Author Topic: Simple mod to 25LA (Read 4087 times)

Brett Buck · « **on:** March 16, 2015, 01:06:07 PM »

If you take a bone stock 25LA, remove the rear needle, add an ST needle and spraybar, guess what great improvement you have made = you lose between 25 and 30% of your power.

Instead of peaked out 14,000+ on a 9-4, you get ~12600-12800. (12800/14100)**3 = ~.75 or 25% power loss. Not to mention the fact you now have to launch it nearly gutted out to get the required in-flight speed, at which point it starts quitting lean in the overheads. Now you need a 9-5 and associated speed stability loss.

As a bonus, it's very difficult to reverse at the field, because you can't just put the original stuff back in, it needs to be sleeved to restore the fit of the much smaller OS spraybar

Brett

Tim Wescott · « **Reply #1 on:** March 16, 2015, 01:22:44 PM »

I can't say I'm surprised, since you're significantly cutting down the venturi throat area. Once again, we find out that the fundamentals still hold.

Did you by chance also do the test with an OS-sized needle, and with a venturi that's bored out to restore the area when you use an ST needle? I very much expect that the correct-sized in-venturi needle will run the same as a remote valve, and I'd be surprised if the ST-sized needle in a properly bored-out venturi didn't (but less surprised than if the correct size needle in a stock venturi showed a power loss).

It does complicate the "put a standard needle valve in it if you want to" instructions.

Brett Buck · « **Reply #2 on:** March 16, 2015, 02:25:50 PM »

Quote from: Tim Wescott on March 16, 2015, 01:22:44 PM

I can't say I'm surprised, since you're significantly cutting down the venturi throat area. Once again, we find out that the fundamentals still hold.

Did you by chance also do the test with an OS-sized needle, and with a venturi that's bored out to restore the area when you use an ST needle? I very much expect that the correct-sized in-venturi needle will run the same as a remote valve, and I'd be surprised if the ST-sized needle in a properly bored-out venturi didn't (but less surprised than if the correct size needle in a stock venturi showed a power loss).

It does complicate the "put a standard needle valve in it if you want to" instructions.

No, this was just the usual "break it before you run it" approach, discovered at the field, also ending in the usual frustration. I expect it would work fine if you altered the venturi to get the same choke area, but, the point being, *it was already right*, so now we are "fixing the fix".

The whole thing was pretty innocent and I know no one starts out to try to mess up, but I see it time after time after time, and no amount of warnings, cautions, pedantic lectures, etc, seems to ever make any difference. I have long since lost count of the people who call or write or post about how the 20FP doesn't work worth a darn and then find out that some "upgrades" were made - like an ST needle, "better" muffler, bored-out muffler, added head gaskets, 10-6 Top Flite, etc.

Another part of the issue, and I knew about it but hadn't seen it recently, is that people are frightened of actually leaning out the engine, the old Fox 35 2-stroke = burn down phenomenon. Just unable to bring themselves to crank it up sufficiently to get any sort of performance out of it.

Brett

Carl Cisneros · « **Reply #3 on:** March 16, 2015, 02:36:01 PM »

Lets face it guys

The smaller motors are MADE to make their power in a higher RPM band, NOT be lugged down in RPM...................(like a Fox)

Leave well enough alone and they run very nicely.

Just make sure you have CLEAN fuel, a GOOD glow plug AND glo-driver.

You will be rewarded with a very nice running little motor.

(there is always the tank, tank position, prop, plane it is in, ect. you get the basic idea folks)

I have come back to CL flying after decades away from it and have discovered there STILL are folks around
that HAVE TO MOD their motors (or think they have to) to get the motors to run PROPERLY, or is that kind of properly sort of.

Brett, I hope I said this correctly..........................

Tim Wescott · « **Reply #4 on:** March 16, 2015, 03:40:11 PM »

Quote from: Carl Cisneros on March 16, 2015, 02:36:01 PM

Leave well enough alone and they run very nicely.

That's the point Brett's trying to make, I think.

Quote from: Brett Buck on March 16, 2015, 02:25:50 PM

No, this was just the usual "break it before you run it" approach, discovered at the field, also ending in the usual frustration. I expect it would work fine if you altered the venturi to get the same choke area, but, the point being, *it was already right*, so now we are "fixing the fix".

I have an LA 25 with a "standard" needle that runs very nicely. BUT -- it's an OS needle with a spraybar that's presumably the same diameter as the spraybar for the remote needle. It came to me that way, it works great, it fits in spots that a remote-needle LA 25 wouldn't -- what's to complain about, eh?

Quote from: Brett Buck on March 16, 2015, 02:25:50 PM

The whole thing was pretty innocent and I know no one starts out to try to mess up, but I see it time after time after time, and no amount of warnings, cautions, pedantic lectures, etc, seems to ever make any difference. I have long since lost count of the people who call or write or post about how the 20FP doesn't work worth a darn and then find out that some "upgrades" were made - like an ST needle, "better" muffler, bored-out muffler, added head gaskets, 10-6 Top Flite, etc.

I know the feeling -- I've got a good friend who occasionally flies CL, and I could not talk him out of putting a Fox combat engine on a Top Flight Score (because hey -- it's a Fox, and they're good stunt engines, right?). We finally got it to sorta kinda work right with half the venturi plugged up with a piece of wood, but it still wasn't nearly as nice a run as you'd get from an LA 46. Now he won't fly CL any more because he doesn't like 4 second laps with a Top Flight score on 60' lines.

I think part of the reason is that it just looks so simple, so the urge to mess around gets irresistible.

I'm actually very likely suffering from the opposite problem -- there are details of my current engine setup that, from reading posts by you and others here, don't sound ideal. But I don't want to mess with it, because it's working well enough for me the way it is. At some point I need to dedicate a month to messing with the power train, and see if I can get more better out of an LA 46 (or at least less slime).

Mark Scarborough · « **Reply #5 on:** March 16, 2015, 10:33:38 PM »

Quote from: Tim Wescott on March 16, 2015, 03:40:11 PM

I'm actually very likely suffering from the opposite problem -- there are details of my current engine setup that, from reading posts by you and others here, don't sound ideal. But I don't want to mess with it, because it's working well enough for me the way it is. At some point I need to dedicate a month to messing with the power train, and see if I can get more better out of an LA 46 (or at least less slime).

uh,, are you doing it the way you were told Tim? do ya really know what "working well " is,, lol,,

Brett Buck · « **Reply #6 on:** March 16, 2015, 11:16:30 PM »

After some figuring, the stock arrangement has ~.0185 square inches. The stock venturi and .157 ST spraybar is .0142 square inches. .0142/.0185 = =.77, or 77%. Or about the same as the power difference.

To get the same choke area with an ST spraybar, you need about a .275 venturi.

Interestingly, just about every successful high rev/low pitch system (piped or otherwise) regardless of the displacement, comes out in about the same range between about .0175 to .0190 square inches. Thats about the equivalent of the stock venturi on an ST46, right down the middle of the range normally used for a 40VF, and is *more* choke area than David runs in his 75 as set up for the 2008 WC winner.

The 25LA only requires about 3 ounces of fuel, while the mighty 75 is over 8. The 75 will suck VASTLY more air, and therefore more fuel, than the 25. Or very roughly 3x as much, or if you prefer, 75/25ths.

Brett

Dan McEntee · « **Reply #7 on:** March 17, 2015, 06:25:48 PM »

The only way I think that you could improve the LA.25 or .15, if you are having any sort or problems, is to replace the needle valve assembly with one from an older OS engine carb or rear needle valve assembly from the FP-series, which is basically the same thing I think. The current production needles have an o-ring in them to seal and they sometimes leak. swapping around o-rings sometimes cures it. But the earlier needle valve assmbly doesn't use any o-rings to seal and works great. On the older OS carbs the needle assembly screws out of the carb body. it's a standard 4mm thread. This needle assembly can be threaded into a venturi body to make a spigot type set up, or mounted on a metal bracket ( that OS has gone back to) and continue rear needle operation. I played with this a lot when my son Sean was little and still prop shy when adjusting the needle. The only "thing" was that you had to really choke it a lot more than normal for flip starting, and it took a bit more care with the tank placement to get a clean shut off when out of fuel. Otherwise, the engines ran the same. I used it quite a bit on OS.40FPs for him back then, and I think I even tried it out an a Brodak .40 just for the experience. I hope I'm making myself clear with descriptions, and just don't know any other way to explain it and can't post photos. One tiny mod that was necessary if using on a rear needle set up was a single wrap of copper wire soldered around the fuel inlet, so fuel line had something to grip when pushed on, like a barb. Of all the OS.40s I've had, that's why I never got rid of the carbs, because I could still make use of the needles and needle valve assemblies.
Type at you later,
Dan McEntee

Phil Krankowski · « **Reply #8 on:** March 18, 2015, 09:34:06 AM »

My venturies have o-rings, but the spray bars do not. I have converted RC with both using the remote NVA and using a spraybar NVA I am nearly positive that the spraybar diameter for the two different setups are identical. The hole clearance seems the same. Easy process once the carb is unscrewed...getting the screws out on a used engine is not always easy though. (heat gun helps)

I wish I could comment on the run differences, but I have only benched these engines. Still building the models, at the covering stage on 2.

Phil

Dane Martin · « **Reply #9 on:** March 18, 2015, 10:02:19 AM »

Hi Brett,
Do you run your 25LA engines on muffler pressure, or suction? I run mine on muffler pressure and a plastic clunk tank.

Brett Buck · « **Reply #10 on:** March 18, 2015, 10:09:11 AM »

Quote from: Dane Martin on March 18, 2015, 10:02:19 AM

Hi Brett,
Do you run your 25LA engines on muffler pressure, or suction? I run mine on muffler pressure and a plastic clunk tank.

Muffler pressure into the conventional vent tube on a Sullivan SS-4 slant tank.

Brett

Brett Buck · « **Reply #11 on:** March 18, 2015, 10:25:07 AM »

Quote from: Phil Krankowski on March 18, 2015, 09:34:06 AM

My venturies have o-rings, but the spray bars do not. I have converted RC with both using the remote NVA and using a spraybar NVA I am nearly positive that the spraybar diameter for the two different setups are identical. The hole clearance seems the same. Easy process once the carb is unscrewed...getting the screws out on a used engine is not always easy though. (heat gun helps)

The remote needle "nozzle" is the same diameter as the front spraybar. The remote needle has only one functional shortcoming -it's a little coarse, and sometimes one click is more than you would want. On the other hand, it prevents you from trying to be too clever with the adjustments.

The conventional OS front spraybar/needle is one of the best ever made, you will be seeing my reasoning on why in a future Stunt News, should I ever get time to write my column. I'd put it in a RO-Jett or PA if there was some clean way to make it fit.

I haven't tried it on the 25LA, but on the 20FP, the front needle and the later rear needle gave nearly identical performance in all respects. I think the 20FP rear needle assembly was generally better than the 25LA version - it was on a separate bracket like the 46LA, but used the guts of an RC carb needle. They used to sell it separately in a read needle conversion kit. I used that to good effect, as shown below.

As supplied, on a profile, the first time you crash inverted, it breaks off, and breaks the backplate in the process. It's possible to rotate the backplate 90 degrees to put the needle up near the head, and it will still turn over without the piston hitting the backplate, but I haven't yet run it that way.

Brett

Phil Krankowski · « **Reply #12 on:** March 18, 2015, 04:50:07 PM »

Quote from: Brett Buck on March 18, 2015, 10:25:07 AM

The remote needle "nozzle" is the same diameter as the front spraybar. The remote needle has only one functional shortcoming -it's a little coarse, and sometimes one click is more than you would want. On the other hand, it prevents you from trying to be too clever with the adjustments.

The conventional OS front spraybar/needle is one of the best ever made, you will be seeing my reasoning on why in a future Stunt News, should I ever get time to write my column. I'd put it in a RO-Jett or PA if there was some clean way to make it fit.

I haven't tried it on the 25LA, but on the 20FP, the front needle and the later rear needle gave nearly identical performance in all respects. I think the 20FP rear needle assembly was generally better than the 25LA version - it was on a separate bracket like the 46LA, but used the guts of an RC carb needle. They used to sell it separately in a read needle conversion kit. I used that to good effect, as shown below.

As supplied, on a profile, the first time you crash inverted, it breaks off, and breaks the backplate in the process. It's possible to rotate the backplate 90 degrees to put the needle up near the head, and it will still turn over without the piston hitting the backplate, but I haven't yet run it that way.

Brett

Funny thing about those plastic backs. I tried every position, spent some quality time loosely mounting the engine and taking it back off, and the original position was the only one that would allow the fuel lines to be routed, and not interfere with the fuselage.

It was on the ARF I am rebuilding after an inverted pancake. Now I have an unattached remote NVA... I think I'll wire it down somewhere so it isn't flopping about. I don't currently have a second NVA spray bar to swap out with, and the engine will get benched at least once before flying, probably on the plane.

Phil

Steve Helmick · « **Reply #13 on:** March 19, 2015, 01:59:36 PM »

My .25LA-S runs nicely with the stock venturi drilled to .272" and a .156" K&B universal NV Assy. I go for the metal backplates and front NV. I can't say I've tached it WFO with a 9-4 APC, or run real 10% in it. I've used a variety of props on it, 9-4 APC and Graupner, and a reworked TF Power Point 10-4 all seems to work about the same...pretty well.

Steve

Edit: I recall that I drilled out the muffler outlet a bit, which in effect improves pumping and acts as tho the venturi is bigger. Also, I was running 10-22 from before the VP purchase of Powermaster, which has been reported to be more like 7 to 8% nitro.

Russell Shaffer · « **Reply #14 on:** March 20, 2015, 04:58:04 PM »

Two screw eyes work pretty well to relocate a broken back plate NV.

Chris Wilson · « **Reply #15 on:** March 20, 2015, 05:35:08 PM »

Quote from: Brett Buck on March 16, 2015, 11:16:30 PM

Interestingly, just about every successful high rev/low pitch system (piped or otherwise) regardless of the displacement, comes out in about the same range between about .0175 to .0190 inches.

Brett

Why is that Brett?

Brett Buck · « **Reply #16 on:** March 20, 2015, 07:34:01 PM »

Quote from: Chris Wilson on March 20, 2015, 05:35:08 PM

Why is that Brett?

Because you need about the same amount of power regardless of displacement.

Brett

dennis lipsett · « **Reply #17 on:** March 20, 2015, 10:08:57 PM »

Quote from: Brett Buck on March 18, 2015, 10:25:07 AM

The remote needle "nozzle" is the same diameter as the front spraybar. The remote needle has only one functional shortcoming -it's a little coarse, and sometimes one click is more than you would want. On the other hand, it prevents you from trying to be too clever with the adjustments.

The conventional OS front spraybar/needle is one of the best ever made, you will be seeing my reasoning on why in a future Stunt News, should I ever get time to write my column. I'd put it in a RO-Jett or PA if there was some clean way to make it fit.

I haven't tried it on the 25LA, but on the 20FP, the front needle and the later rear needle gave nearly identical performance in all respects. I think the 20FP rear needle assembly was generally better than the 25LA version - it was on a separate bracket like the 46LA, but used the guts of an RC carb needle. They used to sell it separately in a read needle conversion kit. I used that to good effect, as shown below.

As supplied, on a profile, the first time you crash inverted, it breaks off, and breaks the backplate in the process. It's possible to rotate the backplate 90 degrees to put the needle up near the head, and it will still turn over without the piston hitting the backplate, but I haven't yet run it that way.

Brett

Brett,

While I agree that the OS needle valve used in a normal front venturi instillation is one of the best made I would offer my opinion that the Enya is superior for one simple reason. It is better made. The needle is a much better fit into the spray bar that OS. I've never had the needle screw cap come loose while adjusting the needle, and I've never had to put a piece of tubing on the spray bar stem to guarantee a non leak air assembly.
Perhaps it is a little more work to buy one but for the time and effort I rate it worth the effort and they are readily available on E bay.

Chris Wilson · « **Reply #18 on:** March 21, 2015, 12:16:58 AM »

Quote from: Brett Buck on March 16, 2015, 11:16:30 PM

..........between about .0175 to .0190 square inches .......

Brett

You do realise that the above dimension is below 1/200th of a square inch?

Brett Buck · « **Reply #19 on:** March 21, 2015, 11:49:52 AM »

Quote from: Chris Wilson on March 21, 2015, 12:16:58 AM

You do realise that the above dimension is below 1/200th of a square inch?

What are you talking about? Someone edited my text when quoting? I will prefer to assume you messed it up when editing, as opposed to removing the words, and then using that to play a "gotcha" game. I note that you DIDN'T edit it in the second post, but you had it wrong in your first post.

Brett

here is the original UNEDITED post:

Tim Wescott · « **Reply #20 on:** March 21, 2015, 12:00:08 PM »

Someone didn't do their math right either when they saw 0.017 as being less than 0.005, or in going from 1/200 to something bigger than 0.017.

Brett Buck · « **Reply #21 on:** March 21, 2015, 12:02:17 PM »

Quote from: Tim Wescott on March 21, 2015, 12:00:08 PM

Someone didn't do their math right either when they saw 0.017 as being less than 0.005, or in going from 1/200 to something bigger than 0.017.

He "accidentally" edited out the "square" from square inches in his first quote, then when I didn't catch it, I was suddenly advocating .019 *diameter* venturis. I prefer to think it was an "accident" since he failed to edit out the "square" the second time.

Brett