I have a Saito .72 in my Funtana and noticed it progressively became harder to tune, I would hover and hold it there for a bit and it would start losing power and backing down, even when I pushed it wide open. Land, re-tach it and tune it, to see that it was still on and ran fine on the ground, but after in the air for about 2 minutes if I pull vert it would go up about 100 ft then start surging, then start falling off all together, no matter what I did with the needles. Valves tight, fresh fuel, inline filters and all. Well, I found while touching the low speed needle it would change the rpm... not turning it, just touch it. Come to find out the little spring loaded screw that holds the carb throat barrel in had vibrated out about 10 turns and while wide open I could slide the barrel back and forth about an 1/8" in the carb body because of it... moving the mixture tube over the squirter and changing the mixture. While wide open, once the engine is hot the barrel lossens up a touch more and slides in and out with the vibrations of WOT and cause the surging... then it would load up and the vibrations would slow down and it would stop moving in and out and just stay out now, exposing too much needle and fall off to about 5000rpm until you pulled it back to idle, push it wide open and start over again. This baffled me for weeks and finally figured it out so I thought I would post it in case it may save somebody else the frustration sometime Was just about ready to order a new .91 for it!

Abraxxas ....

....I saw the same thing a few weeks ago at the field. A guy
and his girlfriend had traveled a long distance to get out
there and fly his new plane. He had another plane that was
OK, but the darned 4-stroker would not run worth a hoot.

Finally out of desperation, he asked me to have a look at it.
I opened the needle way up, and it fired....but the carb barrel
was moving in and out about 1/4", mainly due to vibration.
The "barrel guide/idle stop" screw had backed all most all
the way out, and the darned throttle linkage was the only
thing that was really holding the barrel in....[X(]

We repositioned the barrel and ran the guide screw all the
way in, backed it out a couple turns, and locked it down
with the locknut. After a few adjustments I got the carb
back in the ball park, and it ran fine.

But....good point.....If that barrel is moving in and out....
you can adjust that carb forever........plus, if you lose
that screw....you might be SOL. []

Dave.

I saw this problem a few years ago in a OS Goldhead .60 rear carb. engine.
Not a new problem but good information..

Ok,, so it was 1975 vettdvr

FlyboyDave, have seen the barrel guide screw back out on both my Saito 72s.
Have fixed this by gooping the threads with blue Loctite, and this seems to work.

I've noticed there is still some minimal barrel end-play in the carb body.
It is definititely enough to impact the low speed performance if I touch it.
I've noticed some variation in flight at low speed upright/inverted.
Both engines are mounted sideways on profile planes.
(i.e. Barrel pushing outwards upright, inwards inverted)

Has anyone seen a need for a stiffer carb barrel spring?
Idea would be to keep the barrel under tension in the body so it doesn't move around.

Usually when the needles start falling off on 4s and they don't seem to hold a needle setting, the Valves are out of adjustment. I did say usually!!!!! [8D]