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MVVS 91 quits running after short flight

Old 09-11-2009, 07:26 PM
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Default MVVS 91 quits running after short flight

I have a MVVS .91 that recently has begun quitting in flight after just a brief time. If I move the throtle to 1/2 the engine runs rough and then quits. There is almost a full tank of fuel left. I replaced the plug, cleaned the carb, took out the fuel system (new) cannot find any problems. I am using 5% fuel, the fuel is good as my ohter planes work fine. I am using 14x6 prop. Max rpm 10200 I richened it to 9400 on the high speed needle. Engine transitions fine on the ground. Anyone have the same problem and how to fix the problem.

Old 09-12-2009, 01:46 AM
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Default RE: MVVS 91 quits running after short flight


Let's start with the basic adjustments you are making...

If the engine peaks at 10,200 RPM, richen it so it runs 100-150 RPM less, with the nose straight-up.

I.e. it should spin 10,050-10,100 RPM, when it is pulling the plane up and out-of-site.

Get someone to help you do this, since you cannot both hold the plane securely nose-up, and adjust the engine... And someone must operate the Tx...

If the fuel-tank is properly placed; right behind the firewall, the level attitude RPM will be 300-400 lower than peak - not 800 RPM rich.

There is absolutely no reason for the setting to be any richer than that. It adds nothing to safety, or to the engine's longevity...

It will just waste fuel and make the engine less reliable.

Set the high-speed needle as described above.

As to the fuel you are using; you only mentioned the nitro percentage...
The fuel must be a castor blend, or with a good deal of castor oil in the mix.

Sig Champion 5% (50/50 20% oil), or PowerMaster GMA 5% (50/50 22% oil), are some examples; with Morgan Omega 5% (30/70 17% oil) being the absolute minimum.

Don't use 'Cruel Power', or any other all-synthetic and also stay away from fuels that contain very percentages of castor oil (2%, or less), just for the sake of claiming they do...

Different engines need different fuels.
If your Chevy beater runs on unleaded regular, it does not mean your Cessna 210 Centurion also will...

Does your engine have the new carburettor MVVS carburettor?

The new one has an elaborate intake trumpet, no idle-stop (just a lock-nutted grub-screw) and a flat screwdriver slot at the end of the low-speed needle.
The old one has a blunt-end low-speed needle, which makes the engine incapable of being adjusted perfectly; leaving it too rich in the mid-range.

Make sure the low-speed needle stays put under power (i.e. it does not vibrate and change its setting 'on its own').

It sounds like the low-speed is too lean now.
The low-speed needle controls the mixture curve, from 0% (idle) to ~75% open throttle.

The high-speed needle controls only the maximum amount; not the part-throttle amount.

Trying to adjust a part-throttle condition with the high-speed needle, is like searching for a coin one lost in school, under the street-light near one's home...
...It is more convenient...

Give us the updates.
Old 09-12-2009, 08:01 PM
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Default RE: MVVS 91 quits running after short flight

Thanks I will go through your suggestions when I return next week. I also was considering the low speed being too lean. I do have the new carb and do use Morgan fuel.

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