Briggs v-twin misfiring and puking oil.

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stretch5881

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• Mar 17, 2018 / Briggs v-twin misfiring and puking oil.

• #1

Howdy all,
I was given this Scott's tractor a year ago with a scattered engine. I've collected the parts and did a full rebuild. I am also experienced as a small engine mechanic.
B&S 20hp V-twin. 406777. It starts right up. The rpms bounce around a bit. Random misfires through the intake at all rpms. After it warms up, it pukes oil from the vent into the carburetor. I've been working on this issue, off and on, since last fall.
Parts besides a full rebuild are a new vent and an aluminum intake manifold.
It's not broke in yet and compression dry is 165# on both cylinders. Wet test is 175# on both. Leakdown test is 100# in, cylinder #1 holds 95#, cylinder #2 holds 97#. I don't hear any leakage past the valves.
I have disconnected the grounds from the coils, nothing changes.

Any suggestions?

 

Boudreaux In Eunice La.

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• Mar 17, 2018 / Briggs v-twin misfiring and puking oil.

• #2

Dam nobody get's those high readings on a gauge..... What kind of gauge you have ?? Mostly in the low 100 or 90 lbs. Because of the compression release.

Did you put new head gaskets on both cylinders.....

 

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ILENGINE

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• Mar 17, 2018 / Briggs v-twin misfiring and puking oil.

• #3

The Vanguard V twins can be in the 160 range, but the intek should be in the 70-100 psi range, so something is going goofy there.

 

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stretch5881

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• Mar 17, 2018 / Briggs v-twin misfiring and puking oil.

• #4

Boudreaux In Eunice La. said:

Dam nobody get's those high readings on a gauge..... What kind of gauge you have ?? Mostly in the low 100 or 90 lbs. Because of the compression release.

Did you put new head gaskets on both cylinders.....

Click to expand...

All new gaskets and seals, pistons and rings.
OTC compression tester.

 

Boudreaux In Eunice La.

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• Mar 17, 2018 / Briggs v-twin misfiring and puking oil.

• #5

Well you got all new parts... Is the gov set right ?? Worth a shot on hammering part.....

 

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stretch5881

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• Mar 17, 2018 / Briggs v-twin misfiring and puking oil.

• #6

Boudreaux In Eunice La. said:

Well you got all new parts... Is the gov set right ?? Worth a shot on hammering part.....

Click to expand...

Yup, set the gov 3 separate times. Runs good for a minute, then will lose some rpm, misfires a couple of times, then runs good for 30 seconds, then misfires again and so on.......
After about 5 minutes of this, oil starts gushing out of the vent hose.

 

Boudreaux In Eunice La.

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• Mar 17, 2018 / Briggs v-twin misfiring and puking oil.

• #7

Something is not letting it vent some where.... Bert and IL engine should chime in soon ~!~!

 

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stretch5881

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• Mar 19, 2018 / Briggs v-twin misfiring and puking oil.

• #8

Interesting, compression is too high. It turns over and starts without a problem. I can see the comp release working with the valve covers off. I've had the engine apart a three times trying to figure out what I did wrong. Timing gears are aligned correctly. Muffler is clear. The valve lash I had set at TDC according to my manual (which was wrong). I have reset them 1/4 past now. What else could cause the high compression?
I am 99% sure the misfire is not ignition. While the muffler was off, I had the coil grounds disconnected, and the misfire is from both cylinders and intermittent.
I wish all my chainsaws had compression like this.

 

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ILENGINE

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• Mar 19, 2018 / Briggs v-twin misfiring and puking oil.

• #9

There could be a chance that when cranking without the spark plugs and with a compression gauge in one cylinder the engine could potentially crank fast enough to deactivate the compression release, and then you could see those high readings. The misfire could be potentially from a fuel quality issue, something not quite right with the carb, a small air leak causing a slight lean run would cause the misfire. the oil issue sounds like a blowby issue. Possibly head gasket leaking into the pushrod cavity only when hot. A cylinder that is off round that is allowing combustion gases to push oil out the breather. You could have good compression with a gauge and get good leak down results at TDC but have an off round cylinder that is not sealing about half way down the stroke.

 

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stretch5881

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• Mar 30, 2018 / Briggs v-twin misfiring and puking oil.

• #10

I tore it down again. I borrowed a set of snap gauges and checked the cylinders. Both cylinders within spec. Put it back together. I switched carburetors with a good running engine, and the misfire is gone. The original carb had a slightly worn throttle shaft. After it warms up it still pukes oil. I checked the temp of the cylinders for the heck of it. Both checked in the same spot inside of the v. #1 cylinder was 94 degrees, #2 cylinder was 235 degrees. Exhaust tubes- #1 was 214 degrees, #2 error. My tester goes into error at over 600 degrees.