no compression 1 cyl. still runs 5500rpm

[dcolpitts]

1986 evinrude 140 looper, started motor snd warmed it up. just before leaving dock heard a rattle for about 5 seconds. idle dropped about 100 rpms. drove ran the motor about 14 miles with the only differance being wot was 5400rpm opposed to 5800rpm before noise. check plugs 1 plug wet rest dark brown. I did a compression check and the wet cyl 10 psi, check cyl, no hole or scoring in cyl. Could this be a reed valve problem and how could the cyl have that low commpressionand still run at 5400 rpms. the moter pulsates about 100rpms when running at 4000-5400 rpms. sorry for the long post. confused

 

[OBJ]

Re: no compression 1 cyl. still runs 5500rpm

Okam....did you notice any "clean" areas on top of the pistons? This would be an indication of possible water ingestion into the cylinder most likely by a blown head gasket. Or....you could have stuck rings which is hard to see on loopers.<br /><br />It's also possible you've lost the spark to the cylinder. Have you done a spark test?<br /><br />Several possiblities here. The lost rpm is really due to the cylinder not putting in it's 25% worth. If the head is still off, shake the piston and compare it with the #3 piston. See how much slop is in it. Enough blow by could be putting the fuel charge past the piston and out the exhaust port.

 

[DHPMARINE]

Re: no compression 1 cyl. still runs 5500rpm

Three cylinders were doing all the work if the compression reading is correct.When I check compresion,if the last readings are low,I go back to #1cyl to see if it dropped becaused of battery draw.<br /><br />10 PSI is not a good thing.Pull that head,and check the gasket,hopefully.<br />DHP

 

[dcolpitts]

Re: no compression 1 cyl. still runs 5500rpm

Thanks for the replies, I did do spark test and checked the coil all is well. Rna the motor about 30 minutes at high speed and shut the motor down to check problem cyl, plug was med brown and no longer wet. motor ran at same rpms as prevois post no longer pulsated 100 prms as before in <br />original post. Is there a leak test for reed valves, or is it a pull apart situation. Again thanks for your replies.

 

[dcolpitts]

Re: no compression 1 cyl. still runs 5500rpm

Thanks for the replies, I did do spark test and checked the coil all is well. Rna the motor about 30 minutes at high speed and shut the motor down to check problem cyl, plug was med brown and no longer wet. motor ran at same rpms as prevois post no longer pulsated 100 prms as before in <br />original post. Is there a leak test for reed valves, or is it a pull apart situation. Again thanks for your replies.

 

[R.Johnson]

Re: no compression 1 cyl. still runs 5500rpm

If you are showing only 10 lbs., but still showing 5400 RPM, something is wrong in your test. If you drop a cylinder, and most certainly a 10 lb. cylinder would do that. A drop of one cylinder will most often cause a drop of 50 % in a v-4 engine. I would pull the cylinder head to check. I don't like the sound of that 5 second rattle.

 

[G DANE]

Re: no compression 1 cyl. still runs 5500rpm

I'l second R.Johnson, you could have lost a piece of a piston ring, rattling above piston untill it was spit out in exhaust. Take a look before more damage occurs.

 

[OBJ]

Re: no compression 1 cyl. still runs 5500rpm

And to check the reeds, while the engine is running, check to see if you have any blow back from any of the carbs. This will be most noticable at low speed or rpms. If you do have blow back, most likely reed damage is the cause.<br /><br />But to reiterate the previous posts', check for damage in the pistons first. Running the engine can only make present damage worse.