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Mercury 7.5hp engine stopped when slow down immediately

W

William Wang

New member
Hi experts,

I have an pretty old Mercury 7.5hp outboard 1980s.

It was used on a dingy. It was going at full speed, after I slowed it immediately, the gear stuck and the engine stopped.
It took me a while to get the gear released again by turning the propeller many times.

Anyone know why the gear stuck? and why the engine stopped when reduce the rpm throttle quickly?
Thank you very much.
 
thanks for your reply.
I have change the lower unit oil. It was really bad.
Yes, I have replace the impeller a few weeks ago.
I have been thinking that the exhaust was blocked due the water pressure when immediately slowing down the boat.
Do you think that is a possibility?
 
_Sorry----" it was really bad " does not mean much to me.---------Was it full of water ?-------How much oil was there ?-------Any metal filings ? ------Any metal chunks ?
 
Hi there,

Sorry,I mean the lube must have not been replaced for a long time. The color is really "Black".
There was not any water, probably no leak or anything like that.
No metal chunks etc.

I apologize that I did not describe the problem clearly.

The motor is running fine at the moment.

The problem was that when I took the boat out to the sea last time, it was running at full speed, and I slowed down the boat suddenly and the motor stopped, gear stuck. I had to turn the propeller many times and took me a while to put the gear back to Neutral.
I then started the motor and running fine again.
So, firstly, why the motor stopped when suddenly reduced the speed? Is that because the water pressure blocked the exhaust?
I was also reading some post about the position of the outboard. As I have a small dinghy, the shaft of the outboard in the water is 7cm more than the average range which is around 2.5cm?

Any information would greatly appreciated.

Thanks.
 
"Black" is normal after lots of use.

I suspect the carb is dirty--hence the stalling. With the throttle up, you can't shift into neutral--that's intentional.

Jeff
 
When you say it was "stuck in gear" are you saying that operating the shift mechanism wouldn't change gears for you or was that "stuck" also - like the whole thing was locked up in forward, neutral or just plain locked up period.

You could have a couple of unrelated problems here that just came about at the same time.

These models do have a type of neutral safety switch - a kinda finger on a linkage that pins the flywheel so you can't start up in gear. If that is worn, malfunctioned etc it may have engaged when the motor stalled effectively locking up your flywheel.

Of course that doesn't account for the condition that caused the stall, which also must be addressed, but could explain why it was "locked up"...
 
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