Correct shimming for ys91 w/ viperhead and MP2

[Splattt]

Ran across a discussion in the main forum about this exact same thig with the os91 cspecs but didnt want to hijack the thread. Can anyone tell me what OMI is recomennding for shiming the viperhead on the ys91 w/ MP2???

Thanks,

[DavidH]

Splatt,
I did the prototype testing for the Viperhead on the YS 91.
I was using Cool Power 25% fuel and Hatori SB 16 muffler. We tried several different combinations of shims and the .004 is what worked best for us. That is why the Viperhead for the YS is only shipped with one .004 shim. We didn't test the motor and head with an MP2, but I really don't think it will make a difference. The only thing we have found in last several months is that some fuels require the head to be shimmed a little more than others. If you have the head shimmed at .004 and your getting detonation with your setup. I suggest take the .004 shim out and then put back the stock shim that is .008 and try it. If your still getting detonation with .008, then add the .004 back also for a total of .012 in shims.

The Viperhead for the YS seems to run OK with .004 shim only and Cool Power 30% nitro or lower and also Powermaster 30% nitro or lower. Those were the two brands of fuel that were tested during the prototyping.

David

[Splattt]

Thanks David... I run Magnum 30% which is essentially the same as CP30% so hopefully will be fine with just the .004 shim.

[DavidH]

Splatt,
Magnum is one of the fuels that require the compression to be lower in the motor than the rest. You may have to shim to .012 or even .016 for that fuel to allow the motor to run correctly.
I am not saying Magnum is a bad fuel, it has just been our finding from pilots that use Magnum that the compression setting has to be set lower when using the Viperhead on the OS or the YS.

You can try the .004 first. If you don't hear any pre detonation and the motor runs smooth, then I would just leave it with the .004. If you do, the try the .008 stock shim and see if that is better. If is some better but still detonating, the add the .004 shim also for a total of .012


David

[z11355]

Wonder if that is due to the additional nitro in Magnum fuel that is rumored to be there.

[Splattt]

David,

thanks for the tips i'll make sure to report back as to how things worked out. My viper head should be here tomorrow from Ricks so I'll get it in and if the weather holds out here I'll try and get her up sunday.

Couple of things...I've never had to deal with pre-detonation before...what should I be looking for or listening for? Also what needle settings where u getting on the CP? l would like to know where to start with it or a good approx starting spot.


Z11355:
im sure its probably due to te added nitro, but im not about to switch at $13 a gallon, my engines all run great and im not paying $27 :roll: for CP30. Make my wallet happy as well. :D

[Dino Spadaccini]

After running about six gals of magnum in one of my new Ys with a viper head and enya 3s i still couldn’t get it Wright. Even Todd said there was something up with that engine when he was in a few weeks ago. After that i have been like a wild man trying everything in my powers to tune this motor well i gave up and called Dave at YS and told him all about the motor looking brand new in side after 6 gals of fuel not ever the exhaust side is tarnished. The only thing that set me off was the glow plug coil is getting sucked out little after one flight on a new plug. The top of the piston looks brand new too. Now comes the fun part the motor isn’t giving me any signs of detonation just sounds lean I only get 7mins on a tank. Dave says that the motor is under compressed and not getting hot enough and causing the plug to pull out and running way to rich the only shim i have in is the .004 Dave said pull it out and i will still have .007 clearance. James says the motor is over compressed and to put more shims in ok so what is everyone else’s thoughts :roll: