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 the smoking.
In this post, you’ll learn why your ATV engine smokes and what you can do to fix it.
Oil (valve) 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 require oil to lubricate and cool them. Although clearance between valve stem and guide is snug, oil could easily pass from the cylinder head to the combustion chamber by seeping in between the valve and guide.
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 and allowing for movement. Like all components, these guys only work for so long.
Ten years is about the life span and shorter if oil changing is neglected.
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 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 does it only smoke on startups? The hards worn-out seals 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 immediately 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 seals.
While the head is off, you could de-carbon, re-seat, and adjust valve lash on reassembly. 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.
Worn Valve Guides
The valves as you know slide inside sleeves known as valve guides with the seal sitting over the top preventing oil ingress. Problems arise as valve guides wear and allow for greater free play between guide and valve. The increased free play stretches the seal and damages it-which allows oil to leak into the cylinder.
Replacing the seals won’t fix the problem, the valve guides will need to be replaced and that requires removing the cylinder head.
Other Possible Causes
While a valve seal is the most common cause for smoke on start it isn’t the only possible cause. Here’s a list of a few others worth checking out:
Oil quantity – Too little oil is bad, but so too is too much oil.
An overfull oil level will cause the engine to burn off the excess.
Oil type – Oil type is important, using an oil that’s too thin may cause it to leak past seals and rings. While a thin oil is best for super low temperatures it may not be suitable for warmer climates.
Check what your manufacturer recommends for your engine.
Carburetor issue – A leaking carburetor needle valve or incorrectly adjusted float may cause gas to flood the engine and often results in an increased oil level, and white smoke.
Common symptoms of a worn arb needle seal include:
- Stink of raw gas
- Overfull oil level
- Oil leaks
- Engine smoke
Fuel issue – Contaminated fuel, while a four-stroke engine will run on two strokes mixed gas it won’t like it much. It will cause the engine to smoke and also on startup. Worth checking if the wrong gas was used.
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- About the Author
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John Cunningham is a technical writer here at ATVfixed.com. He’s a Red Seal Qualified Service Technician with over twenty-five years experience. He’s worked on all types of mechanical equipment, from cars and trucks to ATVs and Dirt bikes.