OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Briggs & Stratton Engines Advice on smoky Briggs & Stratton 11.5hp

Hi All,

Recently purchased another older Greenfield for mowing the "rough" parts around our place. I am using it as a bit of a project to teach my teenage sons a bit about engines, mechanics etc. It is also the mower they will use to help me mow.

The 11.5hp engine has the following ID numbers:
Model: 28D707
Type: 0134-02
Code: 951213ZA

The engine uses a bit of oil and blows some blue smoke when under heavy load. The smoke is not obvious when under light to medium load. Engine starts and runs well, both hot and cold, - just a slight "pop" from the exhaust every now and then, accompanied by a "chuff" of blue smoke. Seems to still develop enough power to cut 8ft high grass!

I am asking for advice on what might be a reasonable, cost effective strategy on the repairs when I remove the engine. I am thinking maybe new piston rings, a quick hone of the bore (by hand with sandpaper), and lap the valves. I am assuming the internals are still in fairly good condition, given the engine runs well - obviously I haven't had it apart yet so I don't know for sure.

Any advice will be gratefully received.

Regards,

Andy

Andy, some pictures would help us with model indentification. This is partly because the Briggs site is currently not recognising that model number, which happens a lot.

If the engine blows blue smoke under load it is likely the rings are worn, but you can easily check this by feeling how much compression there is when you pull the starter, then putting a teaspoonful of oil in through the spark plug hole and pulling the starter again. If there is noticeably more compression with the oil in the cylinder, the compression ring is leaking. On Briggs engines, the compression ring is usually worn by the time the oil ring is. (Note this does not work with OHV Honda engines, which have chromed compression rings that easily outlast the oil rings.)

One of the reasons we need to see pictures of your engine is to try to establish whether it has a cast iron cylinder liner. It is best not to hone aluminium bores to remove glazing.

Great point with the honing there grumpy, one of my manuals on the shelf (one shall blow the dust off them one day) specifically says do not hone and aluminum bore but to just replace the rings and leave it alone. ovbiously if there is major scoring and marking a rebore is requited, but at worst a new set of rings would slow the smoke down

Hi Grumpy and Joe,

Thanks for the advice so far. Below is a photo of the engine.



Here is a photo of the mower:



Regards,

Andy

Thanks Andy. Your engine is marked "I/C", which means Industrial/Commercial. I/C engines have cast iron cylinder liners, so it is likely to last a long time, and it will be feasible to hone the cylinder to remove the glazing. However let's discuss that when you get to that part of the process: cheap hones often do more harm than good, and there are better options. Please report what you find when you try the spoonful-of-oil-in-the-cylinder trick.

Hi Grumpy,

Hopefully I will get some time this weekend to have a fiddle with it. As far as checking the compression, adding some oil, and checking the compression again, is it better to do this procedure with the engine hot or cold?

Regards,
Andy

It should work either hot or cold, Andy. I usually do it cold simply because I like to do bench work on cold engines. I only start them when they are on the floor, and because I only have Hondas these days and they are very heavy, I try to move mowers between bench and floor as infrequently as possible. So, all the bench tests happen in sequence, then all the floor tests (which are performed with the engine running or having just run) happen afterward.

Thanks Grumpy. I will probably do it cold then.

Andy

Hi Grumpy,

Checked the compression today. It is much higher when the oil is added to the cylinder.

Regards,
Andy

Combined with the oil consumption and blue exhaust smoke, that means it is very likely that the rings are worn. Adding in the occasional exhaust pop, it would be appropriate to lap the valves as well - however you'll probably be able to confirm that when you start work on the engine: leaky valves usually can be identified by blackening of the seats. The exhaust valve will be the chief suspect in this case.

The only other internal operations that are indicated so far, are very lightly honing the bore to remove the glaze, and adjusting the tappet clearance. Note that tappet clearance can only be adjusted on your engine while the cylinder head is removed. Incidentally I tend to do deglazing of already-smooth bores with a sheet of wet-and-dry paper of 220 to 400 grit, held in my hand and moved in circles of about 2" diameter. A really good professional-grade hone is better, but cheap hones are usually a great deal worse, especially if you do not flood them with kerosene continuously, and limit the spindle speed to 200-400 rpm whilst pumping the hone up and down the bore rapidly.

Of course your engine deserves to have its fuel system cleaned out, its air cleaner serviced, spark plug cleaned and gapped, and the rest of the usual annual service operations carried out at the same time. It will then need new oil, which should be changed after 2 hours or so of running, due to the fine swarf that is carved off and released into the sump by an overhauled engine.

I suggest you do this job a step at the time, posting pictures and updates on what you are doing and planning to do, at each stage. We'll be glad to help where we can. It is useful in a first overhaul to have someone to offer comments or reassurance as you go. Remember, never make a move when you are uncertain: ask first. But don't ask me how I learned that.

Hi Grumpy,

Thanks for all of your information so far. As far as adjusting the tappet clearance, how is it actually adjusted?

Regards,

Andy

You remove the valve chest cover (held with 2 screws, below the muffler) so you can see the gap between the tappets (cam followers) and the ends of the valve stems. Turn the engine to a little past top dead center on the compression stroke (if you are a purist, turn it until the piston has moved 1/4" down the cylinder), then insert feeler gauges into the gap between the tappets and valve stems. The thickest gauge you should be able to fit in without moving the valve should be 0.005-0.007" for the inlet, and 0.009-0.011" for exhaust. Obviously you do this after you lap the valves, not before, because the clearance will decrease due to lapping. If the clearance is less than the minimum specified, you remove the valve concerned and grind the flat on the end of its stem to achieve the middle of the specified range. It is important to end up with the end of the stem square to the cylindrical part of the stem.

Note that there is a good chance your exhaust valve is not seating at present, so until that is rectified you might get an artificially inflated number for the clearance.

Hi Grumpy,

Thanks, once again, for your speedy reply. Fingers crossed that the clearances are within the specified range once the valves are lapped. It sounds like the valve stem grinding procedure requires specialised, expensive equipment.

Andy

Yes Andy, the stem-shortening job should be done with a jig to hold the valve square to the grinding wheel. I think most serious mechanics do this properly with the right jig, rather than doing it by eye. However I think most people working at home on old engines to keep them going, may sometimes do it by eye. Remember you can check the squareness of the stem-end every few seconds of grinding just by dropping the valve back into the valve guide (part of the engine) and inspecting how it sits against the tappet. The engine is a very accurate squareness jig.