I got my G20ei with Walbro carb on it.
I don't like gasser's spring return throttle which seems to give a servo a burden.
I am using ROTO25 with that spring off the stopper so that the throttle moves freely
and it works with no problem.
I wonder why almost all throttle of gassers have that spring even though they are
used for only R/C. I don't think such thing is needed.
Anyone who knows why, please teach me.

Tsutomu Mabuchi

If you use a bowden cable operated lever, the return spring is nice to have. The servo lever can be situated, that at full throttle there is hardly any load on the servo, and at idle the throttle stop is active.
For a different setup, the spring can be unhooked.
You have the options to leave it in place or unhook it. That is good.

Thank you for the input. By the way, do you say air stream going into the carb produce
opening force to the throttle butterfly which reduces the tention to close?
Or do you mean a kind of click position of the butterfly at full throttle where it has less tention?
I might mainly use around 70% throttle during flight so the servo has to fight against the tention
at all times in flight. I will get the spring unhooked since I feel pity for the servo.

Tsutomu Mabuchi

If you position the servo arm so that it is at full travel when the throttle is open there should be very little strain on the servo!
Thats my opinion if you don't like it I have others

Larry S

The carbs are made for weedeaters and chainsaws..The spring is to keep the engine at idle until the trigger is pulled for more rpm...It also keeps the throttle disc centered in the bore..Without the spring the disc vibrates against the sides and wears out both the disc and the carb body...Enough wear and the carb won't shut off at idle....

It is ok to unhook the throttle return spring, buuuuuut don't take it off because the spring tho unhooked will keep pressure on the throttle arm and keep it from vibrating as mentioned.

The spring pressure on the throttle servo is insignificant. If a servo is having problems operating the throttle on an engine I would look for linkage binding and not even consider the throttle spring to be an issue. As RCIGN noted, removal of the spring permits the butterfly to bang side to side and wear out the butterfly and the carb body. It does more good than harm to leave it there, so best to leave it unless you like trouble shooting carb issues after a month or so of flying. Also, some manufacturers have been known to instantly void a warranty if they find that the throttle spring has been removed.

Thanks to all,
I now fully understand the purpose of the spring, not only for chain saw use but also
for anti-vibration.
Yes, the strain to the servo can be reduced by changing the servo horn angle to the
linkage, and less strain at full angle position.
I will leave the spring on the carb. All of your comments helped me very much.
The word " with that spring off the stopper" at my first question meant "unhooked".
Thank you again.

Tsutomu Mabuchi

I like the spring as a back up in case the linkage were to fall off etc. I think you are wise to leave it on. With the spring the servo is seeing very little load. The servo can handle it just fine, very little current drain.

Once measured the load on a DA50 carb. It is about 4-5oz whhen fully open. Even a mini servo can deal with it.

JoeAirPort and nonstoprc,

Thanks for following.

Tsutomu Mabuchi

I have found the spring tension on the carb throttle can vary greatly from unit to unit, some so hard they definately tax the servo, others quite weak and no problem. I have never had a problem as I always unhook the spring but leave it in place so no play on the sleeve bearings of the throttle disk.

you can look at it this way.. The spring is allot less then what you will have on a tail servo.

Just a little up trim will put alot more on a servo then the spring on the carb.. Well it seems that would be the case.

Paul

I called Mr.Kawazoe at Zenoah in Japan.
He said there is no problem on removing the spring out of G20ei carb.
And that the strain would still exist due to that the throttle shaft pushes
the accelerator frange spring when it rotates.
But those are just for G20ei.

Tsutomu Mabuchi