G1K

First off, no its not a MDS... but maybe not much better.

I have a Magnum GP .25, or unknown vintage, but I'm guessing late 80's early 90's. I put it in a CG Eaglet 50 and brought it out to the field today. I tried getting it to run, with fuel of the same vintage.. no go.. It would start and barely idle, but when I took off the glow heater, it died. Also, if I advanced the throttle, it died.

I ran some newer fuel (15%) threw it at home, it would idle ok, but would not rev up. I pulled the hose from the muffler, and I could get it to rev, but then it died. I'm not sure if this was because it was leaning out as there was no tank pressure... I did not try to take the glow heater off this time.

I did notice what appeared to be raw fuel coming from the muffler tip, thinner consistency that the oil that is in the fuel.

Here's the catch, I'm not sure how to tune the carb as I have no manual for the motor. (I have other glow motors, but the have already been tuned and have not been run since the early 90's) I would like to learn how to set it up right... so what can I do. The carb's got 2 screws, (one high and one low needle adjustment?)

Is there a good base line I can use such as turn the for most needle out 3 turns from all the way in and the aft needle 2.5 times out from all the way in and adjust from there?

TIA for any help anyone can provide. I'd like to get this bird into the air...


Ryan

G1K

Here are two pics of the engine. Another thig I wanted to add, I bought a muffler for a newer magnum .25 hoping it would be a direct fit.. no go, but I did bore it out and tap a new hole so it would fit. I noticed a small leak coming from the rear most screw.. I will go back and lap the surfaces to make sure they are flat, and use a better gasket... could this have been the root cause of my trouble?

G1K

Second image:

Dauntae

I got this from another thread and have posted it for a few others also on how to tune an engine.

Dauntae

Here's a scenario: Pilot takes off, plane's engine sounds great. After several minutes of flying, engine seems to lose power, sounds kinda "thin", pilot keeps flying. Engine continues to sag, now full throttle is very weak, pilot now understands that maybe this isn't gonna clear up. Engine dies (what a shock! ), pilot calls for deadstick landing overshoots, tears off landing gear, etc. Never seen this at your field, right?

Here's the way that *I* set mixture on non-airbleed carbed engines (90% of the engines out there fit this category, but the theory is similar for air-bleed carbs). First of all, understand that the high speed needle has its main effect from 3/4 to full throttle, and the low speed needle controls everything from idle up to 3/4 throttle. It thus makes sense to me to spend the biggest majority of my tuning time adjusting the needle that controls the largest portion of engine running, right? Also, remember that there is a proper air to fuel ratio (mixture) that allows the engine to run properly. Too much fuel is rich, and too little fuel is lean. We "richen" the mixture by adding more fuel (turning the needle out, or counter-clockwise), and we "lean the mixture out" by decreasing the fuel (turning the needle valve in, or clockwise).

I start the engine give it full throttle, and lean it to it's highest rpm (peak), then richen it by maybe a quarter turn. Then with the glow plug igniter still attached, I slowly close the throttle to an idle rpm. At the lowest rpm that the engine will still reliably run, I then remove the glow igniter. If the engine dies immediately, I know it's too rich, and I then lean out the LOW SPEED NEEDLE by 1/8th of a turn (don't touch the high speed needle). Start the engine again, (and this is important) give FULL throttle briefly to clear out excess fuel, then slowly close the throttle again. Remove the glow igniter, and this time it may run a little longer before it dies, so lean the low speed another 1/8th turn. Re-fire the engine, give a burst of full throttle to clear it out, and slowly close the throttle again. remove the glow igniter and now notice that the rpm DROPPED a bit when you removed the glow igniter, but the engine kept running. We're getting there. It's still too rich, and you'll prove that by opening up the throttle and hearing the engine "blubber" then die. That's because excess fuel has collected in the crankcase during the rich idle, and when you opened up the throttle, the excess was pulled into the cylinder, making it WAY too rich. Supposed you were on a landing approach, and decided to go around, you throttle up but the engine "blubbers" and then dies (another thing we haven't seen, right?). Yep, the LOW SPEED needle was still too rich, allowing excess fuel to collect in the crankcase, just WAITING on you to try to go around so it could "LOAD UP", blubber, and die!

Keep leaning the low speed needle down until it idles well, but now, when you open up the throttle, it HESITATES instead of BLUBBERS. When this happens, you've lean it down too far, so richen it up 1/16th of a turn and try again. You know you've got the LOW SPEED needle right when you can fire it up, remove the the glow igniter, and the rpm doesn't change AT ALL, and you can open the throttle up, and it doesn't blubber or hesitate, it just runs!

The final thing you do is re-adjust the HIGH SPEED NEEDLE, leaning it to it's highest rpm (peak) and then richening it up maybe 1/8th turn to give it a slightly rich mixture. We also know that the fuel mixture will change in flight when you point the nose up (harder for fuel to travel uphill) and also as the fuel level in the tank changes. In both cases, a leaner mixture results, so we actually need to set the mixture a bit further on the rich side to account for this. While the engine is running at full throttle, CAREFULLY pick the model up and raise the nose to at least a 45 degree angle while listening to the engine. If the engine sags a bit, then you'll need to richen up the high speed needle 1/16th turn. Try it again, and when you can point the nose up and the engine doesn't sag, but maybe shows a slight GAIN in rpm, you know you've got it right.

Now the engine will be happy, and chances are will reward you with reliable running. If you've got one of the few engines with an air bleed adjustment for low speed adjustment, the theory is the same, just refer to your manual to see how to richen and lean the low speed mixture.

G1K

Dauntae,
Thanks for the information.

My situation, I can't get the engine to run much faster than idle with the muffler line on, and when it's off the motor will only run for a few seconds.

What can I base line adjust the needles to (and which one is which) to get the motor to run and rev up. I'm pretty confident I can take it from there.


Ryan

JimTrainor

It sounds like the high speed (main) needle is simply too lean. Try turning it out a few turns, don't be shy with it. Once you've got it running wide open, close the needle until the RPM peaks, then back off a bit.

I suggest leaving the glow driver on in the even it is way too rich to begin with.

I'd back out the low speed needle to start with as well. Once you have the high speed sorted out, you can turn your attention to the low speed.

I don't think the muffler would have much to do with it as long as it is about the right size, and supplying at least some pressure to the fuel tank.

Dauntae

In the pic it looks like a airbleed carb, To richin it up you have to screw the screw in to lean it you take it out. Try setting the high end 2 1/2 turns out and the low end screw it in and back it out 1 turn, Airbleed low end does not adjust how much fuel is going in it adjusts how much air is getting into the mix. With the engine set like this it should be blubbering rich so keep the glow driver on for now, try and get it to full throttle and lean out the high end to max RPM like JimTrainor suggests then go to a lower idle and it should start to blubber a bit, lean it 1/16 a turn counter clockwise and then idle up slowly. It will most likely load up and might stall but might not with glow started still on. Keep doing this until you get a smooth transition but you will have to reset the high end like you did at first every 3-4 low end adjustments. Once you you get a good idle and transition reset the high end and you should be good to go.

Dauntae

MrMotor

Ryan
The first thing you need to do is get a high speed needle valve to turn into the neelde valve body. Both pictures show that it is missing from the carb. It turns onto the exposed threads and is held in position by the clip from turning on it's own. With the needle missing it draws air through the inside of the tube that the needle screws into causing the engine to run way too lean at any setting above idle.

Flyboy Dave

My Feedback: (21)

....Good eye....MrMotor....

Aero330LX

My Feedback: (-1)

EEK!!! I see a disaster waiting to happen. I just have to say something about this...that PC-1 (the thing on the throttle holding that rod with the screw in it)... Loose it! Throw that thing as far as you can throw it. That screw will vibrate loose, and the little keeper that holds it? That's about as reliable too. Please...I beg 'ya...put on a clevis or ball link on that throttle. If there is no threads you can get a solder on clevis. I've seen more of them damn things cause crashes that I care to remember. I've even see them cause planes to hit other planes in the pits. They make setup easy I know, but they will come loose. Why they still sell those things is beyond me. If you have any on your controls you'll want to change those too. Again, I've seen those things cause crashes soooooo many times. Whenever I see those things, i have to tell people...just don't wanna see or hear about anymore of those things causing planes to go in.

lynngordon

Ryan, from your comments in the first post, you have the high speed needle, because you said you adjusted it, even though it is missing from the photos. On an air bleed carb, the low speed needle is initially set to cover one half of the hole in the front of the carb - look directly into the front of the carb and you will see the small hole for the air bleed - adjust the needle until it covers half of it. From this point, you would open up the hole (screw the needle out) to lean out the idle mixture, or close off more of the hole with the needle to get a richer mixture.

See this thread for a complete explanation of how to adjust your air bleed type carb:

http://www.rcuniverse.com/showthread...air+bleed+lynn

Lynnn

G1K

Thanks for all the great info gents. I have spent a few hours looking for the high speed needle, and it seems to have vanished. It was on the at the field, and now it's gone. The little trolls that live at my field must have taken it....

So where can I get a new needle for this motor?


Oh, I will change the method of attaching the rod ends to the servos and motor horn... I wouldn't fly with this setup, I was using these to get the throws set up and motor functioning properly, and needed advice from some of the locals at the field. I don't even have the hinges glued in yet...


Ryan

lynngordon

Since Tower Hobbies carries Magnum engines, I'd check with them first.

Lynn

JimTrainor

http://magnum.globalhobby.com/
... is the distributor. There is a service link on that page.

Matt Kirsch

My Feedback: (21)

That's a Magnum GP engine, which is the exact same engine as a Thunder Tiger. Global Hobby/Hobby People will definitely NOT be carrying the needle valve. Magnum and Thunder Tiger parted company several years ago. Tower doesn't stock parts for the Thunder Tigers either, AFAIK.

You need to find a hobby shop that is an Ace Hobby dealer, one that sells Thunder Tiger products. The needle valve for a Thunder Tiger GP25 will be the exact right one for your Magnum.

If the needle valve fell out, then there's something more wrong with the engine than just the needle setting.

DanSavage

My Feedback: (5)

Get an O.S. .15/.40 FP needle valve assembly from anyone who has it. I don't think you'll be able to order just an needle valve. Done it many times. It's a bolt-on replacement part as were most of the O.S. FP parts on Magnum GP engines.

Also, the Magnums seemed to have problems with quality control for a while and most of the time it showed up in the carbs. Check the play in the carb barrel. As the engine warms up after a couple minutes of run time, the gap between the carb barrel and the carb body increases which causes the engine to die lean.

If it does have a gap, get an O.S. .25 FP carb and bolt that on in place of the GP carb.

Dan