One cylinder sportster?

[Grammaton]

I always did compression tests by bumping the starter. Never with the engine running.
Open or closed throttle won't make any difference.

 

[gs34]

+1.....exactly!

 

[fin_676]

:eam Annnnd....that will do what???????

itll open the carb butterfly and let the air go into the cylinder if you dont get sufficient air into the cylinder then there wont be enough volume of air being compressed to give you a correct reading

Brian

 

[gs34]

OK, I'll accept that to a point, and I figured that was the reasoning. However, I've never found that to be a problem on any engine I've ever checked.
Think about it....if the butterfly being closed restricted airflow that much, the engine would not even idle.

 

[fin_676]

even when the throtle is closed the buterfly is slightly open to allow sufficient fuel/air mixture into the engine to run but any restriction in the incomming airflow will affect the compression test
to get a correct reading you need to give the best air flow possible to eliminate any inaccuracy

Brian

 

[Grammaton]

even when the throtle is closed the buterfly is slightly open to allow sufficient fuel/air mixture into the engine to run but any restriction in the incomming airflow will affect the compression test
to get a correct reading you need to give the best air flow possible to eliminate any inaccuracy

Brian

The same volume of air goes through the cylinder no matter what the engine speed is at the time. At higher RPM it just happens faster.

You will always have the same number of air and fuel molecules in the cylinder on the compression stroke.

 

[glider]

I always did compression tests by bumping the starter. Never with the engine running.
Open or closed throttle won't make any difference.

It makes a very noticable difference in the numbers. The throttle plate should always be open for the truest reading of the compression. In a CV carb the slide should be held up too.

 

[Grammaton]

It makes a very noticable difference in the numbers. The throttle plate should always be open for the truest reading of the compression. In a CV carb the slide should be held up too.

I guess we do them differently (or maybe for different reasons). I don't ever look for a specific pressure. I look for pressures to be comparable with the other cylinders.

 

[glider]

There shouldn't be more than a 10% difference between cylinders but the compression numbers is what determined the internal condition of the cylinder rings/valves etc.. With the carb butterfly closed the numbers aren't true compression numbers and will always be lower than the actual compression which is being done to ascertain the actual condition.

 

[Grammaton]

There shouldn't be more than a 10% difference between cylinders but the compression numbers is what determined the internal condition of the cylinder rings/valves etc.. With the carb butterfly closed the numbers aren't true compression numbers and will always be lower than the actual compression which is being done to ascertain the actual condition.

How does an engine start at all if the cylinder(s) won't fill with air/fuel?
A hot engine will start without manipulating the throttle. What's different about a cold engine? It's not the ability to compress the mixture. Well, maybe to some extent in an air-cooled engine, but I wouldn't think it's enough to be affected by the butterfly.

I've never seen an engine that was a hard-starter cold but not hot because of rings/valves, but I haven't seen everything.

I'm told a leakdown test is better than a compression test for checking valves, but I've never done one. It was always too easy to do a compression test.