Re: Carbs
I know your unhappy but this is the way it is.
we are sitting,looking at a monitor relying on your description.
your describing a lack of fuel in the intake manifold.
why I havent a clue, some educated guess's but no clue as to why.
while its possible to have a bad high speed charge coil you have stated that by spraying fuel into the carb it picks up and runs.
your description eliminates ign as a problem.
I will tell you if its a lean out problem and you continue monkying with it it WILL smoke a piston.
a carbed merc or jonny rude would have long since done it as they feed both the oil and the fuel through the carb.
the yamaha carbed oil injected models inject oil into the intake.
if your not happy with that dealership use the yamaha outboard dealer locator and find a dealership with a master tech,remember 5 star is simply a 20 min video course, and take it to them with the request that only a master tech look at it.
if not you will have to make or purchase the equipment to test the hull fuel system for vacum and the electrical test equipment to test seconday ign output at the problem RPM.
here is where the correct equipment coupled with test wheel YB1626 comes in very handy.
I would start with a compression and spark test.
dont care what compression was yesterday I want to know what it is the day I start troubleshooting.
piston failure is incredibly rapid and can happen in a matter of minutes.
if spark can jump a 7/16th gap then I know the secondary side can supply the correct secondary voltage.
if compression is good I know the engine mechanically can make power.
next I would hook up my boat in a box fuel pump kit.
this consists of an electric pump, a flow meter.a vacum guage and a throttling valve.
its rather expensive but a reasonable subsitute can be made.
I would hook up the boat in a box fuel system intake to the outlet side of the engine fuel filter, place the outlet hose in the tank fill hose.
turn on the pump and adjust flow to about 20 gallons per hour.
we know your 150 can only burn about 15 gallons an hour at WOT on a good day.
then I would look at my vacum gauge, it should show at least 1" Hg and no more than 4" Hg for about 10 min.
to make one use a rotory electric pump UL rated for gasoline with a flow rating of at least 30 GPH.
buy a decent vacum gauge preferably with a 0-10"Hg scale.
T fittings as nessasary.
now to figgure flow.
here is where a gallon jug and a stop watch and some 5th grade math comes into play.
with the pump at open flow from a bucket how long does it take to pump one gallon in one minute.
this gives an open flow reading at the pump outlet.
use a suitable choke and choke the outlet to obtain a flow of about 20 gallons per hour.
then hook the rig to the outlet of the engine filter with the vacum guage t'eed in and measure fuel system vacum at that flow.
this is why I dont like the clear vinyl tubing and high speed running method of teaching.
I have seen techs blow pistons chasing fuel problems.
like I say, two strokes simply dont tolerate lean burns for long and piston failure can happen in seconds at or near WOT.
have you tested your alarm system to insure the RPM limiting function isnt trying to protect the motor?
however by your description of simply spraying fuel down the carbs makes it go tends to eliminate that too.
that description is why I keep saying someplace between the liquid level in your fuel tank and the intake manifold we cannot move enough fuel to keep the cylinders happy. have you tried a known good remote tank plumbed directly to the intake side of the engine filter?
when playing with fuel systems you have to break it into hull systems,everything between the liquid level in the tank to the engine filter intake and engine systems which includes everything from the engine fuel filter to the crankcase side of the reed box assy's.
by spraying fuel directly into the carbs your bypassing all systems except the reed box's.
