Force 50 hp backfires from exhaust

[rgrano]

Engine was running great, one day went to start, wouldn't start, just back fire from exhaust real loud. I rebuilt Carb, used fresh gas 50:1, plugs and coil new with good spark, Thought it might be head gasket, replaced it. Won't even start with starting fluid. Such a mystery since it was running great?
any Ideas would be greatly appreciated. Model 507Y8D
,1988

[eurolarva]

Is it possible you reversed the plug wires? Did you remove the flywheel? If so if you did not torque it down correctly you may have sheared the flywheel key

[JB]

I would bet on the flywheel key.

[rgrano]

I know the wires are not reversed, is it difficult to work on the fly wheel, how does this happen?

[steelespike]

If the flywheel was not installed and torqued to the proper specs it may shear or partially shear the key
The key is an alighnment device not to pin the flywheel in place.Be sure to follow the proper procedure when working on the flywheel.

[rgrano]

You guys are awesome, checked the fly wheel, key sheared off, didn't even need a pulley to get wheel off, why would this cause boat to backfire? Does the boat have timming that now needs to be adjusted?

[Frank Acampora]

The flywheel has magnets in the rim to charge the magneto. When the key shears, the flywheel and magnets shift position on the shaft The key also locates the points cam. The cam shifts and the points open and close at the wrong time, firing the plug too soon. Bang! The engine backfires.

When you re-install the flywheel, make sure the shaft taper is clean. do not lubricate it Borrow a torque wrench and torque the nut to the proper value. My book says 80 foot pounds. That's more than you can do with a simple box wrench unless you are a really strong person.

[rgrano]

Thanks, Mr. Acampora for the education d

[eurolarva]

If you know someone with an impact wrench I would use that . The problem you will have is stopping the flywheel from turning while trying to achieve the torque tolerence.

[Frank Acampora]

Remove the electrodes from an old 14 mm long reach spark plug and screw into the top plug hole, or use a 14 mm bolt. Won't harm the piston or bearings and makes life a lot easier. Personally, I jam a hardwood dowel in then torque the wheel.

[kennywaldrop30]

I noticed on the 50 hp force engines the key has the long pieces on the back side which runs into the collear that also turn the collar for your contacts so the contacts wont open and cose at the same time the motor nits the combustion stroke sometimes it will fire on the exhaust stroke causing the backfire out the exhaust just had the same problem in my motor as well