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Need help with MVVS 77

Old 09-15-2015, 01:38 PM
  #1  
GoNavy
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Default Need help with MVVS 77

I can't seem to get reliable idle and smooth transition...only one or the other.

MVVS 77 (old style but "new" needle and spray bar). FAI fuel (no nitro, 20%castor). K&B 1L or 1S plug. 13/6 Zinger prop. Have 1/3 gallon fuel through it broken in as recommended in other posts here.
Starts like a charm. 10k+rpm top end no problem. Needle valve settings: high speed out 1.7 turns (manual says to start at 1.5 to 2 turns out from closed). The low speed needle is 1.4mm out from closed. (Manual says to start at 1.2mm out which I did). The fuel nipple on the spray bar is parallel to the crankshaft (i.e., "level").

By adjusting the low speed needle valve, I can get an idle speed of about 3700rpm. However, transition is rough to impossible...too rich, even tho full throttle is spot on. Leaning the low speed needle to get smooth transition ruins idle. It will simply die below about 4700rpm.

I found that leaving the glow plug driver on did not help. I see MVVS recommends a Novarossi C6s plug, which some sites describe as a cold plug. Could this be part of the problem? I believe K&B 1L or 1S are "medium".

One other matter about this engine stands out: It is frugal on the glow fuel, like no other 2 stroke glow I have ever run, more frugal than the old black head Webra 60s. Yet I don't believe I am running it lean. The smell of castor is in the air and the blue smoke is visible. I am running it a bit rich I believe.

If this were a Super Tigre, I would adjust the spray bar position to lean the midrange. Will that help with the MVVS?

Your suggestions are welcome. At this point I do not want to trust my airplane with this engine.
Old 02-08-2019, 07:37 AM
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jaka
 
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Hi!
First of all: You have to set all engines by ear! If you cannot do it ...there will be problems. A tach could work but is not that good compared to a the human ear.
The MVVS 12,7cc engine (.77) is set like all other glow engines with a two needle carb. You set the high speed needle first (when you have started the engine and given full power) and then set the low speed needle! The engine should respond immediately when giving full throttle from idle! -If not adjust the low speed needle! -But! remember if you have set the high speed needle too lean from the beginning no tinkering with the idle needle will work! A safe idle speed should be around 1800-2000 rpm with a 13x4 or 13x5 APC prop! Slighly higher (2100.2200 rpm) on a 12x6 or 12x7.
Idle ( drum opening) in the carb should be around 2mm.

Fuel: The older MVVS 12,7cc and 15cc (.77 and .91) engines does not like too much nitro! I have seen and run some that doesn't even like 5% nitro.Those have been impossible to set ! So a good way to start is a fuel containing 20% oil and 80% methanol ...or running 5-10% nitro and at the same time put in a another 0,1mm head shim.

Glow plug: All MVVS glow engines come with either a Nova Rossi 5 or 6 glow plug but equally good is OS 8 or Enya 3-4 or Rossi 3-4, and there are many others that work well too.

Propellers: The MVVS can swing many different sizes of props, from a small 9x8 to a large 15x4 but a 12x6 ,12x7 and ,13x5 are suitable for most sport planes.
Best prop brands are Graupner G-sonic, RAM and APC.

Last edited by jaka; 02-08-2019 at 07:41 AM.
Old 02-08-2019, 10:14 AM
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GoNavy
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Not sure what happened to the many other posts that were in this thread from 2015.
I wont try to recreate the history.
Since then I found an apparent air leak from around the front of the crankshaft which accounts for the problems..
I have ordered a new engine from MVVS and will be running that one up in warmer weather.

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