOT screws are reached from underneath and are engine side of the floatbowls. Use a mirror too see and adjust them. Turning them OUT (counter-clockwise) richens the mixture. (i would start about 1 1/4 turns)They can be tight if never moved before and it's easy to strip the head off. Use a good fitting tool. They are sensitive to adjustments and even 1/8 turn can change the sparkplug color. For a starting point regarding the pilot circuit, I would adjust each pilot screw out 1/4 turn and test. When plug reading for the pilot circuit, ride the bike a few miles in 4th or 5th gear at about 40 mph. Just have the throttle open just past "idle". Do what the plugs/performance tells you. If you find yourself adjusting these screws more than a full turn out further from where they are initially, then you most likely need a step up (2.5) in pilot jet size and of course turn the pilots back in to where you started. You may even have to try larger pilots and turning the pilots out a little. It's suggested you start with fresh plugs or at least clean yours well.
For the SIDE AIR screws...adjust as follows. Put the bike on the centerstand and warm it up first. Turn the screws on both carbs facing you in 1/4 to 1/2 turn increments until you achieve the highest possible rpm's. Fine tune these two screws. Do the other two screws the same way, until you achieve the highest rpm. Now re-set the idle to 1,100 rpm with the idle screwknob. They usually end up about 1 1/2 to 1 3/4 turns out. Turning these screws OUT leans the mixture. Careful, these screws strip easily too if never moved.
Do the pilot screws first and then the side air screws. Jet needle adjustments must be followed with a vacuum tool synch. Then test.
A good carb synch is important with these carbs and re-jetting is too difficult without a good synch. Raising the jet-needles is the most work.
Test for the jet-needles at 1/2 throttle and the mains at 7/8 or wide open throttle.
I forgot to mention, remove the two float bowl vent tubes so the bowls vent better. The increased air intake creates a vortex in the tubes and this cause fuel starvation. Removing the tubes helps the jets draw fuel easier. This is recommended by jet kit makers regarding stage 3 kits, especially with VM carbs.
_________________
And on the seventh day,after resting from all that he had done,God went for a ride on his GS!
 
air screws

air screws

Mentioning the main jets reminded me that the guy I had put these 29 smootbores on mentioned something to the effect of having worn throttle bodies so instead of using the suzuki gs set up with 2.0 slides and 120 mains I believe he installed the kz set up of 1.5 slides and 125 main resulting in a very sensative tune.I will try to set the airscrews as mentioned ,but I have that feeling I won't be happy and will take her to someone else and try to install the gs set up over again with the 2.0 slides.Thanks for the reply.
Aloha Mr.Hand!
 
Your smoothbores don't have any pilot fuel screws (the ones underneath and engine side of the float bowls).
As for the side air screw adjustment, you always adjust these using the highest rpm method. A correctly jetted bike will end up with these set between about 1 1/4 and 2 1/4 turns out.
By the way, to improve on the previous older post... be sure the bike is warmed up completely and ignition timing is spot on. Set initial idle to about 1,000/1,100 rpm's. Adjust one carb at a time for highest rpm, set, then lower the rpm's to initial starting point if needed by using the idle adjuster knob. Repeat to the others.
Some people can't hear much rpm change one carb at a time and it may benefit them to quickly adjust "two screws at a time", the two facing you, to more easily hear the rpm changes. One carb at a time is preferred and more accurate but if you're one of those who can't hear well...this helps. All four air screws should end up very close to each other after correct adjustments.
 
air screws

air screws

Thanks,setting the idle from 1300 to 1000 before adjusting from 1 1/4 was a big help as she was more responsive to the adjustments.Got good top end now but out of the hole still needs help.will try again later after windy cold front passes,it might be easier. out
 
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