4500 rpms is all shes do

E

el_pescador

Member
hi everyone i bought this 1994 cc 20 wellcraft with a 1994 150 yamaha did not water test ran only with earmuff and it sounded great 1st lake run about 4500 rpms with 15x17 prop could trim out to 4800 but bow was really bouncing . i changed props to 15x 15 same e xact thing no change what so ever . the throttle opens the carbs. fullly open at 3/4 throttle and no increase at full throttle. i checked coils to the sparks plugs it has a blue spark on all plugs the plugs appear to be black and a little oily with only 3 lake trips and a half dozen earmuff flushing. the motor idles perfect compression on all cylinders around 118 with no leakdown, good out of hole shot ,run find other than what i stated , my specs. for this motor states rpm range 4500-5500- my prop man believes it should top out closer to 5500 with on 1 person aboard, can anyone clue me in on what i maybe missing here before i take her to a yamaha service dept. i never had a yamaha but have had other motor and in pass full throttles u got max. rpm there thanks
 
Have you checked that the throttle is opening alll the way??  
Might need a cable adjustment?
Decarb the carbs??
Add new plugs??
How old is the gas?
Have you changed the filter??  
How about the tac, are you sure its reading right??
 
last time i chased down an rpm problem on a Yamaha, I asked around, ebveryone told me Yamaha's never have an ignition problem, after going thru the carbs three times, four set of plugs, rebuilding the fuel pump, guess what, it was an ignition problem, teh stator was not producing enough spark at higher rpms. I used a spark checker on all six wires, opened the gap to beyond 8mm(yamaha spec), found the spark becoming irratic, checked the coil output witha dva meter, found the stator was going bad, replaced it never had any more problems
 
pulser coil
resistance 320ohm
peak voltage loaded 2
peak voltage unloaded 2.5
peak voltage @1500 rpm 9.5

low speed charge coil
resistance 740ohms
peak voltage unloaded140
peakvotage loaded145
peak voltage @1500 170

high speed coils
resistance 69 ohm
peak unloaded 40
peak loaded 40
peak @1500 170
cdi output
peak loaded 105
peak @1500 140

ignition coil
secondary resistence 3.8kohms
spark plug boot
resistence 5.0 kohm +- 1 kohm

read thru the rapair trouble shooting guide, you will need a dva meter( explained ont he rapir site)
 
allmost forgot, and this should be the first thing you do before you do any other trouble shooting, is the tach accurate? Check it with a known good tach or a shop tach
 
thank you for all your effort for me spareparts and everyone else who had ideas , hope i can help someone also pay forward. will let all know the results and what will solve it. thanks again
 
If the motor is tuned and all else fails try down sizing from the 15 inch prop, it may be too much . Try a 14 1/4 x 17 prop and see if your rpms come up. That should get you over 5000. One other thing may be the motor is too low in the water. Make sure your cavitation plate is even with the keel of you hull before you switch props.
Dennis
 
A friend of mine had a similiar RPM problem. Finally found out that the fuel line was collapsing. Therefore, restricting the fuel flow. Replaced the fuel line with the correct size,problem solved.
 
thanks again for all suggestions, bit the bullet and took it to the yamaha service man.....rebuilt all carbs. now 5400rpms and 43 mph. yamaha serviceman said worse he's seen in a while....using a 151/2 x 15 s.s. prop
 
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