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1987 Mercury 2-stroke won't start

M

mmac

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Bought this used. Ran but not great. Rebuilt carb. Fuel pump on side of carb. Won't start. Occasional backfire out exhaust. Good compression. Spark at plugs. New stater and trigger. This model has primer and it does spray fuel. I suspect a timing issue however it is a pull start. Not sure where to start on timing concern.
 
Back fire indicates the plug fired the fuel mixture in the combustion chamber at the wrong time.....you had a clear path from the combustion chamber back to the Venturi in the carb. at the time of the explosion, rather than this path being sealed by the piston rings sealing off this avenue......or you have carbon buildup in the combustion chamber that is glowing, setting off the explosion at the wrong time.....but I would think for this to happen, the engine had to have the carbon buildup and have been run enough immediately prior to the event to have the glowing element. I never was associated with your engine series but on larger engines the timing is right at 0* at idle and roughly 20* BTDC or a tad higher at WOT. There should be a marker adjacent to the flywheel that lines up with markings on the flywheel indicating * numbers that are used for timing alignment.

No body jumped on this so I am just trying to help.....surely I'm not the best authority on this series of engines.
 
25 HP Serial B172536. Flywheel is keyed correctly. Plugs were replaced with new ones. I have a manual giving details on setting timing at idle and WOT. However not sure how to check the base timing prior to start. Pull start so not sure how to check timing prior to it running. I did check and timing mark lines up with 0 when #1 piston is at TDC. BTW, it does not even try to start. About ever 20 pulls it will backfire out exhaust.
 
Put 6 drops of fuel in each plug hole.----Install sparkplugs.----Pull starter 10 times.---Report exactly what happens.
 
mmac said:
25 HP Serial B172536. Flywheel is keyed correctly. Plugs were replaced with new ones. I have a manual giving details on setting timing at idle and WOT. However not sure how to check the base timing prior to start. Pull start so not sure how to check timing prior to it running. I did check and timing mark lines up with 0 when #1 piston is at TDC. BTW, it does not even try to start. About ever 20 pulls it will backfire out exhaust.
Click to expand...
Well going out this end tells me that you are getting fuel to the combustion chamber to some extent but you are having a problem getting reliable spark to cause the explosion and the unburned fuel accumulates. Big question to me is what in the world could be wrong with your ignition system whereby it takes 20 pulls on the crank rope to get one firing of the plug(s)?

On "flywheel keyed correctly" one would have a non-damaged key....not even a partial indentation in the key like could happen if you were running at WOT and hit a solid obstruction.....the crankshaft would attempt to stop and the flywheel (weight) would want to keep running (kinetic energy stored in the mass of the flywheel). Altercation not severe enough to completely sever the key.
 
Yes, square button pulled out and left out. When it is pulled out it advances throttle and timing trigger for fast idle. (although it does not start) I am concerned if it is advancing timing correctly. I have not found a measurement for timing link rod to check "static" timing. This motor has the forward reverse control in tiller. I have to leave it in neutral to start. Does not allow me to adjust throttle for start up.
 
Hello again. August got busy and did not have time until recently to mess with this. Did as Racerone suggested. 6 drops for fuel in each cylinder. Backfired through exhaust in 6-7 pulls. Another 6-7 pulls backfired through exhaust again. Then nothing.
I am mechanical but admit no 2-stroke outboard training.
Checked basics again.
#2 cylinder did not have spark. I am certain it did in the beginning. #1 cylinder has spark consistently. (using a tester with plugs installed)
- Switched spark plugs between 1 and 2. Still no spark on #2
- Changed plug wires. Still not spark on #2.
- Switched positive coil wires from switch box. Problem followed the coil wire. Exchanged the switch box. Still no spark on #2.
- Replaced both coils. #2 now has spark, but inconsistent?? Could I not be pulling hard enough? #1 cylinder sparks every time.
- Wiring is basic. Checked for clean contact and good connections.
- With dial indicator set #1 cylinder to TDC. Marks on flywheel matched perfectly.
- I pulled flywheel. Magnets are clean and not loose.
- Stator and trigger ohms check to be within spec and good physical shape.

Question - The backfire still has me thinking timing. It is a pull start. Without it running I don't know how to check past the static timing.
- Is the #2 spark timed by the switch box, (#1 sparks and #2 is timed after that)? Or is #2 triggered under the flywheel?
Reminder this Mercury has a tiller with start position. Pull to prime fuel but throttle is predetermined for start. Timing advances with throttle. I have another outboard where throttle has to be in a sweat spot or it does not start. I cannot "play" with the throttle for start up on this one.
- Would anyone have timing rod static measurements? Manual I have does not list any.
 
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