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yamaha 75 2stroke problems

R

rodey

Contributing Member
well a friend of mine has a 1990s yamaha 75hp pro 2 stroke 3 cyl,

outboard starts ok at first idles ok .
run to fishing ground 20 mins at any troottle no problems,
soon as he goes back to idle engine wont idle and stalls most of the time,
fish for a while go to start engine and it turns over but wont fire untill he puts the revs up quit abit,
put in gear and stalls again, idle seems rough then smoothings out and away we go,
its the same everytime,
just wont idle,

heres what weve done so far,
took carbs apart , clean and rebuilt ,
synced carbs
checked fuel pump,
checked for air leaks
checked thermostat,
changed plugs,
ran engine with spark testers and seems to have good spark ,
clean all grounds,
compression checked out good and even , cant remember the numbers but were very good,

anything else worth checking out.
thought it was defo just a carb problem but after doing the fuel systm dont think it is now,
 
good new gas?.....put an inductive timing light in the boat and decide if it is a fire or fuel problem at the point of failure....
 
Those carbs need to be spotless when rebuilt and all the jets blown out with heaps of compressed air etc. I had two different mechanics cleans some Yamaha carbs once without fixing the problem. Wasted heaps of time looking at everything else thinking the carbs were good until I stripped and cleaned them to 'spotless condition' myself.
 
aliboy said:
Those carbs need to be spotless when rebuilt and all the jets blown out with heaps of compressed air etc. I had two different mechanics cleans some Yamaha carbs once without fixing the problem. Wasted heaps of time looking at everything else thinking the carbs were good until I stripped and cleaned them to 'spotless condition' myself.
Click to expand...
ya i know waht u mean.
i completly stripped the carbs myself today taking real care and attention, used plenty of carb cleaner and compressed air,
even used a stand of wire for the small holes,
still the same . when i took the plugs out there fouled and bottom one was wet,
to me its like its not burning the gas enough and seems very smokey while running,
 
papyson said:
how much was rhe compression and how far would the spark jump?
Click to expand...
can remember compression figures off hand but they were well within range and even.
just tried spark test in the dark,
seemd to be jumping about 1/4 to 3/8 of an inch but to me looks weak .
same on all coils,
was going to try timming but my timming light has packed up.
if it is weak spark on all cylinders what would most likly be the cause , cdi or stator
 
papyson said:
you need to get a spark gap tester and the spark should jump 7/16 inch with aa good snap...testers are cheap and it is possible that your local auto supply store will have a loaner...if it is weak then use this

http://issuu.com/cdielectronics/docs/cdi_troubleshooting_guide_-_2012_1

troubleshooting is the way to go....unless you want a bunch of expensive spare parts

please recheck compression and post readings....
Click to expand...

thanks for link mate,
rrechecked compression.
its 126 ,125, 127,
couldnt get a gap tester so made one out of a plug lead cut with a clear tube over it to see spark.
was jumping a 3/8 gap bunt too me not constant enough and not a real good snap,
tried just the plugs grounded and spark does look real weak . when turning over its jumping around the top of the electrode and very faint, iv a old 6hp evinrude 1970s and the spark is brighter.
all 3 spark the same,
will try the ohms tests but im not great with ohms, what resistants setting do i need to be on ,
thanks
 
depends on the meter...perhaps you have a friend that understands and could assist...accurate readings are a must....
 
Make sure that engine is getting to running temp. If the cyl head water jacket gasket has a rot hole in it, the engine will never get hot and wen you shut it down it will condensate water in the combustion chamber and short the spark plug.

If it starts and runs and idles initially, unless you have dirt before the float needle in the carb, the carbs are probably not the cause of your problem
 
Docksidemarineservices said:
Make sure that engine is getting to running temp. If the cyl head water jacket gasket has a rot hole in it, the engine will never get hot and wen you shut it down it will condensate water in the combustion chamber and short the spark plug.

If it starts and runs and idles initially, unless you have dirt before the float needle in the carb, the carbs are probably not the cause of your problem
Click to expand...

ya engine seems nice and hot,
prity confident the carbs are ok,
just trying to find if it may be a stator problem on the low speed side,
thanks
 
hi check ohms.
stator brown to blue 304
red to blue 222
trigger white red t white black 303
my trigger only has the 2 wires.
flywheel sensor 196
all checked on ohms 2000.
engine yamaha pro 75 ceto 692 L 950406 X
THANKS
 
''when turning over its jumping around the top of the electrode''.....don't understand that statement...hook everything up and see if you can see any fire jumping to ground at night...I would go buy or borrow a spark gap tester...they are cheap and a handy tool to have..as far as I am concerned we still don't know if it is a fuel or fire problem....if the spark is good with a tester then we know that at least at some point the fire is good...if not we need to work on fire..if it is good then we need to hook everything back up and use a timing light at time of failure...to work on fire when it is a fuel problem is a loser..and vice versa...
 
papyson said:
''when turning over its jumping around the top of the electrode''.....don't understand that statement...hook everything up and see if you can see any fire jumping to ground at night...I would go buy or borrow a spark gap tester...they are cheap and a handy tool to have..as far as I am concerned we still don't know if it is a fuel or fire problem....if the spark is good with a tester then we know that at least at some point the fire is good...if not we need to work on fire..if it is good then we need to hook everything back up and use a timing light at time of failure...to work on fire when it is a fuel problem is a loser..and vice versa...
Click to expand...

try and explain it abit better,
when i have a plug in the bplug cap and earth the plug to the body there only seems a weak spark that i can see jumping on the top of the spark plug not a good straight snap of a spark,
am defo going to have to order a gap tester as it would be handy to have one cant seem to find one hady over here.
iv tried running the engine in the dark to check jumping to ground and there was none,
trying to get a lend of a timing light over the weekend as mine is broken,
i think i need the dva adaptor to get the reading for the cdi as it says need to get readings at certain rpm.
thanks for all imput mate and sorry if answers seem abit vague
 
and i am sorry if I came across a bit abrupt...but we must determine what you want to address..fuel or fire..
 
papyson said:
and i am sorry if I came across a bit abrupt...but we must determine what you want to address..fuel or fire..
Click to expand...
Not at all mate . Thanks for all ur help .
its not easy to try explain and the it works vice versa when answering
I'm prity sure its a fire problem as I've been threw the fuel system
and all seem OK . Even tried a good fuel pump
thanks
 
well before i go ordering bits i decided to once more look at the electrics,
was checking out the coils,
took them off and tried to get a reading top one i got a read off but the bottom and middle i couldnt get anything
unscrewed the plug caps there the orange resistor type, and put the meter from the screw were the wire goes and the other side were the plug goes,
top one reads 5.32, middle couldnt get a reading and bottom i couldnt get a reading.
would it be possible the plug caps are gone bad and causing this,
 
is the problem still just a idle or low speed one?...runs well and on all cylinders at higher rpm?....has this motor been used on a regular basis or has it set up for a while?
 
papyson said:
is the problem still just a idle or low speed one?...runs well and on all cylinders at higher rpm?....has this motor been used on a regular basis or has it set up for a while?
Click to expand...
Runs very rough at idle and under say 2000rpm
veey hard to take off in gear at slow speed most of the time the engine will stall
once u get past the 2000 rpm it seem to run fine .
Also very hard to start once u run for say 10 mins . Stop and try to start again it always have to have the revs up .
 
right so my mate had a guy come today ,
the spark now jumps good and bright , will jump half inch no problem.
when i got over to him we started the engine up ran it for a while,
stopped it let it sit for 5 mins,
engine wouldnt start again,
back to the same really hard to start had to give it a lot of trottle,
once started u could hear the cylinders jumping in and out,
got a proper spark tester and spark is jumping very good with a good crack to it,
thing is driving him nuts,
the guy who looked at it was a electric engineer and found a bad wire so replaced it,
 
wel finally i think were getting somwhere ,
spark is now sorted
narrowed down the jumping in and out of the cylinders to the fuel enricher on the 2nd carb,
diaphram is split so must be flooding the cylinders
can i disconect this for now just to see if all is ok or can it simply be turned off by the red switch on the front of it,
 
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