I have been meaning to check the compression of my 240. I have no reason to expect its anything but perfect, but I am an engineer, and I like to know.

Lets see.. standard way to check compression on a small block chevy dictates you unscrew the spark plug, you screw in the compression check. Unplug the coil from the distributor, hold the throttle wide open and crank her about 4 over. Read said gage and repeat 7 more times.

Now with the SU's, you cant really hold open the piston's, so how do you do a proper compression check?

Remove the domes and pistons from the SUs for the duration of the test.

THanks Arne! I was configuring a way to hold those things open, but it looks like just pulling bowls would be easier. As usual...great advice.

...and do it on an engine at normal operating temperature with all spark plugs removed to minimise the load on the starter motor:classic:

Regarding holding up the SU slides (pistons if you will).....to save yourself added work, remove the air filter or filters and prop the slide up with properly sized pieces of soft material (wood, plastic, cardboard, etc., etc.).

Tom

Because the Chevy's used downdraft carb.'s - with throttle plates attached to the mechanical linkage - you had to prop the throttle open, so the cylinder wasn't sucked into a vacuum, prior to compression.

If you were running triple side drafts - with mechanical butterflys you would have to prop the throttle open as well.

With the SU's the vacuum from the cylinder itself will open the pistons as far as necessary - so you really don't have to do anything with the carb's. - just use the idle adjustment screw on the linkage to open the throttle and make sure the choke is full off.

Also run the engine over at least three or four compression strokes on each cylinder. You'll feel the compression tester push back at your hand, on the compression strokes. Some testers screw in and others are held in by hand pressure... If your using the hand held type hold it firm and squarely in the hole.

The reason for turning the cylinder over at least three of four compression strokes, is to assure that the cylinder walls are evenly lubricated and sealed, and to assure you are getting the highest possible reading.

FWIW,

Carl B.

Edited January 31, 200916 yr by Carl Beck
for got to mention the choke

Read said gage and repeat 7 more times.

Why 7 more times ? Have you got a V8 in there ?

With the SU's the vacuum from the cylinder itself will open the pistons as far as necessary - so you really don't have to do anything with the carb's. - just use the idle adjustment screw on the linkage to open the throttle and make sure the choke is full off

Whew! Thanks for clarifying this - I was sure you didn't need to prop open the SU's, but when I saw the other posts, I was second guessing myself!

Why 7 more times ? Have you got a V8 in there ?

I think he was referring to how it is done on a Chevy V8.

Lets see.. standard way to check compression on a small block chevy dictates you unscrew the spark plug, you screw in the compression check. Unplug the coil from the distributor, hold the throttle wide open and crank her about 4 over. Read said gage and repeat 7 more times.

-Mike

If in doubt prop open the throttle to hold the butterfly's open. As was stated the vacuum piston is a non issue. :classic:

Zedyone,

There is an additional "Wet" test if you will, that only takes a few more moments..

Here's a brief write up with an additional "wet" test.

http://freespace.virgin.net/tommy.sandham/compression.htm

Really like the Z, Well Done!

Tuck\o/

And so what are the results??

I was going to check mine as well but the battery was very low on Satuday after being started and driven and back on the truck a few times.

And today - well I just drove it around a bit. Mostly with some trucks about 3 ft off of my tailpipe :sick: (which is fumey to say the least).