GoNavy

My Feedback: (15)

Apparently, I need to adjust pump pressure, and have never done that.

The engine is the UAV Bully PR 1.20, a two stroke. This engine is no longer made, but it is a Webra 1.20 with two plug head, back plate regulating pump, and needle valves modified for the pump.

I have two of these. One runs fine out of the box.
I am unable to achieve that with the second. High speed is no problem, but with the midrange proper, it is too lean at idle. As soon as the engine consumes the fuel in the crankcase, it dies. If I richen the low speed to achieve idle, the midrange is too rich, even though I still cannot achieve idle.

The instructions for this engine state: "If the engine stumbles while going from idle to mid-range the pump pressure relief valve (the brass screw and jam nut on back of engine) needs to be screwed out one half turn. After any pump pressure adjustment the carburetor will have to be readjusted" and "If the engine stops or falters during change over to full throttle, the mixture is too rich and the idle needle should be screwed in a little."

I take this to mean that screwing the pressure relief valve "out" reduces fuel pressure; Is this correct? Is there a particular pressure setting, say in pounds per square inch, which would be appropriate? Should I attempt to measure that, or just use the unmount, adjust, remount and try again procedure?

w8ye

My Feedback: (16)

To screw the bypass out will reduce pressure.

I wouldn't attemp to measure actual pressure.

The usual procedure is to get the pump just strong enough to be able to run the engine very rich if you screw the needle out a ways.

Then try to adjust the idle needle.

Then look at you mid range. If the mid range is rich, you have too much pressure.

Readjust pressure as noted in instructions and start adjustments all over again.

William Robison

My Feedback: (3)

[b]GN:

Expanding on Jim's notes above.

If your poor running engine peaks HS at one turn or less out I'd think the pressure was too high, I'd suppose it should be at least two turns out at peak. You have a definite advantage over most attempting to set up a pump though; you have one that runs fine. Check the HS needle on it, then set the poor runner to the same turn count and adjust the pump to get the engine to peak at the same setting. THis should put you right in the ball park, normal adjustment of the LS and HS ought to bring you home.

Note; You are learning another lesson at the same time - "Unmount, adjust, remount" - do it on a stand, not in your plane. Even if all you have is a notch in a board nailed to a stump, it's a lot better than trying it mounted in your plane.

Bill.

GoNavy

My Feedback: (15)

"....to run the engine very rich if you screw the needle out a ways... the high speed needle, or the low?

BILL: My normal routine is the test stand. I just knew this one would be as easy to run as the first, so it went into the plane instead.
I can see that adjusting the pump will require the test stand.

Thanks guys!

William Robison

My Feedback: (3)

[b]GN:

The judgement point for fuel pressure is at the condition of maximum fuel flow, in other words test pressure with the HS needle, and the engine at full throttle.

Bill.

GoNavy

My Feedback: (15)

Okay.
I reduced pump pressure.
Now I can achieve idle and mid range.
Now my top end is off, from 9,000 down to 8,000 and with surging. No matter how far out I take the high speed needle.

No air bubbles. Is this just too low pump pressure, or do I now have dirt in the pump? I took out both needles and cleaned the carb...nothing obvious.

William Robison

My Feedback: (3)

[b]GN:

If you can't go rich with the HS needle your pump pressure is definitely too low. Crank it up until you can go rich on the HS,

This may make your mid range rich again, out up with it, it's cheaper than ruining the engine with a lean run, but there should be a place with good mid range and enough fuel at full throttle.

I said earlier about two turns out on the HS, if it works out to 4 1/2 turns to get the mid range right and get the HS a little rich that's still OK.

It does sound like you're getting there.

Bill.

GoNavy

My Feedback: (15)

Two hours and 1/2 gallon of fuel and .... no improvement

I again cleaned filters, checked for air leaks, and cleaned the carb.

I took the pressure relief valve out in 1/3 turn increments, and am now about five turns out from factory setting. The high speed is out four turns and is becoming non responsive. The low is now in to about a 1/4 turn, and the midrange is rich enough that you must leave glow heat on to transition, and even then it is iffy.

I replaced the plugs. Fuel is new, Wildcat, but I also tried Morgan just for luck.

Seems to me the pressure relief feature is not working.

William Robison

My Feedback: (3)

[b]GN:

What you're getting is not reasonable, unless there is another problem.

With the engine on your stand disconnect the pump, loop a short length of tubing from the pump inlet to the outlet, plumb the fuel tank directly to the carb bypassing the pump completely. Run the engine and get it stabilized, any particular rpm, doesn't matter. Now undo the looped hose. Any difference in the engine run? While it's still running plug the pump outlet. A difference now? Still running, with the pump outlet plugged attach a hose from another tank of fuel to the pump inlet. Did it suck the fuel up? Unplug the outlet until fuel comes out, then plug it again. Any difference in the engine run yet?

Now, in case you wondered, all this is to check for an internal leak in the pump, and I'm sure you've already realized that. Any change in the engine run while doing these tests to the pump will show a leaky diaphragm.

As a final check, switch first the carbs between the good running and problem engines, then the pumps. If the pump passed the leakage test the further switching will show if the pump or the carb is the problem, if it persists with both the carb and pump from the good engine then it's in the basic engine. If the problem switches with one part or the other, you have at least found where the problem lies.

HTH.

Bill.

GoNavy

My Feedback: (15)

Finally, success!

I took an identical carburetor from another engine.
I reset the pressure relief valve to original setting.
It runs like a charm.

I compared the two carbs visually. The original carb which I removed, with the low speed needle turned all the way in (so I thought), left a gap of about 1/16 inch from the tip of the low speed needle to the "whatchamagigy", the brass tube which extends into the throat of the carb from the side of the high speed needle. In contrast, the carb from the other engine (which had run fine), showed the tip of the low speed needle entering the brass tube, both of these with the carb "wide open".

So I checked to see if the low speed needle in the troublesome carb was in fact turned all the way in. I found that it was not....with sufficient persuasion (brute force) it could be turned in an additional 1 1/2 turns. Now the tip of the low speed needle is about flush with the end of the brass tube.

Either I have cut new threads or this was one tight fitting low speed needle. I shall now switch the carbs and see what happens.

The high speed needle is out about 2 3/4 turns, so I judge from the above comments that the pressure is in the ballpark.

The high end is 8,600 vs. 8,800, but it's only 55 degrees.

How say ye men of the jury?

GoNavy

My Feedback: (15)

Okay.

I put the original carb back on. It ran ok, a fair amount of stumble on accelleration.
I cranked the low in more and it runs well.

The low speed needle certainly turns much harder than on the other carb, but it appears it was just the low speed needle setting all along.

It's amazing what can be accomplished on a Friday afternoon when you take off of work.

William Robison

My Feedback: (3)

GN:

Glad you found the problem.

Bill.

GoNavy

My Feedback: (15)

Bill: Tks for the help.

GoNavy

My Feedback: (15)

Bill and w8ye, one follow up question please.

For lack of better terminology, I'll call the brass tube coming from the high speed needle side the spray bar.

If, at wide open throttle, the low speed needle is partly obstructing the spray bar, will that cause a reduction in top end performance?

In trying to understand how these two needle carbs work, it would seem that the purpose of the low speed needle is to "reduce vacuum" in the spray bar, resulting in less fuel draw, but that's just a guess.

If that were the case, given that the low speed needle is moved further into the spray bar as the throttle barel rotates toward idle, it would seem that the most important part of the taper on the low speed needle would be the portion further from the tip. If so, could one ameliorate the reduction in top end performance caused by the low speed needle, by making the inital taper, on the tip of the low speed needle, more blunt?

w8ye

My Feedback: (16)

Don't know what the carb looks like but most of the carbs that have the High speed spray bar coming half way across the venturi, require the tip of the low speed needle to be about .020" from the end of the spray bar at full throttle.

Some of the ones that go all the way across the venturi have a cat eye shaped slit on the engine side. This idle needle should be half way across the slit at full throttle.

There are other variations

GoNavy

My Feedback: (15)

Looks like I'm not the first person to have this issue with the webra 120 carb:
http://www.rcuniverse.com/forum/m_32.../tm.htm#328677

In order to achieve transition, and in an attempt to have a non- rich idle, you have to turn the low speed needle in so far that there is NO gap between it and the spray bar. This would seem then to interfer with the high speed performance.

Perhaps the pump exaggerates this problem?

Yes, the webra carb has the high speed spray bar ending at the middle of the venturi, as opposed to the cat eye shaped slit of a super tigre.

w8ye

My Feedback: (16)

As a variation, Some of the Saito's have a spray bar going half way with half of a cat eye in the end. All my 91's are that way.

William Robison

My Feedback: (3)

GN:

In response to your questions about the LS needle.

The HS needle sets the maximum fuel rate with the throttle full open, and naturally, as the throttle closes off allowing less air into the engine the amount of fuel has to be decreased at the same time. The restriction of the LS needle is what changes this, and the taper controls the rate of decrease. Changing the shape of the LS needle will upset a good running engine, but sometimes a poorly running engine can be improved by altering the shape.

If you make a study of these simple/sophisticated carbs you might want to experiment, but a blind change will more probably hurt than help.

Bill.

GoNavy

My Feedback: (15)

I queried Lynn Osborn, who ran UAV, the company that made the Bully 1.2, about the low speed needle issue and here are his comments:

Hi Tom,

Yes, the stock Webra lowspeed needles had to be modified. I found inconsistencies on length and taper from the factory. I built a jig that allowed me to shorten the needles to a standard length and steepen the taper. I don't have the jig anymore. It sounds like you might have gotten a unmodified needle. You might try to grind off about .020" from the end of the needle, round the edge with 600 grit and test run the engine. Keep shortening until you get back your highspeed RPM. I found the needles varied from .005" to .085" too long.

Take care,
Lynn

William Robison

My Feedback: (3)

Tom:

Now you will not be making blind changes. Have fun.

Bill.

GoNavy

My Feedback: (15)

Here's my plan of attack: I buy a few spare needles from horizon hobbies. I use the needle from the good unit as a master.
I make a jig from that and try shaping one of the spares to match. (As the instructions to an old Fox 40 said, regarding changing the shape of the needle "If you make a mistake you haven't damaged an expensive part.")

Today low speed needles, tomorrow the world.

William Robison

My Feedback: (3)

Haw!

Bill.

GoNavy

My Feedback: (15)

Well, well, well.... isn't this interesting.
Webra Austria informs me in no uncertain terms that the TNII carburetor is designed to be used only withOUT pressure, of any kind, even muffler pressure. With pressure, either from the muffler or from a pump, they direct use of the Promix carb, which, they point out, should not be used in the absence of pressure.
I am awaiting a response from Horizon Hobby as to why they advertise the webra 120 with pump is sold with the TNII carb!
It sounds as if reshaping the law speed needle on the TNII carb will not make it work properly with pressure.
This also suggests to me that even running this engine with a Cline regulator, which uses crankcase pressure, is not going to give best performance.

William Robison

My Feedback: (3)

GN:

While the Cline and Iron Bay systems pressurize the tank, so far as the carb is concerned either is a normal "Suction" system. The effect is having the tank about one inch from the fuel nipple on the spray bar.

Bill.

GoNavy

My Feedback: (15)

Bill:

In response to your last post, I went back to read some old threads discussing pumps and the Cline regulator. Let's see if I've got this right (whether or not I understand how a Cline regulator avoids delivering fuel to the carb without higher pressure than a line, without the regulator, to a tank that has no pressure other than air pressure). For simplicity, let's say that a line from a tank with an open vent line is "normal" pressure. (I presume it has something to do with air pressure). (I know nothing about the intricacies of the YS system and my comments below do not attempt to include that):

1. The Cline regulates, in the "true" sense of the word, so fuel is delivered to the carb at "normal" pressure. The pressure in the tank is there to ensure reliable fuel flow to the regulator only. (I saw some heated debate about this on another thread, but your last post seems to agree).

2. Pumps may or may not include a regulating effect similar to the Cline. The Varsane/Perry "regulating" pumps do not regulate like the Cline. They allow the user to vary the pressure produced, but the higher pressure generated reaches the carb. The Perry/Varsane oscillating pump does not regulate like the cline. The output, the fuel pressure, however varies more than the "regulating" version, because the pressure increases with increased engine rpm.

How does the modeler know, or how can he test to find out, if the pump in use regulates like the Cline?

3. The engine or carb manufacturer designs the carburetor to operate in a certain pressure environment. Presumably the typical carb, intended for "normal" pressure will perform poorly at higher fuel pressures. Similarly, the carb designed to work with an unregulated pump will also perform poorly at "normal" fuel pressure.

How does the modeler know, or how can he test to find out, when the fuel pressure at the carb is outside design perameters?

I'd better stop here so you can grade my essay, before we tackle the next set of inferences.

GoNavy