1974 mercury 20hp ( 200 model ) won?t start

[morenc00]

I?m trying to repair it for a friend. The story : the engine died while in use on a lake, it never started again. Since this time 2 mechanics tried to repair it without succes.

The facts :

Fresh water outboard with low hours
120 compression on both cylindes
When I remove the spark plugs I get some spark but coudn?t ignite fuel or quick start (ether) poured directly in the combustion chamber.
I checked resistance of the switch box and it?s the same for both cylinders
I also checked the coils both show the same resistance

Could a defective switch box become grounded when there is more resistance on the spark plugs?

 

[Laddies]

Re: 1974 mercury 20hp ( 200 model ) won?t start

wops that was the wrong system does your engine, have a number on the switch box

 

[morenc00]

Re: 1974 mercury 20hp ( 200 model ) won?t start

it's a square aliminum box, there is a paler label on it but it's illisible, given the outboard serial and a lookup on mercruiserparts.com the part number should be 332-4911A2

 

[Laddies]

Re: 1974 mercury 20hp ( 200 model ) won?t start

You can't really ohm the switch box on that engine but you can test the rest of the system and eliminate the other components one at a time to find the problem heres the CDI test for that system

 

[morenc00]

Re: 1974 mercury 20hp ( 200 model ) won?t start

do you have the specs for the CDI components?

 

[Laddies]

Re: 1974 mercury 20hp ( 200 model ) won?t start

They don't list differant spec for their componets so they must be the same in this case

 

[morenc00]

Re: 1974 mercury 20hp ( 200 model ) won?t start

thanks for the help, I do not know why but at the job I did not saw the scan of the shop manual you posted earlier

 

[JB]

Re: 1974 mercury 20hp ( 200 model ) won?t start

Have you done the routine "Outboard won't start" troubleshooting from the Engine FAQs?

NEVER use ether starting fluid in a 2 stroke.

If I was to guess, I would guess that you have a sheared flywheel key.

 

[Motor Boater Bill]

Re: 1974 mercury 20hp ( 200 model ) won?t start

Also try disconnecting the kill switch wire from the terminal block--bad insulation can cause the wire to short to ground, which is just like pushing the kill switch. Had that on mine, grounded under a clamp between the terminal block and the kill switch.

 

[morenc00]

Re: 1974 mercury 20hp ( 200 model ) won?t start

I will check for the flywheel key, I bought a set of inline spark tester on e-bay, put them on the engine and pulled the rope a couple of times, result they light up but some time they dont. (maybe I wasen't pulling quickly enough) I will try to check the timing.

first I will remove the flywheel to check the key, second I will will put a timing light and check timing. What value I sould expect if I pull the rope start by hand with the throttle closed ? also I guess the timing mark is related to the top cylinder ?

 

[8075mercury]

Re: 1974 mercury 20hp ( 200 model ) won?t start

One other thing you might consider after eliminating the other items above is a weak coil (or coils). I had a weak coil that sparked ok with the plug out but apparently would not spark the plug under compression which drove me nuts. I swapped the coils and the 20 worked great. You should have a pretty hot spark just with the pull rope. Also check the coils to make sure you have a good ground on them.

good luck.