40 hp running rough. Compression numbers?

[itsmeagain5789]

As soon as I thought everything was working right on my 40 hp 3 cylinder, it starts missing badly and makes a loud clanking noise, or metallic noise every couple of seconds running idle. It misses and sputters quite a bit when accelerating as well. I'm going to do a compression test tomorrow but was wondering what the average compression numbers were. Any other things I should look at? Thanks
Ed

[boats.net]

All cylinders should be no than 10% difference in compression readings.

[robcole32]

I have the same problem/try taking off one spark plug at a time/it will run on two cylinders/ you should be able to tell if it's just one cylender or a comon electricle problem.

[a387673]

I think my compression was around 120 psi on my 40 HP 3 Cyl. I'm no expert, but everyone points me back to the carbs with problems not unlike yours. I'm preparing to rebuild mine as we speak. If you have the Prime Start, the lever should be in the "normal" position. Seems that lever can make a lot of difference when idling.......

[itsmeagain5789]

It was the carbs, i have an extra parts motor(a 50hp) took the carb off that motor and put it on my 40 and it ran better, but still not great. It sputters a little at higher rpm's and drops down to almost 500rpm at idle and barely stays running. both of the carbs need to be rebuilt.

[99yam40]

the fuel and air jets and maybe other things are different on the 50 and 40 .

Clean up the carbs from the 40 and put back on. make sure to do the link and sync adjustments as the manual calls for after reinstalling carbs

[itsmeagain5789]

i "cleaned" out the 40hp carb, and im confused on how to do the link and sync. I have the seloc manual but it doesnt go in depth on how to do it with this motor. on other motors it says i have to go out and run the engine while adjusting it on the water. What am I supposed to adjust? is it the bolt that has a spring and is on top of each one of the carbs? do I have to visually adjust it or is there someway of knowing its spot on? thanks
Ed

[99yam40]

Here is a thread I started in a different forum on my C40 that may help you. I have no idea what year or model you have since you never said

C40TLRX link and sync procedure need clarification - iboats Boating Forums

[itsmeagain5789]

Oh that's perfect. thank you so much!