Drop in Top end power

[NickMurray]

Hello all,
Firstly, sorry to ask for help on the same age old question, but I've scanned the forums not found a suitable answer so far...

I've owned an '02 KDX200 for the last 12 months or so and have had near perfect performance from it. I've owned a '93, '89, and '88 KDX before that and they were top bikes as well.
Lately I've been noticing a drop in power at high rpm - I've done 25hrs since the last top end rebuild.

The mods - set of aftermarket reeds and a procircuit pipe and muffler. I ride at near sea level and sorted the jetting out when I put the pipe and reeds on (155main, 45pilot) Ive cleaned the KIPS -- a bit of spooge - I also decarboned the exhaust and repacked the muffler, cleaned the carb, filter, and mixed up fresh gas.... The cylinder looks like brand new, the bearings are mint.
I'm really at a bit of a loss....

 

[Houndog]

Hey Nick,

Curious as to what clip position is the needle in?

 

[Matt90GT]

Did you change the plug?

How much compression does the cylinder have?

 

[NickMurray]

Clip position is middle of standard needle - a R1174N, I've tried in all clip positions - little or no difference

I have done a couple of plug chops with a B8ES and the colouring of the ceramic part was a nice caramel brown

Compression is virtually the same as when I purchased it brand new, and when I did the piston and rings back in December - around 180psi on my dodgy homemade compression tester - I don't think the scaling is correct, but all I've been looking for is relative loss in compression - it's down about 5psi which is negligible considering the probable error in my test gear!

Thanks for your help, theres a ride on this weekend, I will give it a good thrashing and see if it fixes itself.........

 

[DRECKFRESSER]

have you gained any weight? :)

 

[canyncarvr]

Warmer temps call for leaner jetting.

That's a simple, inexpensive answer.

Uh-oh. Seeing as you're upside down...I suppose it's getting cooler down there?

OK.

Cooler temperatures call for richer jetting.

;)

 

[NickMurray]

Thanks for replying,
After bashing my head against the wall for many many hours, I tried some new fuel from another gas station down the road - apparently they've been having some real bad batches of gas lately - Instant success, its fixed and goes like a little beauty again..... I'm sure that all my other maintenance was timely though too.

Big ride on tomorrow so I'm getting set up now, did some airbox mods - I'd removed the snorkel when I bought it, but removed the whole lid and relocated the reg - sounds a lot throatier - she was runnign a bit rich, so that might sweeten it some... catch you all later

 

[Michelle]

Out of curiosity, what gas were you using?
I know of a couple of times filling at Shell I've had problems (once in the car & once a bike).

Gull supposedly is heaps better, but never having tried it, have no idea. We just use race gas (my kdx likes 50% 96 & 50% race gas - avgas really). Had to try the BP Ultimate at one stage & the KDX did "ok", but not a gas of choice - it didn't run quite right and was hard to start, but at another pinch, I'd use it again.

Hope you have a great ride.

 

[NickMurray]

Hi Michelle,
I've tried all of the gas combos, 50-50, straight pump gas, all race gas etc, etc. With different brands of fuel, I've mainly stuck to shell, but yeah, that last batch was a nasty one...

Numerous KDX'ers have recommended 50-50 pump/avgas but I find that a real hassle, so I tend to just mix pump gas, and only get avgas for special occasions as I'm quite a long way from an avgas supply....

Cheers
NM

 

[canyncarvr]

Unless you plan on running your kdx around 20,000'el., why would you want aviation fuel?

You might take a read on EG's site on some of RRs technical writeups on fuel.

 

[Braahp]

"av gas" is blended specifically for use in small aircraft. It's also commonly used by many high performance engine owners because of it's high stated octane rating (usually 100-110) and the relatively low price compared to racing fuel. Unfortunately this fuel is not all it appears to be. Avgas octane is rated on a different scale than gasoline's intended for ground level use. What is 100 octane "av", is not necessarily 100 octane "ground level". Besides this, there is also a big chemical difference. Normal ground level race fuels are made up of gas molecules that have a "light end" and a "heavy end". The light end of the molecule ignites easily and burns quickly with a low temperature flame (as a piece of thin newspaper would burn). The heavy end of the molecule is not so easily ignited, but it burns with a much more intense heat (as an oak log would). This heavy end of the gasoline molecule is responsible for the hotter, more powerful part of the combustion process.
Small aircraft are constructed as very weight conscious vehicles. That's because their somewhat weak engines often have difficulty taking off with any extra weight. To help reduce this weight problem, aviation gasoline's are blended with no heavy molecule end. This makes a gallon of avgas weigh substantially less than a gallon of ground level fuel. Since small plane engines turn very low rpms and produce so little power, the omission of the heavy end is not a horsepower issue. However, for high output 2 stroke engines, there is defiantly a compromise in power. This, despite the fact that many 2 stroke owners experience the desirable cooler operating temperatures that avgas offers. In addition, some blends of avgas will quickly separate from some oils used in premix situations.
Despite all this bad news, running avgas (accepting the slight power loss) is usually a better choice than burning down a high output engine on regular pump gas. In this situation, the best choice is usually a 50/50 mix of pump and avgas. That provides "some" heavy molecule ends for the engine.

 

[canyncarvr]

I'd suppose that the 'high output engine' shouldn't be running pump gas in the first place?

Seems a lot of people are of the mind that increased combustion pressures (usually brought about by increased static compression ratio) are across the board an always terrific thing.

THOSE are the people that 'burn down' their engines........would you say?

But then, I know riders that use 89 octane pump fuel cuz they don't want to pay the extra 20 cents for 92. They say, 'it's just as good,' as they ping off into the sunset.

...and at the least end up with ring lands the thickness of a slice of bread in a year. :(

To each his own I'm sure.