Compression Testing

Can someone explain the reason why having the throttle opened up is necessary for a good compression test? Is it because the intake air flow is too low if the carbs aren't wide open? I'm just trying to understand.

DoubleDub wrote:

Can someone explain the reason why having the throttle opened up is necessary for a good compression test? Is it because the intake air flow is too low if the carbs aren't wide open? I'm just trying to understand.

As known, the 4 strokes in a four stroke engine are:
Intake, Compression, Power, Exhaust.

Intake - piston moves down (open intake valve sucks from carb, exhaust valve closed).

Compression - piston moves up (both valves closed).

Power - piston moves down (both valves closed).

Exhaust - piston moves up (pushes spent gases through open exhaust valve, intake valve closed).

At end of exhaust stroke with piston at TDC, there's very little air then remaining inside the combustion chamber, because all of it has been pushed out through the exhaust valve.

Next comes the intake stroke which must first suck air into the combustion chamber to provide air for the next stroke to compress. The air being sucked in comes through the carb throat. A closed throttle slide (in down position) blocks the carb throat and thereby prevents air from being sucked from the carb through the intake valve and into the combustion chamber. No air being sucked in means no air available to compress.

So it would be correct to say that the reason for having the throttle opened up being necessary for a good compression test is because the intake air flow is too low if the carbs aren't wide open.

Now with any luck, this Simple Simon version will suffice without getting into valve overlap, etc. :lol:

Good Luck!

Of course there's always an exception to the rule.

One fellow boasted of his ability to perform a perfectly executed compression test with no concern about throttle position. :ohmy:

And by golly he was right. Because he tested with the carbs removed from the engine. :lol:

When I did it both ways (throttles at idle position versus wide open) I didn't see much change. The compression pumps up over several rotations and the gauge captures the peak reading as it builds up. Even at idle position, it let in enough intake to pump up fully over three spins.

bountyhunter, that just doesn't sound right to me, but GPzInfested wants to do a compression check on his GPz750 Turbo so I'm going to try it both ways to see if it's true.

Consider the number of people that have started threads complaining about their low compression on here, and usually the first or second answering post asks "Did you have the throttle wide open when you tested?", and the person says no and retests, then posts again all happy because they found their lost compression (30-50 psi of it anyway)

I believe you when you say it happened that way on your bike, so now I'm going to be stuck trying to figure out what exactly could cause that the rest of the night :laugh:

I've always wondered why throttle position matters on CV carbs.

KZCSI

steell wrote:

bountyhunter, that just doesn't sound right to me, but GPzInfested wants to do a compression check on his GPz750 Turbo so I'm going to try it both ways to see if it's true.

Consider the number of people that have started threads complaining about their low compression on here, and usually the first or second answering post asks "Did you have the throttle wide open when you tested?", and the person says no and retests, then posts again all happy because they found their lost compression (30-50 psi of it anyway)

I believe you when you say it happened that way on your bike, so now I'm going to be stuck trying to figure out what exactly could cause that the rest of the night :laugh:

Maybe I had the idle set up so the throttle was a bit open? Mine has the hand screw on the left side so you can jack the idle up when it's cold.

What I remember is with it at idle, it took more cranks to get all the way up to final reading. The final reading may have been 5 or 10 PSI lower (like 155 compared to 160) when the throttle was at idle compared to open. This is with new valves, pistons, rings (cylinder just bored) and new gaskets.

Holding the throttle open is definitely the right way to do it. I was just surprised it didn't have a more dramatic effect.

I really dont get why you need to have the throttle open, and its not a 4 stroke deal either, you dont open up a car when you test it.
my guess is the test was done that way in japan, where the compression results were written into a spec book. if the test would have been done closed then that would have been the number used.
In the long run if you test them all the same and dont vary more than 10% from cyl to cyl then you shopuld be ok

BSKZ650 wrote:

I really dont get why you need to have the throttle open, and its not a 4 stroke deal either, you dont open up a car when you test it.

But, I guess you do need to open it. I just rechecked mine with engine warm, both plugs pulled:

throttle closed: 125 psi

throttle open: 155 psi

So, it was a 30 psi difference which is pretty big. I am guessing the measurements where there was a smaller difference may have been when the valves were leaky?

Anyway, it does confirm you need the throttle open to get the max reading.

This also proves the memory is the second thing to go bad when you get old.

You don't want to know what the first thing is.

KZCSI wrote:

I've always wondered why throttle position matters on CV carbs. KZCSI

Tnat's a really good question. And I'm still thinking on it.

Patton wrote:

KZCSI wrote:

I've always wondered why throttle position matters on CV carbs. KZCSI

Tnat's a really good question. And I'm still thinking on it.

Maybe it's because of the reduced volume of air available at inlet when the throttle is almost closed. Remember there is valve opening overlap, so some of the volume of air compressed on the upstroke is lost out the exhaust valve which usually opens just slightly before TDC. It's possible this effect will reduce the amount of air available to compress enough to reduce the maximum compression reading if the intake is closed off enough. In other words, the reduced air volume resulting from clamping off the intake starts to make the "lost volume" from the exhaust valve opening enough to reduce the peak compression reading.

That's about all I can figure.