Plug Chops: Fred's Guide on How to Fix Your Moped

[noiseguy]

All,

This was taken from www.mopedarmy.com. The entire article, "Fred's Guide on How to Fix Your Moped," is posted there in the articles section. It is too long for a forum post. It is an excellent article worth reading in it's own right.

This contains a section on plug chops, which is why I'm posting it here. Plug chops tell you whether your 2 stroke is running rich or lean.

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Fred's Guide on How to Fix Your Moped
by Fred
Read this guide to figure out how to get your moped to run... or maybe run better.

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ENGINE TUNING
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READING SPARK PLUGS and JETTING and PLUG CHOPS

Two stroke engines are sensitive to the level of heat inside the engine.
The main jet size in the carb is the main determiner of how hot the engine runs at wide open throttle.

Too hot and they will seize the piston.
Too cool means they are not making full power.

Engines get the hottest when they are running at wide open throttle making maximum HP.

"Reading" a spark plug is the best way to see how hot your engine is running.
Reading the plug means looking at the ceramic insulator inside the plug and checking its color.
The color will indicate the overall level of heat in the engine.

Reading the plug requires fully warming up the engine by running it for about 10 minutes... Then making a top speed full throttle run for a half mile or more, then killing the motor and stopping and pulling the spark plug on the spot.
That is called.."Doing a plug chop".

When doing the "plug chop"... It is important to chop the throttle and turn the engine off with the switch or key and then pull over to stop... You don't want the engine to idle... that will change the plug color.

You can wait till the plug cools... the color won't change.

The color of a plug on a correctly running engine is a light brown or tan color.
Lighter than that means danger of a piston seizure.
Darker than that is OK... But real dark or wet looking means your main jet in the carb is too big (too rich)... pull it out and look at the number on the side and buy the next size smaller (leaner).

Note... To get an accurate plug reading takes a fairly new plug... a plug with 2 years of crud on it will never really show the true color (or heat) the engine is running at.


JETTING and ADJUSTING the CARB

Jetting refers to changing or adjusting the parts of a carburetor that control fuel flow.

The "main jet" controls the fuel/air mix at top speed.. and is the most important jet to get right.
On most carbs the main jet is a small brass piece inside the center of the float bowl screwed into the carb.
It has numbers stamped on it for size.
A bigger number means more fuel flow... or "richer".
A smaller number means less fuel flow... or "leaner".

There are also usually 2 external adjustments on the carb...they are the idle mixture and idle speed screws.
Idle speed usually opens the slide or butterfly for a little more airflow.
Idle mixture controls the air to fuel ratio (at idle).

They only affect idle ... they don't affect how fast your ped goes or how it runs while going fast.
So you can just play with them till you like the idle... fast enough not to stall... slow enough not to engage the transmission.
Idle mixture screws are usually supposed to be between 1 and 2 turns out from all the way in.

Some carbs with a slide have a needle in the middle of the slide with adjustment slots on the needle for raising or lowering the needle.
Raising the needle makes the midrange richer.
Lowering the needle makes the midrange leaner.


JETTING and WEATHER and AIR FILTERS and FOUR STROKING

Colder weather makes an engine run leaner with the same jets in it.
Warmer weather makes an engine run richer with the same jets in it.

Running with no air filter makes your engine run leaner.
Too much oil (or dirt) on your air filter makes it run richer.

"Four stroking" means your air to fuel ratio is too rich.
Four stroking is this >>.. when you near top speed the engine goes from a smooth high pitched ziiiiiiiiinnnggggg tone... to a lower pitch rougher tone... and the engine acts like it is choking, like it would like to go faster... and sometimes it WILL go faster if you close the throttle a little bit.

IF your engine is doing this... and the air filter is clean... remove the filter and test ride it again... if it goes faster and "four-strokes" less.. then you need to lean it out with a smaller main jet.