Truckracer

My Feedback: (19)

Now days, general opinion seems to support leaving the return spring in place on Walbro type carbs as there is little to be gained by removing or disconnecting it and quite a bit to be gained by just leaving it as supplied.

I've attached a spreadsheet that was prepared some time ago. It shows how little power is actually required for a variety of servo types and models to hold against a Walbro throttle return spring. Some people felt the spring would overload the servo when in fact and by comparison, just the weight of a control surface pushing against the servo when the airplane is at rest can cause a servo to draw more current than the spring does on a throttle!

[ATTACH]1996565[/IMG]

airraptor

My Feedback: (66)

I still remove my springs. Some say that the spring will take it to idle it the linkage fails but what about the other glow engines they don't have a spring. Good or bad I like to take it off because why have the extra force there even if its little. I also hate stiff/hard to move control surfaces. They should drop on their own weight. On larger planes with gas engines I counter balance them.

Did you bring this up start another long thread on this?

Truckracer

My Feedback: (19)

No, I was asked to post this again. Also, lots of new people getting into gas engines and a few facts are nice to have over tired old opinions. Hey at least I wasn't bringing up oil or ethanol in gas!!

earlwb

A standard servo should work fine on the Walbro carb even with the spring in place. I tend to replace the little throttle arm with a longer more useful length arm though. I also tend to not use the spring, but I like to leave the spring on as sort of a spacer to that the throttle doesn't move around as much. Now if you are using a straight length of wire or a pushrod to hook up the throttle with the servo, then using a better servo may be wise. With some airframes the engine vibration can put a lot of back and forth stress on the servo causing it to have problems. Using a flex cable may be better in that case or Nyrod even.

Oh yeah, people are now starting to use the spring to their advantage. You simply use a thin cable to hook up to the servo and let the servo pull the throttle open and the spring forces it closed. So that the servo is only pulling and not pushing then.

Granpooba

My Feedback: (5)

Yayyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy to the oil, ethanol / gas !! But, good question for folks not in the know.

airraptor

My Feedback: (66)

that string idea is pretty good idea. i use straight wire push rods but use a dubro servo saver on them. It allows the wire to slide thru the quick connector but has springs on either side to soak up the vibration. if i cant use a straight wire then I use golden rod with Sullivan plastic ball links.

speedracerntrixie

My Feedback: (29)

I cut the spring when I use Stihl HP Ultra but don't when I plan to run Amzoil. Ok all kidding aside, I have always clipped the piece of spring that engages the throttle arm. This keeps a slight side load on the butterfly to keep it from vibrating. I had been told that side to side play could lead to erratic idle.

ahicks

My Feedback: (2)

My tendency is toward some pretty small servos (12-13g w/metal gears), so for that reason, I unhook the spring but leave it in place. If this costs me a carb in the future, so be it.

bjbellino

My Feedback: (176)

Personally, I unhook the spring tension on the throttle arm, but leave the spring in place as a lot of other responders have said to retain the spring as a spacer to keep the throttle plate from moving side to side. I think that the notion that leaving the spring tension intact might help bring the engine back to idle in case of a throttle servo failure, to me is a little iffy, because if the servo breaks a gear tooth it could lock-up and not be able to move, I don't think I have ever seen a servo that went bad for whatever reason, freewheel when you try to turn the output shaft afterwards. I think the engine manufacturers keep the spring tension on their engines as it is a standard feature on Walbro carbs and they don't make the carbs just bolt them on their designed engines, and taking them off would be modifying a possible safety feature, possibly opening them up to be liable in case of bodily injury crash. This return to idle tension spring aids finger operated levers to return to idle on such applications such as weed eaters, etc., that uses walbro carbs, I would guess other carb manufacturers also employs the tension return springs to broaden the possibly applications of the carbs.

ahicks

My Feedback: (2)

I think the idea of having the spring return the throttle to idle has more to do with a failed linkage scenario.

lean-run

Thanks for "re"-bringing up the issue Truckracer. I'm new to gas engines and always wondered if the throttle servo would be in an almost stalled situation.
Having listened to some fliers of gassers and articles on forums, I have ended up with a set-up much like described by earlwb. i.e.: spring on - longer arm on carb - fishing trace cable
to pull throttle on and let spring return.

Cyberwolf

My Feedback: (4)

I go with the spring in place as designed with a longer arm, I have never had an issue yet. I have seen what removing the spring does to the carb after a while, wears the snot out of things and renders it useless.

I don't get it some folks seem to think that small amount of spring pressure will destroy a servo, but never give a thought to how much wind resistance there is against a flying surface the same type of servo is trying to move in the air.

Bob Pastorello

My Feedback: (198)

cyberwolf - I'm glad to see your testimony about leaving the spring on the shaft, even if tension is not removed. The wobble/damage/running problems that develop VERY quickly if the throttle plate can move sideways in the throat is a very real problem.
___________

I prefer to remove tension, but mainly because I'm old, and it's an old habit. The "pull cable" configuration leaving the spring tension hook in place, even with a longer arm, has been used by some pretty high falutin' name brand pilots through the years, not the least of whom Quique Somenzini. On a plane with a problematic throttle rod configuration, a pull cable can be a great solution, routed over a pulley or two if needed.

airraptor

My Feedback: (66)

The carbs are unchanged from the chainsaw days. Can anyone show pictures of these carbs wearing out because of the missing spring? On all the carbs I have seen there is no slop when the spring is removed so I wonder how it wears out with it removed.

Truckracer

My Feedback: (19)

Sorry I don't have pictures but I can assure it can happen and I have seen the results. Not on all Walbro carbs though, some have positive mechanical means of centering the throttle shaft in the bore where others depend 100% on the spring. The throttle plate wears into the softer aluminum bore near the shaft allowing air leaks, rough idling, inconsistent idle, etc. One example I saw allowed almost 1/8" end play on the shaft at full throttle.

I still don't see any issue leaving the spring in place. It certainly doesn't abuse the throttle servo in any way. Now if you're using mini servos, there could be an issue.

Nhalyn

The carburator command is a very important one ! I've seen the result of a loosen command with half power non-controled engine in flight until there is no fuel left indide, that can last some loooong minuts with lots of problem to deal with...

On walbro carb we have what I consider a security system : a meaning to close down the carb when we break the command. It have to be in place and functional !!!

The stress on the servo is very small, uncomparable to a rudder or an elevator... But as the carb command is a important one, you to put a serious servo on it ! Why put a mini servo on a big plane ??? To gain 10 or 15 grams ???

airraptor

My Feedback: (66)

Nhalyn, why carry extra weight if you dont have to. it is a hobby and do as you please. We all have reasons why we do things and we think they are the correct way. I try to not force my ways on people but sometimes I do.

My thoughts on the spring is that it wasn't put on there for us RC flyers. The spring was on there for the Chainsaw and weed trimmer engines where they had a cable pull set up and was a safety feature on them. The carbs never changed when installed on our airplanes.

Now most of us had started out with Glow engines and they dont have spring returns to idle so what do when do if the throttle rod breaks or servo stops??? nothing we just fly it till it dies.
Now for the gas engine argument on the spring as a safety feature so it will go to idle if something happens. My other thoughts are most of us have a transmitter kill switch installed. So if the throttle control breaks we just kill the engine. A plane will glide better with engine completely stopped than one at idle.

I will always cut the spring.

willig10

If this has not been answered:

Here is what you do. You leave the spring but take the tang and place it under the throttle arm. This way the return spring does not fight the servo and it leaves the butterfly where it needs to be. Very simple and bottom line is you dont need the return spring hooked up. Just need it in place to keep the butterfly centered. That is all

GlennI

MTK

I leave the spring in but change the attachment from higher tension to lower. Easy to do. I simply move the clip to the large hole on the arm, the one close to the shaft, reducing spring tension at least 2-fold. As far as servo power, I'm using Savox micro servos for throttle with less than 20 in-ounce torque capability so it's a non-issue for me

airraptor

My Feedback: (66)

The carbs I have seen with the spring removed, showed that the butterfly didnt move around. Also if the spring is there to keep the butterfly centered then shouldn't there be another spring on the other side?

MTK I use HS-65HB servos of around 28 ounce of holding torque. big ones not needed.

Bob Pastorello

My Feedback: (198)

No. The venturi is shaped by design, and nearly all I've seen are not "on center", and neither is the throttle plate. There is angular movement laterally across the venturi during the movement of the plate. If the spring is missing altogether from the shaft on the exterior of the carb body, that permits the throttle plate to then shift during rotation, and coincident with the vibration frequency. The damage seems to be most severe in that it alters the opening of the primary jet and the air flow/gap around the plate.

Symptoms usually manifest themselves as a very inconsistent idle, and transition up from idle becomes erratic, and is then related to the rate of rotation of the carb arm. That first chunk of opening from idle is the most critical to the performance and consistency of our gassers. Any slop in the venturi when that plate is coming up toward that first jet orifice really, really makes a big difference.

irocbsa

My Feedback: (12)

Yep. My throttle linkage came disconnected once when I was barely two minutes into a flight. Thirty minutes later I was lucky enough to be able to deadstick the plane to the ground safely. I shudder to think how nerve racking it would have been if it came disconnected at 20% throttle instead of WOT. It would not have been fun to try and keep a heavy warbird in the air with barely enough power to keep it flying.

pilotpete2

An even worse possibility for your scenario. At a low throttle setting, but one high enough to keep the plane in the air. You could then take so long to drain the tank that your transmitter battery could die first
While the gassers I fly are far from the giant scale class (15-30cc) I do feel better having an ignition kill on the radio. I thank Truckracer for posting that chart. I do keep the throttle spring connected on my gas engines. I feel it helps to reduce vibration in the throttle linkage. Many insist on using metal gears on their throttle servos, keeping the spring connected, will somewhat slow down wear on the gears by keeping the gear train under a gentle load. That will tend to damp out some of the engine induced vibration in the linkage. At least that's my theory
Pete

Bob Pastorello

My Feedback: (198)

And disconnecting throttle linkages, failed servos, etc. are also one of the REAL reasons to have a transmitter-operated kill switch. The scenario of "failure before fuel tank can run dry" is one that I have personally experienced. At 1/4 throttle, a plane will fly a LONG time....but not when you are able to flip that Kill Switch and deadstick it in.

irocbsa

My Feedback: (12)

Agreed, and I know that point was going to be brought up, but opti-kill switches just strike me as another point of failure in the system. I seem to recall a quite a few guys posting up in this forum about ignitions that wouldn't spark who eventually traced it back to bad opti-kill switches. Not trying to start a debate on them, only explaining my reasoning.