# Compression, spark, fuel, air but not go?



## jerrywickey

a Mercury 9.9 4 to 8 year old outboard 2 cycle.

Been a great motor. But wont start now.

Removed the plugs. See a good spark when I pull. plugs are clean, no water, not fowled. I cleaned them.

Good compression with my finger against the plug hole. sucks at the carb. I feel exhaust at the exhaust port.

I spray starter fluid and turn the crank with a drill. Nothing. Nothing at all. Not after a few minutes of not spraying starter fluid. Not right away. Not with a little fluid not with a lot.

How could this be happening?

Any ideas?

Could it look like a hot spark but the plugs are bad. 

Mind you not even a pop. It is just like I am spraying air in the carb. I even bought a second can of starter fluid just to make sure the first new can wasn't defective.

Any ideas?

Replying with. Never heard of that or that isnt possible it fine too.

Jerry


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## pyro_maniac69

just because a motor "blows air past your finger when rolling it over" does not give a good indication on what you have for compression, you need a compression gauge.


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## 30yearTech

I would suggest trying new spark plugs to be sure thats not the problem.

You can have good compression, ignition, and fuel and still not get a 2 cycle to pop off and run. If you have a major air leak into the crankcase, you will never get it to start and may never even get it to pop. Make sure there are no air leaks around anything that attaches to the crankcase and that the top and bottom seal are in place.


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## jerrywickey

Thank you 30yeartech

A big air leak must be the problem. I have removed the outboard and prepared to remove the housing to look at the crank case.

When I put a piece of plastic in front of the carb intake and pull, the plastic is drawn in. Since the air leak must be small enough to still pull air in through the carb and since the only place for that carb air to go is the cylinders, why is the start spray not poping or puffing or something?

Is air drawn in the carb and no puff from the starter fluid still consistant with your diagnosis?

Jerry


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## Dimark1009

Jerry, 

A 2 stroke engine requires a lot of compression, much more than a 4 stroke. 

you do need to use a compression tester to see what you got. I dont know exactly what the compression should be, but I'm working on a Stihl chainsaw right now that requires at least 150 psi to run. this saw only has 75 psi and it wont even try to start when I put mixed gas into the carb throat. 

someone here will tell you what compression you need to run, meanwhile find a compression tester and see what you have. then we'll go from there...

Hope this help some.


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## 30yearTech

You can have great compression, but if you have an air leak you may never get the engine to pop. 

The reason is that the fuel and air mix enter the crankcase on a two cycle engine and do not go directly to the cylinder, the fuel and oil mix coat all the internal components of the engine and then when the piston comes down, pressure in the crankcase forces the mix into the combustion area. 

If you have an air leak, you will draw extra air into the crankcase, bypassing the carburetor and then it can flow back out of the leak area and not enter the combustion chamber. Air leaks into the crankcase of a 2 cycle engine can cause a big headache.

Compression specs really depend on the design of the engine, ported engines generally require higher compression then do reed valve designs.


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## pyro_maniac69

generally a 2-stroke will run with 90 lbs. of compression. anything below that they normally won't. But this excludes bigger 2-stroke engines (aka snow blower engines) the motor is big enough to run with less compression, that and they do not (that I know of) use a pulse to prime the carb, they run off of vacuum just like a 4-stroke.

since you have an outboard motor, I would believe that if the compression is around 90 or so, that you should have enough compression, but that also depends on if it has a reed valve or not like 30 year stated


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## jerrywickey

I am nearly certain it is a read valve motor. 

Right now I am operating under the assumption that it is a leak in the crank. Probably the lower seal. But I have not taken the motor out of its outboard housing yet. I will have the time to do that probably next week.

Until then if anyone has any other suggestions, I would be glad to read them.

One thing is still bothering me. Considerable air is being drawn in through the carb with each pull. It seems that what ever leak there may be is not big enough to stop air from being drawn by the moving piston. This also means the valves are working. 

This means that at least some of the starting fluid is entering the cylinder as a gas. I am persuaded that 30yeartech is right about a leak. While the crank case leak would greatly reduce the vapor concentration of fuel, it seems unlikely it would reduce it below the point a spark would catalyze oxidation. I should be getting some kind of pop.

If anyone has an explanation for no burn at all, it may help me identify the problem. But unless someone as something to add, next week I will likely tare it open. I wish I could afford just dropping it off at a shop for $80 dollars an hour and let it be someone else's problem. But I don't. So its tools in hand for me.

Thanks. And please if anyone has a plausible theory why no pop at all, let me know. It might help.

Jerry


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## 30yearTech

If it's a reed valve engine a broken reed plate could create a similar problem.


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## G-Mann

jerrywickey: I know you said it has spark but............. I have seen a few engines where the spark plug has spark outside of the engine but at times I have seen where the plug produces NO SPARK when installed on the motor! If that thing doesn't even run, pop or fire in any way with starting fluid, then there is a real good chance it is the plug. Replace the plug with a NEW PLUG!!!


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## G-Mann

jerrywickey: I know you said it has spark but............. I have seen a few engines where the spark plug has spark outside of the engine but at times I have seen where the plug produces NO SPARK when installed on the motor! If that thing doesn't even run, pop or fire in any way with starting fluid, then there is a real good chance it is the plug. Replace the plug with a NEW PLUG!!!


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## jerrywickey

Thank you so much for the information. G-man

I will try new plugs today. That is a simple and easy negative proof. Thanks.

30 year tech. If the reed is broken would I still see the plastic sheet sucked into the carb. I am just trying to support or elliminate this possiblitly.

Jerry


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## 30yearTech

30yearTech said:


> I would suggest trying new spark plugs to be sure thats not the problem.
> 
> You can have good compression, ignition, and fuel and still not get a 2 cycle to pop off and run. If you have a major air leak into the crankcase, you will never get it to start and may never even get it to pop. Make sure there are no air leaks around anything that attaches to the crankcase and that the top and bottom seal are in place.


You have not tried new plugs yet? That was the first thing I suggested. I would try that first before taking the engine down to inspect the seals or reed plates.

If the reed plate were broken you would still get suction at the carburetor, but you would also get pressure there because there would be a back flow as well.

Keep in mind that on a 2-cycle engine fuel is drawn into the crankcase on the compression stroke of the engine, and then forced up into the combustion chamber on the power stroke, the fresh mix in the crankcase is used to help scavenge the burned gas out the exhaust. This is why 2 cycle engines are so "dirty" as quite a bit of the exhaust includes unburned fuel. An air leak into the crankcase hampers this important operation and can prevent the engine from starting.


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## jerrywickey

REPORTING BACK ...PROBLEM SOLVED

The outboard is now performing perfectly.

It was the key between the flywheel and the crank shaft.

It had sheared in two. The flywheel did not feel loose. It took a gear puller to remove it. But once removed, I found half the key in the flywheel and half in the shaft.

Spark and compression but no pop because it was sparking at the wrong time. The timing takes its signal from a magnet on the flywheel.

Thanks all for the help.

I am glad I found the problem. 45 cent woodruff key.

Jerry


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