1991 90HP Mercury not starting

[Global_Conscience]

I am at my wits end. My 90 HP mercury has good spark to all three plugs, the plugs are new, coils tested within spec, has 115 compressions on all 3, and the carburetors are clean. Fuel is getting through to the plugs. All wires are sound and earthed well. I am getting a click from the fuel enriched and the starter is cranking solidly. BUT THE BLOODY THING WILL NOT FIRE, EVEN WITH FULL COLD STRAT LEVER.
It has been running fine for the past 3 months.
Any suggestions will be greatly appreciated.

 

[Faztbullet]

What is good spark ? Will it jump a 1/4 air gap with a bright blue "snap"?

 

[Chris1956]

Give it 1 shot of ether. If she doesn't fire or backfire, you have no spark.

Have a fire extinguisher nearby in case she catches fire.

If she fires and runs for a few seconds, it is likely carburation is the problem.

 

[Scott Danforth]

If you have compression, spark and fuel.....are they happening at the correct time?

Have you checked the timing?

 

[Texasmark]

Give it 1 shot of ether. If she doesn't fire or backfire, you have no spark.

Have a fire extinguisher nearby in case she catches fire.

If she fires and runs for a few seconds, it is likely carburation is the problem.

Since it is a 2 stroker, lubricated by the fuel mixture, the general recommendation on here is to use a spray containing some lubricating oil.....like Sea Foam, or Starting Fluid that contains a light lube....indicated in the printing on the can. Over the years I have seen SF used quite often for starting problem determination.

If you get the fuel to work ok. If it indicates spark and you have the spark that Faztbullet mentioned, pop your flywheel nut and look at the position of the flywheel as compared to the slot in the crankshaft. If the Woodruff key is damaged at all, it could be your problem....have had problems on small engines when the key was only half sheared and prevented starting due to inadequate timing.....usually caused by hitting an obstruction with the blade......on direct drive mowers, not belt driven as the belt absorbs most of the impact.

 

[racerone]

Check the flywheel key.

 

[Chris1956]

Mark, 1 shot of starting fluid is not going to wash off the oil he has been giving it by cranking and choking. Hence the reference to 1 shot.

Racerone, most Mercs do not have flywheel keys. They have a "W" cut into the flywheel and crankshaft. I suppose they could strip, but they do fit together with precision, and even a moderately tight nut should hold them. However, the OP will likely end up needing to check his timing if she backfires.

 

[racerone]

So 26,000 post here.-----Then you MUST check the parts book for this 1991 model MERCURY motor and all the 3 / 4 cylinder looper models!----- From 70 to 125 HP.----And you can't find a sheared key with a timing light either.----Rebuilding a 1965 model 65 HP MERCURY for a dealer this winter .---It sure has a flywheel key as well !

 

[Chris1956]

Gee, my 1973 and 1977 Mercs as well as my 1993 Merc have the "W" machined into the crank and flywheel. The pasts breakout of the OP's motor did not show a flywheel key. While it is true that I never worked on the OP's motor, I think it likely he doesn't have a flywheel key.

 

[racerone]

You are funny.-----Need to look at proper parts book !

 

[Faztbullet]

#6 is the key. If spark is weak it will not jump plug gap due to compression. This would be a battery,starter or stator problem