Backfire - 1990 50HP Force Outboard

[RobEntertainment]

I have a 1990 50HP (507X90A) Force Outboard motor which is backfiring.
Problem: Will not start, as it is now backfiring. The engine ran last year for a brief 10 seconds without a backfire, then quit.

The following have been completed to date:

1. New stator ? wires connected in the right places ? no more misfire (it is a fixed/set alignment)
2. Replaced the CDI Module ? recently bought second hand which is one year old from the original owner.
3. Good compression in both plugs
4. Reeds are shiny clean and not sticking
5. New spark plugs?not fowled. (tested ? good spark with the meter)
6. Fresh gas & oil, new gas treatment (emptied old gas from bowl)
7. Starter works & turns crankshaft.
8. Verified all the wiring according to the wiring diagram ? no breaks
9. New rectifiers ? both work
10. Any suggestions?

 

[jerryjerry05]

2 things: Plug wires on wrong?
Or the flywheel key is sheared or slipped?

 

[RobEntertainment]

Thank you for your response.

Working right after per your advice, my observations are as follows:
1. The spark plug wires were intentionally switched to test whether the backfire would be resolved?unfortunately without success...continues to backfire.
2. The flywheel has been removed, and I can confirm that the flywheel key into the crankshaft is not sheared. I have replaced the old key with a new key (exact key for the 50HP Force).
3. Flywheel is not slipping.
4. The flywheel is aligned with the timing line. When rotated, number one top piston lines up with the timing line (single long line).
5. The pistons are smooth and the chrome is shiny clean.
6. Wiring color codes from the stator all match.
7. The motor ran last summer for a total of 10 minutes (not 10 seconds per my initial query above), 2 times (5 minutes each time) for testing; when tested the third time, it would no longer start.

 

[HotTommy]

I'm no expert, but it sounds like it is not correctly timed. Have you timed it?

 

[Glastron_V210]

Are you sure the triggers to the CDI are wired correctly? IE: trigger for cylinder 2 goes to cylinder 2 etc.

To make sure the timing is on, pull the plugs, mark TDC1, TDC2, TDC3. Remove the plugs, and use a bare copper wire to wire the three plugs to ground so they will fire. I just loop the wire around the metal body of the first plug, then the second, etc, then tie it to the engine case.

Now, time the first cylinder with a light. Is is close to tdc1? Now the second, look for it to be close to tdc2, and the third, etc.



This way you know where each spark is compared to it's cylinder.

99% of backfires are incorrect spark timing. The others are typically due to really lean air fuel ratios.

Hope that helps.

Chay

 

[RobEntertainment]

Thanks to all.
I will take your advice and work on the motor over the weekend.
Will keep you posted on the outcome.
Cheers.

R.

 

[RobEntertainment]

Much appreciated for all your advice.
The crankshaft key was broken....I replaced it with a new one, and now the motor works!

Despite that the motor is working, in examinig the flywheel prior to installing it, i noticed on the inside a slight thin layer scorched (scrapped) off right in the middle of the single trigger line. Will this cause a problem to the timing in the future? This was probably caused by a piece of rust.

Thanks.
R.

 

[Frank Acampora]

The flywheel key does not drive the flywheel--it simply locates it correctly so TDC at the top piston is at the zero mark on the flywheel. The flywheel is on what is called a locking taper and must be torqued properly in order to lock and not slip and break the key. Be certain the flywheel nut is torqued to 95 foot pounds. Do not estimate by hand. If you do not have a torque wrench, buy a cheap harbor freight torque wrench or if possible borrow one.

 

[RobEntertainment]

Thanks for the info. I torqued the flywheel nut to specification. All working good. Much appreciated everyone.

 

[rharr290]

Much appreciated for all your advice.
The crankshaft key was broken....I replaced it with a new one, and now the motor works!

Despite that the motor is working, in examinig the flywheel prior to installing it, i noticed on the inside a slight thin layer scorched (scrapped) off right in the middle of the single trigger line. Will this cause a problem to the timing in the future? This was probably caused by a piece of rust.

Thanks.
R.

So the flywheel was slipping, I'm having the same problem?