Pump with non-pump carb
 

Hi all. Are there any issues that will arise from using a pump with a non-pump carb?

Just for fun (so don't ask why) I've hooked up an old OS PA-104 pump and backplate to a standard SF .40 with the 4D carb. I know this system was designed to operate with the 46 carb, but is there any reason why I might encounter problems?

Thanks,
David

RE: Pump with non-pump carb
 

....most "pump carbs" simply have a larger venturi than their non-pumped friends.
Some have a fuel "over-feed return line" to the tank as well, but those are rare. ;)

Not to worry, Dave....let 'er rip. :D

FBD. :D

RE: Pump with non-pump carb
 

If you can adjust the pressure of the pump there shouldn't be any problem?

One can also plumb the fuel system whereas the pump merely pumps fuel to a point at the fuel inlet to the carburetor. The excess fuel is bypassed back to the tank. This assures fuel is always present at the needle valve with minimal. pressure at the carb itself.

RE: Pump with non-pump carb
 

Well, the pump works great. This is the first pump I've ever used on a glow engine so I didn't know what to expect. The NV has to be set at it's leanest, optimum setting or the engine loads up; that seems obvious now. The NV ended up being set just a hair under 3/4 of a turn open from closed. You also have to advance the throttle smoothly; you can't slam it forward.

I need to tach with and without the pump, but I recently ran this engine without it and my ear can definitely detect an increase in rpm. I'll take it to the field next time I go and get some numbers. I would really like to try it with a larger carb, though. I'll have to keep my eyes out for a #46.

Anyone know the throat diameter of the #46?

David

RE: Pump with non-pump carb
 

Dave,


This is for Perry carburettors only...

Some engines that came originally equipped with the pump, also came with the pump carburettor.
Since a pumped engine is not dependent on venturi depression for fuel suction, a larger carburettor bore can be used...

One bottleneck down, more CFM, so more power is available.
Such were some engines from OS, HB, K&B, Enya and I believe a Super Tigre, or two.

RE: Pump with non-pump carb
 

To my knowledge the SF 40 and SF46 had the same carb?

RE: Pump with non-pump carb
 

Thanks, all.

I should explain this project more clearly... I have a non-pumped SF .40 with the stock #4D carb. I recently picked up a PA-104 pump and backplate without the #46 carb. So, I ran the engine with the pump and the stock #4D carb.


The pumped SF .40 and .46 both came with the #46 carb which has a larger venturi than the #4D. I'd love to locate a #46 carb, but they are extremely difficult to find. Perry still sells a pump carb that fits the SF .40/.46 - the #9300S - so I guess that's another option. But, I'm not even sure if the increase in performance would be worth the cost.

David

RE: Pump with non-pump carb
 

I once drilled out a carb and barrel on a OS 40 FP. It did help the top end some. It made the bottom end awful for some reason but the plane flew wide open all the time anyway.

David

RE: Pump with non-pump carb
 

I ran a K&B .40 with the backplate perry pump with several non-pumped carbs. I had the best results using a .60 sized HB carb. I really liked that combination.

turbo

RE: Pump with non-pump carb
 

I'll keep my eyes out for a bargain carb. BTW, I picked up the pump and backplate for $4.:D

Is it normal for these pumped engines to take a moment to "catch up" with the amount of fuel being delivered, or am I maybe still a bit too rich?

David

RE: Pump with non-pump carb
 

Well, what do you know. Here's the whole outfit: 270167971730

David

RE: Pump with non-pump carb
 

You are still a little rich

RE: Pump with non-pump carb
 

Thanks, I suspected so. But not rich enough to win that auction I listed above.:D

RE: Pump with non-pump carb
 

My guess is your idle needle is a bit rich. The so-called idle needle is really the primary fuel control device over the entire operating range, whereas the high needle ideally should only reduce an overly rich top end. There should be no difference in response time to the throttle stick by using a pump. The major function of the pump is to maintain fuel pressure as the tank empties or the plane goes in a nose-up attitude which forces the carb to suck fuel over more vertical height. With the pump, the level of the fuel relative to the carb becomes almost irrelevant.

For sport flying, a rich low needle is usually nothing more than a mild annoyance, burning fuel a little less efficiently, slowing down throttle response, sometimes drowning the plug at idle speed. For the best setting, the low needle should be only as rich as it needs to be to feed the engine across its entire operating range. If the operating range needs to be smoothed out in spots, that should be done by leaning out the trouble spots.

RE: Pump with non-pump carb
 

Thanks majortom-RCU. I have the idle mixture set up just right for muffler pressure operation. I hate to alter it since I probably won't use this pump with this engine in a plane for a while. When I do use it, I'll definitely lean out the idle mixture a bit.

On the other hand, I do have an extra SF/RF .61 backplate with a pressure tap installed. Maybe I'll try pumping one of my .61's.

RE: Pump with non-pump carb
 

I have been saying this for years, Tom...

Many people just don't get it.


Every weekend I fly, I again see modelers trying to adjust out a mid-range 'stumble', by trying to tweak the high-speed needle...

It is never the other way around...


It looks like tamper-proof idle-needles in full-size car carburettors, or another phenomenon, put most modelers under the impression that 'The idle-needle is factory-set for life'...

So great is the reluctance by many (I almost wrote 'most') modelers to adjust the low-speed needle; even when the engine is running badly... They try fiddling with the high-speed needle, even after it has already been set perfectly for the very top of the range!


It is never the wrong time to say this again; this is the 'low-speed needle', 'part-throttle fuel control', 'fuel metering valve', or any other definition that will emphasize the fact that this needle is the primary fuel control. Not "idle-needle", because this puts people under the wrong impression that it only controls the idle mixture.

The 'high-speed needle' is the secondary fuel control, since it only affects the very top of the throttle's range and the top engine RPM.

Even worse... The Webra manual in the guidance for their TNII carburettor says "...and the idle needle (2) only affects the idle range. The tapered tip of the idle needle (B) influences the idle range in conjunction with the fuel supply opening the spraybar (A). What a poor choice of wording? Incomprehensible and cannot be further from the truth!

You can see it for yourself on page 5 [link=http://www.webra-austria.at/download/en/motor.pdf]here[/link].

RE: Pump with non-pump carb
 

Yes Dar, I know lots of guys as they become familiar with their glow engines are reluctant to poke away at that all-important low-end needle, don't have a screwdriver that fits in the tiny hole, certainly don't get a clue from the nomenclature, don't get much tutelage from the manufacturer (or worse, get misled by statements such as you describe from Webra).

Another feature of glow engine carbs that should be apparent upon casual inspection is the fact that the low needle opens up the spraybar as the throttle is advanced, and reduces the fuel path as the throttle is pulled back. Yet this basic knowledge seems to escape most glow engine users, at least until they find a two-cycle guru to explain it to them. These people are not engineers (nor am I), and they seem to accept that the mechanics of a simple engine with a handful of moving parts is a black mystery and not worth their time to delve into and study for themselves.

I am not dissing these people, because if you take them one by one they are a great bunch of folks. They also have talents in other areas that leave me in the dust. So it's really an issue of education and experience as I see it.

I've bought and studied at least a few of the 'glow engines 101' type publications, learned a lot from them at the time, but don't know that even there where we should find this explanation, that it was plainly stated. Anyway it sure has helped me tune my engines with much less frustration once it became clear to me.

And really, the only way to learn this is at the field or at least the test bench, with an engine and tools, enough patience to try a few different settings, a few different props, different fuels, different glow plugs, and either a good instructor or a publication that I have yet to find. So really, now that I think about it, there are a lot of subtleties involved, and it is a dark forest if you don't have a guide.

Great hobby though!

RE: Pump with non-pump carb
 

I fully agree, Tom.


As to the tapered low-speed needle moving into and out of the nozzle, as the throttle is operated, this is how fuel metering is achieved.
The orifice size changes as a result.

Super-Tigre carburettors have a blunt tipped needle, unmasking a 'cat-eye' slot in the spray-bar, as the throttle is opened.

In slide-valve carburettors, such as the Webra Dynamix and the OPS, fuel metering is quite similar.
In rotary carburettors, where the barrel does not move in and out, like the Perry and Moki, it is different still, yet similar.

Also for the OS 7D...

Enya (not like the OS LA) air-bleed carburettor also have fuel metering, but I don't know how this is done.
I don't have one.

RE: Pump with non-pump carb
 

I have found that the only way to really learn how these r/c carbs work is to disassemble them, study them and simply start adjusting them.

I agree 100% that if you don't understand the physical principles behind the operation of, say, a mixture control valve, you will never really know how or why to adjust it.

My older Magnum XL .46A has the squared-off/blunt mixture needle with the "cat's eye" opening in the spray bar. It is clearly modeled on an ST carb like the one Dar described. Another of my Magnum carbs has a tapered needle that travels into a spraybar that only extends half-way across the throat. You have to love Duke Fox's instructions telling you to file and reshape the needle by hand to adjust midrange! But, then, he expected people to either have some mechanical knowledge or at least be willing to learn.

The OS 4D is a bit different in that instead of a needle that plunges inside the spraybar it has a sleeve that travels over it, revealing and covering the slit as the barrel is rotated.

I recently restored a couple of OS 7L carbs. I had never seen one of these carbs up close before and had to figure out how they work by examining the parts. Their operation is unique, but, has some similarities with Perry carbs in that the entire mixture assembly rotates to adjust the angle of the spraybar's slit. But, the barrel has an outer sleeve similar to the 4D that exposes or closes off the spraybar slit as the barrel rotates.

I recently picked up an old Fox .15 - the one where the fuel nipple rotates with the throttle. Talk about a weird carb! Again, I had to disassemble it and study its mechanics to see what was going on. Now that it makes sense it is easy to adjust.

At any rate, they are all doing the same basic thing which is to gradually increase or decrease the amount of fuel exiting the spraybar in relation to the venturi's air intake opening over a large range of the throttle's travel.

Since the mixture control increases or decreases the opening in the spraybar (cat's eye or whatever) over most of the throttle's range it really can't be thought of as an "idle" control. I believe that the reason it gets referred to as an idle control is because most instructions tell you to adjust the mixture valve with the engine running at idle. Therefore, many folks assume they are adjusting only the idle.

I'm sure many pilots get their engines to run well enough without ever knowing why. I love buying old engines, taking them apart, figuring out their unique quirks and getting them to run. There's a definite reward involved in teaching yourself to repair something like an r/c engine. Since almost every manufacturer has their own unique carb design, their operation isn't self evident unless one takes the time to study the mechanics. My advice, like majortom-RCU's, is to take the time to figure them out on the work bench and test stand.

David

RE: Pump with non-pump carb
 

Holy cow! Look what someone ended up paying for that carb/pump/backplate!

Auction 270167971730

RE: Pump with non-pump carb
 

Darzeelon....

....could you elaborate further why you say the Webra manual is incorrect about the
low speed needle, and why the low speed needle is the "primary fuel control" ?

Are you sure the Webra's manual is as you stated:

"What a poor choice of wording? Incomprehensible and cannot be further from the truth! " ?

FBD.

RE: Pump with non-pump carb
 

Dar and MajorTom,

You can't get a Saito to run right if you don't know this about the low speed needle valve. I'm sure you had no problems with your Saito .72, Dar.

RE: Pump with non-pump carb
 

If I could have found a girl with the personality of a Saito, I'd have married her. A little late for that now, but I still enjoy taking my Saitos out for a fly.

RE: Pump with non-pump carb
 

Dave,


Saying "The idle needle controls only the 'idle range'" is incorrect...

There is no 'idle range'; only an 'idling speed', which depend on the engine and its adjustment by the user.

Supposing the user adjusted a sport engine to idle in a stable manner, at 2,400 RPM.
When this same engine is run with the throttle 3-4 clicks open at 3,800 RPM, it is not idling. It is, however, running at a part throttle setting.
Any other part-throttle position is just that; not in an idling 'range'.

If the writer of the Webra manual meant to say, as I believe, 'The idle needle controls ONLY the idle mixture...', than this person is plain WRONG!

Even at 3/4 open throttle, the mixture strength in nearly all two-needle carburettors, is still dependent on the setting of the low-speed needle only.
If the high-speed needle is closed far enough to significantly affect the mixture strength, when the engine is at 70-75% throttle, the engine will not go to full power when the throttle is advance to full. The mixture will be too lean!

Since the high-speed needle only controls the mixture strength in a very narrow throttle range, it is the less important fuel control, hence; 'the secondary fuel control'.

The low-speed needle, responsible for the mixture setting over 75% of the throttle's range, IS the more important, hence 'the primary fuel control'.

RE: Pump with non-pump carb
 

This discussion takes me back to an explanation in one of the 'glow engines 101' books I mentioned earlier, sorry I don't recall author or title, but his explanation was that the glow engine started out as a non-throttled powerplant, intended to run full-bore all the time. He may have given the example of the Fox .35 U-control engine, which was one of my early and long-time favorites. The trick with the Fox .35 was to start it up with its wide open venturi, since there was no throttle mechanism of any kind, then turn the needle valve (only one needle) to attain a rich two-cycle speed, then fly it around in a circle. Stunts required the nose to go up, which would reduce fuel pressure, causing the engine to go lean, resulting in more RPM, more power to go up in a loop or wingover, then the nose would turn down to complete the maneuver, fuel mixture would go rich, bringing the rpm down. That was how you tuned a U-control stunt engine.

Then, said the author, this perfectly sane mechanism got a bit more complicated when radio control came along and people wanted a throttle, so that power could be added or reduced over a wide range. I remember the exhaust restrictor approach, although I never owned or used one, but that was something that could be manipulated by a third wire on U-control. Anyway, continuing the thoughts of the author of the forgotten name, the two-stroke glow engine became balky and finicky with the effort to adapt a full-range throttle to it, simply because of the difficult challenge of setting the appropriate mixture of fuel to air over the range from 2000 to 15000 rpm (give or take). But some brilliant engineers came up with some pretty good solutions, and we got what we now have--some very good and sometimes not quite so good carburetor/throttle designs.

So here are a couple generations of guys like me who started off with a non-throttled engine, designed to operate wide open or not at all, with one needle valve, and naturally we all viewed the needle valve as the 'primary' fuel control. (Because it was the only fuel control.) I'm sure that paradigm was ingrained in engine/throttle/carburetor designers and users and manual writers, and hard to shake off.

But now we have the younger folks who have always had television, computers, proportional control radios, and throttles on their glow engines. Through no fault of their own, they are missing some historical perspective. But the old paradigm still lives on in the minds and habits of the old-timers who wrote the early manuals and established the traditional way of talking about needle valves.

Now I am just a hobbyist, not an engine designer or historian. I have furrows in my brow from thinking through the new paradigm that with a two-needle carb, the low needle is the more crucial part of the design, since it sets the foundation on top of which the high needle performs its more limited function. On the two-cycle glow engines with which I am most familiar, it is the low needle which moves in and out to open or restrict the fuel flow, and it moves whenever and wherever the throttle arm moves, whereas the high needle stays put no matter what the throttle arm does. That makes the low needle the primary fuel control mechanism. This understanding clarifies a lot of rules I was taught without being taught the explanation behind them--such as, set the high speed needle only with throttle wide open, not at any degree of partial throttle; tune an engine by setting the high needle to a rich two-cycle, then tune the low needle for stable idle and clean transition, then go back and re-set the high needle. The high needle setting in a two-needle carb can only be set in relation to the low needle setting. Any change in the low needle setting necessarily throws off your high needle setting as well.

This is more than just boring theory. Figuring this through with my ossified brain cells made a big difference in my enjoyment of my engines, getting them so they would take off when I want, stay in the air when I want, and come down in one piece.