Briggs 28Q777 OHV single burning oil

[lee]

I pulled the head and expected to see the head gasket blown between the cylinder and push rod area, but nope the gasket looks ok. I don't see any burned spots on the head gasket and no carbon trace on the head or block where combustion gases would be blowing through. Now what? The intake valve has oil all over it and in the valve guide area behind it. It looks like the oil came from the guide and valve stem seal but I'm not sure. It could have come through the breather and intake runner. The intake valve stem does seem like it has a good amount of wiggle in the guide. The stem seal is there and intact but not sure how well it's sealing. This burns a lot of oil on start up but then clears up, still burning oil but not clouds of smoke. I'm thinking if it was the head gasket it would just keep burning oil and not clear up right? The cylinder bore is shiny and smooth with just a few faint hone marks and no ridge at the top. I'm used to seeing some hone marks and I'm wondering if the lack of hone marks might indicate/cause excessive blowby? I would think if the cylinder was worn out I would be able to feel a ridge at the top but I don't. Also wondering if something like a leaky dipstick tube seal might allow air into the crankcase and not allow a vacuum to be formed? I see a lot of oil residue around the dipstick seal and I plan on replacing it anyway. Assuming just replacing the head gasket is not going to cure the oil burning problem what else should I check/replace while I still have the head off?

[Arkie]

You say:

The intake valve stem does seem like it has a good amount of wiggle in the guide.
The intake valve has oil all over it and in the valve guide area behind it.
Smokes more when first started.

Hints are worn intake valve guide.
Might put the head back on and do a leak down test to see if you are whipping on a dying horse. (or sub a donor head if readily available and make a test run)
I'm indicating to do the leak down so as to get idea if the piston/rings/cylinder is ok. (before thinking maybe a good head might reduce consumption) Oil in breather or smoke/oil out the crankcase vent tube would be hint of cylinder seal problems, but usually if excessive cylinder/ring wear you would see carbon buildup on top of piston and you indicated carbon not excessive on top of piston. Might see if Bubba may have not re-installed a intake valve seal at one time. Those OHV rockers/springs/etc are easily removed by finger pressure. Do not get the intake and ex valve springs and hardware mixed up.

Gas getting into the oil on those type engine/carbs will cause pre-mature wear to all. (and when engine starts using oil and little bit of gas is getting into the oil a casual check of the dipstick will not be a immediate hint of gas thinning the oil) and the amount of gas contamination into the crankcase is not as easily detected just by observing the oil level on the dipstick. (engine start wearing out faster)
I always install a manual gas shutoff on those type carbs/engines, especially if gravity feed. If owner does not turn off the manual valve when storing the eq, it's their fault not yours.
AND even if the engine/carb has the anti-backfire solenoid on the bottom of the carb, the pintle valve on this solenoid won't stop gas from seeping into the engine due to a intermittently leaking needle/seat. (the anti backfire solenoid will not prevent gas from getting into the crankcase due to a leaking needle/seat as some people believe)

I'm indicating especially this type of carb having intermittent, erratic needle/seat issues on the Briggs engines and intermittently leaking gas into the crankcase.
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[KE4AVB]

Not much experience OHV smoking as most time it has been bad head gaskets but they not always smoke but excessive oil use always was there. I had an engine few years ago that was burning a quart of oil per tank of gas that didn't even smoke; just could smell it.

A leak down test before tear down would have determine if the cylinder and rings where worn or if it was just a head gasket problem. But from you description it does there is quite a bit of wear but only measurement of the cylinder will tell if it worn out as it could be tapered and/or ovaled. It also just worn rings. Excessive blow by usually pushes oil out the breather into carburetor area. I normally see oil in the areas where the breather tube enters the carburetor.

The intake valve having a lot play may very well be the the source of the problem especially if the seal is harden since area is under vacuum during the intake stroke. This vacuum varies too as it would be greater at idle than at full throttle as carburetor restrict air flow at idle. I just had an horizontal OHV that would smoke excessively at idle but the smoking went away during use. On it the intake and guide were worn out. Since most OHV valve guides are not available separately only head replacement is possible. Didn't get a final solution though as the customer wasn't willing to fix the engine on the worn out tiller.

Now on L heads I have seen either one of valve guides to cause smoking. I got several walk behind mowers gave to me with customer thinking they were completely worn out just to find out the exhaust valve guides were shot. Those go/no go test gauges Briggs have in their valve guide guide repair kits didn't work for me as these engines pass that test. Just felt excessive play so I replace those guides and valves anyway.

[lee]

I took a piece of 3/8" dia plastic tubing and pushed it over the valve stem seal. Then I used a little adapter to connect the 3/8" tubing to the 1/4" line to my Mity Vac vacuum pump. To my surprise was able to pull a 10" Hg vacuum on the valve stem seal and it held. I tried pushing the intake valve in and out and wiggling it back and forth and the vacuum held perfectly. the needle never moved. I was thinking worn valve guide and hardened leaky valve stem seal. Now I don't know? Does the fact that the seal will hold vacuum mean it's good? I replaced the dipstick and leaky seal with a plastic fill plug. I'm thinking maybe I should just replace the head gasket and see what happens?

[38racing]

have you checked the breather for damage?

[lee]

re. the breather another piece of the puzzle is the area where breather hose exits inside the air filter assembly. The air filter looks pretty new and the whole area is dry, no oil. I've seen engines with a lot of blowby and the inside of the air filter housing, carb, and intake manifold are all wet with oil. All these areas are dry in this engine. Since I didn't see evidence of excessive blowby I didn't check the breather. So burning oil without excessive blowby,,,,and the oil is 30 wt and not over filled. If it were head gasket I'm thinking I would have seen evidence of blowby and the whole intake tract would be wet with oil. I know the intake valve guide has wear but the stem seal seems to be working and at least holds a vacuum. Now I'm thinking about how shiny and smooth that cylinder looks. There's no scoring but there is also hardly any visible cross hatch. It looks like it's been polished. Maybe the piston/cylinder is just worn and that's where the oil is coming from. I would think with it burning oil through the rings I would see some blowby as well but maybe not? Heck for all I know the PO changed the air filter and cleaned the housing, eliminating the blowby oil.

[KE4AVB]

From what I have been reading I agree with Arkie that you probably already found the problem and it is with the intake valve and guide.

If it was blow-by there should still some oil present in the intake at the breather tube.

[lee]

I pulled the breather and checked it today and it was fine. I remember when I pulled the head off there was oil in the bottom of the cylinder, on the intake valve and in the area behind the intake valve. I was thinking the oil must have come from the intake side, probably the valve guide/seal as I didn't notice any oil in the air filter housing to indicate blowby. Now I'm thinking the oil might have come from the piston rings as the mower was sitting outside on a slope with the front end down 3-4 inches lower than the back. I think the oil in the sump got past the rings and pooled in front of the piston. The intake valve being low on this engine is right at the bottom of the piston. Now thinking oil may have ran past the rings and through the open intake valve. Let me ask a question: A long time ago a mechanic told me that if you put a piston at TDC and take the palm of your hand and press on it and try to move it up, down, side to side etc., and if you can get it to rock noticeably in the bore then the piston/cylinder is worn out. Well I tried it on this one and I can rock it up and down. I can't rock it side to side as that's where the skirts are. What does this tell you? Time for a new piston/rings and hone the cylinder? The cylinder looks shiny with no scoring and no ridge. This is a well used engine made in 98 so I'm seeing wear everywhere I look. The intake valve guide is worn as well and as far as I know the only repair option is a new head right? Are replacement valve guides available? Even if they were all I can is lap the valves as I don't have the tools to resurface seats and valves. So I don't know if just installing a new guide and lapping the valve would work as I've never tried it. I'm not sure if this engine is worth repairing anyway. I might just put the head gasket on and run it. Let me know what your experience tells you about a piston you can rock in the bore as I described. Thanks to all that have responded! I'm 53 and still learning.

[KE4AVB]

Valve guides not available separately - Head around $150.00 /.020 over piston and rings around $80.00 / Engine gasket set around $50.00. And the honing and resizing of the cylinder you will need a ridge hone and NOT a flex hone. You will also an 1/2 low speed (500 rpm) max drill as you will only able to hone at about 300 rpm. You need calipers for measuring the cylinder diameter. Unless you're planning to a lot engine rebuilds it might be better just find a surplus engine in the 17.5hp (310000 series) to replace the current engine.

Plus you better mic the cylinder before even thinking of a rebuild or ordering parts as it might be already pass the .020 over and the .030 over parts are no longer available.

[Arkie]

KE4AVB wrote:

Plus you better mic the cylinder before even thinking of a rebuild or ordering parts.

Very good info.

Hindsight is 20/20. A leak down test before removing the head would have give a indicator if the bore needed analyzed with gauges to check for wear, taper, out of round, etc.
Worn valve guides even with a good valve seal can really cause a engine to use lots of oil and sometimes not even smoke very much. But not worth investing in a new head.

IMHO: If this is your own engine and you need the mower going just put it back together, stock up on 30W oil and couple of spark plugs and keep a eye open for a replacement engine. (if the mower chassis is worth even installing another engine)
What I'm indicating here is sometimes you can be looking at a motor and it being a 1998 model the mower may need new V belts, (at least 3) spindle bearings, blades, differential is howling and growling, tires bad, and on as vertical shaft engine if the pulley has not been removed completely from the shaft in awhile you may even have to ruin the pulley ($20-$40 for replacement pulley) so as to remove the engine, etc.
The smoking engine will keep the mosquitos repelled somewhat while mowing.

If its a customers tell them they need another engine. (if the mower itself is not worn out)

You can easily get more into rebuilding a worn out engine than cost of new engine and still come up with a bad engine.




And analyzing the cylinder area by rocking the piston by feel is not a true indicator of the cylinders condition. (most generally)