Low compression 25 hp Envirude 1975.

[redfisher2003]

The motor runs well at medium throttle, it does not idle very slow but it will plane my homebuilt wooden clamming skiff (with rocker bottom, 16 ft) easily at full throttle. It will will go for 2 or 3 minutes then it just slows and the moter dies. Almost immediately it will start and run for about the same lenght of time then slow and die. It will run for abit longer if the throttle is kept in the medium range. Just after it stops I pulled the spark plugs and both looked brownish white. Fuel line seems good and works perfectly on my 6 hp motor.<br /><br />I replaced the fuel pump, the impeller, the thermostate(water is coming out the tattletale), the points. and did a carb rebuild before I found out the compression in both cylinders was 75 psi. Then I did a chemical decarbon treatment. The compression did not seem to rise. It actully dropped to 65 with the moter still hot. Also the compression did not rise more than 5 psi when I squirted oil in the cylinders prior to the test.<br /><br />I know both plugs are working a full throttle but in the tank at an fast idle the bottom plug fouls out. I am ready to pull the pistons and do a ring job etc. because it just seems the motor can't handle the load under full power and it does not idle. My hunch is it looses compression as it heats up and then finally just looses all power at some point.<br /><br />Anybody seen low compression the source of full throttle shut downs. <br />Redfisher

 

[Joe Reeves]

Re: Low compression 25 hp Envirude 1975.

If you're using a portable fuel tank, is the vent open? If you're using a built in tank, there could also be a fuel restriction. At any rate, your problem does sound like the engine is drawing fuel from the tank, but air is not entering the tank..... eventually the tank becomes air bound causing the fuel flow to cease.

 

[G DANE]

Re: Low compression 25 hp Envirude 1975.

Hi redfisher 2003<br /><br />The results of poor compression usually is bad starting, rough iddle, and lousy low-end. Not exactly what you experienced. Did you try this gauge on someting else. Gauge could be reading low. Are u sure its a 1975 ? a 1974 would have the old points/condenser ignition, and old style coils which often cracks and goes bad. Did you check spark with a spark gap when problem occurs ? However Joes solution sounds qualified.

 

[G DANE]

Re: Low compression 25 hp Envirude 1975.

I just reread your post. Do a spark check - poor spark will give you exactly the fouled plug experience you have. Rig up a spark gap and make both sparks jump a 5/16 gap. Both should jump with strong blue spark.

 

[redfisher2003]

Re: Low compression 25 hp Envirude 1975.

Joe,<br />I actually tried two portable tanks. My hunch is the loss of fuel stems from the low compression which perhaps in the fuel supply translates to loss of fuel pump suction. However, pumping the bulb does not keep the motor running.<br />Redfisher

 

[redfisher2003]

Re: Low compression 25 hp Envirude 1975.

G Dane,<br />Bingo! no 5/16 spark jump on bottom cylinder. There is a weak spark by comparision to the top. It will not jump 5/16 inch.<br />The motor uses a "low tension flywheel magneto ignition system". The spark plug wire is connected to a coil bolted very near the spark plug which is connected to a seperate primary coil for each cylinder under the flywheel, and finally to single a driver coil. But the driver is obviously working. I have the book and I am going to pursue this tomorrow. Thanks G Dane.<br />Redfisher

 

[G DANE]

Re: Low compression 25 hp Envirude 1975.

Glad you found it.<br /><br />Try to swap the charge wires for the coils. If the failure stays at same coil - you have a bad coil. If failure follows charge wires, its time for points and condensers. Get new OMC, I had aftermarket that were defective from new, points as well as condensers, and OEM will last much longer. Cam is marked "set" , opening 0.020" There is only one charge coil, the other is for light. However each cylinder has its own points and condensers, be aware that opposite cylinder gets ground from the other set when it ignites.

 

[redfisher2003]

Re: Low compression 25 hp Envirude 1975.

Final report on the low compression: the problem ended up being the lack of strong spark in the bottom cylinder. I tracked the problem to armature plate wiggle that caused the points to not remain set at 20 thousandths. See 1975 Envinrude 25hp armature plate wiggle for repair.<br />thanks, <br />Frank Hawley