Mercruiser 120 poor starting

[Scott06]

Just cleaned and gapped them. Will put a meter on them this weekend.

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]

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

 

[Guffer]

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...

Very helpful! Wish my spark look like that

 

[Guffer]

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

I've learnt a lot in a few months. Thanks v much.

 

[Buster53]

there is no IAC on a 3.0 liter carbureted motor

Good to know. Thanks

 

[Guffer]

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!

 

[nola mike]

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?

Are you getting spark to the plugs now?
4' won't matter.

 

[Guffer]

Are you getting spark to the plugs now?
4' won't matter.

Yes, but maybe a bit weak.

One thing that I forgot to mention is i replaced an earthing wire which had degraded. It ran from the fixing point in photo to the fuel sender on the metal fuel tank.

 

[Candy Gramformongo]

If you got nothing with ether starting fluid, and you have spark at the plugs, this is seriously weird. Practically anything will run on ether for a little while, no matter how badly adjusted. Ether is highly flammable (don't ask how I know).

 

[Guffer]

I can only guess that the spark is not strong enough or water is getting in to stop ignition happening.

 

[ESGWheel]

Guffer,
If there is an award for perseverance you have won it 10x over!

For the comment about getting spark at the points > while there is going to be a smallish one, you are correct to think that if its excessive then the condenser (which is really a capacitor) is bad. A strong spark at the points will wear out the points quickly and allow the coil to also have a weaker spark (see this link).

So, change the condenser.

Also, grounding and proper connections are important as well. Make sure the connections from the battery cable to the engine block is nice and bright and free of any corrosion. Remove this cable to clean it as the corrosion will be underneath it.

Please do these things and try again and post results.

Good Luck!

 

[Guffer]

That’s kind ESGWheel! I missed your post and had started calling marine engineers but it may be weeks until they can help Did some more Googling a saw a post re a car engine that was flooding the oil with fuel. Sure enough when I emptied the oil it was full of petrol….so now I have something to work with.

Pulled the mechanical full pump off and testing that. Putting air into the intake whilst blocking the pump to carb outlet it pushes a small amount of water in a connected tube. After that it appears to be airtight. Does this need to be replaced or is this operating as expected?

Many thanks

 

[ESGWheel]

Guffer,
I am not quite following the test you are doing on the fuel pump. You are correct in your thinking that perhaps the diaphragm of the pump is leaking and introducing fuel into the oil. Below is a diagram of a generic mechanical fuel pump. It needs to be “airtight” (i.e. leakproof) in the upper half as shown in this diagram. By applying low pressure air to the inlet (about 5 psi) with the outlet plugged up you should not hear any air hissing out where the lever is. However, I am not sure that is a good test for a situation where there may be a small leak. Would be better to see if it holds that 5 psi for some time which is a different test involving a pressure gauge. Frankly I was suspecting the fuel pump was not working way back in the early posts, and due to its age, I would replace it. Best to simply eliminate that as an issue for both fuel in the oil and lack of running.

As for the fuel in the oil. How do you know and how much? Example: engine holds 4 qts but pulled out 5 qts and oil smelled strongly of gas. And while good that the oil is now changed out, that would not have precluded the engine from running. So still change that condenser!

 

[Guffer]

Many thanks for coming back to me so quickly. Will replace the fuel pump and condenser. I had wrongly assumed as fuel was getting to the carb that the fuel pump was working. I have now pulled it apart and can see crazing on the diaphragm which could have easily caused a small leak.

The 2.5 120 holds 3.8 litres of oil and I pulled c6 litres out yesterday so 2.2 litres of petrol must have been in there which maybe is enough to stop it running? Smelt strongly of petrol.

 

[ESGWheel]

Wow, that is a lot and while may not preclude the engine from running its good that the engine did not run with that mixture! That much fuel must have been coming from the leaking diaphragm of the fuel pump.

So new pump and condenser and lets get it running!!

BTW I meant to highlight that was an excellent summary in your post no. 66