Dead Cylinder

[kennethhigh]

2002 f25 4-stroke has no power in the top cylinder.
The following actions to correct this have not worked.
Clean carburator, (3 times now)
New plugs and ignition coil
New timing belt and valves adjusted
New power pack, (from second f25 I also own)
Engine has good compression
Fuel is clean and carburator is clean
Spark is evident with in-line spark tester
After everything, no change in the cylinder, still dead.
What am I not trying or have I overlooked?
Thanks,
Ken

[boats.net]

Do a leak down test and post results.

[Capt. Barry]

Does the engine start up and run at all?

[kennethhigh]

I have not done a leak down test before.
I believe I would need to take the head off for a leak down test.
Probably would want to take it to a mechanic to be done while the motor still is in one piece and starts and runs, even if only on one cylinder. I really would not want to take this to a mechanic in parts.
Right now the motor starts and runs, it just does it whether the top plug wire is connected to the plug or whether it is not connected. No Difference.

[almetelo]

outboard lealdown test

Performing an Outboard Leakdown Test - YouTube

Leakdown Test Mercury - YouTube

[rodbolt17]

unless you enjoy eating wrenchs, NEVER EVER place a tool on the flywheel when leakdown testing.
if the flywheel moves during the test it can move violently and rapidly in either direction and then you may find out the meaning of Breaker bar.
never personnaly ate one,have witnessed it and saw the video of an aircraft technician that got whacked by a propellor doing a leakdown test on a small airplane.
SAFETY is ALWAYS paramount.
typically the tolerance is 10% on a two stroke and 15% on a four stroke.

[almetelo]

Hi Hi!

In fact was the 1st time I saw someone doing this... (wrench on flywheel durind downleak test)

If is a bigger motor=more compression "eat a wrench" can really happen!

Thanks for keep us that in mind!

[Capt. Barry]

Seems odd. Fuel, air, compression and spark. Should run unless the spark is "sparking" on the exhaust stroke instead of compression.

"Spark is evident with in-line spark tester"

I've never used this device. The way I check for a spark was to hold the boot near a grounded motor part and look for a spark while cranking the motor. Not the best because compression causes more work to make a spark, but a good starting point.

[kennethhigh]

The tester used connects to the plug wire (removed from the spark plug) back to the spark plug. It has a light in line that lights up when spark is present that is visible in daylight. This allows the test to be done at a range of rpm without having to hold the plug wire manually. I am stumped.
The engine has like 128#'s compression in both cylinders.
It ran very well right up until it didn't, (no loss of power over time)
This happened while going from 3/4 throttle to idle while beaching the boat. The engine died while backing off the throttle quickly, and was very difficult to start after, and has run on one cylinder ever since.
I thought perhaps the timing belt slipped, hence the new timing belt and valve adjustments, but it did nothing.
Looks like I will take it to mr good wrench to find out what the problem is.
Thanks for the help.
Ken

[99yam40]

Good compression, spark, and timing are all needed for cylinder to fire properly and are easy to test. Seems you have tested every one of those except timing.But would not hurt to test ignition system like manual calls for with peak voltmeter or dva adapter and regular meter to make sure all in in spec
After that you are left with fuel/ air mix.

Might try spraying some fuel into top carb to see if that will get it to fire.
If it does it means it is starving for fuel. plugged up jet/passages you have not been able to clean properly?Or top seal leaking air?

But too much fuel is just as bad as too little fuel and will cause cylinder not to fire. Might try pinching off fuel line to that carb to see if helps, never tried it myself but have read about it.

I do not know enough about that motor to say if a leaking fuel pump would cause top cylinder problem, but something to think about

Have you tried swapping plugs top to bottom?
I have seen bad plugs straight of of box before, but that was many years ago when I was an auto mechanic

[kennethhigh]

This motor has no timing adjustment. Since I have a second f25 motor that does run well, I have been able to interchange parts from the good motor to try to get the bad motor fixed. So far it hasn't happened. Plugs, ignition coil and wires, fuel pump, CDI
carb is clean, clean and cleaned again. There is only one carb and the motor does run so fuel is getting there. Could there be a flywheel / magneto possible short that would possibly affect only the top cylinder?
Will do the leak down test as soon as I can locate one or I give up and take it to the shop.