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2 Cycle question


Jack B.'s Avatar
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05-26-06, 05:11 PM   #1  
2 Cycle question

Can you succesfully run a compression test and a leakdown test on a 2 cycle lawn mower - Lawn boy model 10227 s/n - 6930493. The reason I'm asking, is that I'm in doubt about the shaft seals. Also, what part would the reed valves play in these tests. I have rebuilt the carburetor and I noticed some spots of rust on the reed valves.

 
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05-26-06, 05:28 PM   #2  
yes you can run a compression test on a 2-stroke its still an engine you want at least 90 psi thats what my parts guy said as a good rule if its less than that toss it.

 
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05-26-06, 07:36 PM   #3  
Bob

The reed valve checks the backflow from the crankcase on a two cycle.

It works like this: As the piston is going up on compression, it creates a vacuum in the crankcase which draws in fuel/air from the carburetor past the reed valve. Combustion takes place above the piston driving the piston down past the exhaust port in the cylinder wall. The exhaust is under pressure and exits.

The piston travels a little lower in the cylinder and passes the intake port in the cylinder. This port leads to the crankcase which is now pressurizing from the piston moving down in the cylinder.

The pressure is checked from backflowing by the reed valve. The pressure from the crankcase forces the fuel/air up into the cylinder which forces the exhaust out from the back side. Then the piston goes up past the intake and exhaust ports, respectively, and the whole thing repeats itself.

The rust you mentioned on the reed valve could affect the amount of fuel/air entering the combustion chamber and consequently the compression reading. But I believe it would have to be pretty bad to affect it significantly. Still, you don't want the rust on the valve so I would definitely clean it.

Hope this helps

 
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05-31-06, 01:37 PM   #4  
I finally got it to start...but it runs like an engine with the choke closed, although it doesn't have a choke but it has a primer bulb. There are absolutely no adjustments on the carburetor. It is also very easy to flood (with gas running out the airfilter). What do you feel like is my problem? I also ran a compression test on the engine and got 70 PSI.

 
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05-31-06, 07:39 PM   #5  
Yeah thats not good you want at least 90psi. A engine will run on low compression just not very good> the only things that cause low compression would be valves,or the pistion rings.Since 2 strokes don't have valves i would check the reed valve and the pistion rings.

 
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05-31-06, 10:56 PM   #6  
I have rebuilt the carburetor and I noticed some spots of rust on the reed valves.
There are absolutely no adjustments on the carburetor. It is also very easy to flood (with gas running out the airfilter).
If you rebuilt the carb and the air filter is getting saturated with gasoline, you made a mistake relating to the float and/or needle valve. The needle valve is not closing tightly and is allowing fuel to flow out of the float bowl, up out the carb venturi, where it collects in the air filter. You will need to correct this before you can accurately diagnose any other possible problems.

What do you feel like is my problem?
I will wager if you fix the carb, it will run just fine regardless of the compression readings you are getting. This type engine is a tried and true design that generally doesn't give many problems. Clean the carb and keep the exhaust ports open and they will certainly run circles around any 4 stroker.

I noticed some spots of rust on the reed valves.
This can occur simply from damp storage. Wouldn't worry about it unless there is damage to the surface between the reed valve and the cage. My family has had LawnBoys since 1962. Out of 5 different machines, I can remember replacing 1 reed valve and we did that only because the machine was approching 30 years old when we rebuilt it. Figured it probably had a couple of billion flex cycles on it by that time. By the way, 4 out of 5 of those machines still work, and I can't remember ever needing to do a compression test.

 
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05-31-06, 11:13 PM   #7  
The rust you mentioned on the reed valve could affect the amount of fuel/air entering the combustion chamber and consequently the compression reading. But I believe it would have to be pretty bad to affect it significantly. Still, you don't want the rust on the valve so I would definitely clean it.
BEWARE: If you decide to remove the reeds for cleaning, it is an absolute necessity that you re-secure everything correctly. Loose parts in this area of the engine mean CERTAIN DEATH. Minor surface rust shouldn't effect the engine in any noticable fashion.

 
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06-02-06, 08:13 PM   #8  
Let me put another idea out there. The float in my carburetor is the cork type. I filled a container about half full of gas and placed the float in it for about 1 day...the float actually sank a little less than half way. My question is, could this cause the flooding condition and preventing the needle from seating. The float seems to have some sort of clear sealer on it.

 
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06-03-06, 10:32 AM   #9  
Bob

A fiber or cork composite float can definitely be a problem if the sealer has deteriorated. Simply put, if it won't float, it can't do its job.

The method you're using is a good way to check the float of any carburetor.

Are you saying it sank to a point of half submerging the float? If that is the case your float could be good. It comes down to a case of bouyancy characteristics versus weight of the object. You may have to compare how it floats with one that's known good to make a laymans determination.

Hope this helps

 
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06-05-06, 03:29 AM   #10  
Replace the float. They are not that expensive. A cork float should float on top of any liquid you put it in. The little attached bracket will try to sink, but should not be able to drag the rest of the float under to any degree.

 
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06-05-06, 05:15 AM   #11  
Thanks JLO, I think I'm going to try replacing the float. By the way, do you know of any clear coat spray that's resistant to gasoline that could be used in an emergency?

 
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06-05-06, 06:30 PM   #12  
There may be a coating available, but the problem is getting the fuel that is saturating the float out. The cells in the cork have broken down allowing fuel to fill the spaces normally trapping air. If you just recoat a half-dead float, you are sealing in your problem.

I have seen brass floats repaired by slowly heating them until the fuel starts to boil or evaporate out of the hole that is letting fuel into the float. After all the vapor is out, the float is allowed to cool slightly and resoldered. You must be careful. No open flame near the escaping vapor. An you must not heat the float any hotter that nessacary to force the liquid out or you will melt the original solder and the float will fall apart.

 
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06-05-06, 08:24 PM   #13  
Thanks JLO for the advice. Never thought about it but everything you said makes a lot on sense.

 
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