Compression Tests

[Tyler6357]

I have done compression tests on both of my bike engines. They are both China Girl PK80, 69cc/47mm cylinder/40 stroke with the short connection rod and the low wrist pin on the piston. However, the bikes have different set ups.

GT Bike--> Ported by my friend who works at a machine shop. He ported the intake/exhaust/transfer ports and decked the cylinder to .03' squish gap while using a mill. The bike is also set up with a .6cc Clone FredHead, CHD-G2 4 peddle reed valve with windowed piston, 15mm Dellorto Clone Carb (HT/RT), MZ Miami expansion pipe, FMF Powercore 2 sliencer. This engine has about 400 miles on it.

Peugeot Bike----> Purchased from Dax about 6 years ago also Duane from Dax ported the engine for me. I decked the cylinder myself using a piece of glass and sandpaper method attempting to get it to .03" squish gap. The bike has a .6cc Real Fredhead, a 15mm shorty intake, stock piston, 15mm Dellorto Clone Carb (HT/RT), Sick Bike Parts Expansion pipe. This engine has over 2000 miles on it.

The results of the test came out:

GT--------> 158 psi
Peugeot--> 195 psi

I gotta say that the Dax engine on the Peugeot has always been just an awesome engine, once I found the right jet (#60) the thing just took off. Not only did it have awesome throttle response and a steady low idle, it has almost no chain drag, the engine sprocket seems to spin endlessly. It has got to be the best engine I have ever gotten. I had the bike running so well that I didn't want to change anything and, you know, screw it up, haha. But perhaps it's just the luck of the draw? I don't know. But I was very surprised to see a compression of 195 psi, a full 37 psi higher than the GT one. Is this normal? Are these ratings above average?
Thank you!

 

[crassius]

my GF rides a heavily modded motor up around 165 or 170 - it's pretty fast

on an unmodded motor I'd be thinking the carb bowl was full of straight oil at that much pressure

 

[Tyler6357]

my GF rides a heavily modded motor up around 165 or 170 - it's pretty fast

on an unmodded motor I'd be thinking the carb bowl was full of straight oil at that much pressure

Do you think I should use a thicker mix for the Peugeot?

 

[Venice Motor Bikes]

I think you're using the compression tester wrong... A healthy Big Block Chevy has about 180-190 PSI... (These little engines are even in that league).

Please describe how you used the tester, (& got those numbers)??

 

[crassius]

Do you think I should use a thicker mix for the Peugeot?

no, I was saying that when someone tries to put oil in tank, then tries to add gas and shake bike to mix, they sometimes get all the oil in the carb with no gas mixed in - this will coat cylinder and rings causing a seal that raises compression significantly

an unmodded bike comes in for repair as a 'no start' & has way high compression usually means they got only oil in carb

 

[Tyler6357]

I think you're using the compression tester wrong... A healthy Big Block Chevy has about 180-190 PSI... (These little engines are even in that league).

Please describe how you used the tester, (& got those numbers)??

Okay, I removed the spark plug, screwed in the compression gauge, got on the bike, peddled and then popped the clutch and held the throttle open and continued peddling until the gauge stopped going up. I did this with a cold engine. Maybe I should try a 2nd test. What do you think the reading should be? How about the GT, 158?

 

[Tyler6357]

no, I was saying that when someone tries to put oil in tank, then tries to add gas and shake bike to mix, they sometimes get all the oil in the carb with no gas mixed in - this will coat cylinder and rings causing a seal that raises compression significantly

an unmodded bike comes in for repair as a 'no start' & has way high compression usually means they got only oil in carb

Oh, well I never mix in the tank itself. I mean, I always make my mix in a gallon container and shake it up completely before pouring it into my tank.

 

[crassius]

as the only repair shop in many miles, I see it all - I do cold compression same way you do except I do it on the build bench

your readings should be right

 

[Venice Motor Bikes]

Okay, I removed the spark plug, screwed in the compression gauge, got on the bike, peddled and then popped the clutch and held the throttle open and continued peddling until the gauge stopped going up. I did this with a cold engine. Maybe I should try a 2nd test. What do you think the reading should be? How about the GT, 158?

I'm not sure what the correct answer is to this, because you're only supposed to crank the engine over a few times in a Chevy until it stops making 'big jumps' to get the correct compression... (not keep going at a high speed with the throttle wide open).
If you were to fire up the Chevy engine & hold the throttle open a little for a second, you'd probably blow the compression gauge up (instead of it just getting a correct reading of 180 psi)



Does anyone here have any ideas as to what the best way to test compression on these engines would be??

 

[Tyler6357]

I'm not sure what the correct answer is to this, because you're only supposed to crank the engine over a few times in a Chevy until it stops making 'big jumps' to get the correct compression... (not keep going at a high speed with the throttle wide open).
If you were to fire up the Chevy engine & hold the throttle open a little for a second, you'd probably blow the compression gauge up (instead of it just getting a correct reading of 180 psi)



Does anyone here have any ideas as to what the best way to test compression on these engines would be??

Yeah, I understand what you mean, when I was peddling it I was just glancing down at the gauge while riding on the street in front of my house. It's possible that I held it too long. Maybe I should set the throttle open and just slowly turn the rear wheel with it up on my dual legged kickstand until it stops increasing. I will attempt a few more tests when I get a chance and see if the results are any different.

 

[crassius]

depending on how long your hose is, it will take time to pump pressure throughout whole gauge & hose system - no matter how much you pump it once it gets to the amount in the combustion chamber, it cannot get higher than that value

if the gauge/hose are pumped up to say 150# no amount of repeatedly exposing it to chamber pressure of 150# will make it go higher

 

[Tyler6357]

Update: Okay, since I did this test neither of my engines will run correctly. I don't understand what could have happened. The GT has simply no spark, I haven't dug into why yet. The Peugeot will start up but will only start with full choke on. It has been sitting for a few months because I've been riding the other bike. I don't know what happened, I'm very bummed that I even attempted this.

 

[crassius]

does the one that starts with choke run OK after it starts

no spark is easy - got to be CDI, or mag, or plug - assuming it is still wired right

 

[Tyler6357]

does the one that starts with choke run OK after it starts

no spark is easy - got to be CDI, or mag, or plug - assuming it is still wired right

No, it will only start up with choke on, once I turn it off it dies immediately. I'm sure I can figure out the spark problem, could the compression tests caused these problems? It seems strange that they were both running great before I did it.

 

[Tyler6357]

Okay, good news. I got the Peugeot running good again. Not sure what the problem really was. I took the carb off, took it apart and cleaned it with carb cleaner and I replaced the fuel filter. Then I put it back on but at first it didn't seem any different, I had to start it up with the choke on but once I got it started I just let it run for awhile. Then I took it out and released the choke and it kept running. I ran it up and down my street a few times and all of a sudden it got all the power back. It still wants to die at idle but once it's warmed up it won't die. Also, once it's warmed up I can start it without the choke now. Maybe it just needed to be run after sitting for several weeks. I had drained the tank and turned the petcock off while it was sitting.
Now I just need to figure out why the GT has no spark.

 

[Venice Motor Bikes]

Your engine cases were probably full of unburned gas & oil from spinning the engines for the compression tests... (& that was fouling the spark plugs).

 

[Venice Motor Bikes]

Also, on another note.... All the oil in the gas (sealing the piston rings) is probably why these little engines have such high numbers on the compression tester.

 

[Tyler6357]

Your engine cases were probably full of unburned gas & oil from spinning the engines for the compression tests... (& that was fouling the spark plugs).

You are probably right about the cases full with unburned gas and oil but how could that cause a no spark situation? Also, what's the best way to clean out the cases?

 

[Tyler6357]

Also, on another note.... All the oil in the gas (sealing the piston rings) is probably why these little engines have such high numbers on the compression tester.

Have you ever tested any of your MB engines with a compression tester? I picked it up because I thought it would be a good tool to have to test for compression leaks.

 

[Venice Motor Bikes]

You are probably right about the cases full with unburned gas and oil but how could that cause a no spark situation? Also, what's the best way to clean out the cases?

It causes a 'no spark' situation because the spark travels through the liquid thats covering the plug (instead of jumping the air gap in the plug).

A good way to to clear out the engine is to take out the plug & spin the engine so the gas gets blasted out of the empty spark plug hole. (Be sure to cover the hole with a rag & avoid making a spark with the loose wire)!!