Hi all, I need some help.

I am putting a new MVVS 91 on my Vector Flight Extra.

I broke the engine in on a stand, it ran flawlessly. I have it on the plane now and it runs well, but from time to time when I advange the throttle to full, the engine will quit suddenly just after it hits max rpm. It doesn't sag and die like it was leaning out, it just quits immediatley. I assume this has to do with my tank plumbing, but I've been through it several times to no avail. I am running a three line system in a Dubro tank. I have used a piece of fuel line, notched at the top, to extend the vent inside the tank to so that it reaches the top. Using the old Mark I Suction Device (mouth), I can detect no leakage in the tank/stopper/lines. I am running a BCM Pitts muffler. I have tried blocking one of the outlets to get a little extra muffler pressure and it still quits. The tank is completley encased in latex foam so vibration is not the issue either. Any ideas? I was hoping to fly this bird tomorrow, but I'm not going to if I can't get things sorted out.

Thanks!

I think you have it backward. When an engine at full speed just stops with a pop it is lean. When it chugs slower and slower until it stops it is rich.

David,

Despite what the instructions say, the starting position for the idle needle is 2.5-3mm from the nut; not 1.2mm (this was in the old carburettor).
Try opening the main needle a few clicks first.
Some MVVS engines (.49 F.E.) do not sputter and do not spit fuel, when the low speed needle is set too rich, just dieing when the throttle is advanced quickly.
They just start behaving well after you lean it down.

The other possibility is that like most glow plugs, yours has taken the long road to eternal life as an ornament..

The break-in deposits particles of aluminium on the element, making the plug, although still lighting, useless.
This happens more often than not, although I do still have the original plug in one of my MVVS engines.

Play with the mixture first.

I did change glow plugs, from the original to an OS #8 with no change in behaviour. My main needle is about 2 1/4 turns out. It behaves normally when adjusting the top end. I might just try a fresh jug of fuel as I had mixed some 5% and 10% leftovers from last year at about 50-50. My MVVS 49 doesn't complain about this however...

Mystery solved. A tank full of 5% nitro and things are all good in the engine bay! I would have never beleived that little bit if nitro would have made such a huge difference!

David,

Although CNC manufactured, your engine may have a slightly lower C/R than the rest of them.
Besides, being sent to North America, your engine may have a thicker/additional head gasket, to allow a little nitro.

My MVVSs run well on either.

You wrote that your fuel may be old/bad. I think this was a more probable cause than the 5% nitro.

I also have had engines quit suddenly when going from 1/2 throttle to full, and found a few reasons:

1) plug The #6 novarossi can be replaced by OS#8, OS-F or Enya #3

2) compression ratio. A talk with MVVS taught me, that their engines are adapted for the US market. This means that lower compression ratios are used at the moment, to cater for added nitro. This is done by adding squish deck clearance. Not the best solution IMHO, which brings us to

3) nitro content, as David already found out. Adding nitro in the modern engines largely solves this problem.

4) When the engine flames out, it is due to fluid fuel surpluss at the plug. Leaning the idle needle to the minimum is the solution, combined with the type of plug.
Using the right plug allows one to reduce needle settings considerably, so that can be used as an indicator.

Using MVVS supplies and some elbow grease, I have constructed a small batch of .25 (3.99cc) Aces 1/12 combat engines, with a squish deck height of 0.2mm, and effective CR of 9.2. These engines run safely at nitro contents up to 15%, and do not exhibit the flame-out that David experienced. Such a tight squish height however is not feasible in MVVS production due to the number of toleranced dimensions to cope with. It only can be accomplishes using the now abandoned system with combustion chamber buttons, which are deck height classed to allow engine blueprinting.
Ah... the good ol' days.

David,

Use a piece of thin solder wire to measure the room left between piston top and cylinder head in top dead centre.
If larger than 0.3mm, you might want to remove the head gasket to lower the head a bit.

I think there is some mis-understanding. The engine did not run whell when I mixed some left-over 10% fuel with some 5% fuel at about 50/50. I think this would give ne a net nitro content of about 7.5% or thereabouts. The engine did not run well at all on this.

When I went back to 5% nitro fuel it was a happy engine.

From this I learned that 5% is the MAX nitro content to run on this engine.

I did take it apart while fitting it to the airplane. There is one aluminum gasket/shim in place from the factory.