ATV Smokes When First Started (This is why)


A smoking engine generally isn’t a good sign, but lets not meet trouble half way.

Hard or worn valve stem oil seals will cause an ATV to smoke when first started. Replacing the valve stem seals will stop the smoking.

In this post you’ll learn why your ATV engine smokes and what you can do to fix it.

Oil Stem Seals

Almost all ATV engines are four stroke with an overhead valve configuration. The valves as you know open and close in sequence and slide inside valve guides.

The valve stem, spring and rocker are all hard working moving parts and need oil to lubricate and cool them. Although clearance between valve stem an guide needs to be tight oil would easily pass from the cylinder head to the combustion chamber through the inlet valve guides.

The solution to this problem is a small nylon seal, engineered to slip over the valve stem and seat on top of the guide.

The seal prevents oil transfer while keeping valve stems lubed. Like all components these guys only work so long. 10 years is about the life span and shorter if the oil changing is regular.

So Whats The Problem?

oil stem valve seal is an unassuming piece of kit, and it’s pretty impressive how something so small can be that durable.

As the seals age, constant heating/cooling cycles and oil contaminates make them hard. The hard seals allow oil slip past, down the guide and into the cylinder.

Buy why only on start up? The seals will allow a small amount of oil into the cylinder constantly but because it a small amount of oil, you won’t notice any smoke. Different story when you let the engine rest for a few hours.

The oil continues to leak in for a time after shut down but instead of burnt it rests behind the intake valve or inside the cylinder.

When you start your engine from cold, the oil is burnt off with the gas mix resulting in a plum of blue smoke.

What’s The Solution

Some have tried an oil conditioner that promises to recondition the seals and while I’ve seen some success with this my inner mechanic would never run for this solution.

The correct fix is to remove the cylinder head and replace the valve stems. While the head is off you could de-carbon, re-seat and adjust valve lash. A new head-gasket will obviously be needed too.

It is possible to replace the seals without removing the cylinder head, if you have the room to work. To attempt this you’ll need a special spring compressor kit and some clean new nylon rope.

Filling the cylinder with rope and moving the piston up to meet the cylinder head prevents the valve falling into the cylinder, it can be tedious to accomplish but does work.

John Cunningham

John Cunningham is an certified mechanic and writer on I’ve been a mechanic for over twenty years, I use my knowledge and experience to write articles that help fellow gear-heads with all aspects of ATV ownership, from maintenance, repair to troubleshooting.

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