Cautionary Tale of 2-Stroke Performance Misdiagnosis

That would have thrown me for a loop. It actually got me to wondering if there's a standardized method of testing compression? Like pedal at a certain speed without a carburetor and the value you get is xx.x? Then, if it's low you could add some oil in the cylinder and see if this raises the compression value, telling you it's the rings.

The asterisks.. and I'm thinking, another Yankee heathen, like myself.
Then I get to the end, and although fulfilling to the reader, as a sometimes lout, I was sorely disappointed sir.


Rings huh? What degree would you guess the bevel to be? Would you think that it was from the factory, or some odd wear?

Thanks, Trey

Agreen said:

That would have thrown me for a loop. It actually got me to wondering if there's a standardized method of testing compression? Like pedal at a certain speed without a carburetor and the value you get is xx.x? Then, if it's low you could add some oil in the cylinder and see if this raises the compression value, telling you it's the rings.

Click to expand...

???

I just use my compression gauge - saved a lot of work one day when the customer's non-starting bike read 175#, so I took the bowl off the carb & saw it was full of straight oil from trying to mix in the tank.

What I'm saying is that there's not a standardized method for testing compression. Sure, I could put a compression gauge on the engine, but at what rpm? What's a good number? How low is too low?

Not a lot of information on this subject.

For doing a compression test, 300 to 500rpm will be quick enough to give an accurate reading, that's basically just pedaling gently and letting the clutch out from about 5mph. What's more important than the rpm is letting the engine cycle over enough times. This is usually 4 to 5 full compression strokes on a 4 stroke engine. The test would be done with all spark plugs out and the throttle held wide open, leaving the throttle at idle is enough to throw off an accurate compression reading.
That's the 4 stroke multi cylinder method using the starter to turn the engine over.
For a 2 stroke engine we still need the engine to turn over several times at a steady pace like pedaling at about 5mph or coasting down a slight incline then let the clutch out for a few seconds. If the bike isn't mobile for some reason, a cordless drill on the mag rotor nut set at low speed would do the trick, but use a wider nut with more threads to prevent stripping the delicate mag rotor nut. It can also be done with a pull starter by giving it a long enough pull to cycle the piston a few times or by doing 2 or 3 full length pulls.

many threads here mention 'cold compression' which is usual test for a non-starter (how someone would run a gauge at a high RPM with only one cylinder is beyond me) - I lift the rear wheel & pedal till the gauge stops going up

All good info. Should be part of a sticky.

I'd like to see what numbers others are getting on their:

a)broken-in engines
b)new engines
c)48cc 2 strokes
d) 66cc 2 strokes
e) Honda clone 4 strokes
f) 79 or 212 cc predators
g) etc...

Each one would probably get different data. Unless there's already such a thread, I'd like to get a collection of that data and compile what people are getting. That way, we'd all know if the numbers are low and in need of a rebuild, rather than guessing and going down the awful path of troubleshooting fuel or spark.

That's assuming we all have access to a compression gauge, which I guess I take for granted a lot.

more to it than firing chamber compression - it should run if it is between 125# & 165# in the chamber, but 2-strokes also need compression in the lower end - with good compression up top & good spark & good fuel, I start looking at case seams & crank seals

What reading on a compression gauge is correct for our 66/80cc China Girl Engine?

Long Answer?


It depends on a few factors.

1. The Combustion Chamber Volume. Usually measured in CC's (I. E. Fred's 5.0, 5.5, 6.0, 7.0 CC Heads). The Stock China Girls heads, as far as I have seen, come in two flavors. An angle fire spark plug, with low (relatively) combustion chamber volume (I haven't measured it). A straight fire spark plug, with high (relatively) combustion chamber volume.

If everything else was kept the same, the reduced combustion chamber volume of the angle fire head would result in a higher compression ratio, and thus, a higher reading on the compression gauge.

Conversely, on the same engine, the increased combustion chamber volume of the straight fire head would result in a lower compression ratio, and thus, a lower reading on the compression gauge.


2. The Piston Dome Volume. This is difficult to measure, and is usually provided by the piston manufacturer (balls!). If the piston had a flat top this measurement would be 0. A dome on the top of the piston (like our lovable China Girl) results in a positive measurement, in CC's, equal to the volume of the Dome Area above the piston crown. Due to loose manufacturing tolerances there are minor variations in this volume, witch would result in minor variations in the reading of a compression gauge if all that was changed was a piston of the same type.


3. The Head Gasket Thickness. The head gasket lifts the head higher relative to the piston crown at top dead center. This means a thicker gasket will result in increased combustion chamber volume, reduced compression ratio, and thus, a lower reading on the compression gauge.

3a. Special Tip: Different thickness head gaskets are often used to adjust compression ratio. This has the unfortunate effect of changing the squish gap, google "squish band" if you want to learn more.


4. The Deck Height. This, in the most basic sense, is a measurement (mm or in) of the distance between the piston crown (the topmost edge of the piston) at top dead center, and the cylinder deck. If the piston crown is below the cylinder deck at top dead center, the measurement is positive and represents "positive deck" (because the deck of the cylinder is above the the piston crown. If the piston crown is above the cylinder deck at top dead center, the measurement is negative and represents "negative deck".

The greater the Deck Height (or the higher the cylinder deck is above the piston crown at top dead center), the more area is left between the dome of the piston at top dead center and the head. This results in higher (relatively) combustion chamber volume, lower compression ratio, and thus, a lower reading on the compression gauge.

Factors that effect Deck Height:

A. Crank Stroke: If using the same piston and cylinder without modification, reducing crank stroke from say 40mm (long stroke) to 38mm (short stroke) would increase deck height by 1mm. 40mm - 30mm = 2mm. 1mm reduced piston travel at the top and bottom of its stroke. This would increase combustion chamber volume, reduce compression ratio, and thus, result in a lower (relatively) reading on the compression gauge.

B. Connecting Rod Length: Even if the actual swept stroke measurement of the piston remains the same (same crank) if a 1mm shorter connecting rod is used, the Deck Height is now increased by 1mm. This results again, in increased combustion chamber volume, reduced compression ratio, and a lower (relatively) reading on the gauge.

B1. A little secret. No two connecting rods (even if marked the same) are EXACTLY the same length. Even these seemingly tiny variances in length will have an effect on the reading on the compression gauge. If there is a tolerance variance large enough to change the position of the piston by 0.010" up or down, it will result in about a 10psi change in the reading on the compression gauge (for example, changing out a head gasket for one that is 0.010" thicker or thinner)

C. Cylinder Height. The height of the cylinder, measured from the bottom face that seals to the crankcase, to the top face that seals to the head. A 1mm taller cylinder than your current one (I have seen MANY different heights in the wild) will result in a "taller" deck height. This will again result in increased combustion chamber volume, lower compression ratio, and thus, a lower (relatively) reading on the compression gauge.

D. Cylinder Base Gasket. The cylinder base gasket (obviously) lifts the cylinder by an amount equal to the thickness of the gasket. A thicker cylinder base gasket will lift the cylinder, resulting in a taller deck height, increased combustion chamber volume, lower compression ratio, and thus, a lower (relatively) reading on the compression gauge.

E. Piston Wrist-pin Location.The distance between the piston wrist-pin hole and the crown of the piston. There are obviously two standard piston wrist-pin locations we have in our China Girl engine, the type A and the type B. If everything else stays the same, and you switch to a piston with a higher wrist-pin location (relative to the piston crown) the piston will be lower in the cylinder at top dead center resulting in taller deck height, increased combustion chamber volume, reduced compression ratio, and thus, a lower reading (relatively) on the compression gauge.

E1. Just like the connecting rod, small tolerance variations in individual pistons means, generally, no two pistons will ever achieve the EXACT same compression reading, in the same engine, at the same time. (Again this relates to manufacturing tolerances. In precision manufactured modern engines from major manufacturers this is not an issue. In our "loose tolerance" machined China Girls, careful selection out of a batch of the "same" part will result in the ability to assemble engines with much finer tolerances from parts that are otherwise not very consistent from part to part. This is what seperates a skilled engine builder -among many things- from someone who just knows how to assemble all the parts together in the right order.)

D. Other Variances. There are other variances that play a role, such as the free-play up and down in the big and small rod end bearings, and their pins, as well as free-play in the crank bearings in the case. The case casting itself, and how low or high the cylinder mounting face is to the crank axle center-line. The higher (relatively) this face is in relation the crank axle center-line, the higher the cylinder will then be. The higher (relatively) cylinder now results in a taller deck height, increased combustion chamber volume, lower compression ration, and thus, a lower (relatively) reading on the compression gauge.


5. The Condition of the Rings. Brand new rings will not give as high a reading (relatively) on the compression gauge as rings that have been properly worn in. Once the rings are sufficiently worn in they will provide their maximum available compression reading. After this point the compression reading will steadily drop as the rings wear, and more and more blow bye is allowed past the rings, reducing compression. As a VERY GENERAL rule of thumb: Once the reading you get on the compression gauge is 20-25% smaller than the reading you got when you installed brand new rings, it is time to freshen up the top end. For us that means new cylinder and rings, I will usually re-ring the piston once before I replace the cylinder though.


6. The Pre-MiX Ratio used for the Oil/Gas Mix. Simply put: The more oil you mix in your gas, for example: 20:1 vs 50:1, the higher the reading you will see on the compression gauge.


Short Answer:


As you can see, due to the different types of parts that are used to assemble our lovable China Girl engine, as well as the various "loose tolerance" variances involved in its manufacture it is literally IMPOSSIBLE to tell you what your engine "should" achieve in a compression test.


The best I can do is give you general guidelines.


If it reads below 80psi on a brand new engine, don't expect amazing performance, but it should totally start as long as it has somewhere around 40-50psi... and there isn't something drastically wrong inside the cylinder.

Anything from 90-120psi is smiles all day long, good stuff for a stock engine, and not too shabby for a mildly upgraded engine either.

120-150psi is performance engine territory, definitely have to keep an eye on engine temps, and it would generally be recommended to keep the oil mix at 32:1 or richer (i.e. 24:1, 20:1, 16:1).

150psi+ is extreme performance engine territory, and you had better know what you are doing.

I run my engines anywhere from 230-250psi, but they are strictly race engines, they are not run on the street, only at race events. I run a big Jake Head, 16:1 oil mix, 0.025" squish gap. I tune the ignition advance and the carburetors judiciously, run oxygenated race gas (oxygenates help cool the cylinder) and I keep a SHARP eye on my spark plugs.

After all of this, I get roughly 10-20 hours of run time before I have to rebuild the top-end... If it makes it that long.

Luckily I always have a new porting idea I want to try. When I finally tackle exactly what I want out of the port arrangement I am going to pony up for some Nikasil. Gotta love that stuff.


.... I could go on for a few more pages on the subject, but I think I'll leave it here for now. If anyone has anymore question don't hesitate to ask... I like typing.




Sean Davis
Apollo Moto Racing
https://www.facebook.com/ApolloMotoRacing/