mysterious power loss, spark plug only clue

Joseph Lisi Iii /

I've recently built a derbi varient sl. After I put it together it ran at about 40mph and was very powerfull off the line. Then after about 2 hours of riding the speed dropped to around 10 to 15mph. I've done everything I can think of and everything seems relatively ok, but the sparkplug keeps getting blacker and oilier. I' ve tried tuning, I can actually run it lean and the plug stays black, I've even cleaned the gas tank to no avail. Does anyone have any other ideas why my spark plug is black and oily?Could it have some thing to do with timing or points?

Re: mysterious power loss, spark plug only clue

what mixture are you using????

too much oil, running too rich maybe???

Re: mysterious power loss, spark plug only clue

if you are running too lean then the plug will not be black. so that really confuses me.

but paul could be right, could be starving it of fuel. waay too much oil.

Re: mysterious power loss, spark plug only clue

First try a brand new, properly gapped spark plug. Cleaning plugs is often a total waste of time..

If the new plug fouls, there are a few reasons why.

Gap is too small or too large. Too small gap is easily bridged by a drop of fuel or oil. A too large gap is more difficult to spark across.

Double check how much oil is being mixed with the gasoline.. sometimes temporary brain-lock can cause mixing mistakes.. 14 ounces looks like 4 ounces..

" I can actually run it lean and.. " How would you know you are running lean? What did you change to make it run lean?

You could have developed any number of carb malfunctions.. too high float level, a loose main jet.. some junk under the intake needle so it leaks.. An intake vacuum leak due to a loose nut..

Although it sure sounds like a carb problem, a weakness in the ignition can cause a weak spark, misfires and fouling. Electrical is at the bottom of the list of suspects, imo.

Re: mysterious power loss, spark plug only clue

Jason Luther /

yeah, how do you know its lean?-jason

Re: mysterious power loss, spark plug only clue

Don Pflueger /

after reading all the replies, i have one question. how much compression do you have?

Re: mysterious power loss, spark plug only clue

good point donp.

Re: mysterious power loss, spark plug only clue

Joseph Lisi Iii /

when I run it lean the plug stays black but , little pieces of aluminum stick to the spark plug

Re: mysterious power loss, spark plug only clue

Joseph Lisi Iii /

I know I'm running it lean because I removed some of the air filter and a fine aluminum dust builds up on the plug after a chop.

Re: mysterious power loss, spark plug only clue

Joseph Lisi Iii /

The compression seems good. It has a decompression valve type start and even with the decompression valve activated it is a bear to turn over. I can also start it, run it and hit the decompression valve, and it still runs.

Re: mysterious power loss, spark plug only clue

Tom Winters /

Ya, I was going to ask the same. Going from a good solid 40 down to max of 20 mph seems like a piston, cylinder head problem. If your plug is that black I bet the head and top if the piston is thick with carbon black. I also suspect that some of this gunk probably flaked off and may have scored your piston and or cylinder. Its pretty hard stuff and doesn't come off very easily.

The black stuff is from incomplete combustion. For complete combustion you would get out of the pipe, water and CO2. This requires the correct amount of air, so either there is too much fuel, or not enough air, getting into the cylinder head.

At any rate, you need to know if you have the right amount of compression. If its too low, you probably have a messed up piston. After this is fixed you will have to tune the carb, so that the plug looks grey, rather than oily black!

Take a look at this url:

http://www.scootordie.com/article/articleview/55/1/2/

Tom W

Re: mysterious power loss, spark plug only clue

Don Pflueger /

aluminum?/ sounds like you need a new top end.

Re: mysterious power loss, spark plug only clue

Don Pflueger /

pull your exhaust off and look to see how scored the piston sidewall is by looking up thru the exhaust port. sounds like your top end is shot and your compression is low.

Re: mysterious power loss, spark plug only clue

Don Pflueger /

you want to see a nice even tan color, not grey. grey is bad and means you are too lean.

spark plug only clue.. need more

Explain "I’ve recently built a derbi varient sl." Provide details about the engine rebuild.

Re: mysterious power loss, spark plug only clue

Steamboat Aka J. R. Stevens /

Aluminum dust on the plug means means mechanical breakdown. Are you mixing a good 2-stroke oil? Bad oil won't burn or lubricate correctly.

Re: mysterious power loss, spark plug only clue

Steamboat Aka J. R. Stevens /
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