Merc 6hp four stroke...compression values?

[Mas]

I'm doing some diagnostics for a friend. I did a compression check and got rather low numbers...55# Is this a low compression motor, or is this number low?

Also, throttle should be all the way open when testing a 4-stroke?

Thanks for the help & Happy Motoring,

Mas

 

[Texasmark]

Yes on throttle. 55 is way low like half of a normal low reading which is 100. Should be on up around 120 or so per my manual for a different engine but engines are engines sorta. Do you know if your gauge is accurate. Reason for the question is if both are reading that low, you should have had problems operating the engine long before now, like back when it was falling past 100.

Mark

 

[Mas]

Motor runs and idles good...is there a "decompression valve" for easy starting. My gauge is good...getting 120+ lbs on my other motors. I'm not a merc guy, so I do not know the features of these newer motors. I stick with the old stuff.

Thanks,

Mas

 

[JustJason]

42 psi +/- 14 psi on a static compression test.

Yes there is a decompressor to allow for easier starting. There is a nub on the camshaft that just cracks the exhaust valve open on the compression stroke.

 

[Guest]

I had a 96 merc 6hp my compression was around 70 on both cyl. that thing ran like a beast. Some small outboards 60s is normal. I am not sure about 4strokes. Were you testing the comp on a warm or cold engine? It is recommended to test on warm for more accurate results

 

[bilge rat jim]

Because of the automatic decompression feature on these small outboards, your compression test numbers may appear to low compared with other engines, however the manual gives the following numbers- 8.5:1 compression ratio, 42 psi +- 14 psi Looks like your outboards' compression is normal.

 

[Texasmark]

Well I apologize for the disinformation but I did say it was per my manual for a different engine. Glad your numbers are ok.

Mark

 

[Mas]

Thanks guys...the info is a lot of help! I just have to work on cooling issues. Does the motor have a thermostat? If so, where is it. It is not too obvious.

Thanks,

Mas

 

[Texasmark]

Sales brochures from 2 strokes of the late 90's says they are optional for 6 and 8 hp engines. On 4 strokes probably standard equipment as they help the environmental cause and that's what the 4 strokes are all about. The water surrounds the cylinders and the hottest part is the head area up where the spark plugs are located. You should have a bulge about the size of a nickel coin either on the rear of the block or on top rear. There will be a cover with a couple of screws and the stat is under that. On larger engines the bottom of the pellet is stamped with the opening temp and they use 143F. You can test it on the stove in a pan of water with a candy thermometer. Manually open the stat pushing against the spring and insert a sewing thread. Holding the thread with the entire stat in the water when it starts opening it will drop from the thread.

HTH,
Mark

 

[bilge rat jim]

Your outboard should have a # 804540 thermostat- 126 degree F.