runs for about 30 mins then loses power

[brewer06ky]

i have a 1984 70 hp evinrude i have cleaned the carbs put new plugs in gaped at .030 the fill mix is 50:1..... in the water the boat runs fine for about 30 min then i lose a lot of power i have no take off but it dose stay running. its kinda like a cylinder is not working or something i thought it might be a coil pack any ideal on what it might be i am new to boating and need some help plz....

 

[slasmith1]

Re: runs for about 30 mins then loses power

the first thing I would check is the vent on the fuel tank.

 

[brewer06ky]

Re: runs for about 30 mins then loses power

i not for sure about how that works it has a built in vent... however it dose build up pressure but i have taken the top off to let the pressure out and it did not fix my problem..

 

[tashasdaddy]

Re: runs for about 30 mins then loses power

check your compression and spark, when the motor is cold. then immediately when it fails to perform, check the spark again. i believe you will find that you are loosing spark on one cylinder. replace that coil.

 

[brewer06ky]

Re: runs for about 30 mins then loses power

thanks that is what i will do

 

[brewer06ky]

Re: runs for about 30 mins then loses power

well i took the boat out aging and i changed the gas cap so i no its venting good after it started messing up i checked all the plugs when i puled the middle or bottom one it dies when i pulled the top one it has no change.. but it is firing good it has a good spark any ideal what could b going on.. also the engine is getting hot but the water from the water pump coming out the back is not even warm is this right.....

 

[kenmyfam]

Re: runs for about 30 mins then loses power

well i took the boat out aging and i changed the gas cap so i no its venting good after it started messing up i checked all the plugs when i puled the middle or bottom one it dies when i pulled the top one it has no change.. but it is firing good it has a good spark any ideal what could b going on.. also the engine is getting hot but the water from the water pump coming out the back is not even warm is this right.....

Change the coils out and see if the issue follows the coil. Need more explanation on "running hot" How hot ?? and how soon ??

 

[tashasdaddy]

Re: runs for about 30 mins then loses power

the motor has a tstat that should keep temp in the 140-160 range, tale tell stream, is not necessarily hot. most of the hot water goes out the lower unit. so no change on the top cylinder, but good spark, you either have a compression problem, or carb problem on the top cylinder. check your compresson on all 3, should be above 100psi, and with in 10% of each other, if this is so, then clean and rebuild atleast the top carb. preferable all 3 carbs, and adjust.

 

[Randybeall]

Re: runs for about 30 mins then loses power

Just a quick thought, in my years as a mechanic I have run across more than one bad spark plug. The worst I have ever had was on my wifes own car. When hot one cylinder would die. Turned out to be a broken insulator on that new spark plug, expansion of the ceramic would allow quite a crack to open up when hot. I put it in a toaster oven and observed after replacing it. Swap two plugs around and try it again. No cost, minimal time.