Here's my story.
I have a 3-w 150. Had it many years with no issues at all. This Sunday had two perfect flights. Then on the third flight as soon as I do the clearing turn on takeoff and level off the engine sags to near idle. Land the plane and check it on the ground, it idles good and no problems to WOT. Take off and the same thing. Open the high speed needle. Same thing.
Can't get it to go bad on the ground.
It starts easily every time. If i pull the throttle back to idle it will die, but restarts every time. will run good on the ground, but not in the level
Where do i look? Dennis

When was the last time you went through the carb? If it has been some time it may be the diaphragm has finally worn out. Although that little hole in the cover doesn't pass much impact pressure it's possible it may now be enough to be a problem. There may be just enough pressure the diaphragm can no longer perform like a pump.

There may also be issues with the check valve flaps in the diaphgam on the other side of the carb. I'd replace them both for good measure.

The die off at idle sounds like fuel flow. Two places to go here. One is the carb screen. If it's loaded up with crud the engine will run very badly or die at low throttle positions. May get some missing or stuttering at higher throttle settings.

Next is a worn out set of reeds. They have the most impact at low RPM. At high RPM the crank rotation has more effect. Most reeds do not function at a rate equal to the RPM above a certain point. Unless they are ultra high performance stuff of course. Reeds rear out over time so a check of them once a year is not a bad thing to do. If creacked, chipped, or frayed at the corners, replace them. If they fail to lay flat on the cage either flip them over or replace them. A little lift here or there is not a problem but if an entire petal doesn't seat, get another one.

Finally, 3w had a bit of a problem with warping carb isolators for a period of time. They went to an aluminum version to correct leakage. If you have a plastic one, remove it and re-surface it flat by laying a sheet of 220 wet or dry on a very flat surface. Like glass. Lay the insualtor of the paper and slide back and forth, both sides, then check the finished surface with a known quality straight edge.

Thank you for your reply. (1) I have replaced the carb block. I did check the screen and found some dirt, but wasn't able to test the engine after cleaning it. If i remember right the reeds were replaced two years ago. I will check the front diaphragm today. did check the tank and the clunk was in the right place. Dennis

Check all electrotechnical stuff. The failure may be vibration relates, loosing proper power to the ignition unit when in flight. Check battery leads, solder joins, switches and plug cap. Replace the plug.

Thanks pe I thought about that. but the engine doesn't quit just loses power in level flight. and it will restart and work perfectly on the ground. the plug caps are tight on the plugs. Dennis

Check the plumbing. I've been seeing some tanks with old tygon and blue colored pick up lines developing splits in that line inside the tank. They work great on the ground but experience a lot of problems in flight. With no gravity load on the tank the fuels remains covering the split. Once the plane starts moving and tank volume drops a little that split gets exposed and the engine sucks air.

I plan on removing the carb today and giving it a good look. I will also remove the tank too. the plane is new and all the lines are new. started to fly this plane this spring. Dennis

ive had that one happen to me... it was a new tank setup, so i never thought it couldve been that.... i was already blaming the carb...out of desperation i took the tank and found it

The two I saw were in fairly new tank installations. Leads me to believe there has been a delivery of defective line to some of our hobby suppliers.

If the plug caps were loose how would this affect the running of the engine? Dennis

Hayes seamless has never failed me, is immersed in gas for monthe in my test stand tank...No barbs needed...It holds so well it will break off at the fitting when pulled off...
Your results may vary, there must be some other neoprene being used...
Maybe Northern Arizona gasoline is less destructive..NO MTBE either...

Sure it is not your radio fialsafe setting making it go back to idle???

Rick

Loose caps could trigger an idle set failsafe if they were generating interference. They will generate interference if not secure on the plug. A loos cap in and of itself will not smoothly reduce power. Any power interrupts would be exactly that, an interrupt not a reduction.

Ralph,

I'm only telling what I saw. I do not know who the manufacturer was, nor am I questioning your experience with a particular product. If we want to talk specific products I could say that in 6 years and a great many engines and fuel tanks I have yet to see a Viton fuel line leak or fail unless it was chafed through mechanical vibration with a hard and sharp object or melted because of contact with a muffler. From those observations I could make an objective statement that there is no better fuel line, or one that could be expected to last longer. However, that's not my intent here.

I have recently seen two relatively new fuel lines inside a tank split longitudinally. The why of it I do not know. The engines ran badly and after correcting the lines they ran good. The purpose of this thread is to isolate, identify, and correct a problem and hopefully that's what we are all trying to do.

JUST A NOTE:
A fuel tank in the airplane that is mounted to high and full of gas will cause the engine to miss a little. I found this out by rolling to the inverted flight and flew several laps around the flying field. Yes I do have a carb cover tube to dead air way back in fuse. I know it is the tank mounted to high. The tank is too long to lower it...so i may change to a shorter tank. Capt,n

John,

That tank height delta shouldn't make any difference. The carb can pull gas 3 to 4' straight up from the floor, and squirt it another few feet above itself. Down is even easier. Less head pressure. I'd be looking at the vent tube as the culprit.

the line i got was 1/8 id Viton from a tank i ordered from AI ... i used some leftover tubing on 5/32 tubing i might have over stretched it

I am using a JR 11X. If I was going into fail safe I wouldn't have any control of the plane. The radio works fine. I'm able to land it every time. I have complete control of the plane. Dennis

Nice radio!!

OK I took apart the carb and this is what I found. When I removed the front plate the diaphragm was stuck to the plate and not hooked to the spring. I will replace it. I looked at the reeds and they have a very little roughness on the edges but close tight. I do believe the diaphragm is my problem. Now all I have to do is suck it up and test fly the plane. I did check the tank and everything was OK.
Also found out that a gasket that goes on first before the diaphragm is also missing.
Any other ideas are welcome. Dennis

Captain John, how is the engine mounted in that airplane that "misses" when inverted? I have seen people have cylinder UP mounted engines have this problem if their mixtures are set too rich. Fuel can accumulate in the crankcase during upright, (and cylinder upward) flight. When they invert the plane, the engine goes rich because the accumulated fuel then gets into the mixture flow.

I had a friend that had this problem recently. He had his low end set too rich, and it would occasionally burble at around half throttle. He would fly around for a while, then go inverted and the engine would puff out a bunch of smoke and go rich. Setting his mixtures properly solved the problem.

Other than that, tank placement will have no difference with the pumped/regulated Walbro carb or it's copy cat cousins.

AV8TOR

If the diaphragm is stuck to something and not hooked into the plate you would definately have a problem. If it runs on he ground it wil run in the air. It did before Give it a go.

Well it turns out that no one here in town has a carb kit. There goes the week-end. I'll have to wait a week to found out if the fix is in. Dennis

OK Vent from fuel tank or vent line fro carb to back in fuse to dead air???

List what to look for in what you call vent tube...please

Too long?
Too short?
Loop around tank position? horizontal or vertical?
Vent tube inside tank?
Vent dia?

In other words....what the_ _el do I look for in vent tube?????????????????????????

Well, I just make a loop slightly above the level of the tank, then run my vent tube straight down and out the bottom of the fuselage near the tank. Some people do all kinds of fancy loops, turns, and such, claiming to need to do so to stop fuel from coming out while on downlines. I've just never seen that problem myself, and my airplanes are very rarely horizontal when in flight...

Echo yard machines use a one way vent valve that a person could use, but then you would have to take it off each time you fueled the plane. All and all I believe the fuss about vent lines is in truth a "non issue".

AV8TOR