OutdoorKing Repair Forum Forums Repairs and Maintenance Questions On Ride On Mowers Questions On Ride On Mowers Frames MTD B&S struggles to crank engine

Hi Steve, these extracts from the Intek shop manual show you what to do to check the decompressor:





You have to open the crankcase, remove the camshaft, and check it carefully. This is quite a hassle, and replacing a defective camshaft is expensive, so it makes sense to be very sure you have checked the tappet clearance very carefully before doing it.

If you turn the engine slowly by hand (spark plug lead disconnected for safety, of course) with the valve/rocker cover removed, if the decompressor is working you should be able to see the exhaust rocker move - just slightly - during the compression stroke. If it doesn't move, the decompressor is not working. I only know of two ways that can happen: too much tappet clearance, or a defective decompressor mechanism on the camshaft.



If you look at the red circle, you can see that the decompressor (the top edge of the steel stamping that sits edge-on beside the cam lobe) is almost in line with the cam lobe: it only rises above it by a tiny amount. If there is excessive tappet clearance, it simply doesn't stick up far enough to lift the tappet sufficiently to take up the clearance and lift the valve at all, and therefore it doesn't relieve the compression.

Note that your Vanguard 28Q777 uses the same camshaft design - the decompressor is identical to the Intek one.

To find out definitively whether your decompressor is working, without dismantling the mower, I suggest you try this. Adjust the exhaust valve tappet to zero clearance, or even less than zero clearance. Then rotate the engine slowly by hand with the spark plug removed (both for safety and to make it easier to rotate). When you find the compression stroke (by putting a finger over the spark plug hole), rotate the engine slowly back and forth, up and down the compression stroke, watching the exhaust rocker for slight movement near the middle of the stroke. Any such movement is the decompressor operating. Remember, the question is not whether it relieves the compression, but whether it moves the rocker at all (you should only expect a few thousandths of an inch - you may have to use a dial indicator to detect the movement). If the rocker moves at all, the decompressor is functioning. Then step by step increase the tappet clearance until it stops working. Measure the tappet clearance at that point. If the maximum clearance at which the decompressor will work is less than 0.004", your decompressor has failed, perhaps by sticking in the "on" position and wearing itself out, or just by normal wear on the exposed edge of that steel stamping that pushes the tappet up. If the decompressor will not work at all, even with a negative tappet clearance, it is probably stuck or broken.

Bear in mind that your engine is now 17 years old, and that decompressor is a tin-pot mechanism to begin with.