MTD engine valve lash setting

[Mr Mower Man]

Anyone know what the valve clearance is supposed to be on those cheap 420cc MTD engines? I think it's a 4P90HU model number. It's on a Craftsman lawn tractor, model 247.203726.

Exhaust valve is very loose, and this thing is running horrible. I need to get the valve setting right before I try to blame the problem on something else.

[KE4AVB]

IF its a P90.

Info from the P90 Service Manual.

Intake valve lash (top valve) should be 0.004” - 0.006” (0.10 - 0.15mm).
Exhaust valve lash (bottom valve) should be 0.006” - 0.008” (0.15 - 0.20mm).

Use a 10mm wrench to loosen the jam nut, and a 14mm wrench to adjust the rocker arm fulcrum nut.
Tighten the rocker arm fulcrum nut to close-up the clearance between the end of the valve stem and the contact point on the rocker arm.
Loosen the rocker arm fulcrum nut to open-up the clearance between the end of the valve stem and the contact point on the rocker arm.

Hold the fulcrum nut with a 14mm wrench, tighten the jam nut to a torque of 80 - 106 in-lb. (9 - 12 Nm).

Install the valve cover, tightening the valve cover screws to a torque of 62 - 80 in-lbs (7 - 9 Nm). Don't over tighten the cover as it will leak.

[Skywatcher]

Hi M³

Here's a trick I learned from an old Cummins mechanic. When you have a setting range like .004" - .006", take the next feeler gauge below the minimum and the next gauge above the maximum, so this would be .003" & .007". After each adjustment, check with these two gauges, if the thinner one slides in but the thicker one doesn't, you're within spec. All the best,

Sky

[Mr Mower Man]

Thanks for the info, guys. I have a feeling we're gonna have to replace the carburetor on this one, since it was running rich (AFTER the carb rebuild ). But I need to get this valve issue right first, before I pull the trigger on ordering a carburetor.

That's a pretty good tip for being within spec, Sky. I never tried that before. The way I've always done it, if the spec is between .004" and .006", I make it a tight .005". I know it's gonna get looser over time, so I adjust it toward the tight side of the spec.

[KE4AVB]

Did you make sure the air bleed is open and that o-rings are good on the metering plug? The SM has a good chapter on cleaning and rebuilding the carburetor.

If you haven't already download the SM and review it. It posted on my Google drive via link in the downloads forum under MTD Service Manuals.