Force compression readings...interesting

[Bigprairie1]

I was just reading in my newly acquired Cylmer manual that the compression readings in the Force engines are not necessarily designed to be the same on all cylinders. I thought this was pretty interesting info because it would have a bearing on the interpretation of results from any compression testing. Particularly if one was assuming that the reading should all be about the same.
On some 3 cylinder engines (ie: 90hp/120hp) the 1st cylinder are supposed to be about 10-15 psi lower than than #2 and #3 cylinders. On the 4 cylinder engines the lower compression should be on the #1 and the #4 cylinders.
This is pretty interesting...anyone know of this or heard of this on the Force engines?
I guess if I'm getting a slightly lower compression reading on my #1 cylinder....it might just be designed to be so?
I'll have to recheck my serial/model no's to see if it applies to either of my Force 90hp's before resolving my compression tests.

 

[bertthedefender]

Re: Force compression readings...interesting

I have the 70hp motor, and interestingly for me in the Clymer book, the compression values table doesn't actually tell you what the readings should be, just that they should be within 15psi of eachother!

 

[Bigprairie1]

Re: Force compression readings...interesting

I saw that and it is odd. You could probably safely assume its up around the 150psi point tho'.

 

[rudedude]

Re: Force compression readings...interesting

That is rather odd. I wonder why it would be any diffrent from other motors.

Glad you posted that I was going to do a compression check on my 125 tomarrow. Thanks

 

[eedwards]

Re: Force compression readings...interesting

On my '75 90 hp all three are different but within 10 psi of each other.

 

[oleliner]

Re: Force compression readings...interesting

all the shop manuals, and my working experience, has been any cylinder that was 15 lbs or more, difference in cylinders, was time to find out why,

 

[CVX20SPRINT]

Re: Force compression readings...interesting

This is the second time I,ve heard of this.Yamaha 2-stroke 3-cylinder snowmobile engines are built with lower comp. on one cylinder.I believe it is a cheap way to make the eng. appear smoother without the extra weight of a balance shaft.I know people who have walked away from used ones after a comp. check when there really was nothing wrong.I owned one of these sleds since 03 and have approx. 25,000 klms on it without a rebuild and it still gets to 100+ mph pretty quick.I can see Force doing this on a 3 cyl.,but I don't understand why on a 4 cyl.

 

[Frank Acampora]

Re: Force compression readings...interesting

I never paid attention to actual position, but if you look at the marks from the bendix on the flywheel teeth, you will see that the engine always or almost always stops at the same position. Lower compression on that cylinder could be an effort to lower initial cranking power necessary to start.
1 and 4 on the 4 cylinder engines are 90 degrees apart so both would affect cranking.
A lot of my Chrysler engines will crank slowly for the first turn then pick-up speed. That's what I base my guess on.

 

[moparman]

Re: Force compression readings...interesting

manufacturers do some weird things some time to fix problems,I know on an old air cooled vw ,the #3 cylinder timing is retarded 3-4 degrees in the distributor cam to make that cylinder run cooler[ oil cooler is right on top of that cylinder]Just one way to get around things some times cheaply????........MOPARMAN ROLL TIDE

 

[oleliner]

Re: Force compression readings...interesting

gosh i never knew that, i lived & worked in south florida , a long time before returning to co, my force engine is a 125 hp , 1988 and the compression is a even 105, on all cyls, close to overhaul,but feel because i have a low spark issue my cyls, are washed with fuel mixture, once i correct low spark and retime engine, compression will increase, is my experience..guys

 

[Boatdrinks]

Re: Force compression readings...interesting

Interesting topic to be sure. Never thought of that either.