Trouble with a 2008 Yamaha 40hp 4 stroke

[emorgini]

I bought a Zodiac with what appears to be the dreaded 40 hp Yam 4 stroke. It hadn't been run for a few years so totally cleaned gas tank, new gas lines, new battery, new cables, clean gas. Now, I am up against the wall. I installed the 25 hp jets as everyone recommended. Have had the carbs apart 5 times, cleaned and recleaned all jets. Flushed with carb cleaner and blown out with air.
The motor starts good now but the top cylinder isn't firing. Here are my findings so far:

1) Good spark to all cylinders
2) Top cylinder compression 180, middle 200, bottom 200
3) Somebody had taken off the idle mixture plugs. Top was only out 1/2 turn (I moved it out 1.75 turns) middle and bottom out 1.75 turns
4) Fuel is obviously pumping to engine as it runs on bottom 2 cylinders. But if you advance throttle, it dies.
5) I hooked up the carb synchronizer and the middle and bottom cylinders both showing 22 (after i adjusted #2) but top showing barely any vacuumn. Don't know if that is because top not firing or a problem. (I am a total novice at this but have watched probably 50 you tube videos so I am dangerous!)
6) New spark plugs

I am on an island with no Yamaha mechanic so I basically have to bring the next logical part to solve the problem. My guy that knows something about outboards sprayed some fuel in the top carb while it was running but the top cylinder didn't fire. He thinks it is running rich already since it didn't seem to be a fuel problem.
I hate to plop down $700 for a new top carb as I am not sure it is a problem.

Any advice is greatly appreciated about what to do next. It is close to becoming a good boat anchor! Thanks.

 

[Sea Rider]

Welcome to Iboats,

OK what's good spark on all plugs, having a spark is no indication that the spark is strong. Will need a spark gap tester to check if the spark jumps the length that should jump factory stated which don't know which distance should be..

Does that motor have individual ignition coils, if so, check upper coli state, probably misses when hot, works ok when has cooled down to fail again when hot...?

Happy Boating

 

[emorgini]

Yes it has separate coils and we swapped top and middle and top still not firing. It appears to have a real good blue spark.

 

[raczekp1]

are you sure that first carb is geting fuel?
if fuel bowl is with fuel then
check timing- check if timing belt is instaled in corect way
and next is to check valve clearnce valve clearence

 

[rejesterd]

Try getting the service manual from yamahapubs.com (hopefully they will ship to your location). There's lots of good content on YouTube, but lots of terrible content as well.

When you check compression, make sure you warm it up for 5 minutes first, stop it, remove the safety lanyard, then fully open the throttle before you do the test.

Assuming you performed the test correctly, I think the lower compression in the top cylinder is the thing to focus on. It's within 10% of the others, but barely, and the others are both exactly the same. A fuel issue (like a bad carb) wouldn't cause low compression in one cylinder like that. There's either some internal engine damage or a leaking head gasket.

I would squirt some engine oil into that cylinder and check the compression again after you do that. If you get a higher reading, that means it's something related to the piston, the piston ring or cylinder wall. Then I would check the valve clearance for all cylinders. The service manual will tell you how to do that. If that checks out, try some gasket sealant like BlueDevil.. if that doesn't work, replace the gasket.

 

[emorgini]

Thanks. We are going to check that this weekend.