ATV Smokes When First Started (This is why)


A smoking engine generally isn’t a good sign but let’s not meet trouble halfway.

Hard or worn valve stem oil seals will cause an ATV to smoke when first started. Replacing the valve stem seals will stop 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 hard-working moving parts and need oil to lubricate and cool them. Although clearance between valve stem and 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. Ten 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 seal’s ages, constant heating/cooling cycles and oil contaminates make them hard. The hard seals allow oil to slip past, down the guide, and into the cylinder.

But why only on startup? The seals will allow a small amount of oil into the cylinder constantly, but because it is 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 after shut down, but instead of burning, 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 plume 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 be needed too.

It is possible to replace the seals without removing the cylinder head if you have 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 from falling into the cylinder. It can be tedious to accomplish but does work.

You may also find these posts helpful:

Air-cooled ATV white smoke

ATV blows blue smoke

John Cunningham

John Cunningham is a certified mechanic and writer on ATVFixed.com. I’ve been a mechanic for over twenty-five 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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