tom222

Hi

I'm having troubles with my MVVS 152 twin spark. The first issue has come about recently whilst the second I have had from new.

1. At full throttle it is really unstable, there are violent pulses as if it keeps cutting. I have tried adjusting the needles and it seems to have have some effect but it does not go away wheteher rich or lean..

2. The tuning seems to vary dramatically depending on whether I am upright or inverted. I know on the 58 you run a pipe from the breather to the back of the fus to provide a constant pressure but there is no nipple on the carb of the 152 to do this.

Please can you help?

Regards,

Tom

pe reivers

If ignitions are programmed, both should be the same, either short or long. I would not program anything until the engine is well run in (about 10 - 15 gallons of fuel). In principle, the items are plug and play, and should be left alone until you are fully familiar with the engine. Reprogram them both for short muffler.
 
I fit a carb reference pressure line on all my planes, so I advise you to do the same.

The Walbro carb is quite sensitive concerning idle needle settings. First set the idle needle for best (prolonged!) rpm and clean running at 30% throttle. Make only minute adjustments at a time!

Then advance to high needle setting, best rpm at 100% throttle. Tune for max rpm on the richest possible needle setting.
When done, program an exponential throttle curve in your transmitter, so that half stick is about 30% throttle, and 75% stick is 50% throttle. This would be about 45% exponential.
 
If the carb will still not tune well, it is possible that the membrane engagement with the lever fork is binding. This engagement is not required. Just snip off the engaging part of the membrane stem that pulls on the lever, so that it only can push.

tom222

thanks very much! The first problem was my own fault not realising one of the plug caps had lost a clip. I think the tuning is much better thanks to your tips so i'll see how it flies tomorrow.

Thanks,

Thomas