B&S 130000 series L-Head intake valve issue

[kharrisma]

Hi Folks,

Have a circa 1991 130212-3250-01 (code 91061910) 5HP horizontal shaft engine (happens to be mounted to a chipper-shredder). Pull start rope: zero compression (based on pull feel - no 'stack up'). Pulled head; both valves stuck open. Freed up exhaust easily, and it cycles and seals well, and 'in time.' Intake, however took a LOT more to free up (penetrating oil, soft-faced hammer, many cycles of open, tap down, open, tap down; evidence of water corrosion) until it was moving easily. Put head back on and tested; still zero compression.

Pulled head again, this time noticed that the intake is opening when it should (few degrees before TDC at end of exhaust stroke), stays open until piston reaches bottom, and as it 'turns the corner' and just begins to head back up, it *seems* to close... but it isn't fully seated; it's easily rotated with twisting thumb pressure atop the valve. It remains easily rotated until the piston is just about an inch from TDC/compression stroke, and only then does it seat sufficiently that it can't be twisted with thumb pressure.

Hmmm. Pulled valve springs/keepers, pulled valves (yucky), and cleaned up with a wire wheel. Cleaned seats with hand-drill mounted wire wheel. Lapped intake valve until valve and seat showed a good sealing band. Went to put spring/keeper back on, and noticed that the exhaust valve spring is not only a thicker gauge wire, but must also be compressed slightly by hand before it can be introduced into the tappet box. The INTAKE valve spring is a lighter gauge, and sitting on the floor of the tappet box, is about 3/8" from the 'ceiling' of the tappet box... sits *much* shorter than the exhaust valve. Seems weak by comparison, too.

Anyway, spun the starter again, and the intake valve is doing the same thing: seemingly opening and closing when it should, but when 'closed' it's not a firm closure, easily rotated until piston is approaching TDC.

Have not yet checked tappet clearances along intake valve travel cycle; I'd like to verify that the spring is normal; if it's supposed to be 'tall' like the exhaust, I'll just order another spring before proceeding any further. Related question: is it a bad idea to just install an exhaust spring on the intake valve to get a tighter seal? Oh... come to think of it, maybe I should put the exhaust spring on the intake and see if that solves my problem!

Anyway, thanks for reading and any input you may share!

 

[Hammermechanicman]

That operation of the intake valve is normal. That is what briggs calls easy spin. It acts as a compression release for easier starting. Eliminates the need for a mechanical decompression device. Leave the springs where they are.

 

[kharrisma]

Thanks Hammermechanic man,

I did run across mention of something like that elsewhere; wasn't sure it was included on this engine. If this is correct, I guess I don't understand how it manages to achieve enough compression to work; if it's pushing out air/fuel charge as the piston is coming up (and not compressing it in the process), how does it manage to start & run? Is it one of those things where it becomes a non-issue when it's actually started & running?

 

[Fish]

Check your clearances...

 

[Hammermechanicman]

Thanks Hammermechanic man,

I did run across mention of something like that elsewhere; wasn't sure it was included on this engine. If this is correct, I guess I don't understand how it manages to achieve enough compression to work; if it's pushing out air/fuel charge as the piston is coming up (and not compressing it in the process), how does it manage to start & run? Is it one of those things where it becomes a non-issue when it's actually started & running?

Yes.

 

[bertsmobile1]

The carburettor is mixing the fuel to the air at the same ratio
So all that is happening is your 500cc engine with a compression ratio of 7:1 ends up being a 100 cc engine with a compression ratio of 4:1
The lower the compression the wider the range of air : fuel ratios that will ignite.
The lower the compression pressure the better spark you get because the resistance between the electrodes drops .
This is why the 4 stroke 150 cc lawnmower starts a whole lot easier than the 25 cc line trimmer motor.
Also why the old side valve engines are easier to start than the equivalent OHV engines.
I have 2 almost identical model L BSA's
Both have the same carbs and the only difference between them is one isa 350 cc OHV @ 7:1 and the other is a 350 cc SV at 5.5: 1
The SV will start if you fart on the kick starter but the OHV requires several strong kicks before it will even attempt to come out to play .
A match in a petrol tank will cause the contents to burn and there you have a compression ratio of 0

 

[kharrisma]

Hi bertsmobile1,

I am absolutely NOT arguing with you... just trying to understand the 'mechanics' of this thing. Perspective: I was a forklift mechanic for nearly 30 years (electric lifts, unfortunately in this case). I used to HATE working on the propane-fired lifts!

So by leaving the intake valve open by 0.001" (according to an explanation of 'easy spin' I found) for most of the compression stroke, B&S is effectively dropping the compression (to that last little bit of piston travel, about 1 inch or so, where the valve is finally tightly sealed), reducing both the trapped volume of air/fuel charge, and compressing it only that little bit? Thus allowing for a wider range of fuel-air mix and therefore easier starting, both by virtue of lower compression resistance to pull, and more tolerance of air:fuel ratio? Makes sense.

Where I lose it is since this works because of the shape of the cam lobe, isn't the same thing happening when the engine is started and running? How does it achieve tull compression with that valve being open a bit until it finally fully seats for that last bit of piston stroke? Same sequence of events, same effect (reduced compression), just higher RPMs. Not understand this.

Other thing that made me blink is: "The lower the compression pressure the better spark you get because the resistance between the electrodes drops." Again, no argument here, just curiosity. Wouldn't the air-gap resistance be *lower* in a higher compression situation due to the greater density of the fuel charge which is being compressed, creating more fuel molecules jammed into the same space than a smaller, lower-compression fuel charge? More particles closer together = better conductor = lower resistance? You seem to be saying exactly the opposite. Most likely I'm missing something or have some basic 'understanding' wrong, but that's what my 'logic-train' is telling me. Or is it the actual pressure of compression that's the issue here, regardless of fuel charge? While I *do* understand electricity very well, spark plugs in a high-compression situation with an explosive fuel charge present isn't something you find in an electric lift truck.

Thanks for your explanation, though! No such thing as useless knowledge. Every bit counts.

 

[Hammermechanicman]

Voltage to fire plug goes up as pressire increases.
You can defeat the easy spin feature by opening up the intake lash. This makes the engine harder to start. Will give you a slight increase in low end torque but above about 2000 rpm the easy spin makes negligible difference.

 

[kharrisma]

Thanks again, hammermechnicman,

Got it. Never thought about it much; quick search produced a nice little graph that shows the relationship between voltage, compression, and fuel-air mixture temperature. Explained a lot. Thanks for that tidbit of knowledge!

Nope, I have no desire to defeat the easy spin feature; I was simply very puzzled by the intake valve behavior, racking my brain for some kind of malfunction, wear or damage that would produce such behavior. Never occurred to me that it wasn't a glitch, it was a feature! Knowing this, I can ignore that aspect of what I see, and go on with seeing if I an get this almost 20 year old engine to run. Much thanks for your help!

I also saw that in order to test the compression of this engine, I need to turn it opposite of the normal operational direction, otherwise the easy spin feature will produce really low readings. Sound accurate?

 

[Hammermechanicman]

A better test is what is called a leakdown test. Turning backwards won't give you a true compression reading