Please help - Stihl MS021 won't start

[Anchorman]

Picked up an old Stihl MS021 on the cheap and had it running at one point but not smoothly. Wanted to get it fully restored so I took it apart, cleaned the carburetor (didn't look bad, diaphragms looked fine, needle valve too), and cleaned just about every other part with compressed air. Drained the oil and fuel and replaced both. Gas is ethanol free and mixed at 50:1. New spark plug, gap is .020"per the manual. Chamber looked pretty good but I got it even cleaner, and replaced one piston ring which had a break it it. I've never seen that before but replaced it with an oem ring. No scoring damage on the inside walls; compression is about 90psi, which I assume is normal for a baby saw like this. Removed air filter & cleaned it. Removed spark arrestor screen from exhaust (looked clean but I torched it anyway to remove all carbon residue). When I pull the spark plug and check for spark, I ground it to the block and it sparks just fine. I put a small amount of fuel in the chamber and with the spark plug installed I can't even get it to turn over a few times. With the air filter on, and off. No start. I am following the manual's starting procedure to a T, with no luck. What am I missing? I pull and pull and not one time has it ignited. I also used a feeler gauge to set the flywheel gap to .01", which I've researched and seems to be correct. I also disconnected the kill wire from the ignition coil to ensure that wasn't the problem.

No idea what to do next. Any help would be greatly appreciated!

 

[Bob Hedgecutter]

There is no MS021, so either you have an 021 with screw on fuel/oil caps, or an MS 210 with flippy caps.
90psi is not going to run nothing in the small engine 2 stroke game- 120 is about bare minimum, but it all depends on how true the gauge reading is- if using a standard automotive gauge it might read 90 where actual true compression is nearer to 150psi.

Could be a million and one things- incorrect reassembly of the carb or major saw components, if you blew the carb out with compressed air through all the tiny galleries- thats not good, if you honed the cylinder to get it cleaner- thats not good, if it has bad seals, bad impulse, bad fuel lines, thats not good, if you used Asian parts not OEM, thats not good. If you put fuel mix directly into the cylinder that is not the correct mixture of vapour and air- thats not good.

Pull the plug, pour some straight mix oil, engine oil, or bar oil into the cylinder on top of the piston- about a teaspoon full. Swill the oil about a bit, refit the plug and associated wire/boot and try to start the saw as per the manual.

 

[Anchorman]

Thanks for your reply Bob. Yes, it's an 021 with the screw on oil & fuel caps. I used an automotive gauge; didn't realize that'd give a different reading but that's good to know.

I did not blast the inside of the carb with high psi. Regardless, to rule out it being a carb issue I removed the spark plug and put about a teaspoon of 50:1 fuel in the chamber.

I did not hone the cylinder. Just a little brake cleaner on a rag to clean some light deposits.

I've completely dismantled and reassembled a 90hp 4 stroke Yamaha outboard before so by comparison this is child's play. For good measure, I used exploded diagrams of this model chainsaw during disassembly and reassembly, so I'm confident that it was reassembled properly.

I put 50:1 fuel mixture in there on top of the piston via the spark plug port as I have, successfully, with other chainsaws. Just doesn't seem to do anything on this one.

I'm not clear as to why you are suggesting putting "straight mix oil, engine oil, or bar oil" into the cylinder on top of the piston; can you please clarify the reasoning behind that?

Any other suggestions?

Thanks in advance.

 

[Bob Hedgecutter]

The oil give a temporary yet effective test of compression- or rather lack thereof.
The oil gives enough of a temporary coating to the cylinder to piston clearance to initially start the fuel/air charge being introduced from the carb via the transfers.
If having done so and the saw starts, runs like a bug fogger for a few seconds and allows you to throttle up- compression (a lack of) was your problem.

 

[Bob Hedgecutter]

Another possible problem is timing.
If the flywheel is misaligned due to a munched key or sheared/missing key- the plug will spark outside the cylinder when earthed- but will be out of time when refitted and not firing near TDC, so the saw will not run.

 

[Anchorman]

Ok that makes sense, thanks for the explanation. Will try that.

I was wondering myself if it was a timing issue but couldn't see how that was possible. But if the key sheared off after reassembly that would indeed be another possible culprit.

Appreciate the recommendations, thanks Bob.