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Discussion Starter · #1 · (Edited)
Hello all, I am in the process of checking into a rough running (backfire from exhaust, no power at elevated rpm) John Deere 316. I did a compression test and got 65 on the left cylinder and 95 on the right. I also did a leakdown test on the same cylinder and definitely got some air leak from the exhaust and also oil coming out the fill tube. I pulled the engine this weekend and got the exhaust valve clearance checked and it was .015. Was def full of carbon so I pulled it and got it cleaned up. Brass brushed the seat and popped the valve back in. The issue now is that I can tell just visually the the valve isn't sitting totally flush on the seat. To test I shined a flashlight into the valve spring location and while I push the valve in tight I could still see light from around the seat.

Any help would be appreciated here.... Should I try lapping in or maybe check valve for flatness?

Thanks
Josh
 

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Roll the valve on a glass(very flat) table and look for wobble. Then try lapping. After a brief lapping, look to see if the valve seat is being polished in one spot (seat is the issue) or the valve is being polished in one spot (bent valve).

** make sure the valve is not sloppy in the guides **
 

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Discussion Starter · #5 ·
I couldn't really see any wobble while rolling it on a surface plate at work. I can see when i put the valve in the engine and rotate it with the flashlight shining in the spring area that the light escaping does change while I rotate which I am guessing means the valve stem maybe bent or the valve face is out of wack somehow.

I had to leave for the day but I will bring the valve back in tomorrow, get it in a V block and measure the runout on the edge of the valve face. That should definitively tell me its the issue. The valve seat in the engine looks pretty perfect from what I can see.....

SO if this does prove to be the valve itself, options are regrind at machine shop or replace the valve and lap it in right?

Thanks!!!

Josh
 

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Based on Bob's post, I don't know about the lapping part. It doesn't mention it, but it would seem to say that you grind the valve and the seat?
 

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You might check the valve clearance,it might too tight and not colding
 

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Discussion Starter · #8 ·
I ordered a new valve from onanparts...

We'll see how that seats. Biker, clearance can't be the issue...I have no springs in and am just holding the valve against the seat with finger pressure. I'll get a few pictures in a min...

Definitely need to decarbon while I have things open because this thing is pretty black!
 

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Discussion Starter · #9 ·
Strange, now that I look at it again I don't see the light gap changing when I rotate the valve. I looked at the seat pretty close and it looks pretty perfect on the face that the valve contacts.

2498034
 

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Dang,weird,lap it and see what happens
 

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Discussion Starter · #11 ·
Got a new valve in yesterday and it seems to fit better, but not perfect. I can still see a tiny bit of light escaping through the interface when pushing hard against the valve face. I'm thinking tonight I'll reinstall the spring and see how it looks.
 

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Before installing spring(s), go to auto parts store and get a tube of "Hi-Spot" bluing. Apply a thin coat to valve (and wipe most of it off!), install valve and rotate 1/8-1/4 turn back & forth 2 or 3 times, and remove valve. The bluing will transfer to the seat and you should see a complete light blue circle where valve seats. Any "openings" in circle means valve is not touching seat.

If you decide to lap, keep in mind that lapping compound will fall into seat area of block and may get washed into crank case when engine runs!!!! I don't know any way to prevent that! Bob
 

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Or just use "magic marker" to check the valve to seat fit.
Do not lap it, it sounds like the seats needs to be ground. (one section of the seat is either low or widened)
Once the seat is ground, a very light lap is Ok.
 

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Discussion Starter · #14 ·
This is what I got after some dykem on the seat surface. It's strange because the two places that kept the marker were just about on exact opposite sides of the valve.

I just gave it a pretty quick lap with the permatex and everything seems to be pretty good now. I may repeat the dykem check to be sure. Will post results...


2498778
2498779
 

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Discussion Starter · #15 ·
Got everything put back together, new points, Transdenser, new plugs. Compression now at 95 (was 75) and she purrs like a kitten.

Thanks for the help guys
 
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