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Dominick
Dominick, Marine Mechanic
Category: Boat
Satisfied Customers: 1009
Experience:  25 Years Experience as Mechanic, 20 Yrs Boat Yard Owner/Mechanic, Factory Certified
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I have a 1977 mercury 850--I just got back from

Customer Question

I have a 1977 mercury 850--I just got back from shop--mechanic said only way it would run is at 1500 rpm-is this possible
Submitted: 1 year ago.
Category: Boat
Customer: replied 1 year ago.
they changed out a plastic choke piece--rebuilt carbs--boat would not go anywhere--returned to shop they set idle at 1500 rpms and said motor sucking air and fuel and needs rebuilt--is this possible
Expert:  Jeff G. replied 1 year ago.

Possible but not hardly.

I need the serial number to ID the engine.

Customer: replied 1 year ago.
the boat was running but rough--would die at full throttle-but run slow-took it in-guy said choke broke and ordered a plastic part they do not make anymore--was 95 dollars for part. I got motor back---they said carbs were rebuilt and filthy-
and told me put in in line filter which I did--Took fishing -started once then nothing. Took back---they said only way motor would work is 1500 rpm amd on last legs--needs rebuilt engine because sucking air why idle so high.
Customer: replied 1 year ago.
I test drove and yes it worked-but OMG shiftinf from neutral at those rpms--is ridiculous. I am just trying to make sure if this explanation is plausible or stupid.
Expert:  Jeff G. replied 1 year ago.

While the engine could have problems it doesn't sound right.

Start the engine and reduce the idle via the idle stop screw on the throttle link get it down to about 1000 rpm. Let it run for a couple minutes. Spray some WD40 or carb cleaner around the engine. Spray around the carb mounting and around the intake covers, anywhere there is a seam. If the rpm changes there may be a leak.

After running for a couple minutes, never above 2000 rpm, stop the engine and check the compression. You'll need a compression gauge for this which if you don't have one may be borrowed from some auto parts stores.

If the compression is around 100 psi and within 15# ***** each other, the pistons and cylinders are OK.

Next we will need a spark tester. Put the tester on each spark plug wire, all spark plugs out. Crank the engine and check each cylinder.

Do you have a timing light? We use it to diagnose ignition problems.

Your ignition has both a low speed side and a high speed side, we use the light to check this.

Do these checks and get back to me and we can carry on from there. Sounds like the shop is not competent on these engines.

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