Sounds like a burnt or sticking exaust valve, should never feel any back pressure in the intake side. As far a valve lift goes, no idea unless you have the cam lift notes someplace. How are you observing the valve lift, head off or just looking at the rocker or pushrod? Could be carbon keeping the exaust from closing 100% so it would seem to lift less if it never closes.

Oops that should have been 'sticking intake valve'
Having a duhh moment!

I'll check that, your explaination makes sense.

I've been told that the headgasket has been replaced recently trying to solve this***are the valves clearly visible in the head when it's off the block? I'm wondering if they would likely notice that.

Is it somehow possible the cam and crank could have gotten out of sync/time?

Kevin,

Not likley that the cam is out of synch unless someone was deep into the engine. Have not looked up the engine, if it's a OHC motor it could have skipped a notch on a timing belt or just a worn timing chain. Can you post the engine parts/service manual so I can give a better answer without having to look it up?

Armed with the motor diagram I can offer more suggestions but without seeing the valve train it's just a guess, like I said there should be no back flow into the intake so something has gone wrong, either a stuck/burnt intake or a valve timing issue.

I only have the owners manual. There is a parts explosion in the back, but it's hard to read in real life, let alone take a pic.

I can tell you it looks like its a gear drive. It is an OHV. I have also noticed it seems to have good compression that doesn't leak off very quickly at all (when turning it over by hand). I haven't done an official compression/leakdown yet, I didn't feel like it was necessary (unless you think I should)...?

Well a quick check may be putting compressed air into the sparkplug hole('s) mid cycle and see if it's leaking through the valves to the carb or exaust. If it;s a twin cyl, it could have a warped head/block and be blowing from cylinder to cylinder even after a new head gasket.

Just some thoughts, and man I feel you pain being third one to get saddled with that job, I usually decline anything 'someone else' has tried to fix as you never know what they may have messed up or mis-assembled.

Sorry, I should have told you...it's a 1 cylinder.

I'll do a leakdown tomorrow and let you know if there's any major discoveries.

No problem, if you can locate the on-line manual post a link, still sounds like a leaky/sticking intake valve to me, if it's a gear drive to the OHV train it would have to be well worn to be that far out of synch. I think the pressure test will tell the story, if you get it to mid stoke and it's leaking out the intake it has to be a bad or sticking intake valve. Well one more thing is possible, could the key in the magneto have gotten tweeked? That motor probably fires a spark every stroke so if it was firing early before the intake closed it could pop, may be time to pull the flywheel/mag and take a look.

i.e, one bad backfire along the way could have half sheared the mag key and the timing could be too far advanced causing all the problems.

I did pull the crank bolt and can see the keyway (having not removed the flywheel) and everything looks lined up. Do I need to remove the flywheel to get the full story??

Sounds like a worn cam to me. They seem to show up on occasion on these.

kevin, send me an e-mail and I will send you some lit in PDF.

I pulled the cam out and found the exhaust lobe is almost completely worn away to (almost no lift at all). Now I'm wondering...Should I try to replace the cam, clean out the engine, put it back together and hope there's no bearing damage anywhere else (from all the metal shavings in the oil), or just tell this person they need a motor?

One more question: When I was looking over the cam, I noticed what looks like some kind of centrifical (looking) weight that turns a pin that is half round, half flat. This pin is connected to the weight on one side and the other side fits into a knotch on the intake lobe. The weight is lightly sprung to be normally in and makes the round part of the pin stick up from the cam lobe, making the intake valve jump (I think this is what was causing the popping through the carb) about halfway down on the lobe. Does anyone have any idea what I'm describing and what is it for/how does it really work?

It will all come as an assy. The dealie of which you speak is the compression release. Clean it up, put it together w/ a new cam, gasket and tappets- set the valve lash properly and it will run like top. And oil. Oil would be good...

Tomplum,
I'm really impressed that you could tell what I was talking about, really!
Why is that dealie there and how does it work? Is it possible that it was sticking and causing the popping through the carb or is it more likely that was the excess exhaust gas that couldnt get out due to the flat cam???

Your popping was likley caused from the worn lobe- too much in and not enough out. The little metal release slightly holds the intake valve open to release the compression at cranking speeds.

For buzzard flats -- Quoting from above: "As far as valve lift goes, no idea unless you have the cam lift notes someplace." Is that a table you ordinarily find in the specs for a particular engine? I have been looking for that type of information.

I have not ever found where Briggs actually gives a "specification or numerical value for lift".
Briggs instead prefers to measure the camshaft lobes and compare the dimension to the specs on the "reject chart".
You can, however, measure the lift of both lobes with a dial indicator and compare the 2 readings. Since both lobes have the same dimensions given on the reject chart, any disparity would indicate a worn out camshaft.

{{gwi:322870}}

Just so I'm on record...I did replace the cam, cleaned out the engine and put it back together.
I've been told at every family gathering (it's my wife's grandma's lawnmower), that the tractor runs perfectly.
Thanks again for all the help, you've made me look like a hero.

Another Happy Ending ! Thanks Kevin for updating the Forum Experts , They do a Great Job within Assisting us All .:)