cylinder not IGNITING

[dthx]

25 HP...SN# 0B338109........I believe made around 1988..... Mercury 2 stroke...100 psig compression on both cylinders....carburetor perfect.....timing set correctly.....replaced old plugs with new plugs.......spark getting to both cylinders....BUT lower cylinder not igniting....engine only running on one cylinder....starts good...idles fine....and runs at top speed ....just on one cylinder
Friend suggested a problem with the reed valve for the lower cyl.
Any ideas..?

 

[flyingscott]

How does it run at top speed if it's only running on one cyl you should lose at least 1/2 your power.

 

[dthx]

Sorry I took a little poetic license....as a country boy sometimes does.
When I say "full power" it's a generic term.... I realize that HP is a function of Torque and HP....and is the Rate of doing work.
The engine is in the driveway of my shop and is on a water clamp and is not in the water.
With an increase in the throttle the engine develops full RPM....it obviously cannot develop full torque because of the 2nd cylinder not igniting.
I say igniting because to say that it isn't firing may confuse.....as it is getting a spark.
Do you have an idea of what could be wrong?

 

[flyingscott]

Stop revving it up in the driveway is not a good test of anything. Did you use a spark checker to test spark and did you try switching the coils around to see if the problem follows it.

 

[dthx]

Well...I don't have a big tank to put the motor in...and the lake is 6 miles away and then all the tools are in my shop.
I dont have a spark checker....just a screw driver and my hand.....the spark is enough to show up in bright sunshine.....I didnt change the coils around....but could.....and will if that's what the next step would be....
Any idea how much voltage the coils are supposed to produce....?

 

[flyingscott]

The spark needs to jump a 7/16" gap. Regardless of where the lake is revving up in neutral or no load on the prop will not tell you anything and is hard on the motor and can cause damage. Switch the coils around and what do the spark plugs look like also do a cylinder drop test. Pull each wire off as it is running and see what happens.

 

[dthx]

The lower cylinder is not producing power....I pulled the plug wire off the plug as the engine was running and there was no change at all....that's how I figured the lower cylinder was not "firing"... I replaced the spark plug with a new one....no change
I didnt actually use a spark plug as a spark checker....I see that I can make one or buy one..I've taken the insulator off the plug and inserted a screw driver "to ground" and the spark is blue and makes a noticeable "tick"....but I didnt use a checker and couldn't see a long number of sparks....probably should do that.
I'll change the coils around and see if I can rule out a bad coil.
The old plugs look normal. they're the "surface" type of plug with no "hook" on the plug.
I thought the gap should be about .030 so I put a plug with that gap in the lower cylinder with no change.
Thanks for the advice...I live in a very rural area and the nearest certified mercury mechanic is 80 miles away....and they aren't very good..... we're pretty much stuck working on our own stuff.

 

[flyingscott]

I don't think you have a bad reed valve you could have a leak in the gaskets by the reed valves or a lower seal going out.

 

[dthx]

OK....,,,that would be great if it's only a gasket or two.....
2 questions...
why don't you think it's a broken reed...(not holding the gas in the cylinder on the compression stroke)?
where is the "lower seal' are you talking about?

 

[racerone]

The reed valves have nothing to do with compression in the cylinder.----They have to do with compression in the crankcase.----The crankshaft has a seal on the bottom to keep crankcase compression from escaping.

 

[Faztbullet]

That motor is a chrome bore and likely the exhaust side of piston is scuffed....this would cause low crankcase pressure and cannot receive a fuel/air charge.

 

[dthx]

thank you,racrone.....Crank case pressure, huh?....I have since learned from your comment how a 2 stroke works......thank you...I was ignorant of actually how the engine works.....
question to all and Faztbullet......would a scuffed bore affect the compression pressure...or would it be too slight to measure on a compression gauge....and would still be enough to affect ignition?
AND....if the lower seal or inlet manifold/reed box gasket or seal is leaking...would there be any tell tale effects that I could see from the outside of the engine?

 

[dthx]

When it stops raining., etc. I'm going to take the carb off and look at / remove the reed box....BTW....as I have learned...I understand that the "lower sea"l is the seal around the bottom of the crank shaft....correct? Keeping combustibles from escaping from the crankcase there...yes?
I dont want the problem to be there...
I also dont want the prob to be a scuffed cylinder bore.....
I'll let everyone know what I find....
The little engine is not mine....it belongs to a friend that doesnt have a computer ....Now I find out that he bought it from a guy that said it was just "running rough and prob. needs a tune up"....
The seller prob. knew something was up...if it's a scuffed piston, I bet that the engine is beyond economical repair.
Thanks to all.

 

[schematic]

Switch the plug wires. If it runs the same, check each cylinder with a timing light to see if both are firing at the same time. If so, look to wrong coil installed or bad switch box.....(been there)