Logo
FULL SITE MENU

2001 mercury 40 hp 4 stroke missing on botton cyl under load

S

siscoken

Contributing Member
With air intake off ... it will pick that cylinder up if air is restricted... choked .... have had carb appart... cleaned , but see nothing wrong... am about to check valve adjustment.
Anyone had similar problem, with possible suggestions... thanks.
I am a honda guy ... Haven't had much to do with mercury... helping a neighbor out ... he has already spent over 1000 $ on problem at shop and now they want to put a cdi on it because they are out of ideas.
 
Decades ago, when a teen and starting my enthusiasm with internal combustion engines, I recall a comment concerning engine compression and spark plug firing. Seems in air a defective (how I don't know) plug will fire but under compression it wouldn't. Maybe you are looking in the wrong place for your solution. Spark plugs are cheap!
 
You are right that just because a plug fires in atmosphere doesn't it will fire under compression.... plugs and coils have been swapped out.... have also ruled out valve adjustment now .... carb is coming off again to see what I missed .... thnks for reply
 
Update..... finally got time to look at this again and last test I did was swap carbs between #2 and #3 cyl .... not easy as it sounds...and the problem followed the carb.
 
Update .... finally forced to revisit this .... with no other options and already to much time sunk in .... i just asked myself , what would I do if it were my motorcycle.?
Reamed out the main jet from a 95 to what I believe would be a 100 and raised the float level.
Just lake tested and all seems good .... called owner to come and get it!

I know resizing jets is not recommended but no other sizes were available.
I know these floats are not adjustable.... but where there is the need you find a way!
 
siscoken said:
Update .... finally forced to revisit this .... with no other options and already to much time sunk in .... i just asked myself , what would I do if it were my motorcycle.?
Reamed out the main jet from a 95 to what I believe would be a 100 and raised the float level.
Just lake tested and all seems good .... called owner to come and get it!

I know resizing jets is not recommended but no other sizes were available.
I know these floats are not adjustable.... but where there is the need you find a way!
Click to expand...
".......but where there is the need you find a way!"

Welcome to "Farming 101!
 
Texasmark said:
Decades ago, when a teen and starting my enthusiasm with internal combustion engines, I recall a comment concerning engine compression and spark plug firing. Seems in air a defective (how I don't know) plug will fire but under compression it wouldn't. Maybe you are looking in the wrong place for your solution. Spark plugs are cheap!
Click to expand...
If you’re really bored one day, search Paschen's law.
Long brfore we were born he discovered that as compression increased, so did resistance, & therefore the voltage required to jump the gap. A weak system may very well show a good spark at room pressure, but not be strong enough at higher pressures.
Another fun fact of his studies was that voltage required decreased as the gap decreased, but only to certain point. If the gap was smaller, the required voltage increased.
Hence proper gap is also important.
 
Back
Top