running rough

[Capt dave 258]

Just purchased a pontoon boat with a 2001 80hp Yamaha 4 stroke. Motor looks very clean. Knew it had a problem when purchasing. Previous owner said boat was running fine for months then problem occurred where it wouldn't get over about 1200 rpms. They changed the fuel filter and fuel lines, cleaned the tank and checked compression. The numbers they gave me for compression were 142,138,140 and 141. They firmly believed it to be a carb issue. I brought the boat home and drained the little gas that was in the tank and noticed it to look a bit cloudy. Disconnected the fuel line at the fuel filter and used the primer bulb to get any remaining gas out of the line. Put in fresh gas. Changed the fuel filter after getting as much gas as I could out of the fuel line that runs from the filter to the fuel pumps. Removed the drain screw from each on the carbs and using the primer bulb, squeezed it to get any bad gas out of the carb bowls. Noticed that carb #2 would not allow and gas through and out the drain screw. Took a small piece of wire and bent the tip and pushed it up into the hole for carb #2 and wiggled it around. Must have lifted the float with it because gas started coming out the hole. Thought I had found the problem with a stuck float and was excited to start the motor. Got it running and it still ran bad, very rough and couldn't get the rpm's up. Did a compression test with the following results, starting at cyl #1; 185, 120, 170 and 160. Does this motor need a rebuild or could the difference in compression be because the carbs are out of sync? Read somewhere that the compression test needs to be done with the carbs at WOT? is that correct? My test was not done at WOT. Is there something else that could be the cause of the variance in compression or is the motor just tired? Like I said earlier, the motor looks super clean. Thanks for any advice.

 

[saltchuckmatt]

Just purchased a pontoon boat with a 2001 80hp Yamaha 4 stroke. Motor looks very clean. Knew it had a problem when purchasing. Previous owner said boat was running fine for months then problem occurred where it wouldn't get over about 1200 rpms. They changed the fuel filter and fuel lines, cleaned the tank and checked compression. The numbers they gave me for compression were 142,138,140 and 141. They firmly believed it to be a carb issue. I brought the boat home and drained the little gas that was in the tank and noticed it to look a bit cloudy. Disconnected the fuel line at the fuel filter and used the primer bulb to get any remaining gas out of the line. Put in fresh gas. Changed the fuel filter after getting as much gas as I could out of the fuel line that runs from the filter to the fuel pumps. Removed the drain screw from each on the carbs and using the primer bulb, squeezed it to get any bad gas out of the carb bowls. Noticed that carb #2 would not allow and gas through and out the drain screw. Took a small piece of wire and bent the tip and pushed it up into the hole for carb #2 and wiggled it around. Must have lifted the float with it because gas started coming out the hole. Thought I had found the problem with a stuck float and was excited to start the motor. Got it running and it still ran bad, very rough and couldn't get the rpm's up. Did a compression test with the following results, starting at cyl #1; 185, 120, 170 and 160. Does this motor need a rebuild or could the difference in compression be because the carbs are out of sync? Read somewhere that the compression test needs to be done with the carbs at WOT? is that correct? My test was not done at WOT. Is there something else that could be the cause of the variance in compression or is the motor just tired? Like I said earlier, the motor looks super clean. Thanks for any advice.

Compression test at WOT.

Carbs need to be removed and cleaned/rebuilt.

 

[99yam40]

Leak down test should tell if cylinder leakage is occuring and if it is by rings, or valves

 

[Capt dave 258]

Update. Let the engine run for a couple of minutes and did another compression test with the idle lever fully lifted. Compression was 190, 200,190 and 190. Will order rebuild kits for the carbs and attempt to clean them? Any pointers on that would be helpful.