OS Max 15LA Won't Run Upside Down

[EvilC57]

I've recently finished this little Marquess model (pics below) based on the old KK Marquis.

It has an old OS Max 15LA fitted, which was my first engine from 20 years ago, dug out of the loft and bought back into commission again.

After de-gunking the engine after years of disuse, it started first time and ran fine on the test stand - so I built the model to put it in. However now it's in the model, it's playing up. It'll only start the right way up (I.E. with the model upside down); although in my experience this is not unusual - even for my other bigger OS engines. Once started with the model inverted, if the model is turned right side up again, the engine cuts out at any more than 1/2 throttle.

It's been treated to new fuel, a new carb, and a new OS No.6 (was A3) plug, but still doesn't like running upside down.

A number of the experienced 'engine whisperers' in our club have had a go at fettling it, but it's beaten us all. If you start it upside down, and get it running perfectly at the top end, and the bottom end with the air-bleed screw, then turn it over, all appears well until throttling up as if for a take off run, when it stalls pretty much every time despite numerous attempts at tweeking the needle valve.

It's running with the usual exhaust pressurised system, and I've checked (several times) that the tank clunk pickup is free, clear and of the correct length.

It has been suggested that the tank may be too low in the fuselage, however looking closely at it, the top is within 5-10mm of the level of the needle valve, as suggested in the handbook.

Help! I'm running out of ideas, other than ditch the engine and convert the model to electric.

Engine installation, tank removed.

[Richard Wills 2]

Sounds very much like a tank height issue. The centre of the tank should be level with the carb spraybar, not the needle valve it may or may not be the same as it is remote.. Also once the needle is set for full max rpm at full throttle leave it alone. Transition can be tuned by slightly adjusting the air bleed to give either a richer or leaner idle.

Edited By Richard Wills 2 on 05/09/2020 19:09:56

[J D 8]

As with Richard, tank centre level with spray bar. If feed pipes coming through engine mount as it looks then tank to high in fuselage .

[Denis Watkins]

Am with JD8, and that tank height can definitely siphon when the model is upright.

Siphoning produces excess fuel which will help the plug go out

Can you get the tank onto the floor of the model

Edited By Denis Watkins on 05/09/2020 19:40:31

[EvilC57]

Posted by J D 8 on 05/09/2020 19:14:58:

As with Richard, tank centre level with spray bar. If feed pipes coming through engine mount as it looks then tank to high in fuselage .

I take it you mean too low in the fuselage. Don’t forget in the close up picture above, the model is upside down.

[EvilC57]

Posted by Denis Watkins on 05/09/2020 19:35:25:

Am with JD8, and that tank height can definitely siphon when the model is upright.

Siphoning produces excess fuel which will help the plug go out

Can you get the tank onto the floor of the model

Edited By Denis Watkins on 05/09/2020 19:40:31

The tank is already on the floor. I suspect that it actually needs raising to get it up more level with the carb spray bar, which won’t be easy as it’ll need a number of mods to the model.

[Denis Watkins]

.

Edited By Denis Watkins on 05/09/2020 20:32:55

[EvilC57]

As can be seen in the photos below, if anything, the tank is lower than the carb.

The general concensus on our field is that it shouldn't be that critical, after all we all do a 'nose up' full power check before at least the first flight of the day (don't we?), where the carb is well above the tank. And engines should be happy to blow/suck enough fuel up to continue running OK then, and during prop hanging manoeuvres etc..

Position of the tank

Tank in situ

[Jon H]

The mixture is likely too lean judging by your description.

Look at it like this. Most of the tank is below the needle when the model is upright, this is as it should be so d0ont alter it. When you turn the model over to start it most of the tank is now above the needle. If you then tune it for this configuration it will be tuned in such a way that it needs a gravity assist from the high tank. When you then flip the model back over the tank will drop below the carb and gravity is now resisting fuel flow. As a result the engine will run lean and snuff it as it has not only lost its gravity assistance but its gained a gravitational resistance as well.

By all means get it started with the model inverted, but once its going flip it over and then give it a tune.

The nose up test is a pointless exercise as well, just ignore it. 

Edited By Jon - Laser Engines on 05/09/2020 20:49:25

[Richard Wills 2]

The general concensus on our field is that it shouldn't be that critical, after all we all do a 'nose up' full power check before at least the first flight of the day (don't we?), where the carb is well above the tank. And engines should be happy to blow/suck enough fuel up to continue running OK then, and during prop hanging manoeuvres etc..

never

[EvilC57]

Jon - Laser Engines: “By all means get it started with the model inverted, but once its going flip it over and then give it a tune.”

Yep. We’ve been doing that, but it still won’t run reliably with the model the right way up, despite some of the most experienced old hands in our club looking at it.

”The nose up test is a pointless exercise as well, just ignore it.”

Not sure I can agree with that. I’ve had many an engine run fine when level, and then lean out and needed richening by a couple of clicks when the model goes vertical.

[Jonathan W]

Also, try running it with and without exhaust pressure to the tank. In my experience, using air bleed carbs with/without exhaust pressure significantly alters the relative mixture control from WOT to idle/mid.

What I mean is, if you have exhaust pressure and tune for top end power, your needle setting will be say 1.5 turns (example). Then if you remove the exhaust pressure and re-tune, you would need say 2 turns to get the top end correct. Now, when the engine is idling, there is no (or very little) exhaust pressure in either case, so in the configuration with an exhaust pressure pipe, the idle/mid mixture is leaner than when the carb is tuned without the pressure pipe.

I hope this convoluted explanation makes sense, but I have experienced this effect myself in practice.

You might find the whole thing would work better with a twin needle carb.

PS Nice looking model, I hope you get it in the air soon!

Edited By Jonathan W on 05/09/2020 21:14:09

[PeterF]

Is the plug getting wet with the inverted engine, could a hotter plug help. It has been a few years since I ran glow, so I may be writing nonsense.

[EvilC57]

Posted by PeterF on 05/09/2020 21:13:13:

Is the plug getting wet with the inverted engine, could a hotter plug help. It has been a few years since I ran glow, so I may be writing nonsense.

An OS No.6 (formerly A3) is fitted. That’s as hot as they come.

[Keith Berriman]

Has anyone in the club likely to have an old Taylor glow plug with idle bar

It worked for me on my only inverted set up

[Jonathan W]

Enya No.3 might be a little bit hotter, or you could even try a 4 stroke plug. Anything is worth a go rather than re-construct the whole thing for a different power unit.

[EvilC57]

Posted by Jonathan W on 05/09/2020 21:07:31:

Also, try running it with and without exhaust pressure to the tank. In my experience, using air bleed carbs with/without exhaust pressure significantly alters the relative mixture control from WOT to idle/mid.

What I mean is, if you have exhaust pressure and tune for top end power, your needle setting will be say 1.5 turns (example). Then if you remove the exhaust pressure and re-tune, you would need say 2 turns to get the top end correct. Now, when the engine is idling, there is no (or very little) exhaust pressure in either case, so in the configuration with an exhaust pressure pipe, the idle/mid mixture is leaner than when the carb is tuned without the pressure pipe.

I hope this convoluted explanation makes sense, but I have experienced this effect myself in practice.

You might find the whole thing would work better with a twin needle carb.

PS Nice looking model, I hope you get it in the air soon!

Edited By Jonathan W on 05/09/2020 21:14:09

Yes I kind of see what you mean. We’ve tried connecting & disconnecting the pressure feed, and the results were somewhat inconclusive.

Thanks for the compliment. I hope I get it in the air soon too. It was finished last November, but with the lockdown and engine problems, it’s yet to have its maiden.

[EvilC57]

Posted by Jonathan W on 05/09/2020 21:21:29:

Enya No.3 might be a little bit hotter, or you could even try a 4 stroke plug. Anything is worth a go rather than re-construct the whole thing for a different power unit.

Good idea, thanks. I have a new Enya No.3 plug in stock, I’ll try it sometime...

[PeterF]

I always used Enya no3 when I flew glow, hence my suggestion about trying a hotter plug.

[J D 8]

I ran an OS LA 15 for many years in an o/d funfly, it was mounted nearly inverted and ran fine at all strange angles the aircraft got into. Ran with Enya No3 plug.

Tank position looks good. Could be an air leak or vibration issue when the right way up. With it running look at remote needle to carb pipe for bubbles.

I never bother with the nose up test. Start up and run and if it holds full power on the ground for 20 second's for the first flight without dropping off then good to go.

[Jon H]

Posted by EvilC57 on 05/09/2020 21:00:10:

 

”The nose up test is a pointless exercise as well, just ignore it.”

Not sure I can agree with that. I’ve had many an engine run fine when level, and then lean out and needed richening by a couple of clicks when the model goes vertical.

On the ground yes, in the air no. Once in the air the engine will perform fine when nose up. due to its forward motion causing it to run a shade rich. This brings us back to my original statement, nose up tests are pointless as they do not accurately represent the conditions the engine will see in flight. If the engine does change in flight and leans off in a climb or as the fuel is used, the tank is likely too high in the first place and should be lowered. 

As for the rest, if it still wont wear it even when tuned the correct way up then that is a little strange. Assuming it was tuned correctly there is no reason it should not carry on until it runs out of go juice.

You did mention it was treated to new fuel, which fuel did you get? My 15fp 2 stroke didnt run brilliantly on the southern modecraft fuel i used to use but went like a train on the model technics laser 5 fuel. I use an OS8 glowplug.

The 4 stroke plug trick is a good one but be very careful as some 2 strokes will not have the space needed for the longer plug and you can punch a hole in the piston if you just crank it over. GO easy the first time if you try this.

 

 

Edited By Jon - Laser Engines on 05/09/2020 22:14:49

[EvilC57]

It’s Model Technics Formula Irvine 5%.

[Jon H]

Posted by EvilC57 on 05/09/2020 22:18:30:

It’s Model Technics Formula Irvine 5%.

its not my favourite fuel (quite messy) but it should not be causing a problem

[EvilC57]

Posted by Jon - Laser Engines on 05/09/2020 22:20:44:

Posted by EvilC57 on 05/09/2020 22:18:30:

It’s Model Technics Formula Irvine 5%.

its not my favourite fuel (quite messy) but it should not be causing a problem

I agree with you about the mess. Models end up covered in black gunge .

[Martin Harris - Moderator]

Black gunge? Usually a sign that the silencer hasn't been tightened properly and is fretting the aluminium.