Saw fires & yanks starter handle out of my hand. Why?

[NewToStihl]

***NOTE, if you have a short reading attention span feel free to skip to the last paragraph.***

Scenario:
Put the saw on ground, left hand on the wrap handle, right foot through the rear handle, pull starter cord with right hand. Saw pops once and violently yanks the starter handle out of my hand and sucks it back to the starter. Hurts like hell - not interested in having that happen anymore!

Awhile back I had this happen with an 064 once when I swapped coils and it wasn't the correct flywheel. I did get it to run, but for a short time and it ran like crap due to the timing being off.

Today I had it happen on an 090AV. I started the saw and ran it a bit, but needed to dial in the idle speed . . . gotta shut 'er down for that due to way too much vibration to get in there and turn the LA screw! Fired it again, and still a little fast. When I went to fire it up the third (or fourth?) time it started to yank the handle out of my hand when I went to start it. The only reason I knew of was timing, so I pulled the flywheel to make sure the key was stock and in tact - which it was. The problem repeated several times until I needed to walk away and rest my hand. Not to mention I was afraid it was going to snap back and break a fin off of the starter cover eventually.

I searched around on AS and only found some references to an old Mac that's notorious for this and the fix was essentially to pull the starter like a man instead of a mouse! No explanation why though...

I called my buddy Joe (ELECT6845) and asked if he had any ideas and he immediately went to timing first, but also had a few other stories & suggestions. Then told me to try using the decompression valve (which I normally don't). Sure enough that made the difference! The saw started right up and ran great. I'm glad that allowed me to start the saw, but I don't understand why that made a difference or what causes the problem. I'm the kind of guy who doesn't consider a problem fixed until you understand the root cause.

So, I said all that just to ask the question - can someone explain the details of what's happening when this occurs? I get the timing issue that can do it, but what causes it when timing isn't (or doesn't seem to be) the problem?

 

[nmurph]

Tldr
Sent from my C771 using Tapatalk 2

 

[cobey]

high compression? if it has a decomp use it, its to save the operator and the recoil

 

[z71mike]

Anything with high compression will do that if you don't get it to TDC before you rip the cord.

I've seen it one other time. Almost tore my fingers off. A coworker had an MS361 that wasn't running right. Long story short, I told him his coil is bad. He don't have much $ so I told him to watch fleabay for a while until a good deal comes up on an OEM part. He doesn't listen. Buys a chicom coil from his local shop for like TEN BUCKS. We put it in and try starting it for about 10 pulls. Then it rips the HELL out of my hand and starts running backwards! It was 180 out. Everything normalized when he finally got the right coil for it.

 

[Venomvpr900]

Yeah buddy the 075 will do that to me on occasion and when it does my forearm is sore for a week. I deleted the decomp valve because it was missing and I'll take a sore arm over the $40 for the valve.

 

[bucknfeller]

I had the same thing happen with an MS440 one time, tore up some nerves in my wrist. Had to get it operated on, and was on pain killers and nerve meds for over a year. Finally got sick of dealing with all the doctors and insurance company BS, and quit going to see them. It's been about 4 years and it still bothers me all the time.

So my advice would be to use the decomp, and keep it working properly. Learn how to drop start safely.

 

[blsnelling]

It's not because it has higher compression, it's because it's a larger engine compressing greater volume.

 

[GrassGuerilla]

Yeah buddy the 075 will do that to me on occasion and when it does my forearm is sore for a week. I deleted the decomp valve because it was missing and I'll take a sore arm over the $40 for the valve.

$40 decomp valve will seem incredibly cheap, easy and painless when you need surgery to repair the damage done to your wrist, elbow or shoulder. That "sore arm" is trying to tell you something. I'd listen. You'll get old and decrepit soon enough. Don't rush it.

 

[Venomvpr900]

$40 decomp valve will seem incredibly cheap, easy and painless when you need surgery to repair the damage done to your wrist, elbow or shoulder. That "sore arm" is trying to tell you something. I'd listen. You'll get old and decrepit soon enough. Don't rush it.

Lol I know, I think I'm going to fit it up with something similar to this motor

 

[NewToStihl]

Okay guys, thanks for the responses. The saw is right where I like these big boys to be, at 150 psi. I always get the piston on the upstroke and building compression (get starter to 'stick') before going for the fast pull.

Can anyone tell me why this happens though? What is it about good compression in a large volume top end that causes this? It's not that I can't overcome the compression when the piston approaches TDC, it only happens when the saw fires. I'm really curious what the dynamics are?

 

[old-cat]

I have an MS440, when I first got it it had a real bad kicking problem. Then someone told me to gap the coil .016", that stopped the kicking. A few days ago when I had the f/w off, I noticed there was quite a bit of slop in the f/w keyway, so I set the f/w all the way retarded and set the gap to specs. It runs GREAT now!

 

[blsnelling]

You're compression a much greater volume of air to that 150 PSI. Just a thimble at 150 PSI wouldn't feel like anything.

 

[NewToStihl]

You're compression a much greater volume of air to that 150 PSI. Just a thimble at 150 PSI wouldn't feel like anything.

Yep, I get that. There is a lot more surface area to accumulate that 150 pounds per square inch on with a saw of this size.

It's completely beyond me what would causes this specific situation though. My simple mind can't wrap around how the starter cord could be forcefully retracted back onto the spool. Given how starters work it seems like the only way for the flywheel (not the recoil spring) to pull the cord back it would have to be turning backwards.

To be clear, it's not that I'm hitting the high compression and the handle is just pulling out of my hand and coiling back up from the forces of the spring. It is literally snapping - with great force and speed - the handle back in. Absolutely not by means of the recoil spring. It was yanking the thing back with such force that I was genuinely worried it was going to break off a fan cover fin.

 

[Venomvpr900]

What is happening is the engine is firing just before the piston gets to TDC then trying to spin the engine backwards. Could be caused by a few things, could be the combustion chamber has a lot of built up carbon in it. Also check your muffler and exhaust port for excessive carbon buildup. Could be the flywheel has slipped? My 075 has only done it after reassembly and first start after

 

[NewToStihl]

What is happening is the engine is firing just before the piston gets to TDC then trying to spin the engine backwards. Could be caused by a few things, could be the combustion chamber has a lot of built up carbon in it. Also check your muffler and exhaust port for excessive carbon buildup. Could be the flywheel has slipped? My 075 has only done it after reassembly and first start after

I had the muffler off and it's clear as well as the exhaust port. Also, I removed the flywheel to verify the existence/condition of the key and everything is just as it should be there. The piston doesn't appear to have excessive carbon build-up, but I would need to pull the cylinder to check out the combustion chamber.

I'm not trying to look a gift horse in the mouth, but I have to ask, how would excessive carbon buildup in the combustion chamber cause the saw to fire before TDC?

 

[fossil]

I would suspect the tiiming is advanced a bit too much. The saw will fire sooner and at a low piston speed it will kick the piston back down. The flywheel dogs aren't made to release with the piston going down before it goes past TDC. The fast pull will usually beat that. Try starting with the piston just past TDC so it has a good speed going into compression. It doesn't seem to have anything to do with high compression. I have an 85cc Poulan that is a ***** cat to start, compression at 170 lbs, I have a Homelite 902 AM (77cc) and a Jred 801 (80cc) that you almost need to kneel on to keep them from flipping over. The Jred is around 195 lbs and the Homelite is at 150lbs. The Poulan is the strongest runner of the three.
As Brad says, displacement is the killer on kick back.

Looks like kitty cat with a p is a bad word

 

[firefighterwolf]

This is probably why stihl developed the elastostart handle with built in shock absorber.

 

[machinisttx]

What is happening is the engine is firing just before the piston gets to TDC then trying to spin the engine backwards. Could be caused by a few things, could be the combustion chamber has a lot of built up carbon in it. Also check your muffler and exhaust port for excessive carbon buildup. Could be the flywheel has slipped? My 075 has only done it after reassembly and first start after

Firing before TDC gets my vote. When I first started working on small engines, my uncle showed me that retarding the timing a couple of degrees eliminated the kickback on four cycle equipment. You lose a little power, but don't have sore hands or arms from it jerking back.

 

[fossil]

A friend of mine collects old Lawnboy mowers. He said starting backwards was a common complaint on the old Iron Horse models due to a very advanced timing.

 

[workshop]

I would consider it the nature of the beast. For me, big saws are just harder to pull over. While you can work with the timing by modifying things, they were built with the timing in a fixed position for a reason. You shouldn't have to mess with it unless you're looking for more or less power. Sounds like you need to adjust your starting technique. Try easing it past TDC slowly, then let rope retract again. Then pull like there's no tomorrow. If you give that rope enough speed, you won't have a problem.