WALBRO CARBURETOR PROBLEMS ?
 

My spe 26 has been running great for about 25 flights and several hours breaking in , but now after some minutes working fine it always quits.
The engine starts very easily, can hold idle at 1700 rpm , the transition is good , top is 8000 with 17x6 ( it can even reach 8200 rpm too), but after some minutes running in the field, suddenly it begins to hesitate when throttling up at low rpm, transition turns bad and quits. I tried all kind of needle settings with the same result. Spark looks ok.
My first suspect was dirt in the carb, so I filled up the tank with new gasoline and clean the carburetor the best I know, cleaning the needles ,blowing the needles hole, cleaning the grid filter inside, but no signs of dirt. After that seems the engine is taking longer to quit, even I was able to fly for a while.
But the problem persist, the more trials I do the sooner the problem comeback and is a bit harder to restart.
I need help. Is dirt the problem ?, is the pump membrane ?, how to clean the carb properly ?....

RE: WALBRO CARBURETOR PROBLEMS ?
 

I'm no expert, but tend to read alot.
A guess without being there is either an air leak, meter arm heighth, or pop off pressure.

Here are two sites that have a boatload of good things on gas carbs and tuning.
[link=http://tech.flygsw.org/]Walbro Stuff[/link] Go to the Engine section, and then to the "Gas Carb Tech" link

Here there is lots of good info on tuning -> [link=http://www.geistware.com/rcmodeling/glowpower/index.htm]Geistware[/link]

And here is a link to a good thread on another board on tuning -> [link=http://www.flyinggiants.com/forums/fg68/427-gas-engine-tuning-newbies.html]Tuning[/link]

Get a coffee or a beer and spend some time. It's worth it

Good Luck
Bob

RE: WALBRO CARBURETOR PROBLEMS ?
 

Guess I did a No/No by referencing another board [X(]

If this doesn't work, send me an email and I'll give you the link.

Sorry RCU...
Just trying to help!

--> """http://www.flyinggiants.com/forums/fg68/427-gas-engine-tuning-newbies.html"""

RE: WALBRO CARBURETOR PROBLEMS ?
 

I really did it now.

TYpe in wwwdotflyinggiantsdotcom where all the astericks are.

RE: WALBRO CARBURETOR PROBLEMS ?
 

Gpar,
I'll take a shot at your problem.... It sounds like a possibility that the engine is getting hot. Once it gets hot you may be "chasing the needles" and you won't get a good setting on the carb no matter how hard you try and your engine will run like garbage. If you start the engine and it runs good the first few minutes but then everything gets worse this could be a sign..... Set the needles when the engine has been run for about a minute or so and leave it. Also make sure you have good air flow to the engine... Bill

RE: WALBRO CARBURETOR PROBLEMS ?
 

Sounds like it's to rich on the low end needle. After idleing for a while the engine tends to load up with fuel in the crankcase, then when you open the throttle it's to rich. In order to tell, open the throttle slower so it doesnt quit. you'll see a lot of ex smoke and raw fuel coming out of the muffler. Lean it out a bit at a time about 1/8 of a turn till it will maintain an idle as long as you want without quiting.

RE: WALBRO CARBURETOR PROBLEMS ?
 

When you say "some minutes of running in the field" do you mean on the ground or in the air??

RE: WALBRO CARBURETOR PROBLEMS ?
 

This is all done on the ground. Properly set up, it will sit idling all day without quiting. Another sign is if there are black spots of oil on the fuse, another sign of being too rich on idle mix.

RE: WALBRO CARBURETOR PROBLEMS ?
 

I go fot the "to hot" theorie :D.
Just check everything, from ignition to exhaust blocking ( carb was already done), or just replace the carb with a borrowed one.
I don't tink it's a carburettor problem.

RE: WALBRO CARBURETOR PROBLEMS ?
 

In fact I tried a new carb wich I had available with a much bigger venturi (the original was 9.57 mm and this is 13.7mm). This is much harder to setup since in less than an 1/8 needle turn you can go from too rich to too lean.
Anyway, now I´m getting more power, like 400 or 500 more rpm and no quitting at all, even after many long test at very different needle settings, trying to find the best and as i said before, was a long and hard job.

Concerning the original carburetor I´m planning to buy a repair kit and replace the pump membrane and keep searching. I don´t think the engine turns too hot neither is a needle setting problem nor a popoff pressure, since the engine performs perfect at all ranges and transitions, for 3 , 6 or 9 minutes before turns crazy.

RE: WALBRO CARBURETOR PROBLEMS ?
 

1700 RPM is very low. 2000 is average. This is for mag. ign. Batt ign will go down to 1000 RPM. This would make it difficult to get a prolonged idle too. Carb diaphrams should last for 3 or 4 years before replacing unless there is something visibly wrong with them. What colour is the centre electrode of the plug? What is the pop off pressure.? Needles become more sensitive as you go to a bigger carb, as the pressure in the throat is lower. Ran a small motor shop for 35 yrs before I retired so I know Walbro and other chainsaw carbs inside out and backwards. Still have quite a few mod. motors of all different makes.

RE: WALBRO CARBURETOR PROBLEMS ?
 

Thats what I would like to know, what is pop off presure? What pops off?

Mike

RE: WALBRO CARBURETOR PROBLEMS ?
 

There is an arm connected to the valve inside the carb which has a spring holding the valve closed. The diaphram makes contact with the other end of the arm. When the vacuum in the throat of the carb is low enough it pulls on the diaphram, opening the valve and letting fuel in. The higher the RPM, the lower the pressure, the further the valve opens. The pressure it takes to open the valve is the pop off pressure. It can be raised or lowered by lengthening or shortening the spring. Pressure should be between 12 and 18 lbs. Can be tailored to the engine if you know what your doing. Has the same affect as lowering or raising the float in a float bowl type carb.
Gord.

RE: WALBRO CARBURETOR PROBLEMS ?
 

Check post number 2 in this thread --> [link=http://www.rcuniverse.com/forum/m_1866098/anchors_1866098/mpage_1/key_pop/anchor/tm.htm#1866098]RCU Thread[/link]

and check the links I posted above. It takes some time and reading, but it's worth it to understand what these carbs are all about.

RE: WALBRO CARBURETOR PROBLEMS ?
 

Thanks chaps, having never taken one apart (and I don't want too) I was at a loss to what you where talking about, now if you had been talking Webra, Delauto or even Bosch Mechanical Fuel injection I would have been with you all the way.:D:D

Mike

RE: WALBRO CARBURETOR PROBLEMS ?
 

These are a fairly old design. Chandler-Grove made them in 1939 for aircraft engines. Worked great for aircraft as it would run just as well upside down.

RE: WALBRO CARBURETOR PROBLEMS ?
 

After many trials i finnally got the new carburetor (the one with the big venturi) perfectly tune up in flight, excellent power , excellent flight , and good engine performance, after landing I refuel for another flight, but this time the engine turns richer and richer as it flies, all by itself (I did not change anything), until it finally quits .

I think this was too big carb for the engine and consequently unreliable. So I go back to fix the original one , the wt-500.

I don´t know much about the pop off pressure, but the problems came at less than mid-range (3000-4000 rpm), so according to Flypaper 2 ,I should make the spring shorter in order to retard this pop off effect
How about canceling forever this pop off effect ?

RE: WALBRO CARBURETOR PROBLEMS ?
 

Unless you have a pressure gauge, I wouldn't cut the spring as it would just make things worse if you don't know what the pressure is. Try running it at a little higher idle RPM. As I said 1700 is a little low. 2000 is average. Can't cut out the popoff affect. It's part of how the carb works.

RE: WALBRO CARBURETOR PROBLEMS ?
 

Go back to the original carb.

Assure that you have good airflow into and out of the engine compartment for good cooling. The engine will not be cooled by adjusting the needles. Through the engine is the best by installing a home made baffle. I'm with the group that thinks the engine was over heating, not going lean. Now set both carb needles to 1-1/2 turns open and start the tuning process all over again. Run the engine up to max rpm and peak the high needle. Then back off the high needle just a "skosh". Now drop the rpm down to about 3,000 or a little less. Use the low needle to peak the rpm. Check the high needle again by peaking and backing off justa touch. No numbers here, your ear will do. Now go fly and see if the low needle needs a little more adjusting. If it falters when you advance the throttle the low needle is too lean. If it blubbers it's too rich.

That wasn't hard to do at all now, was it?;)