1998 C90 new powerhead tuning

[boscoe99]

At an ambient air temperature of 45-50 degrees I can see the motor taking quite a while to warm up. Possible 10 minutes even.

Now keep in mind that the carburetors have different circuits in them. It is possible that the idle/low speed circuit is fouled in which case it won't idle or run well at low speed while running like a raped ape at high speeds.

Many many Yams will benefit by going up an idle mixture jet size or two, or three.

[flounder daddy]

Its like it wont come off the high idle. It was idleing at 1500rpm for 10-15minutes and it will bog if you give it any throttle and have no power. The pee water is warm, and if you turn it on and off several times waiting 5 minutes between, it will finally fire up and idle at 900 and have full power and performance like normal at all ranges. Wont do it at all in warmer weather above 70deg. Runs great, just the warm up/fast idle period is really annoying in cold weather. Plugs all look great, nice brown color on them.

[rodbolt17]

all that enrichment valve does is allow for extra fuel when cold.
it wont raise idle speed at all.
the fast idle is a function of the CDI module .
index the flywheel to insure the timing marks are correct, observe the marks when initially started until it idles down.
remember without actually moving the throttle plates there are only a few ways to raise OR lower engine RPM.
actually about only 1 way.
that 1 way.
advance or retard ign timing.
by advancing the ign 10* or so on a cold engine we can get away with adding some fuel to increase speed without opening the throttle plates.

the enrichment manual valve should be left in the auto position.

the on or open position is typically enrichment needle bypassed full enrichment all the time.

valve to off closes ANY enrichment all the time.

I can agree if the engine has been bored at the rebuild sometimes the jetting needs to be altered as that engine has no adjustable A/F method.

[flounder daddy]

The powerhead was replaced with a stock OEM brand new unit from yamaha. Stock bore/stroke, etc..

The CDI unit uses a fixed linkage correct? Theres not a air temp sensor or anything like that, that I can see or am aware of. I checked the timing with a light and it looks good according to the settings in my seloc manual. Maybe take a little more timing out of it at idle? extend the timing linkage 2 turns?

[robert graham]

Well something is not just right....about the coldest I've cold started my motor was about 28 degrees F. and the motor started right up and ran like normal. I'll be real interested to find out what this little mystery turns out to be, so keep us posted!

[99yam40]

I am not sure how thatC90 is set up to read when the motor is warm, but my C40 has a temp switch on the head that when warmed up will send a signal to CDI to tell it it is warm. Then the CDI changes the Timing to normal.
At start up the idle timing is advanced until the warmed up signal is received ,then it drops back to the idle specs in manual.
Have you tested the stat and temp switches to make sure they operate in spec?

Not sure why you had to replace the power head, but if you did not find and correct what caused it to fail, you will be hurting the new one.

If the carbs were not gone through at the rebuild, they probably need to be done along with the fuel pump

[flounder daddy]

all that enrichment valve does is allow for extra fuel when cold.
it wont raise idle speed at all.
the fast idle is a function of the CDI module .
index the flywheel to insure the timing marks are correct, observe the marks when initially started until it idles down.
remember without actually moving the throttle plates there are only a few ways to raise OR lower engine RPM.
actually about only 1 way.
that 1 way.
advance or retard ign timing.
by advancing the ign 10* or so on a cold engine we can get away with adding some fuel to increase speed without opening the throttle plates.

the enrichment manual valve should be left in the auto position.

the on or open position is typically enrichment needle bypassed full enrichment all the time.

valve to off closes ANY enrichment all the time.

I can agree if the engine has been bored at the rebuild sometimes the jetting needs to be altered as that engine has no adjustable A/F method.

You are exactly correct. I just in from the driveway, checked a few things on the timing situation. I took the carb side linkage off the main control arm and just moved the timing arm with zero change in timing on the light/pointer showing with the engine at the high idle again around 1500-1600 rpm. So it was acting like the box had zero effect and the engine was in "limp" mode sorta speak. I just made a call to Chris Carson Marine (excellent reeds btw) and asked them what the heck could cause this at only cold temps. First thing out of his mouth was the engine thermo switch sensor on the head is bad. He then explained to me that the thermo switch isnt only a hot alarm buzzer switch, but its kinda like a O2 sensor on a modern car. When the sensor sees heat, it allows the timing box (CDI unit) to retard the idle timing and take over timing control. He said it will add 10deg approx of timing to the normal idle setting which is almost exactly what was happening. I was showing 4* BTDC at fast idle and it should be 6-7ATDC. Gonna get a new switch on the way right now.

[flounder daddy]

I am not sure how thatC90 is set up to read when the motor is warm, but my C40 has a temp switch on the head that when warmed up will send a signal to CDI to tell it it is warm. Then the CDI changes the Timing to normal.
At start up the idle timing is advanced until the warmed up signal is received ,then it drops back to the idle specs in manual.
Have you tested the stat and temp switches to make sure they operate in spec?

Not sure why you had to replace the power head, but if you did not find and correct what caused it to fail, you will be hurting the new one.

If the carbs were not gone through at the rebuild, they probably need to be done along with the fuel pump

Pump is brand new. The old powerhead top cylinder had a tiny crack in it (my fault, got it hot once and blew a head gasket after a bad water pump rebuild) and something got in the top cylinder or it broke a ring. There is peppered damage in the top of the piston and chamber on the old head. Im putting my money on that thermo switch being bad.

[boscoe99]

I am not sure how thatC90 is set up to read when the motor is warm, but my C40 has a temp switch on the head that when warmed up will send a signal to CDI to tell it it is warm. Then the CDI changes the Timing to normal.
At start up the idle timing is advanced until the warmed up signal is received ,then it drops back to the idle specs in manual.
Have you tested the stat and temp switches to make sure they operate in spec?

Not sure why you had to replace the power head, but if you did not find and correct what caused it to fail, you will be hurting the new one.

If the carbs were not gone through at the rebuild, they probably need to be done along with the fuel pump

What is the part number please?

[flounder daddy]

688-82560-10-00 YAMAHA THERMO SWITCH ASSY

[boscoe99]

My question is to yam about his C40.

[99yam40]

See if you can find a diagram with the temp switches on the head, I cannot find it on here.
1999 C40TLRX Yamaha Outboard Parts

I know mine has 2 switches one for overheat and the other of motor warmed up, closes at lower heat

[flounder daddy]

See if you can find a diagram with the temp switches on the head, I cannot find it on here.
1999 C40TLRX Yamaha Outboard Parts

I know mine has 2 switches one for overheat and the other of motor warmed up, closes at lower heat

1999 C40TLRX Yamaha Outboard ELECTRICAL 1 Diagram and Parts

items 11 and 12.

[99yam40]

Thank you I thought I was going blind or just crazy.

The one with pink wire at the top I think is over heat
the one with orange wire is the warmed up switch

Apparently your C90 is not the same, as I cannot find another switch on your motor, so I have no idea how it is suppose to know when the motor has warmed up

[boscoe99]

Appears the C90 uses only one thermoswitch for over temperature warning.

Appears the C40 use two thermoswitches. Each of a different kind. One for over temperature and one for telling the ECU if the motor is cold or warm.

Yamaha is certainly willing to try every different type of motor known to man. Maybe more. No two models can ever be counted upon to be similar.

One thing about this website it is a hellava learning tool. Wish I had come here about ten years ago.