Runs rough, no power under load...

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  #1  
Old 12-18-10, 08:43 PM
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Runs rough, no power under load...

I have a 9hp Briggs Vanguard that was leaking fuel if the fuel wasn't shut off. It filled the crankcase with fuel, but I didn't realize it at first and I ran it a few minutes before I realized what the problem was. I took the carb apart and cleaned it to try to fix that problem. The needle looked OK and I cleaned the seat. After I put it back together, it doesn't run right.

Under no load, it runs rough, and it seems like it is running rich because it kills as soon as I put the choke on. However, when I put it under even a small load, it kills. Or if I put the choke on under load it'll stay running, but still run rough. So it seems rich under no load, but kills when I put a small load on it like engaging the PTO or drive wheels. (This engine is on a BCS 2-wheel tractor with a snowblower attachment currently.)

So it seems like either (1) I didn't get it put back together right somehow, or (2) I damaged it while running it with fuel in the crank case. Any idea what might be wrong? I am going to borrow a friends compression tester...what kind of compression should I have? Do I maybe have an air leak around a carbuerator gasket? How tight do the nuts need to be to hold the carb on? I tightened them quite snug using a 1/4" rachet.

What should I try next?
 
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Old 12-19-10, 06:20 AM
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First make sure you have the oil changed and at the proper level.

The problem is at the float valve/seat. It simply isn't doing its job to regulate the fuel level in the carb bowl. Often the rubber tipped valves play out with the neoprene tip aging and distorting to the point it can't properly seat although it may look fine. The all brass valves, just wear to the point of not sealing. The brass ones can be reseated, but it's better to just replace them.

The best thing to do, since we're into winter and you'll likely be using the machine a lot, would be to replace the float, valve and seat, if the latter is removable. If you have a permanent seat it can be touched up with a cue tip and a little Comet cleaning powder or something similar.

With the float and valve in place, invert the carb and set the float parallel to the carburetor casting.

When you mount the carb, tighten the mounting nuts/bolts to wrist tight = thumb on center of the ratchet and twisting the wrist. That will work in place of a torque wrench. Never tighten one side then the other, but do one side about 1/3 tight, over to the other side, then back and forth. This is much the same way you would torque an engine head or intake manifold on a car.

When you get it all stuck together, use 1 1/2 turns out from lightly seated on the mixture screws as the base for tuning the engine. Adjust from there. If all is well you won't have to go much to have it nailed down.
 
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Old 12-19-10, 02:10 PM
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marbobj

Thanks for the advice. I've got the oil changed and filled to proper level. I assumed the needle and seat were the problem originally with the flooding. However, would a worn needle and seat be causing the rough running problems I'm having now?

Thanks.
 
  #4  
Old 12-19-10, 02:36 PM
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It certainly would cause problems. Worn needle & seat will not properly seal off gas from coming into the carb and can cause the engine to run rich (too much gas..kinda like with the choke on). I would also replace the spark plug.
 
  #5  
Old 12-19-10, 03:02 PM
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Any ideas on what the compression should me on this engine?
 
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Old 12-19-10, 03:22 PM
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80 psi or above should run ok, 120 would be good. Even though there is no real spec that I am aware of. Small engines are difficult to get an accurate reading on. If you can feel resistance in the flywheel when turning it by hand, IE: turn it until you feel resistance then look for it to spin back when you release it, that is what the old briggs manuals state anyway.
 
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Old 12-19-10, 07:05 PM
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You can not and will not get an accurate compression reading using the compression gage since the engine has a compression release for easier starting.
 
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Old 12-19-10, 07:16 PM
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Originally Posted by indypower View Post
You can not and will not get an accurate compression reading using the compression gage since the engine has a compression release for easier starting.
That was basically my point, however, if you test with a gauge and get a reading of 100psi, I would say compression tests good
 
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Old 12-20-10, 08:39 AM
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OK, thanks for the insight on the compression. I didn't realize that it had a compression release. It pulls as hard as it ever has, so I'm guessing that isn't the issue.

So I suppose that leads me back to an air/fuel issue? Should I start by replacing the needle valve, or something else? The float is plastic, so I don't think I need to replace that.

Thanks.
 
  #10  
Old 12-21-10, 08:01 PM
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Unless the parts store is just across the street you're better off replacing the float, needle, and seat (if removable). Then set the mixture screws and see what ya got.
 
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