Intake valve stem seal is item # 868 in the IPL for the 28N700 series engine. Briggs part # for seal is 690698.
The valve stem seal only helps to control oil from being sucked past the valve stem/valve guide and is featured on the intake because the intake valve guide is subjected to engine induction vacuum whereas the exhaust valve is not.
It is not likely that a defective seal is causing the popping back through the carb.
You may just need to add some Seafoam fuel treatment to your fuel supply and run the engine in normal use for a while to see if the popping clears.
You might also add some Marvel Mystery Oil upper cylinder lube to the fuel.
The intake valve stem gets bathed with gasoline in the intake draft and this sometimes causes that valve to become sticky. MMO adds just enough lube to help offset the washing effect of the gasoline.
You might also want to confirm that the pushrod for the intake valve is made of aluminum and not steel.
The intake pushrod is aluminum but the exhaust pushrod is steel.
Blowing oil out usually indicates a defective head gasket but surely you put a new one on when you had the head off, right?

@mowie..thanks for the fast reply,this thing is driving me nuts, i have read so many posts in different forums that my eyes are crossed, and yes i replaced the head gasket, after cleaning the head and grinding the valves, i also went over the valve stems with a brass brush, and cleaned both mating surfaces with the brass brush before installing the head gasket, about the push rods,can you confirm that the exhaust valve is on top when looking at the engine from the front? if yes the rods are correct, i also checked them for warp and excessive wear, they look great,i have ran the motor for about 3 hrs now on and off over the past 2 weeks, and i did add marvel myst oil in the beginning to the fuel, its a good idea any time a motor has sat for a while, this is why i am stumped, i have worked on small engines for many years and usually have no problem fixing them but this beast is illusive,i think i will pull the head again and give it another good inspection, im really out of options at this point, in my experience it all points to the intake valve, but it appears to be fine, and the lash is staying set at 004, the guides seem good too , no excess play in them, here is a list of things i have done, cleaned fly wheel, replaced key way, torqued fly wheel to 100 foot lbs, gaped coil at 010 and 012 tried both, cleaned gas tank, new fuel line, new filter and clamps, cleaned carb twice,used cleaner and a guitar string (old trick i learned from a old mechanic years ago)removed the head, cleaned piston,was caked with carbon, cleaned head,and valves, and lightly lapped the valves, installed new head gasket and torqued it to spec, then adjusted valve lash to, exhaust=006 and intake=004, i am a true believer in cleaning surfaces that require gaskets, and a small brass brush on aluminum is just the thing, it will clean to a polish finish without harming the aluminum surface, also cleaned all intake surfaces the same way, any more ideas would be great, you seem to be a guru on these engines and i am no pro by any means,so think hard, and thanks for your time and help

Yes, exhaust valve is the top valve.
Tell us, does the engine exhibit the popping as soon as you get the engine going from a cold start up, or does it require running at least a few minutes before the popping begins?
When you had the head off and the valves out......did you check the fit of the valves to their respective valve guides.....for "lateral play" or "side ways" play?
This is done after removing the valves and cleaning both the valve stems and the valve guide bores.
The valve is then inserted back into the valve guide and held with the fingers so that the valve is just barely open (valve face is not touching valve seat).
With the valve in this position, rock the valve stem sideways (perpendicular to stem length of the valve) using your finger tips.
Generally speaking, if you can detect any motion (play) in the valve in its guide, the wear may be excessive.
Unless you take the head to a Briggs shop that has the proper "plug type measuring pin gauge" to determine if the wear is excessive, it would be necessary to set up a dial indicator to tell if the wear is more than Briggs specification. The Briggs wear limit for this dimension is a mere .002".
Briggs does not offer replacement guides for this engine.
Head replacement is their offer.
How wear in this area might cause a pop back would be that the intake valve may not be seating squarely each and every time it closes and on those intermittent instances, compression/combustion may be slipping between the valve and the seat.
The fact that the combustion chamber was heavily carboned may also bear out wear in the guide. Was the intake port of the head where the intake valve stem emerges caked up with oily carbon? Sometimes the build up there may be a single sort of "ridge hump" of build up on the lee side of the valve stem where the incoming draft wash gets blocked by the valve stem itself.

One point to add would be to verify and compare valve lift as some of these did have soft camshaft lobes. As they wear the engine seems to feel weak and pop. A worn intake lobe is normally noticeable due to the excessive valve lash, but not so much on the exhaust. These will also loose the valve seat on occasion. Hopefully the attention to detail that you put into the head work, it would have been noticed.

@mownie, i have removed the head again, the valve guide seal is there, in good shape, i have super cleaned it, the seats look good, im gonna give them a good grinding this time, i just lapped them lightly the first time, the push rods were in the correct position,is there a difference in the valve springs? is one stronger than the other?, i inspected the new head gasket that i had installed, there was no sign of it being blown,gonna put head back on tomorrow and see if its better, oh ya the valve guide did have a slight amount of play, but i dont think its enough to matter, i noticed that the guide is brass, so in theory one could have it knurled to tighten it up, and not have to buy a new one, i have knurled valve guides on car heads and it works well, also i have another question, today i found and bought 3 riding mowers, for $50 couldnt pass it up, one has a engine like mine, it slung a rod, but didnt bust the block, any way can you tell me if the head will work on mine? i think they are the same, it is a B&S 15 hp commercial diamond plus, mod#=28n707...type=0026-01...code=9503134A, it looks like the same animal, also got a yard machine 13hp tecumseh 38'' cut 7 speed, it runs and cuts but the belt jumps off when you reverse, should be a easy fix, also got an old rusty sears from the 70's from the look of it and another 12hp b&s motor and a spare front tire and wheel and a rear transaxle with good tires, i could not believe it when the guy said $50, oh ya will the cam from above b&s work in my motor as well? just in case i need to change it

@tomplum, the seats look great, nice and tight, also the valve lash stays the same,it dont loosen or tighten any, i check it often in thinking it was ''playing'' due to wear, and the engine is strong, its a real head scratcher, should the valve lift be about the same for intake and exhaust? could i measure the push rod travel to determine if the cam is worn?

Van, valve springs are the same number for intake and exhaust.
If one is weaker than the other, that could be part of the problem, but how are you determining the "power" of the spring?
You can use push rod travel to evaluate cam lobe lift, but Briggs does not provide a specification for actual lift. Briggs only provides that the cam lobes standard dimension is 1.240" (31.49mm) and the reject limit point is 1.234" (31.34mm).
Of course you can still set up a dial indicator to check travel of each push rod/rocker arm and determine whether both lobes are providing equal lift. If one has greater lift than the other that would signal a problem in itself.
But it is possible that the wear on each lobe might be near equal and the cam lobes still be smaller than the reject dimension.
Only way to know for sure would be to remove and "mike" the cam lobes.

@mownie, i was just wondering about the springs, i thought they might have got mixed up the first time i ground the valves, but i dont guess it would matter, you didnt say if the new engine i bought would would be ok for a parts doner

Well now that mownie has written two books (inside joke) I just had an engine doing the very same thing and changed the carb and it was fine.

Yes, all the parts from your two 28 series engines will interchange, including the carb. Very much doubt valve spring problem. 28 series OHV was not generally prone to camshaft failure.

Walt Conner

@walt2002, hey i was wondering if you were still here, i have read many of your replies and was hoping to hear from you and mownie, you 2 seem to have a wealth of knowledge on these things, i will try the carb first, because it is the easier than the head, but how would the carb cause fire to pop in the carb? im no genius but it seems compression is coming through the carb, and with the few parts in the carb, even no diaphragm, i cant under stand how it would be bad, do they have a ball and spring behind a welch plug like the old model walbro carbs?, i remember years ago when i worked on them a rule of thumb was if you shook them and heard no rattle they were no good, it had something to do with a barometric pressure adjuster valve, you didnt have to adjust the carb in higher locations as you did a b&s carb, just wondering, because i cleaned the carb twice, i put it in a bowl of water while disassembled and used a small straw to blow air through every hole, got bubbles on all, but i will swap them anyway and see, i just want to know what it is, thanks for your advice man

"but how would the carb cause fire to pop in the carb?"

Just reporting my experience from two days ago. (Lean mixture)

Mownie is the expert. I have a short attention span and tend to go to sleep after the first 50 lines. Cleaning the modern day carbs is a problem. I have some that have been thru ultrasonic cleaner and still have to hold my finger over half the carb throat to get it to run right. Probe all orifices with tag wire.

Walt Conner

ditto walt/mownie's carb cleaning advice/comments. To help find those SMALL holes, a good magnifier should help. Compressed air and rattlecan carb cleaner will also help along with an overnite presoak to loosen things up. Like walt noted, use small dia coppper tag wire to open up small carb holes/orfices.

I tend to think that when these carbs create a popping like condition, it comes back to something that creates an air leak. On these, bowl/ seal or worn throttle shaft is my first guess. Seems different to me than a backfire from a valve or timing issue when you hear it though.

" im no genius but it seems compression is coming through the carb,"

Just as a point of info, MOST B&S engines relieve compression thru the INTAKE valve when starting thus you are going to get a little blow back, panting, thru the carb at cranking speeds. The exception is the Japanese built Vanguard V Twins which do relieve compression thru the Exhaust Valve.

Walt Conner

hey could one of you fine fellows please send me or tell me where to find a diagram for the torque sequence of the head bolts on my motor, and the torque values in ft lbs, thanks, well i got the other carb cleaned out, wasnt very dirty, and got head set on engine,then the rain started from the tropical storm, we have tornado warnings for tomorrow (mon) we dont need any more of them, our area got hit hard in the spring, also i noticed the aluminum push rod (intake) has a notch about a inch long in it where it passes through the head, is this normal? or supposed to be that way? i re ground the valves, they are sealing super good now, i cleaned the valve stems good too, please send or tell me the answers to above questions so i can continue, thanks guys

For detailed instructions on your valves, head bolt sequence, pushrod and more, email address below, put in proper format and remind me.

Walt Conner
wconner5 at frontier dot com

I had an intermittently firing ignition module on my 16 HP Vanguard that caused this behavior. It made sense once I realized small engines fire twice per cycle (the piston is top dead center (TDC) at the start of both the power stroke & intake stroke; and the magnet passes over the ignition module about TDC). The intake stroke is normally a "dead" fire as there's no fuel. However, if the spark plug doesn't fire on the power stroke, fuel will be dumped into the exhaust on the exhaust stroke (causing a backfire through the exhaust and/or black smoke). Since not all fuel will be pushed out on the exhaust stroke, any remaining fuel can combust at the start of the intake stroke (next time the magnet passes over the ignition module). The intake valve opens shortly after and the burnt fuel pushes out the carb. If the burn is long enough you could see fire in the carb. I only figured this out after I replaced the carb, reset the valve lash, and was just short of rebuilding the engine.