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can bad valves do this?


rbig's Avatar
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09-16-06, 02:09 PM   #1  
can bad valves do this?

Is there any way that an intake or exhaust valve on a twin cyl flathead which isn't seating properly can cause crankcase pressure?

 
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09-17-06, 01:15 AM   #2  
Not realistically. What's the problem you're having?


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09-17-06, 07:33 AM   #3  
Posted By: cheese Not realistically. What's the problem you're having?
Got healthy crankcase pressure, and trying to determine the source. Engine runs strong, even, and good. Some steady white smoke out the exhaust. Strongly indicative of ring blowby.

Problem I'm having is resolving the tests run by an engine shop, and what I'm experiencing.

Shop ran compression test, and it shows 120lb each cyl. That's good as new. Leakdown test showed no leakage.

No leakage on leakdown test tells me rings should be be good, valves are seating/sealing, and there are no air paths out of the combusion chambers through blown seals.

Yet, I've got strong crankcase pressure, and noticeably oil burning.

Just trying to consider all possibilities so I'll know what to tackle to fix the problem. I'm thinking more and more that the leakdown test wasn't done, or done properly, and I'm seeing ring blowby.

 
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09-17-06, 09:30 AM   #4  
bad valve stem seals and possible valve guides.

wouldn't show up on a comp test or leak down but could casue both of your problems.

 
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09-17-06, 12:58 PM   #5  
Posted By: nap bad valve stem seals and possible valve guides.

wouldn't show up on a comp test or leak down but could casue both of your problems.
Correct me if I'm wrong, but don't think my old flathead has valve stem seals. Believe those are in OHVs.

I'll have to check out the valve guides. My horizontal opposed cyl configuration would tend to make it easier to get oil out through the guides.

 
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09-17-06, 01:18 PM   #6  
Very few have seals, Some (older) tecumseh intake valves have seals, no Briggs.

 
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09-17-06, 01:27 PM   #7  
The most common valve problems usaully end up being "loss of Clearance".
Other problems can be, "burnt valve seat and face" Carbon build-up around the valves can prevent proper seating.

Some years back it was not uncommon to find a "loose valve seat" on a Biggs and Stratton.

 
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09-17-06, 01:36 PM   #8  
Posted By: mikesmalleng The most common valve problems usaully end up being "loss of Clearance".
Other problems can be, "burnt valve seat and face" Carbon build-up around the valves can prevent proper seating.

Some years back it was not uncommon to find a "loose valve seat" on a Biggs and Stratton.
Are valve seats and guides the same things?

 
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09-17-06, 01:46 PM   #9  
Posted By: rbig Correct me if I'm wrong, but don't think my old flathead has valve stem seals. Believe those are in OHVs.

I'll have to check out the valve guides. My horizontal opposed cyl configuration would tend to make it easier to get oil out through the guides.
The valve stem would have to seal in some manner, even if it is only the clearance of the stem/guide being minimized. If the guide is worn, it will allow pressure to be passed into the crankcase and it can allow oil to be brought into the passageway the stem passes through.

seats and guides are not the same thing. The seat is what the valve seals against and the guide is the tube the valve stem passes through to keep it straight.

 
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09-17-06, 02:16 PM   #10  
No, Valve seats are what the valve face contact when closed.
Unless you`ve never changed to oil, I would`nt worry about the valve guides.

Check your valve clearace, Briggs twin Cyl, Int. .06 Exh. 0.10

 
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09-17-06, 03:30 PM   #11  
Posted By: mikesmalleng No, Valve seats are what the valve face contact when closed.
Unless you`ve never changed to oil, I would`nt worry about the valve guides.

Check your valve clearace, Briggs twin Cyl, Int. .06 Exh. 0.10
and this explains the pressure in the crankcase and burning oil how??

 
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09-17-06, 03:59 PM   #12  
I don`t understand what your trying to say about crank case pressure and burning oil.

You`ve never changed the oil?

 
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09-17-06, 04:58 PM   #13  
The OP has pressure in the crankcase that he suspects is from blow by of the rings and the engine is burning some oil.

Based upon the test results it would seem the rings and valves are all sealing well. The only other avenue I can think of that would allow what the OP is experiencing is worn valve guides.

Do you have any ideas as to a cause mikesmalleng?

 
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09-18-06, 03:09 AM   #14  
rbig, your excessive crankcase pressure appears to be from fuel dumping into the crankcase due to a carburetor in need of reconditioning. The white smoke is indicitive of this. Black smoke = rich fuel condition, blue smoke = oil in the combustion chamber, white smoke = fuel diluted oil in the combustion chamber. Smell a sample of the oil for any gasoline indication. Have you noticed the oil sump overfilled from what you had in the crankcase? If so, it's likely gasoline. Be sure to rebuild the carb using a genuine Briggs rebuild kit and any necessary mounting gaskets. Be sure to change the oil before running the engine any longer as it can do some serious damage to the engine.

 
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09-18-06, 06:42 PM   #15  
How are you checking the pressure in the crankcase?? This is a closed system & if you run it with the dipstick out, you'll have oil everywhere. If you cover the dipstick opening with you're hand & feel a vacuum, the P.C.V system is O.K. Try this. May be a blocked P.C.V. valve?? Roger

 
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09-18-06, 06:49 PM   #16  
Posted By: hopkinsr2 How are you checking the pressure in the crankcase?? This is a closed system & if you run it with the dipstick out, you'll have oil everywhere. If you cover the dipstick opening with you're hand & feel a vacuum, the P.C.V system is O.K. Try this. May be a blocked P.C.V. valve?? Roger
The crankcase vent has been re-routed to atmosphere. Blocked the carb entry port where the crankcase vent line was previously. Got a pretty good air flow coming out the vent tube opening, even at idle. Sure looks like ring blowby to me. But, I wanted to explore all possibilities.

 
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09-18-06, 06:59 PM   #17  
I would be very hesitant to blame the valve guides, which is the reason I replied "not realistically". The guides seldom wear out on these engines. Chances are you have ring blowby, or fuel contamination of the oil as Puey61 suggested.


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