Problem with MVVS 58
 

Dear Sirs:

I hope you can help me with a strange problem I've. MVVS58. BCM muffler. It's inverted mounted in a Chip Hide's Vision. Menz 24x10, 98 oct gas and Mobil 1 2T racing at 2.5 %. About 6.100 rpm.

Everything is perfect, power, transition, idle (about 1.700 rpm) and very smooth most of the travel of the stick, only a very little 4 stroking in the first clicks from idle, but really little, almost smooth. This is the situation in the ground, in normal flying, during the torque roll which can be easily done as long as you want without any small sign of hesitation from the engine, verticals, everything except ... inverted flying. It doesn't matter how you get inverted, neither if it happens in the beginning or the end of the fly.

As soon as the model becomes inverted (this is with the head of the engine upward) the sound changes to a very rich way, to the point that it seems to die (It's not stopped yet, but I'm afraid of it) I can't close more the L needle without disturbing the normal operation.

The fuel tank is 50 cm far from the carb, and it only has two lines, one to the engine (with a filter on it) and one free for breathing. May be this length is too long, but I think this must affect more to other maneuvers, like torque roll.

Do you have any suggestion?

Thank you very much

Best Regards,
Javier

PS: It has been running for 10 - 11 hours nowdays

RE: Problem with MVVS 58
 

This is the standard question of many gas fliers. It is not engine, but carb related. The carb needs a reference pressure for the fuel inside the plenum chamber. To that end, it has a vent hole in the plenum membrane cover. The reference pressure can change with plane attitude, which can cause the mixture to go lean or rich. The 58cc engine has provisions to attach a tubing line to that cover just for that purpose. This tubing can be routed to where the problem vanishes. A closed balsa box inside the fuselage gives excellent results. The balsa breathes just enough to stabilize the pressure.

RE: Problem with MVVS 58
 

Thank you, Pe:

Do you mean a completly closed box, only with a hole for the tube? Any volume required? Thickness?

Thanks a lot.
Regards,
Javier

RE: Problem with MVVS 58
 

no need for a complete construction drawing. regular 1/8" balsa sheet, matchbox size will do. glue in a short brass tube to connect the pressure line.

RE: Problem with MVVS 58
 

Thank you very much!!!

I'll do it

Regards,
Javier

RE: Problem with MVVS 58
 

Dear Sirs:

I followed your instructions and built a balsa box as you can see in the attached picture, but it didn't work. After testing it, I did the small hole you can see in the front of the box, only to see if this could help. There was no difference, without the box, with the box, with the hole ....

I would like to enphasize that the problem doesn't appear with the altitude changes but with the fuselage position (normal, knife edge both sides, ok. Inverted, too rich) and it's much more improtant in negative G maneuvers, when it becames really concerning.

Please, could you have another tip for me?

Thank you in advance

RE: Problem with MVVS 58
 

The balsa box should be inside the fuselage, not under the cowl. Inside the cowl pressure differences play havoc with your fuel delivery sttings. There should be no holes in the box.

BTW,
The fuel tank is very far aft, and the tank vent line exit is very far forward. The vent line should exit as close to the tank as possible. Just a thought.

RE: Problem with MVVS 58
 

Dear Sir,

please kick-out filtr. You can have problem there.

RE: Problem with MVVS 58
 

Dear Sirs:

Thank you very much for all your tips. I think the solution is on the way. I couldn´t read your post until today, but on friday y took away the box and put the tube inside the fuselage. The resut of this is that the engine worked much better on saturday and sunday, not asbolutly fine, but much better, so:

I'll chage the vent tube to a new possition as close as possible to the tank.
I'll have a look over all filters. There is one specific fuel filter whic can't be seen, but it's there.
I'll built a new box (without holes :D:D) and I'll put it inside the fuselage.
I'll repport you

What a great and powerfull engine !!!

Regards,
Javier

RE: Problem with MVVS 58
 

one box type that should work extremely well, is a small tin box, soldered airtight. Before a flight, vent to atmosphere in order to be sure of equalized pressure, and then close it again. There is no way that flight attitude can change the carb reference pressure with such a contraption.

RE: Problem with MVVS 58
 

Thank you for the tip.

First I will try with the balsa box, which, if works as expected, will be easier to use (no use, in fact) than the metal box. If the result is not good enought, I'll try the tin box.

I'll tell you.
Regards,
Javier

RE: Problem with MVVS 58
 

Dear Sirs:

The engine was running fine on the ground

I made a new balsa box 50 x 25 x15 mm, 2.5mm thickness without holes and put it inside de fuselage some centimeters behind the firewall. I also moved the vent tube output to a closer position to the tank. Now it is 5 -6 cm far from the tank.

Then, I started the engine. 7.000 rpm with Mezjik 23x10. I tried to get some more rpm so I close 10 min the H needle, but this made the engine too lean, and max rpm couldn't be reached again. Stop and re-open 10 min. Start again and ... still lean. Open 10 min... and still lean. And so. Everything seemed to be messed then. The high speed was open about 2 and 1/2 and L also a couple of turns.

I thought something must be dirty, so I dismounted the carb and cleaned it carefully, but I didn' feel brave enough to disassembly too much. On one side, there is a small thing moved by a spring. This part was alrady completely clean so I didn't anything there. In the opposite side, where there is a small hole with a filter in, there was a bit of dirty, not very much, not solid things, only something like small bubbles or foam. I cleaned it carefully with normal fliying mix and reassembly. I also took apart both needles, being very careful don't exchange them. Cleaned both, reinstalled and moved as per book position: H 1h 10min. L 1h 45min.

Started the engine. H was far lean.
Disconnected the reference pressure tube from the balsa box, but remain the tube inside the fuselage.
Open and open. Now it works on the ground nice, but the H is open 2 and 1/2 turns, and L is about 2 turns. With this setting working fine, I connected the reference pressure line to the balsa box and L and H became lean again.

Now it is disconnected and working on the ground, but with all this tangle I hadn't time to check it on the air. I'm confused about how this carbs work. I really don't understand them. I'll check the model in the air next weekend.

Thank you for your help
Regards,
Javier

RE: Problem with MVVS 58
 

For a better understanding of the carby, see the sticky post "tuning the walbro" at http://www.prme.nl/forum/viewforum.php?f=7
I have linked to manuals etc. so you can get a good impression on the inner construction, and how to adjust the carb properly.
For cleaning a carb, WD40 is very good. This is sold with a thin spout, that can reach into the carb. I have noticed "roaming debris" in several weed appliance carbs that I serviced, so I would not exclude that possibility. This kind of debris in the carb can drive you mad, because one moment it runs well, and the a few minutes later it will not tune at all.

In your case, the pressure in the fuselage obviously is lower than the in cowl pressure when the engine runs. I think that the cowl pressure is higher than atmospheric, so the engine should tune well with the pressure line in the fuselage. The higher cowl pressure helps the carb lean condition. This should be cured first.
Take the carb apart, and clean it well. You may even have to clean it several times.

RE: Problem with MVVS 58
 

... until it runs properly with (more or less) the "per book" settings, if I understood correctly.
Well, I'll try cleaning it again with that product and, of course, I'll look your link.

Only one comment: The engine is only 10 mm far from the firewall, so the carb passes throught the firewall and cames inside the fusselage.

Thank you, Pe.
I'll tell you

RE: Problem with MVVS 58
 

Dear Sirs:

Following your instructions, I cleaned the carb again, and the result was a easier regulation. I have also made several test positioning the reference pressure line and now I have got one good enough. This is attached to the firewall and pointing to the open air in the bottom of the fuselage.

This new position has totally changed the operation of the engine. Now, the behavior is much better. When I get the model inverted, the idle or half throttle is perfect of even becomes a bit lean. There is a very small difference to the normal flight, and, in fact, it sounds better, smoother.

Thank you for your help.
Regards,
Javier

RE: Problem with MVVS 58
 

Dear Sirs:

Unfortunatly, here I am again.

The previous issues related fuselage possition are solved with the new location for the pressure reference tube, but I have a new problem. I hope you can help me as with the others.

Compression is excellent
Mezjik 23x10. at 7.000 rpm
98 Oc + 3% Mobil 1 rancing 2T
When cold and with choke first explosions after 4 or 5 flips, choke off and starts at 2nd flip. This is for the first start in the morning.
After that, every time start at first flip without choke
There is a long fuel line (40 - 50 cm) with a dubro valvle on it and also a filter betwen the vlavle and the engine. I know it's long, but the fuel runs and drops freely if I disconnect the line from the carburetor and several (10 or so) drops fall per second, so no problems here.

As you can see in the attached file, the plug (for me) shows a rich adjustment but you must consider both, the flight conditions and my problem:

Flight conditions: I hardly use full throttle, only in vertical climbs and only for a few seconds. Most of the time I use idle or high idle.
The first problem: I never could get my engine rich at high speed, It's the same what you do wiith the HS needle. 2 turns, 3 turns ... the only way to keep the engine runnin at full throttle is having the L richer than normal. As I use the idle range most of the time, the plug shows this rich condition, but it isn't like that at full throttle. I have having this problem since ever.

But now I have a new problem.
Start engine. Idle high, but ok (for me, it sounds lean). Can't get less rpm.
If I advance smothly the stick, engine dies inmediatly.
If I advance quickly to mid range, hesitates a bit and continues runnig at mid range.
If I reach full throttle, dies.

From this point, I started messing the needles. After several test, the conclusion is that behaviour does not change with the needles. It's the same what you do:
L 1 turn + 3/4 H 1 + 1/4 than
L 2 turns H 2 turns + 1/2 nothing changes.

I have cleaned the carb dismantling both covers and taking aparat all membranes in the next ways:

1 time submergin it in to normal fuel and cleanning it with a brush
1 time with my air brush
1 time with WD40 as your recomendations
1 time again the same
1 time connecting the fuel tap to a compresed air supply set at 3 kg and again with WD40

All times everythig appeared clean since the beginning.

I don't know what else can I do, but I feel frustrated. Meanly because the engine runs great (when it runs) If it was an ugly engine It would be easier for me to discard it, but I really love its power.

I want to use this engine as a "all days trainning machine". Before it, I was using a YS110, theorically much more complex engine and difficult to use, but in the practise, and after 500 flights without major issues, that engine became me used to "fly all day and forget the engine" and now I feel frustrated with my MVVS because I never can forget the engine. There is always somthing wrong. Please, help me.

Thank you very much.
Javier

RE: Problem with MVVS 58
 

No ideas?

RE: Problem with MVVS 58
 

I had a twin ZDZ 80 that was very difficult to get the needles correct, would go lean in flight. Even lost one of the cylinders to the problem.
Turns out it was a inline fuel filter between the carb and the tank, I will NEVER again run a filter on the airplane. I put one in my field tank so the fuel is filtered going into the airplane.
PROBLEM cured, can set the needles and the adjustments stay put.

RE: Problem with MVVS 58
 

Dear Sirs:

Last problems related to carburation were solved changing the carburator for a new one. Now I can set it correctly, but one of the problems is still there.

"Two post" ago I talked about a small hesitation just leaving the idle condition. This hesitation is still there. Considering that the carburator is new, I'm starting to think something about the ignition. What do you think? Once, this hesitation changed the direction of the propeller. Fortunatly, she was on the ground, not flying. My idea is that the ignition may be a bit advanced, but there is no way of adjusting it ...

Any suggestion? What can cause the reverse of the propeller? A lean mixture? A wrong ignition advance setting? Propeller is Mezjik 23x10, not too light ...

Thank you in advance.
Regards,
Javier

RE: Problem with MVVS 58
 

Dear Sirs:

As I haven't got any asnwer for my lasts post, I think that, for any reason that I don't understand, you're not interested in helping me more. It's obiously your chance, so I'll continue alone.

In any case, I want to thank you the previous help you gave me.

Regards,
Javier

RE: Problem with MVVS 58
 

Javier
Sent you a PM.
Mike