By: Author John Cunningham. Published: 2023/09/15 at 5:37 pm
Stupid chainsaw!…sooo frustrating!!
Now for the good news – I’m a mechanic, and you’ve already eliminated two common chainsaw faults: spark and fuel. I’ll bet you and me together will have this figured out in jig time.
A chainsaw with spark and fuel but refuses to start commonly suffers from flooding; drying out the cylinder solves the issue. However, there may be other reasons, they include:
- Bad gas
- Carburetor issue
- Blocked air filter
- Blocked spark arrester
- Compression issue
In this post, we’ll troubleshoot the crap out of this saw until we hear music. So let’s get stuck in!
I start troubleshooting in my workshop by checking all the basic stuff first because, over the years, I’ve learned making assumptions often ends with me looking like a total bellend. As Sherlock Holmes might say – “We’ll check a fact is a fact.”
Basics include checking that I’m following the correct starting procedure – switch On and choke On, the air filter is clean, the spark plug is clean, gapped correctly, the correct type and tightened down, the plug wire is on, the gas is fresh, and plenty of it, and the saw doesn’t exhibit any obvious issues such as gas leaks, etc.
Not to worry if you haven’t covered all of these; we’ll cover them all in this post and some.
An engine needs three things to run: spark, fuel, and compression. So, you’ve done some great work testing the first two. That said, I can’t tell you the number of times fuel is at the root of a no-start issue, especially if the gas is over a month old.
Yep! I did say one month… I know, crazy, right? Modern gas is so problematic for small engines.
Ethanol gas goes stale really quickly, and small engines hate it. If this sounds like this may be your problem, draining the gas and filling it with fresh gas usually solves it.
So, in the workshop, when I meet a customer who complains about a no-start, I usually ask how old the gas is.
If the answer is more than one month, then repeated cranking of the saw has likely flooded the engine with bad gas, so you might say the engine now has two issues – bad gas and flooded.
To check for flooding, we remove the spark plug and check the plug condition, but as you likely know, flooding usually gives you a heads-up as it’s accompanied by a stink of gas.
Anyhow, go ahead and pull the plug, and we’ll check its condition. The plug tells us a ton about what is happening or not happening inside the cylinder.
A wide range of issues may cause flooding; bad gas is the more common cause; other reasons include:
- Excessive choke use – choke off as saw warms up
- Blocked air filter – clean filter
- Wrong spark plug – check and fit the correct plug
- Failing coil – check and replace
- Wrong gas type – drain and replace
- Wrong gas/oil mix – drain and remix correctly
- Bad compression – test compression and evaluate
A flooding condition is easy to fix; allowing the saw to sit for ten minutes with the spark plug removed does the job.
You can speed up the process by cranking over the engine a few times with the plug and air filter removed and the throttle held wide open to expel excess gas from the cylinder.
But remember, if our gas is bad, attempting to start the saw will only wet the plug again. If you are in doubt about the gas, drain it out and replace it—more on bad gas below.
If, on the other hand, the spark plug is dry, then we may have a fuel supply issue, and we’ll test that below in the carburetor issue section.
If the spark plug looks oily, that may indicate too much oil in the mix or the wrong spark plug fitted. Check your fuel mix ratio.
Gas isn’t what it used to be! Really, it isn’t! I remember when gas was all gas if you know what I mean; now it’s blended with alcohol and called ethanol.
While our cars and trucks have no major issue with it, the same can’t be said for small engines. The alcohol content attracts moisture, and if that ain’t bad enough, the 2-stroke mix separates from the gas, meaning the first start (if it starts) runs without proper oil protection.
Depending on where the gas is stored will dictate how the fuel reacts; in some carburetors, it turns to a sticky sludge, which can be difficult to clean.
It’s known unsurprisingly as carburetor gumming; it can be prevented by using a gas stabilizer in the gas.
Manufacturers recommend using no more than E10, ethanol blend, with a minimum octane rating of 89. That said, if you want true piece of mind, then buy canned gas from your local chainsaw store; the gas is a premium high octane, treated, meaning it will keep, and you will notice the extra Oomph.
So, if your gas is older than one month, try draining and rinsing it out and replacing with fresh gas.
If fitted, press the primer bulb to displace the old stale gas inside the carburetor and replace it with fresh.
I wrote about ethanol gas in more detail here, including storage tips.
Carburetor issues are very common, but as already said, the real cause of the carburetor issue is bad/old gas. In most cases, if we diagnose and confirm a carburetor issue, cleaning usually solves it.
And when inside the carburetor, it’s customary to replace rubber diaphragms and gaskets, and we’ll look at that shortly.
First, we’ll need to diagnose a carburetor issue, which we’ll do next with the gas shot test.
Gas Shot Test
The gas shot test is a simple, quick test. Go ahead and remove the spark plug, dump a shot of fresh gas into the cylinder, refit the spark plug, and attempt to start the engine.
Two outcomes are likely:
1. The engine starts or attempts to start. (attempting to start may be observed as smoke from the exhaust or a partial engine fire-up). If this sounds like your result, then you have confirmed that your carburetor is likely the source of the no-start.
We’ll need to remove it, clean it, replace the gaskets, refit, and tune the carburetor. I’ve covered the whole process previously step by step with pictures, and you can check that out right here.
2. The engine made no attempt to start. If this sounds more like your result, then an ignition system or a compression are among the more likely issues. And since ignition system issues are more common than compression system issues, it makes sense to double-check the ignition system first.
I’ve covered ignition system testing step-by-step, and you can check that out right here.
If the ignition system tested OK, then we’ll need to look at engine compression, which I’ve covered below.
Blocked Air Filter
The air filter is an obvious one and, ideally, should be at the top of our list, but as you know your way around a saw, I’m thinking you’ve checked this already.
If that’s not the case, go ahead and remove the air filter and attempt to start the engine; if she now starts, then our air filter is at issue.
Chainsaw air filters are commonly made from foam, nylon mesh, flocked material, and paper. All but one may be washed in detergent – the paper filter may not be cleaned in detergent.
Paper filters may be cleaned by tapping them on the ground or using compressed air (reduce pressure to 10 psi). Using solvent or detergent to clean paper filters tends to block the filter, and that causes the engine to flood.
We’ve covered flooding previously, and if this sounds like your problem, then it is best to order a new paper air filter.
I’ve covered air filter cleaning previously, and you can check that out here.
Blocked Spark Arrester
The spark arrester, fitted in the muffler, is designed to catch hot exhaust embers and prevent forest fires, etc., especially important as our summers seem to be getting dryer. In 2-stroke engines, the spark arrester often gets caked in oily deposits as some users may be slightly more generous with the 2-stroke oil in the mix.
That said, spark arresters need to be cleaned every 50 hours and are often forgotten or not even known about.
A blocked spark arrester prevents the engine from breathing, and that may prevent the engine from starting.
Cleaning the spark arrester is a 10-minute job, and I’ve covered it here step by step with pictures.
Compression is last on our list because it is the least likely cause and, unfortunately, the most expensive (usually) and hardest to fix. Compression is, as you know, an essential ingredient in the 2-stroke cycle.
The cylinder piston and rings must be able to compress the air/fuel mix into the top of the cylinder (combustion chamber), where the spark plug ignites the mixture.
Compressing the mixture serves to heat it and atomize it, which is critical in preparing for a perfect ignition. Low or no compression means poor performance or no start.
Worn rings are the usual cause of low compression; we can test for that condition by running a compression test.
Check out the step-by-step compression test here.
You may find the following links helpful:
- About the Author
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John Cunningham is an Automotive Technician and writer at Lawnmowerfixed.com.
He’s been a mechanic for over twenty-five years and shares his know-how and hands-on experience in our DIY repair guides.
Johns’s fluff-free How-to guides help homeowners fix lawnmowers, tractor mowers, chainsaws, leaf blowers, power washers, generators, snow blowers, and more.