Topic: Air screws moving around from heating and cooling vibration

[PFribley]

Having trouble with the air screws moving around on their own. 1974 cb750. Bike runs good for a while. Then gets hard to start. Runs bad. Check air screws have moved from 1.25 turns out setting. Air screw looks like it might use an o-ring. Parts book shows none. Any suggestion on using some type of thread locker. Or solution to stop the moving around.

[ekpent]

 No o-rings on those if stock for the year but there should be a spring inside there where the air screw goes in. Never had any move before on many 750's I have had.

[evinrude7]

not sure what carbs you have.  the round tops, 657a's i have, have springs. 

[PFribley]

Yes springs are present. But I can feel no resistance from the springs. The air screws turn pretty free.

[evinrude7]

stretch and see if that helps.  if not, replace. 

[pjlogue]

Those springs should quite stiff and you should feel resistance after threading the air screw in ~2 turns.  You should feel the "slop" in the screw disappear when the spring starts to compress.

-P.

[PFribley]

Yes I agree. Or even hear the spring rubbing on the screw. Mine don't. The springs look like the right ones. Not too happy buying four rebuild kits to get four springs. Try heading to the hardware store.

[carnivorous chicken]

You can always dab a bit of silicone to keep it in place once you have it dialed in to keep the screw from backing out.

[PFribley]

yes. thought of that. or even blue Loctite. Parts list doesn't even show air screw and spring. Got wise idea  to order 4 springs. Not!!

[TwoTired]

IIRC, the stock Pilot screw springs are wound and have ground flats at each end.  This makes the end of the wire very thin at the flat.  Bending this very thin end outward provides a little "dig" into both the carb body seat, and the pilot screw spring flange.
Do this after you have stretched the spring slightly so the screw don't turn easily.  It should require enough force to break a thumbnail if you use it to turn the screw head when at the properly adjusted position.

Cheers,

[PFribley]

Purchased 4 nice used original keihin air screws and springs off evil bay. Strange. These air screws have a small bleed hole drilled through the screw just up from the end. What is this all about???

[TwoTired]

Those holes, along with the hollow tips are limiters, so you can't close off ALL the air to the pilot system path and make it totally inoperative.

Cheers,

[PFribley]

Thank you.

[PFribley]

Stretched the springs. Now resistence. 3/4 turns out running fine again. I am beginning to think that an o-ring and the small washer needed to be in there. That was why the slack!! What ye say???

[TwoTired]

Air screws do not need an oring.  Idle Mixture Screws do, as found on the PD carbs, as they meter a fuel/air premix, and could weep when the engine stopped, as well as alter air inlet past threads.

If air screw threads leak air, it doesn't matter as the tip is where the metering of air (only) occurs.  Any thread leakage would still be controlled.

That's my analysis.   And, I have no reason to suspect the carb designers made a mistake in that area.  They have about a million examples out there that work pretty well.

Cheers,

[PFribley]

Sounds good to me. Thanks.

[Airborne 82nd]

not sure what carbs you have.  the round tops, 657a's i have, have springs.

Maybe I missed it but still don't  know if you have the correct carbs for your engine. Thought you may be using the correct screw on the wrong carb bodies.