79 140hp Johnson

[cvfmom]

We have a 79 140hp Johnson that we had fully rebuilt in early 2000 because we had to use it evacuating from floods and it caused some damage to the motor. Since the rebuild it has ran fine (maybe 30-40 hrs) but this weekend it died. It seems that the ring broke on the 4th cyl and the piston look somewhat melted. We keep this boat in tip top shape and treat it like a baby. Any suggestions except to rebuild again? It is difficult since we just did it.

 

[Dhadley]

Re: 79 140hp Johnson

Why did it die the first time? Tell us exactly what happened both times. What spark plugs? What fuel? How old? What timing at 5800 RPM? Is the top of the piston burned on the exhaust side?<br /><br />More info please! Good luck!

 

[cvfmom]

Re: 79 140hp Johnson

both times the same thing happended but different cylinders. Not sure which cylinder the first time because we took it straight to the marina and had to have 2 and 3 head put on (after the flood). This time #4 cylinder burnt on the right hand side of the cylinder (if you are facing the motor behind the boat). Plugs were Champion and fuel 93 oct. Fuel was the other week. The marina marked timing on the flywheel at 28 so we don't know timing at 5800. When the motor was rebuilt the man that built it told him to run it on double oil for 20 hours. Which we did. Then we put 2 sets of plugs in also (it burned the others out). We switched down to regular oil and it had been running good. Last weekend the boat shut off while running wide open at about 4500 rpms. Then it started back up and ran good 3 miles then it stopped again, cranked again and would not plane after all. Put new plugs in again, ran good for 4 miles and it burnt one side of the cylinder. Any suggestions is really appreciated. Thanks for all help!

 

[Dhadley]

Re: 79 140hp Johnson

The right hand side of #4 is the exhaust side which indicates too much combustion heat. "Burning out the plugs" indicates too much combustion heat. 93 octane will contribute to too much combustion heat. WOT at only 4500 will also contribute to too much combustion heat. <br /><br />You must have used some very tough pistons to have lasted 40 hours.

 

[cvfmom]

Re: 79 140hp Johnson

The marina that rebuilt the motor told us to use the 93 octane. So we were using to high of an octane for the motor?

 

[cvfmom]

Re: 79 140hp Johnson

Hate to ask another question but we have already spent so much money on this motor after the flood that it is really upsetting to have all of this happen again. Any suggestions why it would have burn only on one piston and what causes so much combustion heat? When running on a plane he usually is running 4500 (Sketter boat). Is this to many RPM's? Thanks again.

 

[Dhadley]

Re: 79 140hp Johnson

Not all combustion chambers operate at the same temp even in the same motor. #4 is (obviously) running hottest in your motor. Under normal operating conditions #4 will live. We have to get back to normal operating conditions. This includes the correct WOT RPM's. At 4500 RPM WOT the motor is "overpropped". A bad term but easy to understand. You have to relieve the strain ("overpropping") on the motor. Get the WOT RPM's to 5800. Get FRESH 87 octane fuel. Make sure the timing is set at WOT -- not static. Use Champion plugs only. When they rebuilt the motor the first time did they cut the deck or heads? If so, how much? Were the carbs rebuilt?