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mid 70s 2stroke Mercury 40hp Blue line misfires

markmcintyre

New member
Hi,
I recently bought a boat with this motor on it.
It misfires under load at lower rpms but seems to run great when opened right up.
Here is a list of the things I have tried to get it running good but hasn't solved the problem:

Carby cleaned out with carby cleaner and air compressor
New needle and seat
The float floats in fuel
Replaced visibly worn fuel lines
New spark plugs
Both plugs throwing blue sparks
Checked compression, both cylinders producing 135psi
New fuel
New fuel tank
New primer bulb
I'm not an expert at mechanics so just wondering If anyone knows if there's anything else that I should check.
 
Are you positive it's a misfire as opposed to a cough/sneeze?

Often if you have a bad seal or gasket (so either sucking air or blowing out some of your oil/fuel/air mix) it will run crappy at lower rpms, sneezing and sometimes stalling, but if you hit the throttle it "seems" to run better.

I toasted an intake gasket while away at a remote location. Firing it up and immediately giving it throttle would keep the motor running well enough to make it "usable" (didn't have the tools to even jury rig a fix and the closest location for parts was a 3 hour drive away) - it did burn more gas than normal and it still lacked top end power but if you were unfamiliar with the motor it would have "appeared" to be running OK at higher rpms, even though it wasn't.
 
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Terms have different meanings to different people which causes some confusion on boards such as this.

"Cranking", in my books means "turning over but NOT firing" - so the starter is turning the flywheel fine, but the motor is not "starting", yet "cranking" is often used to describe "starting".

To ME a "misfire" is when there is no/intermittent SPARK to a cylinder - as in "failed to fire" or "failed to fire at the correct time".

A Sneeze would mean that the cylinder still fired fine but lacked either enough compression or enough fuel/air/oil to produce a proper power stroke.

In the case of the intake gasket failure that I noted the cylinder was still firing fine - so the whole "ignition system" was sound (no "misfire") but because of the bad gasket, while the piston was on the compression stroke a fair bit of the air/fuel/oil mixture was getting blown out of the port leaving precious little to actually "burn" once the the piston cleared the intake port. It still "fired" fine but didn't have much to burn and was far more noticeable at low rpms than high.
 
It sounds like the cylinder isnt firing to me. Cause it kicks pretty violently when it happens and the rpms drop down a fair bit.
Is it possible for the coil to produce a strong spark but not spark when it is supposed to?
Ive checked everything in the fuel system except the fuel pump cause im guessing if that were the issue it would be having noticeable trouble at high rpm.
 
I had a look, there are three wires from the coil to the cdi switch box.
Two green wires and one black.
Only one of the coils has a black wire connected to it.
There is however a black wire that reaches the other coil (unsure of whether the wire is for the coil) but isnt connected, its connected to one of the bolts that holds down the cdi box like an earth.
Both coils are earthed already so what is that black wire for?
 
Not sure, but your motor is definitely dropping a cylinder, and it's ignition not fuel related.

I'd tie that black wire to the motor (ground it). Merc used lots of ground wires when they went to rubber isolation.

Jeff
 
the black wire is grounded btw but im not entirely sure what it was attatched to. mightve been attatched the coil but i cant see why since the coil is already grounded.
 
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