Have a new to me 2003 yamaha 40 4 stroke.
I boat the engine yesterday and ran it for the first time today. It was winterized prior to the sale.
It runs great at a low idle and can get up rpms under no load. When in the water, when I throttle it up it bogs down and cuts out. If I nudge it up slowly, eventually I can get it up on a plan and run like heck. If I power down and try to power up quickly, it bogs down or completely stalls out.
sounds like its time to clean the carbs paying speacial attention to the acceleration enrichment circuits.
you should also check for fuel restrictions with a vacum guage and a T fitting installed just in front of the fuel pump intake, vacum should be less than 4" Hg and at least about 1/2" Hg at or near WPT.
Now I get it. Thank you.
The engine is an 04, not and 03. The carbs were just cleaned, new fuel filter and a fuel/water separator added. When I recieved the boat, the tanks were dry. I started with fresh fuel, also filling the separator prior to hooking up the fuel lines.
test spark, test compression, then go chase fuel.
I dont care if the carbs were brand new this morning,if they got trash in them this afternoon Guess what ?
its Suck,Squeeze,Bang and Blow. been that way for over 100 years.
when a gasoline engine,especially a 4 stroke, has a rapid opening of the throttle you MUST add extra fuel or a POP or total stall will result regardless of anyother issues.
as you stated with a gradual throttle opening it runs well I dont suspect compression or spark issues but they should be tested.
the chemically correct burn mix of gasoline is about 14/1 air to fuel. at about 16 to 1 the mix becomes to rich to ignite, at about 13to 1 it becomes to lean to burn. as its a constant volume fuel you must add or subtract fuel with the throttle opening.
diesel is a constant combustion type fuel and does not require air throttling and will burn in a 300-1 range from high to low.
gas is about a 3-1 range.
so based on your description when you open the throttle rapidly either there is not enough fuel in the fuel bowls for extra enrichment or the acceleration circuits are defective/dirty.
I'm considering rebuilding the triple decker carbs. I have rebuilt lots of carbs on lawnmowers, chainsaws and chevy engines, so I am hoping that I am not getting involved in something that I shouldn't. Any suggestions before I start. Can I get a carb kit from this site.
Thanks Bob
Have implemented all suggestions.
Spark check out
compression checks out
Fuel pump pressure is good
Carb rebuilt
Changed all filters, 6 gal tank, checked little titty on fuel line (and it is nice and firm when squeezed) - just like the old days.
How about timing? I have a light, but not the know how.
Or, is it time I bite the bullet and bring it in?
Thanks again,
Bob
The motor still dies out when doing a "hole-shot" (really fast acceleration).
The only thing I have not done is timing. I have a timing light, but
2003 Yamaha 40hp 4 stroke bogging
Linear Mode
