This may apply to other mid-range or pro chainsaws which have carburettor high and low mix and idle adjustments.
Stihl 028 AVS bogging down under load - carburettor tuning Edit
Not having previously owned a 'pro' type chainsaw I was unaware of the importance of careful carburettor adjustments and how dramatically this could affect the performance of the saw, such as 'bogging down' under load and even to the extent of causing damage if incorrect. Please note I do not own a tachometer so the tuning process was performed paying attention to advice on keeping inside the safe limits of mix adjustments to avoid piston damage from overheating but ultimately limiting the saw to slightly less than optimum performance.
I purchased a used 028 AVS from a junk shop, in nice condition, no signs of abuse or overuse - for a 30 year old saw , but which after filling with fresh fuel mix bogged down severely under load and I quickly became concerned that the saw may need a major work. I had the option to return it .. but, hey, a nice 028 my first decent chainsaw ... And I like fixing things.
I initially suspected a hardened diaphragm, gummed up carburettor or leaking fuel lines, blocked vent. Compression seemed OK - not that I have a built in compression tester. The point was it ran - it started fine and ran smoothly, sounded good, ran fine a high revs without load. But when cutting would bog down and seize immediately. I had experienced this on a cheapo Mcculloch leaf blower - turned out to be a hardened diaphragm, on a machine that was only 5 years old .. I naturally suspected the carb - varnish, blocked filter, hardened diaphragm etc. I checked the fuel line - soft and no cracks, and the file tank vent (albeit that the the tube was disintegrating..) What next? Go online, before pulling the thing apart ..
Enter the whole new world of carburettor adjustment. I have an MS250 since 7 years but have never needed to touch the carb settings, in fact I was not aware that it even had aadjustments. The 028 certainly does and to cut a long story short this article nailed it - only available now via web archive - thank you so much. There was a whole process, procedure which I had no idea was a regular part of setting up the saw, and which which for any other contemporary "factory set", no adjustment tool would mean a certain repair or even end of life. I won't repeat it all, but in case it is ever lost I will summarise.
A few terms:
Mixture - motors work on air (oxygen) and fuel. I general the 'mix' has to be quite precise - whether this is a motorcycle, chainsaw, car or industrial plant. There are consequences to incorrect mixes ..
Rich - means too much fuel - causes blockages from unburnt fuel - carbon, gumming, smoking, uneven running, loss of power - general uneven running and flooding and cutting out. Fairly even to diagnose as the motor simply sounds uneven, spluttery and blubbering. This is OK in small amounts especially on idle.
Lean - too much air, motor too hyper - engine will run fast and hot. But lack torque. This will damage your motor. If the motor sounds too hot, metallic, "pinking", too smooth, fast or just wild - it is too lean and could overheat and damage the engine. It is not a Ferrari - there is no oil jacket to cool the engine down - these are simple basic 2-stroke engines and need oil/fuel mix to perform correctly. They give you performance but only if you feed the motor the correct mixture. They live on the edge. Pros have tachometers to check the engine is not running too lean and fast at high speed - will overheat and cause damage. At idle a too lean setting will make the saw crotchety at best. It will run too fast and cut out if you look at it. So run it slower to be safe, not too slow of course ..
- Clean everything - filters, exhaust, spark arrestor, bar, chain, fuel filter etc. Adjust chain tension so you can almost drag the saw on flat ground. If the chain is loose, it will get looser when hot - dangerous. You should tighten when hot (the chain stretches) and again when finished as it cools down in order to avoid putting the chain sprocket under tension. I would like to know if pros really do this or just find an optimal cold/hot chain adjustment. Add fresh mix, chain oil. Work clean - eliminate all other factors!
- Warm up the saw for a few minutes - after all that's how you will use it.
- Set idle screw so chain does not spin without brake. I found this difficult - the chain would twitch, or engine would cut - so I left it twitching every few seconds. If I put the saw down I put on the brake, so not sure if the odd twitch is OK. Never rev the motor with the brake on. But you know that .. a lot of people do not.
- How does it sound? It should sound happy and content. Like after a big dinner. So go ahead and adjust the low speed mix setting - too lean and she'll rev up and eventually die, to rich and sh'll bog down and die too. Somewhere in between, maybe a touch on the lean side, is OK. But a few belches and burps too. I set mine so that I would be happy to leave her running all alone for an afternoon* in case I got distracted ... :) *Never leave an unattended chainsaw running - it was a figure of speech used to illustrate how comfortable and stable the motor should sound on low idle.
- OK so here we go. Rev her up. If the motor sounds lean and metallic at high revs make it more rich. One and a quarter turns anti-clockwise from closed (be gentle closing, the seats are delicate) if you get lost and need to start again. If the saw is bogging down go back and check 3. and 4. If the slow idle is too rich the saw will load up and have too much fuel in the system. Run the saw at full speed and adjust the high mix screw until the saw sounds comfortable but not too hot and metallic. This is where you need the tachometer. If you do not have one make the mix slightly rich - you do not want to risk overheating, but you are losing some power and efficiency.
- You are cutting wood. Some of these adjustments require 1/8 of a turn. Or less. Take your time, when the saw is doing what you want it to do be happy and leave it till you need to adjust again. But check point 1. before you do that 6 months from now.
- Please wear the safety gear and saw safely.
My new/old saw is now running like a champ and I expect her to outlive me. It did not need any new parts or major repairs. Just a new fuel tank vent tube for the one that was perished.
So go visit these folks in WA, if you're passing, they seem to know what they're talking about .. :)
Why Can't Carbs Be Set When They Are New & Then Leave Them Alone? Edit
Some people wonder why their saw can't be adjusted when it's new and then not need any other tuning. They don't understand that saws can't be set at the factory and stay correctly adjusted for the life of the saw. The fact is, some manufacturers think that way, too, and have tried "fixed jet" and "semi-fixed jet" carburetors. These types of carburetors have never worked well. The reason is that elevation, fuel type, and a host of other factors cause saws to need periodic carburetor adjustment. In the future, fuel injection units will probably be developed for chain saws. Sophisticated electronics will be able to monitor and meter air/fuel needs of two-cycle engines and saws will be able to "self adjust." Until then, keep your adjustment screwdriver handy.