Compression Check for OB?

[merc245]

What is the proper way to preform a compression check. I've heard and read differant things or ways.I just looking for the proper way!Thanks merc245

 

[Will30]

When I check compression I pull the coil wire and ground it to the block . Pull the plugs one by one screw the gauge in and spin it over . Put that plug back in and go to the next . Thats how Ive always done it and havent had any problems .

 

[merc245]

i've read you open the trottle when cranking?

 

[justincase]

If its a merc i heard it has to be warmed up first to get a proper reading, just what ive been told.

 

[Will30]

That my be true but the last check I got on my inline 6 was in the 140's on all 6 Ive never opened the throttle when Ive checked . Well try both ways and see if your readings change . I dont see where it would make a difference tho ?

 

[merc245]

Yeh, I did one at the start of last yr and I was happy with the numbers 137-132 lbs. But than I read this, heard that so I was just questioning my own findings!

Tomorrow I'll let get good and warm before I start and I'll do it both ways I guess.And I'll lengthen the throttle stop and keep myself out the 7000rpm for the rest of the season!

 

[TheNerd]

Numbers should go up when the boat is warm, dont worry about the throttle position as that makes no difference. Last time I did it, I had two guages, measured the bottom, mid, then the top cylinders. Was pretty quik to do.

 

[Vandalizer]

I think the important thing is letting it run til it's warm, partly to get a good reading, and partly to make sure everything gets good and lubricated before you start cranking it over dry. Instead of grounding to the block, I'd just use your kill switch to totally disable the ignition system.

 

[Enticer1]

Don't forget to

Even though you are only turning the motor over for a short time you should still have a hose running on the lower. It doesn't take much to damage your impeller when spinning over dry. I did address the issue of having the throttle wide open with a professional engine builder and he told me yes on that. Common sense would tell you that you'll have a better reading with a warm motor but its not necessary as long as all cylinders are done at the same temp.

 

[Mark]

Cold or warm, open or closed, the main thing is that the cylinders are all within 10%. At all mid 130's you are good to go on that motor.

 

[JW]

The Merc service manual says warm the engine, pull all the plugs, open the throttle all the way, pull the kill switch, maintain a strong battery.

so many opinions, and only 1 merc service manual

 

[livelegend]

The Merc service manual says warm the engine, pull all the plugs, open the throttle all the way, pull the kill switch, maintain a strong battery.

so many opinions, and only 1 merc service manual

Theres your answer. And thats how we do it at work, although warming it up isnt totally necessary. I've always done mine cold, and i think thats key. To do it either cold all the time or warm all the time for comparing reasons

 

[TheNerd]

Well, I agree with the kill switch and strong battery. The other plugs out, well, that will make it easier to turn over the engine. Also for comparison reasons always doing it either hot or cold is a good idea. However, I dont agree with the throttle NEEDS to be wide open. Thats just my opinion, and its not going to change.

 

[LilSki]

Supposedly by opening the throttle it allows it to build pressure faster by allowing more air to enter the cylinder. Either way you will come to the same number. It may only take 1 or 2 rotations with open throttle, and maybe 2 or more with closed throttle.

That is just what I have been told and it sort of makes sense. But I've never put it to the test.

 

[livelegend]

I havent found much change in opening the throttle all the way vs. leaving it closed either. Last fall I got 115 on all 6 closed and this spring got 120 on all 6 with it open. I was also using a different gauge tho, some sears brand and this spring a snap-on gauge. I also turn it over 4 times
It doesnt hurt to fully open the throttle tho and thats how I was told to do it, but it is a PITA when ya have a foothrottle