I've got a quick question about a Walbro HDC carb off an XL chainsaw. I converted the engine and put it in a Four Star 120, and have 4 days of flying with it now. The first two sessions went great, but the 3rd time out, on the second flight the engine started to lean out in the air. I landed, and flew my other plane for the rest of that day. Later, I took the carb off and checked the screen, and it did have a bit of trash in there which I cleaned real well. I took the rest other side of the carb apart as well, and checked under the diaphragm for a second screen, which this carb doesn't have. Everything looked good so I put it back together, then took the plane out for some more flying. I dialed the low and high speed needles in, and had it running great. About 10 minutes into the flight it started to lean out again in the top end. I landed and checked the needle, which hadn't moved. I opened it a little and it was running great again for a few more minutes, then started to lean out again. I checked the screen again, and there's no trash in it now.

Do any of you have any ideas on what could be making the carb lean out like this? I think I'll just order a kit and rebuild it, unless there's some other simple fix. Everything looks ok though, I don't know what could be causing it. It's just in the top end though. The bottom end always stays perfect, with a great idle. Even when it starts bogging out at the top, I can fly it back on 1/2 throttle to land it just fine.

Check to make sure your vent tube on the tank isn't plugged.

Then, I would say throw a kit in the carb in case the diaphragms are getting bad.

AV8TOR

The vent definitely isn't plugged. I always fill the tank until the fuel drips from the vent. The carb kits are cheap, I'll put one in there and see what it does I guess!

Pop off pressure could be too high. Every Walbro I've ever checked had too high pop off pressure readings. It's fixable, but you have to make a tool to pump up the carb and actually check it. The fix is to clip 1/2 turn off the spring and check the engine. Continue to check pop off pressure and clip the spring until it stops leaning out. This would be my LAST RESORT. I'd check everything else BEFORE I clipped the spring and fiddled with the pop off pressure. Once you clip the spring and shorten it, you can't go back.

Did you check the fork on the main jet? Is the top of the fork level with the flat body of the carb? If it's too high, it can cause flooding. Too low and it can cause the lean condition you describe.

Air leak? Things get heated up and then the air leak shows up. Happens sometimes. Check carb gaskets around the phenolic heat insulator. Check backplate. Check cylinder gasket.

Air vacuum on the hole in the side of the cover? If you're getting a vacuum over the hole, it could be sucking the diaphragm out and causing the engine to starve for fuel.

How about the hole in the diaphram cover? Sometimes it needs a small brass tube soldered to it and run the tube into the cowl for "dead" ambient air. It has worked for me.

I've read about needing the vent the diaphragm into the fuselage or cowling on some setups, but I left that for last, being that it ran great for a few tanks of fuel on the first outings, then started acting up.

I just pulled the diaphragm off and checked the little lever, fork, whatever it's called. At rest, the back of it is level with the carb body, but it has to move a bit before it starts to lift the valve open. I'm not sure which way to check it, at rest, or when it starts to pull the valve open.

Either way I'll check the local shops for a kit, and order one if nobody stocks them. If that doesn't do it, at least can rule that out...

It's pretty easy to solder a brass tube in the vent hole. I've done it several times. Put a piece of fuel tubing on the tube and run it inside the airframe. That diaphragm only leaks when it's broken. I ran the fuel tubing from my vent to a little plastic pill bottle with a cap on it and a hole in the top. If the diaphragm wears out, it won't leak fuel inside my plane. I mounted the pill bottle where it is visible for inspections.

I've seen guys simply cut up a Coke can and punch a few holes in it to match the vent plate screws. Remove the screws from the vent cover and put your "air dam" on top of the plate. Run the screws back in. It deflects incoming air from passing directly over the vent hole.

Hmmm. What size prop do you run and what RPM is it turning wide open. Kind of sounds like it may be overheating as you state that back to half throttle it picks back up. Could the high needle be too lean still? Your plane isn't cowled in stock configuration so the hole in the cover may or may not be an issue. you would need to run the tube inside the fuselage.

No, no cowling. I'd like to get the fiberglass cowling, but I doubt the engine would fit very well, so I'll just leave it as is. Anyway, I'll include a picture of how the carb sits. The vent holes (it has two if them) are both facing forward. I would think if they were the cause, it would happen all the time, not after being in the air for some time.

It's got a Master Airscrew Classic Series 18x6 prop. I don't have a tach, or access to one, and don't know the max RPMs. I also don't think it's heat, once it leans out, I can let the engine coll off completely and start it back up, and it's still lean. I open the needle to get it running right, take off, and it starts 4-stroking like it's incredibly rich, land, lean it out, and fly with it running great for 10 minutes or so, until it leans out again. It's almost as though the needle valve is moving, but I've checked it and made sure it's right where I had it when I took off.

I'm going to start with the kit, it surely can't hurt anyway...

For what it's worth, I ran this engine a number of times over the past 3 or 4 years on an airboat, and it never gave me an ounce of trouble.

MMMMMMM. Could be a blob anywhere in the fuel system. Bad real engines have that symptom when the fuel filter starts to get blocked. Fits you to a T.

Bad fuel mix load ?

blobs come in clear & long skinny sizes also. no joke. That opening the needle routine sounds like slime going back & forth. Next time it does it . leave it running fast on the ground. open the high needle more & more till it is almost loose. Put a piece of flat stick SAFELY over the carb inlet. It should suck the slime thru or clog it so bad you will know where the problem is.

Post when you find the problem.

I'll definitely update once I find the cause. I just haven't had time to mess with it yet. I'll probably get the carb kit next week if I can find one local, of not I'll order it and go from there. I did pull the carb off and look at the gaskets around the intake, they look fine to me. I'll put them back on with some sealer if I can't find new ones.

Also, this is a reed induction engine. Could the reed cause this behavior? I don't know how many hours is on this engine, but I do know that someone had been in it before I got the saw second hand. There was silicone on the gasket where the case splits. I don't even know if I can find the reed for it though...

You may have found the problem with Silicone. Not sure if it stays FIRM at all seal lines & joints when warmed up over 10 minutes & a re-wetting with fuel mix. The WOT vacumme & heat softening the crankcase seal lines may be the cause of leaning till a cool down occurs.

The more you describe the 10 minutes +- , the more it sounds to be heat related.

Can you afford a NEW wacker for about $60 ? I would pull this one & play with it on the bench. It should heat up quicker to make heat a problem.

No not tight fuel line ends ? Any old kinked spots? They will rekink when heated up. sometimes.

WHO said 2 strokes are MUCH simpler than 4 strokes?

The Ryobi I have has a BAD fuel line design into the tank. It has a piece of line to the tank & then another seperate piece of line inside the tank at a pretty good bend rate to the bottom of the tank.

I live with it. But do not like it.

I agree that the symptoms do indicate a heat related problem, but like I said I ran this engine on the airboat numerous times without a bit of trouble.

I think I've solved it though. I downloaded the manual from Walbro for the HDC series carbs, and read how to set the metering lever. I took the diaphragm out and checked it again, and it's spot on where the manual says it should be. While I had the carb off and apart though, I took the high speed needle out and cleaned the passage out with carb cleaner. I noticed though when I took the needle out, it was set way to rich. (I counted the turns in so I could put it back to the same setting.) The manual said to start with both the high and low speed needles one turn out. Both of the needles on my carb were just over two turns out. So, I took the rest of the carb apart and cleaned everything really well, even though I didn't see any trash anywhere. I put it all back together and set the needles to one turn out, and went and started the engine. It ran well enough, and after closing the low end a bit and opening the high about the same, it's running great on the ground. I'm hoping to have time to fly it tomorrow and see how it does in the air. It looks promising though!

I'm guessing there must have been a piece of trash somewhere or something limiting the fuel getting into the engine, which is why the needles were so far open. It's running great now with the needles where they should be.

I'll be sure to update once I get it in the air!

Just an update to say that the engine is running great again! I got a flight in this evening, and it seems to be perfect now. It was a little rich when I took off, so I landed, leaned it a bit, then flew nearly the whole tank of fuel.

On another note, you gotta watch these big props! I had also adjusted the linkage on the throttle servo, and the idle was a bit high with the trim all the way down. I landed and taxied up to my flight box to refuel, and forgot about the idle. It kept rolling right into the box and made three clean cuts right through the 1/2 plywood handle on my flight box, cut the fuel line on my pump for glow fuel, cut the wire for the pump, knocked the pump off the box, and cut the wires on my starter. That was just at a high idle. I'd hate to see what would happen to a hand in one of these things while it's spooled up! [X(]

Now I gotta go shopping for a new flight box and solder some wires...

I took a metering diaphram and sprayed it with silicone spray on both sides 2 days ago. It was one I took out of a engine that did not seem to run good at all. The silicone spray made it flexible as can be...and looks like new. I will place it in my test engine and see what happens...that is if it is the same as one in engine! Anyway, if you guys got a diaphram laying around...try it also. I have sprayed carbs before and stored them for future use. Seems to work great. It Is $1.98 at Wally World. Take Care, Capt,n

Interesting experiment/discovery Captain John. I'll have to try that!

KeithLuneau Yeah, be really careful with the props!! There was a pic in a mag a while back of a guy's hand missing a couple of fingers and the rest of the hand mangled, all done by a prop. Years ago, I was adjusting a little .40 size glow engine. The engine was at idle and I decided to give the carb one more tweak and reached right through the prop arc. Even though the engine was small and only idling, it cut three fingers to the bone, one of them right through the finger nail. OUCHH!! Taught me a great deal of respect though. I also once did something similar to what you just did. I reached for the idle trim to shut down an engine as I taxied into my pit area. In doing so I bumped the throttle and the airplane ran into my friends Dynaflite Butterfly motor glider and tried to eat the wing. OOPPSS! (Quite embarrassing too!)

With both model airplane engines and full size airplane and ultralights, I also follow a practice of never revving the engine up if anyone is anywhere in front of the prop arc, even if they are standing off to the side.

AV8TOR

I'm just glad it was my flight box that took the hit and not me! It's a nice reminder though... Hey, at least I have an excuse to get a new box out of the deal too! lol

It's been almost 10 years now since I stuck my hand in the prop while removing the glow igniter from a .91 four stroke. It only scraped my hand a little, guess I was lucky then too.