Konrad

There has been a lot of talk lately about the various attributes of the different carb design. I'd like this thread to be an open discussion about the merits and constraints of the various carb designs we see on today's and yesterday's engines. I encourage that the discussion be heavy on the technical side. After all, my objective is to drop the veils of ignorance from my eyes. I do want to limit the discussion to the carb. While I know that often times the inlet timing and exhaust system can have an effect, I'd like to keep these out of the discussion whenever possible. But if you feel the need to discus these aspects please feel free to do so.

All the best,

Konrad

Konrad

I'll start the discussion off with discussing False Needling"

This results when the flow through the fuel jet ( both high speed and low speed) changes even though the cross section area of the jet has NOT changed. This happens because the boundary layer along the surface of the needles and jet walls can change size as the needle and jet move closer or further away from each other. Please note the velocity of the flow is low in the boundary layer and if this layer gets larger (in area) the engine will go lean.

Many better needle designs are what are called single sided. That is it looks like a ramp has been grounded on one side. This allows the body of the needle to pilot in the jet and limits its radial movement. Also the advent of dual fingered needle springs helps keep the needle centered as the engine vibrates. And the barrel style needle holder often with its O ring also helps greatly making the false needling issue a thing of the past (well somewhat). High performance engine manufactures often had this needle profile as an option. Jett, Nelson and Fox are three manufactures that I know of that offer the single side (flat) needle.

pe reivers

cut it twice, still too short. Now, what did i do wrong!
When you want to study carbs, you will need to know a thing or two of fluid dynamics. Like the fluids in gaseous form, and flow when liquid. Bernoulli's law comes to mind, but also flow across weirs and through orifices, both of gasses and liquids. The general gas laws of Boile, and Boile Gay-Lussac would be needed as well.
In addition to that, you will need knowledge of combustion, and how it can be influenced.
In short, you want to have a complete phd in a nutshell. It took me a life time. I wish you good luck on your yourney.

PS
Please do not think my comment is in any way meant negative. gaining knowledge is a wonderful thing. But once on that path, there is no end. The journey itself is the end.

Konrad

Your points are well taken. And yes if the discussion needs to go in that kind of detail I'm more than willing, and actually prefer, to follow the discussion in that direction. Again I would like to stay away for issue of combustion or evaporative cooling and the sort. We can go into those issues in other threads.

All the best.

Konrad

Heck, I cut it twice and it's STILL too short!!!

Broken Wings

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Isn't an evaporite a rock...

Konrad

Thanks, I'll correct the SP.
Now please correct me if I'm wrong, but isn't this site form about engines and not an english tutorial site. Now if my grammar and spelling are so atrocious as to cause a lot of confusion please bring it to my attention otherwise please don't try to up ones post count at my expense!

All the best

Campgems

If you want to discuss pratical issues vs theory, I think this is a good topic.

Also I'm assuming that for the most part, you are focused on the Glow engine carbs, not the gas carbs with diaphragms.

If we are on the same page, I'll dig out some rough drawings I did to explain the balance between the high end needle and the mix control needle.

Don

w8ye

My Feedback: (16)

There's spell checker in the editor.

Right click and then left click on spell checker.

Konrad

Thanks I often use it. Unfortunately it is not a technical dictionary and I have a propensity to mis spell the wrong word correctly. English and I have never gotten along well together!

All the best.

P.S.
Right click left clicK, what is this stuff?

Konrad

Yes, I was thinking of the none pumped carbs (Non Diaphragms). But they might make a good side bar.

I'm thinking like the merit of the airbleed carb verses the automatic mixture control (AMC). How does changing the placement of the discharge port effect fuel draw enabling some midrange adjustment (i.e., Super Tiger or OS) Also what are the limitation of the twin needle , verses the tube over slot or even the slotted disk (i.e., Perry and Enya), be them manufacturing, theory or operation. I'm even willing to go as far back as Ernie Kratzhet's Bramco carb on how it work and what were the nuances one needed to extract the finer point. I'd like to explore some of the unpopular carbs like the Fox butterfly. (by the way I had great success with then).

I'd even like to get into model specific issues. Basically if it is carb related I think it is fair game for this thread. One of my concerns is that so many sport fliers really don't know a thing about their carbs. Looking at what has died in the market place because of the ignorance of us the modeling public has me very concerned that in the end all we will have on the shelves is mediocrity. I point to the loss of so many 3 mixture carbs and the inflight mixture adjustment. With the fact that the piece part cost of many carbs is often just as high as the cost for the entire engine system I think a dedicated discussion is warranted.

All the best.

Konrad

Broken Wings

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What a wonderful opportunity to bash a product that everyone knows you don't like.....[sm=cry_smile.gif]

Air bleed carburetors work fine. They're less expensive to produce. You'll find them on less expensive engines.

I really can't understand why you're so "concerned" as you only fly electrics. You gave up on carburetors long ago.

If this thread doesn't end up as some sort of "Bash" on O.S. Engines I'll be surprised.

Konrad

I have to question what I often read as folks using silicone fuel tubing to seal the needles. I don't see how silicon can seal a thread as it can't reach down to the minor diameter.

This leaves much the same air path there was before one added the tubing. I suspect that what is really happening is that the tubing is dampening the needle's vibration and limiting the side to side movement that causes the flow disruptions (leaning out) of a false needle. If the needle does't vibrate in the jet and change its position the fuel flow won't change giving the impression that there is an air leak (lean) It looks to me that the silicone tubing does help, it is just that the mechanism has been wrongly identified.

Konrad

Do you have an issue with OS or the carbs found on the OS engine that you would like to get out in the open now?

As for me I think in the above posts I have laid out my motivation quite clearly!

Konrad

Broken Wings

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Class of fit...

Cutting tool life management...

Design...

Material...

Quality Control...

I would think that all these things determine whether a length of silicon tubing is necessary.

It works very well for a lot of people. "They fixed it" Now they can fly their model.....

Broken Wings

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Sounds like a group therapy session of some sort....

No issues with O.S. or any other model airplane engine that I feel the need to get out in the open....

I like them all.

Some better than others.

Konrad

But the thread has never been a sealing feature. Higher end carbs often have proper sealing "O" ring lands machined into the needle and holders to address this very concern, of not being able to seal the thread no matter its class or finish. You often find these is application were the RPM (Vibration frequency) varies a lot. Unlike C/L carbs (fixed venturi) were the RPM is rather narrow (Yes I know about the FAI C/L speed engine and the huge jump in RPM when they come on pipe) that often just have a collet the fasten down the needle after adjustment.

I was surprised at how little the thread clearance (class of fit) had on fuel flow variability. Even though air is about 6 times easier to get to flow (leak) than fuel, it was ease to compensate for if it was leaking at all. What was the bear to control was the variability in the settings (fuel flow) as the needle was pushed around (manually forcing a false needle).

This work was done for the engines of my design and manufacture.

All the best,

Konrad

earlwb

Well, I used to always think that the short piece of fuel tubing sealed something. I had a couple of engines where you could see a large quantity of fine airbubbles being sucked in via a remote needle valve and heading to the carb on the engine. The engine ran like crap, no adjustments worked, false needle settings, too rich or too lean, nothing in between. But after sticking on a piece of fuel tubing, the air bubbles went away and then engine behaved like it ought to.

But in retrospect now, maybe it was just centering the needle better than stopping the air bubbles. so maybe the air bubbles wasn't really the problem, it just looked like it was.

If I get rambunctious, I'll hookup a GMS 1.20 engine with remote needle valve and see if I can take close up videos and pics of the remote needle valve on it causing the problems. Then try it with a piece of fuel tubing and see what happens.

Anyway, the piece of fuel tubing fixed several ASP 61 four cycle engines that I dealt with. But in that case the needle is on the carb and not remotely mounted. But at first I thought it was an air leak, but maybe it was really simply keeping the needle centered better. These engines all use a single detent on the needle valve.

You can see in the video I made of the "Generic" .36 engine earllier where touching the needle valve without turning it would change the engine speed as the needle moved or the throttle barrel moved ever so slightly. It has the single spring detent too. So that might turn on to be a problem on some engines. Including the one I made a video of too.

Konrad

earlwb,
As you know I'm a bit of a troublemaker in that I ask, and I hope I get, folks to think. This is exactly the kind of thinking I hope to instill, both in the readership and I hope for me. I really enjoy it when somebody can teach this old dog new tricks.

Now to be clear I didn't say that the tubing didn't solve some problem I just don't think the mechanism credited to solving the problem was correct. But the fact that you saw bubble without the tube and none with the tubing does led credence that you did seal something if nothing other than allowing control of the clearance to allow some oil to pool in the valley to make a seal. I can't subscribe to that as the flow would be away towards the discharge port and not allow replenishment of the valleys with oil.

I too think that the single leaf spring detent, or holder, that puts a side load on the needle is a very poor design.

All the best,

Konrad

captinjohn

anyone try a simple thing like wrapping the threads of the needle valve before installing it with 1/4 inch wide teflone tape? Thanks capt,n

Broken Wings

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As long as the thread isn't truncated beyond it's class of fit any thread can/will act as a sealing feature in some respect.

The question is how much is enough.

If Rolex made a glow engine........

earlwb

The teflon tape idea is a good one. But the teflon tape is very fragile, just a few turns of the needle valve causes it to fragment and break up and now you have little bits of teflon tape getting into the fuel ports and orifices on the carb.
I have used a little silicon grease or even mineral based grease on the threads and that works fairly well. But you have to be careful and keep the grease off the tapered parts of the needle valves. I usually use both a short piece of silicon tubing and a little dab of grease on the threads when I was fixing leaky needle valves before.

Campgems

Konrad, A question for you.  Is this gathering reasearch for another "article?

"I had some of my words published in MAN and FLY R/C but on the subject of aircraft and as a reviewer of some aircraft kits. I'm sorry if I left the impression that I was published on the subject at hand."

The quote is from one of your other post on another thread.  Just want to set the playing field straight. and level.

Don

Konrad

Yes,
The limitation with that is that the tape tends to sherd with the back and forth action needed to tune an engine. These shreds can cause havoc in tuning and trouble shooting as they block or partially block the metering ports inside the carb. The best thing I have found is some dry film lube that is baked to a chemically resistant coating then allow the needle holder's threads to cut this rather soft coating making a rather nice knife edge seal. I have also used electroless nickel plating in an attempt to build up the thread. While nickel is soft it was still too hard to allow the thread holder to cut a seal into.

All the best

Konrad

earlwb

Here is one of the ASP 61 four cycle engines I messed with previously. In hindsight I think it was centering the needle valve more than sealing the threads up. But the little piece of fuel tubing worked and the engines ran fine afterwards.
But on the remote needle valves it may have helped with two problems, a needle valve that wasn't centered and a air leak as well.


Before


After with the tubing in place.

Konrad

Good lord, No!

I'm so far outside the Gaussian curve it would take three months of articles just to explain why I don't subscribe to what is normally excepted. I'm always questioning the status quo and my assumptions.

Besides I have a nasty tendency to speak what I see as the truth, and those that pay for ad space don't like to hear it.

To everybody else please remember that what I say here is worth what you paid for it. If my points (arguments) don't make since you are free to ignore them. Remember this is user generated content.

All the best