A Word On Decompression Valves and Mopeds: Lapping and Sealing

I've done valve jobs on small engines before and not had any issues with the following described method.

To get a metal on metal valve to seal, part of the issue is getting the valve lapped properly into the head - the other is having adequate spring pressure.

I more than amply lapped the valve in my vespa, yet it still leaked. upon disassembly of the head, I noticed the spring was incredibly weak and barely applied any pressure once the uncompressed valve height was reached. This is what I attributed my leak to, and after reassembling, have confirmed this was the case as it is gone with a new spring.

The best way to achieve a properly seated valve is to start out with a coarse and then fine lapping compound. Chuck the valve up in your drill and spin it against the valve seat.  Occasionally push the valve out, and allow fresh lapping compound to splatter onto the valve and seat. If you are working in a tight spot, fuel line can be used to spin the valve as a flexible drive - hose clamps or zip ties may be needed to retain the fuel line on the valve. After doing the coarse compound, inspect to make sure that the rings etched into the head and valve are even all the way around the valve. Once they are all even and consistent, move onto the fine compound. Look for the same thing here, except this time, the rings around the valve and head will be smaller. Make sure during this process to keep lapping compound out of the valve guide.

This is not traditionally how a new seat is cut. It is by far the best way, but not the fastest way.

Traditionally, a cutting head is used to cut the angle of the new valve seat into the head when new and also when new valves are installed in a new head, however, you can get away without the cutter when resurfacing a valve seat. Lapping compound is applied to seat the valve in both processes. As long as the new valve is a replacement and not over-sized and the seat is not too far gone - this almost always applies to 4 stroke valve jobs. If there is any kind of seat angle what-so-ever on a decomp seat - this method will work fine.

When you're going back and rebuilding, it's usually sufficient to resurface the seat with aggressive use of coarse grinding compound.

When you do it without the cutter, you are sort of cutting the new seat with the coarse grinding compound. Don't be afraid to spend a pretty good amount of time cutting the new seat.

Once you've got a good seat cut, you'll need a good spring to put ample pressure on the decomp.

blocking it is super easy. just tap and plug with appropriate hardware. decomps are awful handy though and you can end up stranded with a bike that has a loose belt and no decomp.


this is an example of what the sealing surface should look like all the way around the valve and seat (notice the consistent and small grooves\rings).

Here's what the dirty valve looks like after about 600mi on my vespa with a weak spring, notice the grooves that are visible.

Here's a shot of the lapped valve seat before assembling. The wrong valve was sent with the head, so that is what is in the bottom of the decomp bore. once again, notice the grooves.

Other thing to keep in mind is that it may not seal perfectly and that good enough will have to be ok.
any type of seat like that is going to leak a little bit.

There's just an acceptable amount of leakage (which isn't much). I suggest running the motor and seeing if spraying carb cleaner at the decomp causes any fluctuation in idle. If it doesn't - don't worry about it. Nothing will ever be "perfect" just within "tolerance". So long as it's in tolerance is all you need to worry about. On 4t motors, you're going to want to do a leak down check or transmission fluid check to check the seal.

Leak down is pretty straight forward, you need the leak down tester and you look at percent leakage at different pressures.

Transmission fluid is pretty straight forward too. Place the head on a level surface and fill the dome of the head with fluid so that valves are covered. Let it sit over night. If there is no change in the level of the fluid over night - you're good to go. Transmission fluid is highly detergent and will seep past any imperfections worth of paying attention to.

Perhaps the next time I do a valve job or lap a decomp valve I'll make a video to make this a wee-bit more comprehensive.