Yamaha C90 prime start not working?

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blackshear

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I have a 1999 Yamaha C90 that requires me to flip the prime start switch to the on position it on cold starts. After the first start of the day the engine will starts fine on it's own. I took the prime start plunger assembly out and did my best to clean that area but that doesn't seem to help. Does anyone have any experience with this sort of problem?
 
Flip on? You mean the toggle switch for it? Every Yamaha I ever had and this was a 'momentary' switch, not a On or Off switch that stays where set.

Do you prime the primer bulb firm before starting? Keep the OB end UP so as to ensure the check valve works correctly. Sometime bulbs get old and don't work as well anymore. If you replace it, buy the Bombardier/OMC designed bulb, as they are the best in the OB business, so much so that even the Mercury Racing Teams use them on their Merc OBs.

Prime bulb firm, turn key ON, flip & hold that prime switch for a slow of count of 6 or 8, release the prime switch ... and crank. I bet she fires right up!
 
Sorry I didn't explain that very well. The switch i am referring to is on the side of the middle carburetor. It has 3 positions for fuel enrichment; auto, on and off if I'm not mistaken. My motor will not start in the Auto position so I have to take the cowling off and move the switch to the on position to get it to start. Once the motor is started I flip it back to auto position and it will start fine for the rest of the day. I have been dealing with it like this for years.
 
Then it sounds like the connection from the key switch (when you push the key IN to prime) to that primer enrichment solenoid is faulty. You should be able to diagnose it once you identify which wire color should be energized with the key switch pushed in.
 
Check what Dale asked you to check and if you find that you have voltage at the solenoid then you may have to pull that carb. You may find a crack in a diaphragm for the enrichment circuit. Have found several although usually on smaller engines. I also have found that, if no voltage is present, that you may have a rectifier issue.
Start with the simple stuff first.
 
They rarely fail, and many don't know how it works.

The prime start system does nothing when you push the key in or pull the "choke" lever on the control box.

The way it works: the plastic white colored plunger mechanism has a type of wax inside of it. At cold air temps, the plunger is drawn up into the body, which uncovers a couple passages (one for air one for fuel). Once the engine is started, the alternator (or generator) supplies AC voltage to the two wires on the prime starter which warms the wax inside of it, and as it warms, the plunger drops down to cover the two passages.

One can test the primer mechanism by removing it from the carb and throwing it into the freezer for a while. Measure how far the plunger is up inside of the body. Let it warm up a while to room temp and see if the plunger drops. If it does, it's working--and I suspect that it probably is since I have never once seen one fail. BUT what I have seen VERY often is the passages in the carb(s) get plugged, which then causes all sorts of problems that people blame on the prime start mechanism. I know, most say well I've had the carbs apart and they're clean as a whistle....but on some of the carbs you can't get to the passageways that feed the prime start's fuel; they are close to impossible to access, and to actually get to them requires drilling out a plug in the bowl, then tapping it for a screw/plug. No fun.

On C90, I don't remember "exactly" how your particular motor is set up. Some have electric choke, which is an electric solenoid that kicks all 3 choke plates shut when you push the key in or pull up on the choke lever at the control box. Then they changed it at one point to use true prime start, which works as I described above, it's got a red lever on one carb that you can use to manually "choke" it-to get it started. Later ones didn't even have that. On prime start motors, you do not advance the throttle to start a cold engine, nor do you use the push to choke or pull lever up to choke as they do no good. Advancing the throttle on cold stat makes it even harder to start because there ain't no choke, so you're just adding more air, which makes it even harder to start. No throttle, crank until it starts--assuming the fuel passage(s) are clear.

The prime start has been a source of many frustrations from owners to techs; and once the system's operation is learned, it then becomes almost EFI-like in cold starting. I've known techs to give up on them and tell the owner to sell it and/or try to buy it from the owner.
 
Turbotodd...I think mine is the true prime start. It has the little red lever on the side of the middle carb and I have to flip it CW to the middle position "on" to cold start my engine and back CCW to the "auto" position once it gets going. If I leave it in the middle position the engine will start to smoke and idle rough. It sounds like the passages may be blocked. I can take the little plunger off and test it but if you have never seen one fail I'm probably wasting my time. If I have the carbs professionally cleaned would that work?
 
turbotodd said:
The (sic: Yamaha) prime start system does nothing when you push the key in or pull the "choke" lever on the control box.
Thanks TT, I was under the impression (wrong one at that!) that it operated like the solenoid function primer assists of other OB makes.

Good info there sir!
 
blackshear said:
If I have the carbs professionally cleaned would that work?


Sometimes. If they have an ultrasonic cleaner, that works better than manually, BUT if the passages in the bowl(s) are plugged, sometimes there's no cleaning them-as mentioned earlier, at least not without drilling out a plug and then tapping the hole for a screw. The "plus side" is that if they get plugged again, simply removing the screw(s) and cleaning the passage(s) is a LOT easier.

When Is said "sometimes", not all techs want to (or can) spend the time to check the primer passages. Hence, a "proper" cleaning of a carburetor (or a set of carburetorS) isn't a quick/easy/cheap job. I've had those working under me before. Those seem to jump from shop to shop fairly often.

The worst carbs are the early 2000's F40 and F50 4 cylinder. Prime start, only primes on some of the set of carbs, the way they're built they are a royal PAIN in the backside! .....and folks gripe about the cost of course. A C90 should be considerably easier.
 

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