STG

My Feedback: (26)

I think that is the only way I can go--sure do like the way the motor meters fuel now though (accept for down-line breaking /wind milling). No problem with motor going quiet or any hint of motor wanting to quit on a long elevator or 45° down line. Most likely I am just on the edge & need to go bit richer--might not change much overall. [&:]

What do you set your RPM @ for in flight?

Thanks for the help.

pe reivers

The low setting differs a bit from engine to engine. In general I find, that if the engine needs to warm up a few seconds before it will accept throttle well, the setting is about right, though there is little room for error left. Slightly richer (1/8th turn), and the cold engine will accept throttle immediately after starting. This too is a very acceptable setting, and I feel more comfortable with it.

As for the mixture going richer when windmilling:
The nature of a two stroke engine is, that on each cycle the crankcase pressure is "reset" by the open transfer ports. High rpm will not cause a higher vacuum inside the engine, because the cycle start pressures in the engine will not change . The carb however will see more vacuum pulses on the idle jet, so will go slightly richer because the fuel flow does not change the same way as the air flow. Not much, slightly.
A well tuned carb will not stumble after a downline, but accept throttle without any hesitation.

See this topic in my forum for better understanding the carb:
http://www.prme.nl/forum/viewtopic.php?t=10

rmh

so--if the engine has carb which has air running across the inlet or a very open exhaust--the downline rpm will be different--as compared to a carb which sees constant flow-pressure.
also a tuned exhaust engine will typically react differently under a open throttle, down line run
My 80 starts to "back up " (go rich) doing this --limiting rpm - a BRAKING effect, as the pipe AFFECTS the exhaust and intake .
switching to in cowl Pitts mufler, the revs easily increase.
the carb/muffler / prop load, all interact.

STG

My Feedback: (26)

I was just starting to get some understanding and then you have to take it up a notch and confuse me. [&:]

rmh

awe- you can easily see these things - just swap prop loads -and and see how the engine either has better -or worse midrange--
change mufflers - to one which has zero restriction - again it runs differently - howsomeever --- changes from the air past a side draft carb -is harder to see , as some are inside the cowl -some outside but at different angles of flight - the engine may easily see difference.
guys who run at wide open throttle from take off to landing ,don't see these problems as readily.
So won't even know they can exist .

Tired Old Man

My Feedback: (1)

Good LOrd, Dick, Now you done it. You went and told him about the "jake" brake on your 80

pe reivers

Yes, the muffler will have an effect, because it influences the pressure in the cylinder that the scavenging ports (AKA transfer ports) will see on each cycle. In a two stroke engine, <everyting> interacts.

rmh

Jacobs brakes not permitted within city limits.

karolh

Why do they have to be so damn noisy.

Karol

Gerry77

Hello,

I use the walbro WB and WG series mounted on 15hp engines for a people-carrying hang glider or powered paraglider. Your diaphragm carb seems smaller but the principle is the same.

Seems to me you don't have a transition problem between low RPM to high RPM but only at idle. Your engine runs too rich at idle -and dies- because the throttle butterfly does not allow enough air in at idle. There's a notch on the throttle butterfly outer edge; Some flaps have the small indentation and some do not. It should be on the adjustment needle side of the carb close to the lowest pick-up hole (idle hole). Almost appears as though someone accidentally nicked the butterfly.
Here's the problem. That little nick in the butterfly plate accomplishes the entire airflow for the idle circuits. This is why your idle stop screw and spring are mashed all the way in while your Low speed screw open a mere one turn out. Your engine is starving for air while you’ve cut its idle fuel supply to nothing. Please take a look at my web site :http://www.geocities.com/farellus/idle_adaptation.html


I'm willing to discuss it further if you have any questions.

Cheers,

Gerry

STG

My Feedback: (26)

I am beginning to see how dynamic this carb tuning is. [&:]

In theory only--Would you think that a piston port motor (I guess this would depend some on the port timing) would be more apt to have a lesser low pressure (Less vacuum on the idle jet ) while braking/wind milling than a reed or rotary valve motor--meaning less apt to be drawn into a slightly rich condition?

Thanks for your help here!

rmh

your engine will react much the same as other - reed/rotary- to getting correct carb for your use. the side draft setup tho can be a problem-from air going past the inlet - and some part of the cowl maging flow problem.

- some carbs have that nick as part of the calibrating for a given engine -and some also have a hole drilled in the throtle plate as a bleed -- some have one --two--and --three , low speed circuit discharge points (I have called the next two "high speed" as they actually supply the fuel for "off idle" running -but they are fed by the idle needle ' -The Walbro chart and other Walbro charts are generic for their carbs - for example the 76 has ONE low speed discharge -- the 201 has THREE-and works a sh-it pot full better in my ZDZ's all of em using small carb (40/50cc)
I have a dozen or so different types of Walbo and they all work - some just work a little differently - what came with your engin - is unknown to me --I am not in the engine busines - I simply accumulated carbs from various engines starting with Zenoah - then others brands -

Gerry77

Walbro Diaphragm Carburetor Service Manual
(PDF document): http://www.wind-drifter.com/technica...viceManual.pdf

Modification for smooth idle:
http://www.geocities.com/farellus/idle_adaptation.html

Cheers,
Gerry

pe reivers

I am with Dick all the way on this issue. As far as I know, there is not a single Walbro for model airplane engines that is tuned by Walbro for the purpose. Instead, engine manufacturers choose a carb that works well at their location. (read: height or elevation). A user that is at sea level may require a carb that works better at richer settings, and Dick, being at Las Vegas level, may require a carb working better at leaner setting.
STG, this is not to confuse you. There is a very tight relationship between all parameters in a carb, that can be open to fair reasoning.
A side carb tends to stick out in the airflow, introducing not a can, but a bucket full of worms because of the complex airflow at the carb entrance. The number of variables becomes so big, that no normal human can tell you exact what happens there. Back to the good ol' cut'n'try. Just don't blame the engine. It's all part of the big game of model airflight.

@Gerry77
The notching of the butterfly valve requires profound knowledge of what one is doing. It certainly is not a cure-all, and cannot be recommended as such. We need to have idle as low as 1200 rpm in larger engines. At these rpm, some four stroking, or stumbling as some call it, is not to be avoided, because the two stroke cylinder just cannot be filled with sufficient undiluted mixture on each cycle. This is easily confused with overly rich mixture settings. Trying to correct it will invariably result in too lean idle mixtures and engine cut-outs. This irregular engine running is a fact of life below 25% power on a two stroke engine, when all further settings are correct.
Ultra light airplane engines often have a centrifugal coupling, that only activates the prop at higher rpm levels. That is not the same as our application.

STG

My Feedback: (26)

I don't mind muddled water. Confusion is part of the learning when the subject is this dynamic. Thanks for the help.