Mercruiser 120 poor starting

Scott06

Scott06

Admiral
Joined
Apr 20, 2014
Messages
6,778
Guffer said:
Just cleaned and gapped them. Will put a meter on them this weekend.
Click to expand...
compare spark gap test before and after setting with dwell meter. Dwell is what determines the amount of time the coil builds up spark energy prior to firing.

this is a great video explaining how points work and a fantastic example of what a good spark looks like. The money shot is at 5:07...

 
Scott Danforth

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
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50,314
Gapping alone may get you home , but it isn't accurate it in the +/-40% range

Dwell needs to be correct to get coil saturation so that when the points open, the magnetic field is as strong as possible when it collapses and energizes the secondary windings providing the coil output

The meter will tell you your dwell..... And the health of the condenser
 
G

Guffer

Seaman Apprentice
Joined
Nov 1, 2024
Messages
31
Scott06 said:
compare spark gap test before and after setting with dwell meter. Dwell is what determines the amount of time the coil builds up spark energy prior to firing.

this is a great video explaining how points work and a fantastic example of what a good spark looks like. The money shot is at 5:07...

Click to expand...
Very helpful! Wish my spark look like that :)
 
G

Guffer

Seaman Apprentice
Joined
Nov 1, 2024
Messages
31
Scott Danforth said:
Gapping alone may get you home , but it isn't accurate it in the +/-40% range

Dwell needs to be correct to get coil saturation so that when the points open, the magnetic field is as strong as possible when it collapses and energizes the secondary windings providing the coil output

The meter will tell you your dwell..... And the health of the condenser
Click to expand...
I've learnt a lot in a few months. Thanks v much.
 
G

Guffer

Seaman Apprentice
Joined
Nov 1, 2024
Messages
31
So, this is becoming a proper 2 pipe mystery. Had hoped to be back to the salty stuff a month ago but my MCM 120 still won’t start so am stuck on the river! This where I am up to:

Compression
  • Before winter the engine was running fine but cylinder 4 was low on compression
  • Had the head reworked and now have c150 psi on all 4 cylinders
  • Engine put back together with a new exhaust manifold

  • Ran a few times for 30-90 seconds then died.

Fuel
  • Old fuel line replaced to include and antisiphon valve and a water separating fuel filter, using Permatex # gasket sealant
  • Fuel tank is clean, pick up is clear and fuel is fresh
  • Carb has been through the ultrasonic cleaner and rebuilt with a new kit
  • Fuel is getting to the carb
  • Have tried Easy easy start through flame arrestor / fuel down carb throat…

Spark
  • Coil is good (new), powered and getting spark though centre cable (don’t know if it is strong enough)
  • Plugs are new and gapped to .035
  • Points have been filed and gapped to .018
  • I see what I perceive to be a good spark at the points
  • Timing is set to 8 degrees BTDC
  • Dwell is set to 32 degrees
  • Cap and rotor replaced with quicksilver 393-9459Q1 (tried old cap and rotor also)
  • Battery appears strong and engine turns over fine (tried a second battery)
  • Shift interrupt has been disconnected for now

Questions:
If I’m get spark at the point that suggests the condenser is doing it’s job?
If I set timing to 4 degrees BTDC will this materially change anything?

I am guessing that I have made a mistake somewhere and it feels like this is most likely to be on the spark side…..any thoughts on things I should consider / checks to do?

Many thanks!
 
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