Briggs 19.5 hp motor won’t operate past half throttle / overly rich

[slomo]

Slomo, not sure what you mean about the shut off valve but I do have a remote tank hooked up to bypass the fuel pump and it has a shut off.

But even if the carb was rich I would expect to see black smoke but would I expect to see mist and feel pressure coming back through the carb inlet?

With that much blow-back, either the needle is dumping fuel constantly flooding the cylinder or you have a cam/valve/head gasket issue of some kind.

You can install a valve. that shuts off the fuel, after you mow, inline to the carb. Doing this, you never have to worry about your carb needle dumping fuel into the cylinder and then into the crankcase contaminating your oil with gas.

slomo

 

[slomo]

Question. I don't know if this engine has a plastic cam but if it did, would the valve clearances at TDC always be correct like they are?

Don't forget there is more than 1 TDC. I was shocked that you found the valves IN SPEC. I've never see any used mower with proper spec valves. Course I get them thay way and not running LOL.

slomo

 

[Baymee]

I think I found it. Should have done this sooner. The intake valve rocker moves the valve stem inward 7/16" in a cycle while the exhaust valve only moves 1/8". Cam lobe?

 

[StarTech]

I think I found it. Should have done this sooner. The intake valve rocker moves the valve stem inward 7/16" in a cycle while the exhaust valve only moves 1/8". Cam lobe?

Sound like it.

StarTech found you have a normal steel camshaft. Forget about a plastic camshaft.

You might be 180 degrees out on your valves or a flat lobe as StarTech said.
slomo

Impossible to have the camshaft 180 as the turn ratio is 2:1 (2 crankshaft rotations to 1 camshaft rotation). Even if you actual put in a 180 out to the timing mark the very next full crankshaft the timing will line up. Plus being a wasted spark magneto system, the plug fire on every complete rotation of the crankshaft.

Now that said you can install the camshaft up +-90 degrees and have the engine out of time. I have time a few old 12 hp L head engines out by one tooth of camshaft gear and have more HP.

 

[Scrubcadet10]

I think I found it. Should have done this sooner. The intake valve rocker moves the valve stem inward 7/16" in a cycle while the exhaust valve only moves 1/8". Cam lobe?

I'm sure you know what you're doing, are you sure it isn't the exhaust vale opening for the compression relief on the cam? Just wanted to point this out to further help confirm a cam problem.

 

[Baymee]

Here are the pics. Look at the timing marks, then see the piston position. How did that happen? Then see the cam.



The word "cam" was mentioned enough and that was correct. Thanks to all that opined.

 

[Scrubcadet10]

To me the closest lobe, (intake?) Looks worn more than the other

 

[slomo]

I think I found it. Should have done this sooner. The intake valve rocker moves the valve stem inward 7/16" in a cycle while the exhaust valve only moves 1/8". Cam lobe?

Yes, valves should have similar

Sound like it.

Impossible to have the camshaft 180 as the turn ratio is 2:1 (2 crankshaft rotations to 1 camshaft rotation). Even if you actual put in a 180 out to the timing mark the very next full crankshaft the timing will line up. Plus being a wasted spark magneto system, the plug fire on every complete rotation of the crankshaft.

Now that said you can install the camshaft up +-90 degrees and have the engine out of time. I have time a few old 12 hp L head engines out by one tooth of camshaft gear and have more HP.

I was talking about setting valve lash and being on the wrong stroke. Sorry for the poorly written info LOL.

slomo

 

[slomo]

Here are the pics. Look at the timing marks, then see the piston position. How did that happen? Then see the cam.



The word "cam" was mentioned enough and that was correct. Thanks to all that opined.

Was I correct with cam timing off? Sure looks like it. That cam lobe resembles a drag racing solid roller, minus the missing lift LOL.

When you have blow back out of the carb, you are dealing with a huge racing style camshaft. Lots of valve overlap on those. A mower uses the exact opposite cam profile. Time to call Briggs and get a new camshaft.

slomo

 

[slomo]

StarTech and ScrubCadet10, could THAT cam gear rotate on the shaft and cause this faulty cam timing? Or someone got it off by mistake? I see one lobe splined up a bit. Those lobes shouldn't move unless stripped.

slomo