surging in flight
 

2 clubmembers & me built 3 of the RCGuys Treehoppers. It is my first gasser. We have 3 identical conversion 26cc engines. All 3 engines were brand new. 2 of us converted to EIC & one remains magneto. We all changed from the stock carb to a Walbro WT. Mine has a problem.
In flight it surges or pulses & seems rich. On the ground it runs perfect. Last nite we stayed till dark working on mine. We put in a rebuild kit, tuned carb. Sounds great. good idle, transition, and hi rpm. Once I take off it sounds good as it's climbing out, once I bank around & level off it starts the surging at full throttle. I can reduce to mid throttle & it seems OK. We made minor adjustments the carb multiple times last evening. I've read about the vent hole, & pressures etc. I do NOT have a cowl, can this still be a problem. If so why do my friends engines not do the same.

My carb is a Walbro WT-45A. one guys is a WT-645, not sure of the others.

I noticed at home last eve that my throttle linkage (ball link) is very sloppy. I wonder if under a no-load situation (such as level flight, the plane is a floater) the butterfy might be moving around. on the ground it is always under full load with the plane fixed as it is also under a full load on takeoff.

I'm planning on soldering a tube to the vent hole & running a hose into the fuse thru the firewall to see.

Driving me crazy...almost there anyhow.

Ken

RE: surging in flight
 

Tighten the linkage up and eliminate that. I had a G62 on an Edge with no cowl doing the same thing. Put the vent line on and no problem.

RE: surging in flight
 

You might also look for an air leak around the crank case. That would affect the pulse pressure to the carb. If that were it, it sounds like a very minor leak since it takes full throttle to start the surging. A shot in the dark!
Edwin

RE: surging in flight
 

without a cowel you dont need the vent tube. If if you put the airplane in a climb and it doesnt surge and it does surge in stright leval flight three things I would check is 1 is the prop unloading (try another prop less pitch) 2 adjust the low end 1/8 turn at a time (bring it down to half throttle and adjust to max rpm and then lower it some more and adjust it again keep doing this untill it is all the way to idle 3 check the carb gasket might have a small air leak there michael

RE: surging in flight
 

MrB
I will definately fix the slop

Edwin,
at full throttle on the ground it doesn't surge at all. I will check anyway

Michael,
don't understand the term "prop unloading" but will try again to tune as you describe.
BTW, my prop is 17x8, we flew a buddies last nite which I set up for him, it had an 18x10 (same engine & plane) ran great.

RE: surging in flight
 

I think it could be too lean on the high end needle. Rapid surging of engine RPM is the tell-tale sign of a lean engine.

It doesn't surge on the ground because it's under a static load. It's got enough fuel.

When you take off, the prop unloads and turns more RPM because it's moving through the air. It's now a dynamic load. It's moving, so it'll turn more RPM.

If it's turning more RPM, it'll need more fuel.

Fatten the high speed needle 1/8th turn and try it.

RE: surging in flight
 

I might be wrong but I think its where the prop does not put a load on the engine but kinda free wheels due to the low pitch I think that was how it was explained to me years ago. I use ryobi 31cc with a 18/8 and tuned the engines the way i said and works great. I read it on a post here on rcu and it worked for me michael

RE: surging in flight
 

Try switching props with your buddy, see if the problem goes away or switch to his plane, then go from there.

RE: surging in flight
 

not so easy. his has a 10mm hole & mine has 8mm hole. enines are same but prop adapters are differrent. I could bush his down I suppose.
I plan on working on it tonite but won't be able to get in the air till Thurs eve. Think I'll do one thing at a time so I know what fixes it, although that will drag it out a bit.

I am also thinking of switching to his carb to see.

RE: surging in flight
 

you did not say which plane has the magneto. Magneto's on garden appliances often have rpm limiters. Combined with a smaller prop you may easily run into the limiter.

RE: surging in flight
 

sorry! the one we flew last nite has the magneto. It flew great. He purchased a brand new carb & b&b muffler. It ran sweet. Mine actually had a magneto at first & flew well as I recall but I had my electronic igniton ordered to make it lighter. Had an incident broke the prop so I waited till I got the ignition installed before flying again. thats when I started having problems. I cant think of any reason the ignition would perform differently in the air...but its my first gas engine.

RE: surging in flight
 

Or switch carbs AND props? That would cancel the main differences. If the problem remains, the engine might be sloppy in the bearings or something like that, so that at higher rpm, air can bypass the carb system, or the pulse line from crankcase to carb is obstructed / loose.

RE: surging in flight
 

Well GOOD NEWS!!!:) Last night I soldered a pc of brass tubing to the diaphram cover & run a pc of fuel tubing thru the firewall & into the tank area of the fuse. I started it, tweaked the carb a wee bit & flew several flights. Not once did it surge. Absolutely blew the guys at the field away, who would have thought an 8" pc of hose could help so much, we all learned something for sure. This first gas engine has been quite a learning experience but now well worth it.

The other 2 guys have 18" props & seem to out power me. (we had 2 of them up together tonight) I am gonna get a bigger prop, but it runs GREAT!

Thanks guys for all the input...that's what it's all about.

Ken<><

RE: surging in flight
 

That's why I lately make a static reference tube in all my gassers.

RE: surging in flight
 

yeah, I did it on mine just for good measure, much easier to do when the engine is on the bench instead of in the plane... mine was a little different as I was using the carb off a G230 car engine that has a primer bulb (VERY handy on a non cowled plane) which makes it more interesting because the atmospheric reference is under the primer plate, a little carving with an exacto knife and I got a brass tube in there and a line running behind the engine into the fuse.

RE: surging in flight
 

This is the static reference system that I use, based on the Prandtl tube AKA pitot tube. But just the static part of it.

RE: surging in flight
 

Sad followup to my thread:(:(:(
Took my treehopper out again this evening got several flights in. Me & another guy was flying both our treehoppers following each other around. I went into a dive for a low highspeed pass & stripped out the elevator servo. I no longer own a treehopper. Nothing but splinters. destroyed the engine, broke the case I'm sure the crank is bent too. From the rear of the wing back the fuse is fine...other than a stripped servo. Sadly I didn't have metal gear servos on the tail. Lesson learned on a big plane. I spent most of the winter on this one.


I did get the surging fixed though[:o] [:o]

Ken

RE: surging in flight
 

That is sad news indeed. Finaly ready to rumble, and then BOOM.
Only metal gears will do if the going gets rough.