12.5 HP Flat Head Valve Timing

[Ihategrass]

Hi Everybody,
My engine is a vertical shaft 12.5 hp side valve. Model 28T707, Type 1180-E1, Code 020211ZD. It will not run and backfires through the carb. Can anybody tell me what the what the valve timing should be?

I don't mean aligning the dots on the crank and cam gears. I mean the actual crank angle degree values before and after TDC & BDC for the opening and closing of the inlet and exhaust valves.

With a degree wheel on the fly wheel and a DTI on the valve faces, my engine measures IVO 28 BTDC, 53 ABDC. EVO 68 BBDC, EVC 15 ATDC.

The valves have been lapped and pressure tested and the clearances set to the maximum specs I can find of 7 thou on the inlet and 11 thou on the exhaust.

For an 'industrial' engine these figures just don't seem right. The overlap is massive and seems too advanced.

The flywheel and key are in perfect condition and also apparently is the camshaft (press fit metal lobes and gear). I suspect something might have slipped but nothing is obvious. The compression pressure is low at 60 psi but I'm sure it's due to the late closing of the inlet valve, the cylinder is in fine condition and the piston blow-by is well within standard specs. The spark is healthy and steady at 20 BTDC.

Hard data on these engines is difficult to find. Any help much appreciated.

Regards, Len

 

[cpurvis]

Using your angle measurements, I come up with a duration of about 260 degrees, which would be a lot of a low speed engine. But those angular measurements are usually taken with the valves open a small amount. For car engines, these measurements are typically with the valve .050" off the seat. I'm guessing you did not take that into account. That would get the duration down to a more 'lawnmower-like' value.

Lawnmower engines differ in that the spark plug fires every time the piston approaches TDC. So if the intake valve timing is too early when the spark plug fires at the beginning of the intake stroke, that's a possibility for a backfire through the intake.

I'd make sure the timing marks are aligned.

 

[Born2Mow]

You don't need all that. The way I read your question is that you want to check the cam/valve timing without assuming the "dots" are correct.

Presumably, you have a single camshaft engine. Because both valves need to be nearly closed at TDC, and because one gear tooth will typically throw that into ciaos, all you need to do is set the camshaft to have both valves equally open at TDC.

True, the intake may have a longer duration and/or more "lift", but that is always added to the side of the intake lobe furthest from TDC.

IME a mechanic only needs to degree in the cams and measure valve lift when DUAL camshafts are present. And that type design is simply not present on "industrial" engines, like mowers and garden tractors have. High end cars and motorcycles 'yes' ; mowers 'no'.

 

[StarTech]

Plus those old camshafts use ramped decompression system which would also throw the timing off a bit using a degree I would think. Now years ago (20+yrs) I did intentional miss timed a 12.5 L head by one tooth. Just don't remember if I advance or delayed the timing (most likely delayed) and greatly increased the topend (3600 rpm) torque which one thing that can change the torque power curve.

 

[Hammermechanicman]

I think sometimes people overthink valve timing on these engines. The engine in question, like star said, has a bump on the intake lobe that holds the valve open very slightly for about half of the compression stroke with normal valve lash. If the gears are timed properly and the valves seal properly and the lash is set properly then on this engine there is no problem with the valves. Engines with a mechanical compression release can be a different story. Folks want to do compression tests and wonder why manuals don't give a spec. Engines with some type of compression release make that test irrelevant. Back in the 90's when i was building 5hp briggs for WKA cart racing i spent many many hours with a degree wheel many different cams a couple dozen sets of valves and lifters and another guys engine shop with a dyno.

 

[Ihategrass]

Hi Everybody,

Thanks for all the received wisdom. However, it does seem that nobody has yet answered the direct question about the factory spec for the valve timing.

Regards, Len

 

[StarTech]

If you are wanting factory specs then you will need to try Briggs and Stratton directly but there is slim chances of getting that info on a 2002 engien.