Mercury 650 Ran great, until today : (

Okay guys, I have been working on a merc 650 for a while, and I got it running great with some sea foam and a lower unit overhaul. I had never had the motor in actual water, but on muffs it always started up easily, and idled great once I got it warmed up.

We took it out for the first time today, and the motor would not start. The only difference was that the motor was submerged in the water, rather than placed on muffs this time, and we drove about an hour when the temperature was around 40 degrees. We decided to head out with our electric motors, and they did fine. I tried to crank the motor over numerous times throughout the day and one time it sounded like it wanted to go, so I tried a few more times and it cranked over. I let it idle for about 10 minutes. Then we head off and eventually I hit about 3/4 throttle. The motor sounded and ran great. It pushed my 16' starcraft so well that I did not feel safe pushing the throttle any further. It was flowing water without an issue at all. After about 15-20 minutes, we slowed down to do some trolling, and I put the motor into neutral and let it idle to set up our gear, and it died. I could NOT get it started again, for the rest of the day.

I was wondering if you guys have any ideas. I was thinking that the added back pressure of the exhaust being in the water reduced the "scavenging" effect of gas and air, and basically shows that I need fuel pump/carb work. Has anyone on here had the issue with the motor not starting in a.) the cold weather or b.) only when the motor is submerged??

Re: Mercury 650 Ran great, until today : (

Hey sutor623,
Did you run the motor on muffs since you had the "issue"?

Re: Mercury 650 Ran great, until today : (

kax2000 said:

Hey sutor623,
Did you run the motor on muffs since you had the "issue"?

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Hahaha I knew someone would ask that. No sir I am going to get at that tomorrow morning. Just wanted a nice head start for my saturday. Just a quick FYI, the motor has compression of 125 in all 4 cylinders.

Re: Mercury 650 Ran great, until today : (

If you haven't replaced your fuel lines and cleaned both carbs, you don't want to trust it on the lake. Check to see if it's firing when it doesn't start. If it's getting fire, then it's in the fuel system. Also, disconnect every wire, clean the connections, and reconnect. Make sure the battery is fully charged.
Just some suggestions----I own 2 of the 650's.
Wait a minute---I see you have a 4-cylinder 650....... I'd still do the same thing to it. I hear the 4-cylinder 650's are some smooth running, powerful motors. Much better than the 3-cylinder 650 that I have.
JBJ

Re: Mercury 650 Ran great, until today : (

The merc 650 (3cyl) I have did the same thing for the previous owner...
He spent a ton of money trying to figure it out until his wife forced him to get rid of it.

The issue for mine was the plug that connects the internal and external wiring harness.

If you have a volt meter check the ohms from your key switch to the switch box.

On my engine it would start and run great, loose power, then die..
checking the spark right after it died I found a very weak spark.

Re: Mercury 650 Ran great, until today : (

Has anyone on here had the issue with the motor not starting in a.) the cold weather or b.) only when the motor is submerged??

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Do a leak-down test on cylinders, engine can have good compression but bad leak-down. A compression test is only a test to see if cylinders have enough psi to support combustion and if out of range/balance and tells you little about internals of engine. I have seen engines with good compression and have scuffed cylinder and rings, which a leak down will catch.

Re: Mercury 650 Ran great, until today : (

Has anyone on here had the issue with the motor not starting in a.) the cold weather or b.) only when the motor is submerged??

Click to expand...

Do a leak-down test on cylinders, engine can have good compression but bad leak-down. A compression test is only a test to see if cylinders have enough psi to support combustion and if out of range/balance and tells you little about internals of engine. I have seen engines with good compression and have scuffed cylinder and rings, which a leak down will catch.

Re: Mercury 650 Ran great, until today : (

Faztbullet said:

Do a leak-down test on cylinders, engine can have good compression but bad leak-down. A compression test is only a test to see if cylinders have enough psi to support combustion and if out of range/balance and tells you little about internals of engine. I have seen engines with good compression and have scuffed cylinder and rings, which a leak down will catch.

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Ok not really sure how to do a leakdown on a 2 stroke motor. I got a chance to finick with the motor last night. And it would NOT start. I was running fresh gas to her(with the muffs on) So what I did, since the carbs are hard to get to in this motor, was put some fuel/oil right into the spark plug holes. It then started. I cut the motor off, and tryed to restart, and nothing. Then I filled a trash bin with water under the motor, put a little more gas into the plug holes, and she fired right up. So I no longer believe that it has to do with being in the water. I ran sea foam through the motor for about a half hour. The motor would not idle well for more then 15 seconds. Then I cut it off, let the sea foam sit in the motor, and went to start it a few minutes later, and nothing. If I had run some moist gas through here is there a chance that there is still water in the carb bowl?

Re: Mercury 650 Ran great, until today : (

I had almost the same problems with my 75 hp. Ran great at home on muffs took it to the lake, hard to start ,did not want to idle, ran great accross the lake went to troll would not run at less than 1200 rpm. Took it to the shop tuned it up set the idle mixture (on muffs)idled at 650-700 rpm ran great. Took it back to the lake (same elevation as the shop) would not start, choked it got it to start then died, tried over and over would not run. Took the cover off used the manual choke and got it to run with choke half on. Adjusted the idel air mixture richer and it ideled great. Ran it accross the lake slowed down and trolled for 3hrs. never missed a lick. The whole problem was it was running to lean (on muffs) turning the idle air screws 1/4 turn fixed it for me. Give it a try

Good Luck

Re: Mercury 650 Ran great, until today : (

truhl_58 said:

I had almost the same problems with my 75 hp. Ran great at home on muffs took it to the lake, hard to start ,did not want to idle, ran great accross the lake went to troll would not run at less than 1200 rpm. Took it to the shop tuned it up set the idle mixture (on muffs)idled at 650-700 rpm ran great. Took it back to the lake (same elevation as the shop) would not start, choked it got it to start then died, tried over and over would not run. Took the cover off used the manual choke and got it to run with choke half on. Adjusted the idel air mixture richer and it ideled great. Ran it accross the lake slowed down and trolled for 3hrs. never missed a lick. The whole problem was it was running to lean (on muffs) turning the idle air screws 1/4 turn fixed it for me. Give it a try

Good Luck

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Okay thanks truhl, I will definately keep this in mind. I have a little corrosion in my main power wires, I am going to change them out today since my battery was getting hot as HELL when the motor was running.

Re: Mercury 650 Ran great, until today : (

So dudes, check it out. Here is the prognosis. I ran a gallon of fresh gas and oil with seafoam in it at about 1200 Rpms last night. It would NOT idle at all, and if I shut the motor down, the only way to get it started back up is to pour gas into the carbs. This morning, I cut off about 6 feet of corroded power wire and put in a bar splice with 4 gauge battery terminal wire with a beefed up battery post terminal and she started up immediately(running in a bucket). It idled after a minute or so and I let it burn a full gallon of fuel idling today. It took about and hour and a half.

So I guess between the not so stellar power wire (and the cold) and also possible moisture in the gas, the motor just decided that it did not want to run for me Friday. Hopefully the next time I have her out I will have no problems. I knew I should have started that motor up the night before the trip.......:facepalm: