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1996 Mercruiser 5.7 litre wont start

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bosch

New member
When i was out the other day the engine would not go above 3100rpm and would only get up to 18 Knots eventually, when usually it does 24 Knots on the plane and about 30 Knots wot. we arrived at our destination and spent a few hours anchored up when i went to restart the engine wouldn't start. it turns over fine but it wont run.

Any help would be greatly appreciated
 
It is a carbed engine and has new plugs and leads and nearly a full tank of fuel. i will have to get the serial No tomorrow
 
I have found the problem this morning it was a faulty kill switch, i have replaced that and the boat starts fine and runs up as usual to 4700rpm.

But now there is another problem when the boat is out for a run and on load at wot it only goes up to 3200rpm and 18 knots (eventually) where she used to reach 4700rpm and cruise at 24 knots with a top speed of about 30 knots.

Any ideas anybody please as as usual the warrenty has run out 4 weeks ago
 
Have you changed your fuel filters? There should be 2.
There is a bigger one that's also a water separator, it looks a lot like a spin on oil filter but it's not. Should be near the front of the motor on the starboard side between the fuel pump and the carburetor. That should be changed yearly.
There is a smaller one under the fixture where the fuel line from the separator enters the front of your carburetor, it is a basket shaped screen common to most carburetors.
 
Yes the fuel filter that looks like a spin on oil filter was changed during the service in april this year. i havent seen the other smaller filter, i shall have a look for it
 
Is this a saltwater boat, if so do you have a lot of marine life growing on the hull and drive? Did the boat run normally one day and worse the next? Or did it deteriorate gradually?
 
yes it is saltwater and kept in a berth in a marina. there is hardly no growth on the hull due to the antifoul and a little on the leg but not much which comes off once moving.

it was fine when i took it out and the next time it was like this it wouldnt go above 3000 to 3200rpm
 
New means nothing. Pull the plugs and check the burn pattern...you want to see a medium brown tip. Test for good spark on the end of the wire at each plug.
 
A simple way is to use a good spark plug inserted into the HV wire end and grounded to the block. You want to see a blue tinted spark that fires with a loud snap. A white, yellow or orange tinted spark is weak and won't completely burn the compressed fuel/air mix in the cylinder.
 
bosch said:
Yes the fuel filter that looks like a spin on oil filter was changed during the service in april this year. i havent seen the other smaller filter, i shall have a look for it
Click to expand...

Ayuh,.... Time means Nothin' to a fuel filter,... Ya mighta gotten a cup of Water, just last month, or in May....

Pull the filter, 'n inspect it's Contents, then change it for a new 1...
 
the compression and vacuum tests are easy all you need is a compression gauge and a vacuum guage. to do the compression test remove all plugs open the throttle to wide open place the compression guage on a cylinder and then crank the engine watch the guage when the needle moves 5 times stop cranking and record the reading then move on to the next cylinder. all cylinders should be close to the same reading if there is more than a 10% difference between the highest and lowest cylinder you have an internial engine problem that will need to be fixed. For the vacuum test start the engine and let it warm up install the vacuum guage on a vacuum port on the intake. You must use a port that has constant vacuum on it when the engine is running. this is a good link on how to read a vacuum guage and what the readinds mean. www.secondchancegarage.com/public/186.cfm I hope this has helped you
 
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