rmh

pont40 -is describing a different type of carb than we use in the model engines
no bowl in these -and the discharge hole is even with the bore - no low pressure bump. so -the little bugger which makes these holes work is the diaphragm (not the pump diaphragm-t' other one with the little vent that screws up the bottom/side port engines .) This one is doing a fine balancing act all th time --it is known as a "demand regulator -and when the bore pressure in the carb bore drops enough -it lets fuel flow thru the passages to the low/high speed discharge ports . the location and sometimes the shape of the throttle blade ( some have a wee cutout) lets the low speed hole flow then the high speed hole flows as it sees low pressure .
all very simple yet can be all very screwed up .
changing pressure at the inlet of the carb -or- the vent on the diaphragm can knock this balancing act on it's ear.
the pressure changes with the AOA of the fuselage. not from air blowing on the diaphram -but from the ambient pressure change as air flow changes in the cowl (or over the cowl). In many cases -it all works just fine -on some setups tho - bad news

On engines with relatively larger bore carbs - this all gets more sensitive -if all is working as intended, the carb works -if not --- bad news
for example -small engines (25-26 use small bore carbs -- 60-100 use larger bores - and 100-160 large bores .
put the big carb on the small engine and you likely get not enough vacuum (the low pressure needed to make it all work correctly.
here comes the rub
in all of the bore sizes - there are a jillion discharge hole/ throttle blade / etc., variations - all lovingly done by Walbro and Bing and Tillotson - to match a particular engine and engine task .
For guys who run their engins at full tilt and turn left -- the problem is different -as compared to guys trying to slide/twist flip the planes as they juggle the power . so as frightening as it sounds -if you really want to get the best results --be prepared to do some swapping and adjusting to carbs, needle settings - prop sizes - exhaust systems . if not
stick to flying a computer sim .
BTW-- the early ZDZ80 is a 6 port -open port - but six channels . the Super 80/160 has paired angled ports blowing inward n upward . different ideas -they all can work extremely well when properly setup.

Pontiac_40

My Feedback: (3)

I guess we (as in high performance seekers and high performance engine manufacturers) are mating components that are off the shelf with a completely different use in mind to a 6 port high performance engine...not a good mix. There is so much you can get out of the basic Walbro design, on the other hand BME has introduced some radical changes in the 115 to boost performance and reduce weight. I think an electronic fuel control unit just like OS engines came out with for there 160 is in order here.

RTK

My Feedback: (1)

Same principle. Some of my other carbs have a low pressure bump, SDC 80's I believe.

I agree that these carbs are not really designed for our intended use, but like you have said Dick, with a little trial and error we usually find what we need.

Antique

My Feedback: (4)

Part of the dreaded bog might be the ignition....An electronic ignition works by varyi8ng the timing according to the rpm...The tail is wagging the dog...The spark advance occurs when the rpm changes...If the low needle is too rich the crank doesn't accelerate very fast and the igntion stays retarded...It happens in microseconds, but if you purposely set the low needle very rich you will see what happens...Slow crank acceleration, slow spark advance, bog.....
You could test this with an ignition that has fixed timing...Unfortunately all the collective wisdom says you NEED electronic advance ignitions, so no one makes fixed timing any more....The bog problem was not as pronounced when we made mechanical advance, or no advance at all....My 26s have fixed timing....
All magneto engines are mostly fixed, with the possible exception of the Fuji with the magneto....

rmh

to add-
some use an advance which comes on (hopefully) out of phase with the bumpt/bog transition in the carb AND the inherant out of balance phase on singles.
some come on early -I checked out the new 3M 53--that engine had one small vibe at 2200- very low - then smooth runnin right on up. Not an endorsement -just noting .
most of my single engines have the "vibe at 2900-3000rpm -and no bog . the new ZDZ40F3A has a very very low vibe node -but it is at 3300 . --fascinatin--

Everone has their own idea as to which and when and to whom-

swapping prop LOAD -big prop --little prop - will change the whole darn character - the vibes stay the same place but engine transition and "bog" moves and/or improves..
if you think there is a magic setup which eliminates these problems - guess again - yo just shift em around.

Tired Old Man

My Feedback: (1)

The ignition "bog" problem was one we experienced with RTK's 115 when it still had the original carb. Once everything else was dialed in with regards to airflow disturbances, baffle designs, and carb tuning, what was left was a hesitation that appeared to be caused by the ignition timing. Everything else was about as peachy as could be. Believe that anything that could be experienced in the beginnings of the 115 have been experienced by RTK. I was there and got to watch along with him.

Ralph, I agree. All the wisdom that went into the electronic advances have left us in the cold in some areas. The mechanicals were very nice and easy to work with.

RTK

My Feedback: (1)

As of now (on the ground, haven't flown with new carb/block) There is no hesitation from any throttle setting. You can stab it from any throttle setting and it will instantly respond. The mid range has almost no detectable 4 cycling, just when backing down. We will see this weekend.

Tired Old Man

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You still have that Falkon or did you make the change to the CH?

RTK

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Falcon until it quits again, or causes problems, then CH.

Tired Old Man

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I put that Falkon right up there with your servo operated choke. It's not an if, but a when...

rmh

there is a problem with servo operated chokes?
ok --what?

RTK

My Feedback: (1)

Although I have never killed my engine in flight with it (close), Silversurfer has done it more than once

rmh

problem not the mechanics --
problem - the mechanic.
A local friend who was recently doing hi low n slow hover -- hit his throttle position /cutoff switch -with his big ol 40%
KABOOM--
I use retract switch as choke switch - on all models-- back is ON so I can't pull it accidently wif my finger.
so far so good
same reason I use no switches in flight - just lots of throw and lots of expo and lots of practice .
I have flown with guys who are constantly clicking switches - good flyers- but I would never remember what was on -off.
I can barely remember my way home --
(one woof for right turn -two for left )

Pontiac_40

My Feedback: (3)

Here is an interesting read. If only this system could be redesigned to be light-weight to work for our gassers it would be great.... Keith check it out.

http://www.envirofit.org/files/publi...ofit%20Kit.pdf

Tired Old Man

My Feedback: (1)

I use 3 switches fo flight. Dual rate switches for aileron, elevator, and rudder. Any more than that and I can't remember what was where and why. RTK tosses in a servo operated choke switch and about a zillion different conditions. It's usually very close to where one of my rate switches are with a different make transmitter. I killed the engine twice, he almost did it with his own transmitter once. Me, I use a very simple mechanical (read that as human) operated choke rod. If I'm really in deep doo-doo in flight and desperately need to kill the engine I figure the elevator is still funtional and the ground is still hard. That engine is GUARANTEED to stop!

Being a simple guy I figure if I need more than a couple of rates it's time to sit down at a ground control station and let the auto pilot do all the work

altavillan

My Feedback: (1)

trolling for #4 kinda question; I count the rev's it takes to fill the 24oz. tank feeding my 115. Using a dubro pump it takes 98 rev's to fill it up.
Carb 1, took an average of 65 cranks to refill after an average flight. Timed flight 14 minutes each.
Carb#2 the "new carb" takes an average of 75 cranks to refill the tank. Timed flights 14 minutes.

So I usually use 18 onces per flight of 14 minutes.
My question; Anyone else know and want to share how much fuel they are using per flight?

altavillan

My Feedback: (1)

You need to slap Auto if he gets that close to a fence again

krayzc-RCU

My Feedback: (31)

what type of flying for those minutes?

altavillan

My Feedback: (1)

hard 3d practice, basically areobatic stuff, oh and chasing the bulls off the field. I've been warned to stay away from one mean old one.

RTK

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altavillan--Never counted the turns, might have to next time I am out. I run a 32 oz tank and I think I run a little more rpm's than you, I would be almost dry in 14 minutes

AmpAce

Wow! Thats about 100% difference in fuel usage between those two engines! I realize that higher rpms will cause higher consumption, but it sounds like the type of flying is not all that different. There must be some real tuning, exhaust system, etc, differences to account for Altavillan's better economy. I have to believe that one of these engines has to be running pretty rich and the other pretty lean, to have this much difference.

Altavillan's difference between the two carbs is not too surprising. Obviously, the new carb likes running a little richer. Actually I would have expected it to be the other way around----the new carb should have allowed leaning out the mid-range to get rid of those transitional splutters that carb no. 1 was suffering. ??


AmpAce

krayzc-RCU

My Feedback: (31)

any comparision with h/l needles settings there?

Tired Old Man

My Feedback: (1)

I don't think RTK has run a tank dry in flight with the 115 yet. I know we fly pretty close to a 12 or 15 minute flight when it's up. RTK, how much gas is it taking to fill her up after those flights? BTW, on the way home in the morning and will try to make the Pow Wow. Sunday for sure, but Saturday is iffy. I'll e-mail at arrival home time.

RTK

My Feedback: (1)

AmpAce--The new carb (when adjusted) should be able to run more miserly as you have described, because you should be able to have a leaner mid range where most people fly at.
I was kinda joking about my tank running dry, but when I was purposely trying to see what some in-flight temps were, I pushed it real hard. WOT passes with WOT vertical with WOT flat spins. That really suck up the fuel.
The last few times I have been out with Silversurfer I have been flying a lot more like normal people

Silver--drop me a line when you get home, see you at the pow wow. You gotta fly that new little electric job I have!!

Krazy-- I would imagine with the new carb most people will be about 1+ on the H/L

FreedomFlier

My Feedback: (8)

OK.. I'm ready to swap carbs again.. maybe a webra, rochester or holley...or something

So I followed you advice RTK, and put baffles in front of the cowls . granted, my not be the best design, but I was able to get the engine down from 300 to 250 per my Raytek IR gun this Saturday. BUT, it acted like a completely different engine this Saturday, .. I guess airflow/pressure had an impact. it was a very humid day (80%) but of course none of the DA boys had to tough their engines, so I got a hard time about that... The engine acted VERY lean on the ground during run up.. fall on it's face.. opened the low end up almost .75 turns! got it to run up.. but during roll out it would load up terrible. In a harrier it was supper sloppy rich and in fact and did not have the mid-range power I thing it should have. each flight progressively got worse. "too lean" in the pits.. "too rich" on take off and in the air. Finally landed it, pulled the carb off, and had the help of about 5 guys offering support to the issue at hand. reset the needles to 1 turn High, 1.5 low. change out gas. changed pump arm angle. etc etc.. an hour later got it to transition OK, but still sounded very wet in a hover.. I have not problem with wet, but I don’t want the thing to load up and die when I need the power at a critical moment (don’t we all) It did not die on me, but it did load up terribly a couple times...

And still running 240-270, depending on where the measurement is taken. Dry Ice???