It is interesting to note that the BMW F650 runs 11.5 to one piston and uses the same cam. This computes out to 215psi and runs pump gas. I would say that anything over 210psi requires higher than pump octane. Either the cams are degree'd differently or the actual compression is lower because of production spec allowancies.

I would blend a 50/50 ratio of 93 pump with 103 aviation.

I think you should lower the needle one notch in your carb and jet till rich.

OMR

 

<div class="FTQUOTE"><begin quote>Originally posted by: OMR

It is interesting to note that the BMW F650 runs 11.5 to one piston and uses the same cam. This computes out to 215psi and runs pump gas. I would say that anything over 210psi requires higher than pump octane. Either the cams are degree'd differently or the actual compression is lower because of production spec allowancies.


OMR</end quote></div>

Those BMW F650's are fuel injected, and have detonation pings at lower rpms where no appreciable damage occurs due to low hp/cubic inch at low rpms on EFI. The cams are also different, but don't know how much (I don't care - it is difficult changing ANY EFI stock engine performance wise). It is a jerry-rigged (cyl. head for) EFI on the DS650 carb-based engine, that never ran as good to this day: except on top end like all EFI's. The BMW G650's are totally redesigned EFI engines, but are made in China now. Again, who cares, but something for BMW F650 and Canam DS650 customers to look out for in the future - Rotax moving ALL mfg operations to China :-(

http://www.mcnews.com.au/NewsA...er/bmw_speech_geri.htm

 

I lowered the needle to the leanest position and installed a 175. It bogs and falls on it's face at WOT, but when I check the plugs they are bone white. I went to a NGK dr9ea plug thinking a cooler plug may help. The heat mark on the strap is right about where the curve is, but the porcelien(sp?) is white. I dont think an even cooler plug is a good idea, but I really don't know what to do. I'm running a 50/50 mix of avgas and our highest pump which is 91. It seems spot on with a 165 main, #97 needle 2nd notch, pump activated, using the snorkle. It runs real strong, pulls hard with no hesitation, no knocking or pinging, but the plugs are still bone white. Could the AVgas be causing this? I've seen airplane plugs and they are tan/brown just like any other plug, so I don't think this could be it but I'm at a loss.

 

Compression ratio..bring piston to top dead center, using a cc syringe fill combustion chamber to first spark plug thread ( bottom of plug hole ) Take the amount of cc's it holds add that to the cc's of motor, them divide that by the cc's the combustion chamber holds = compression ratio.
If you have a 250 cc,1 cylinder motor, chamber holds 21cc is 12.91 compression ratio. 250 + 21 =271 divide 21 in to 271 + 12.91-1
This is asuming the spark plug hole is on top of head.

 

<div class="FTQUOTE"><begin quote>Could the AVgas be causing this? I've seen airplane plugs and they are tan/brown just like any other plug, so I don't think this could be it but I'm at a loss. </end quote></div>
IMHO I think your still lean. Jet it up a bit.
Race gas or AV gas will not color the plugs like pump gas will. Pump gas has no lead in it anymore so what is showing up is the tars in the gas. AV gas and race gas is VERY clean. If you get unleaded race gas you get very little color on the plug. We run leaded race gas and it's still hard to color a plug.
When we run straight Trick, I set the plug color to a VERY, VERY light gray.

 

<div class="FTQUOTE"><begin quote>Originally posted by: Doctorturbo

<div class="FTQUOTE"><begin quote>Could the AVgas be causing this? I've seen airplane plugs and they are tan/brown just like any other plug, so I don't think this could be it but I'm at a loss. </end quote></div>

IMHO I think your still lean. Jet it up a bit.

Race gas or AV gas will not color the plugs like pump gas will. Pump gas has no lead in it anymore so what is showing up is the tars in the gas. AV gas and race gas is VERY clean. If you get unleaded race gas you get very little color on the plug. We run leaded race gas and it's still hard to color a plug.

When we run straight Trick, I set the plug color to a VERY, VERY light gray.</end quote></div>

Thanks for the insight. I jetted up to a point where I knew it would be rich and then jetted down until it ran right. I know it's not the best way, but I feel pretty good about it. I may bump it up some and play with the needle a little more. I figured running 50/50 with pump 91 would let my plugs color some. I'm not real happy having to run high octane and may look into getting some gaskets to lower the compression back to where pump would be okay.

When I bought the piston I was under the impression I could use pump until I talked to Eric. Then with a little help from the nice people on the forum we figured my true compression (12.25 to 1). I may get the gaskets and try to get it around 11 to 1, so I can run straight 91 octane.

 

Sounds like your real close. I would jet up a tad and see how it feels and see how the plug looks.

That's the trouble with power. It's like booze or drugs. You get as much as you can and then the next day you wake up and say, why the hell did I do that???!!! LOL

By the way, sounds like you did a good job on the tune.