Merc 7.5 - only one cylinder running

[Medvind]

I have a Mercury 7.5 (-84) that only runs on one cylinder. It always starts and runs quite well, even though it is weak.

I have noticed that if I remove the cable from the top-plug it runs as usual, but removing the bottom one makes it impossible to start. I can see that both plugs have sparks, but one of the cables from the switch (CDI) box to the coil (for the failing cylinder) is damaged (and have had a connetion to the chassis, now there is some tape around it d ).

I am quite new to engines, but I guess that either the cylinder doesn't get fuel, or there is something electrical. Do anyone have any tips on what to look for?

I have some specific questions:
- Even if there is a spark; can the spark be not strong enough to light the fuel?
- Can the timing of the spark be wrong? How do I see that? Can I do something about it?
- Is it worth the effort to check the reeds? (It seems quite complicated to dismantle them)
- What's in the switch box? Does anyone have a schematic diagram for switchboxes?

 

[Laddies]

Re: Merc 7.5 - only one cylinder running

If the 1 cyl was shorted out for awhile it could be a plug, if you have spark to both cyls. it could be a reed if it only misses at idle a bad reed will run at wot. Is the compression equal on both cyls? It could be a bad coil, swap them and see if it runs on the othe cyl. sometimes flywheel magnets come loose look under the flwheel and see there 180 degrees apart and yes the switch box could be bad on 1 side and cann't be repair only replaced hope this is some help

 

[Medvind]

Re: Merc 7.5 - only one cylinder running

Thanks for the fast reply!

I bought a cheap compression tester and tested both cylinders. It seemed like the compression was equal at both (but tester was a little tricky to use as it was a little too big for the plug hole).

The motor seem to run as bad at idle as at wot, however it has happend twice, and only very shortly, that it got more power (I assume that the faulty cylinder started).
Why will bad reeds 'work' at wot?

The magnetics at the flywheel seem very interressting! I didn't realise that they could be loose. I have one problem though; how do I get the flywheel off? I have tried in several ways.. Is it a 'counter-clockwise screw'?

I shall also try swiching the coils (switching the plugs didn't change anything).

 

[Laddies]

Re: Merc 7.5 - only one cylinder running

Above about 1200RPM the reeds are held open by the fuel/air flow and aren't closing so the engine don't realize that thet are broken till it slows down to the point it needs them to operatr again

 

[perkdp]

Re: Merc 7.5 - only one cylinder running

If you are getting a good run on occasion it sounds like a bad spark plug and / or the damaged wire is touching ground to kill the spark. When it bounces away from ground it allows the spark

I had an old Elgin once that had two or three reeds per cylinder. One cylinder had enough reeds missing that it wouldn't run (flooding or something) created new reeds out of sheet brass and fixed it.

Oh.... it is possible for you to see spark, certainly at night, and it not be hot enough to ignite. Depending on compression and fuel mixture.