don't stop a saw with the choke

Woody Weeder

I'm no expert on chainsaw mechanics, but I learnt something new the other day and because it seems obscure (to me anyway) I thought I'd post it.

My 009L stopped oiling, and I did the right thing and took it down to the friendly local stihl dealer to have a look. The first thing he asked me was "does the stop switch work?". Well, yes it works cos Dad fixed it before he gave it to me (hand me down, he got a 290). But it does have a history of being choked to stop it.

Anyway, I can't exactly say I was keeping up with him, but the dealer said that choking it makes it run rich and and sends a pulse through the crank case and petrol ends up stuffing the diaphragm, thats about where he lost me.
Can anyone explain it a bit better so it actually makes sense?

Anyway, new diaphragm, problem fixed, moral of the story is fix the stop switch and don't use the choke to kill the engine.

Eric Frei

I know a lot of people choke a saw to kill it.

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sharkey

Shutting an engine off with the choke every so often helps to clear the carb of any varnish or particulate matter stuck in the jets. For instance, Walbro HDA and related WT carbs. Generally a carb by definition works off barometric pressure created in the venturi where the baro pressure is beneath atmospheric. On the HDA carb, the low side feeds 80% of the engine needs. When at full throttle, the check valve under the high jet is designed to flow from vacuum rather than baro to supplement the additional needs of high rpm load.

Bruce Hopf

What the dealer said, dosen't make any scence at all. Shutting your chain saw off with the aid of the choke, won't harm the carburetor, or the diapgrams, in the carburetor.
As for the pulse, it is there all the time, when the chain saw is idleing, and working.
Your flooding the engine, to shut it off, with the choke on. It wets the spark plug, to where the spark won't ignite the fuel, because there is too much fuel, in the firing chamber, of the engine. It's all common scence.
Same with starting a chain saw with the choke left on, and we all knows what happens then, it gets flooded.
It has nothing to do with the diapgrams of the carburetor acting up. Like sharkey said, about helping clear the jets out.
Reasons for the diapghram needed to be changed, was either it was too old, stiff, and brittle, covered with too much varnish, and not pumping right, or worn out, from years of use. Just my thoughts on the matter. Bruce.

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McCulloch chain saws
1- Pro Mac 60, 1- Pro Mac 700, 2- Mac 10-10 Automatic's, 2- Mini Mac 30's, 2- Mac 110's,
2- Mini Mac 35's, 1- Mac 140 with Automatic Chain Sharpener,
1- Pro Mac 10-10, 1- Mac Cat, 2- Eager Beaver
2.0's, 1- Mac 1-10
Stihl chain saws
2- 044's, 2- 034's, 2- 024's, 1- 064, 1- 084,
Strunk chain saws
1- Busy Beaver, 1- SpeeDemon Special
Stand Back, I Have A Very Extreme Case of CAD (Chain Saw Addiction Disorder).

Bill Becker

I believe the dealer was referring to the oil pump diaphragm. This sounds plausible. The oil is pumped by a diaphragm that pulses with changes in vacuum and pressure in the crankcase. This is also why these type of oilers pump oil even when the chain is not spinning.

Bruce Hopf

Still chain saws are operated from either the clutch drum, or they are gear driven, from the crank shaft. There is no diaphgram, for the oil pumps, for the oiler, of the bar, and chain.
Now for different chain saws, like the old McCulloch chain saws that I have in my signature, that part is true, about the diapghrams. Bruce.

__________________
McCulloch chain saws
1- Pro Mac 60, 1- Pro Mac 700, 2- Mac 10-10 Automatic's, 2- Mini Mac 30's, 2- Mac 110's,
2- Mini Mac 35's, 1- Mac 140 with Automatic Chain Sharpener,
1- Pro Mac 10-10, 1- Mac Cat, 2- Eager Beaver
2.0's, 1- Mac 1-10
Stihl chain saws
2- 044's, 2- 034's, 2- 024's, 1- 064, 1- 084,
Strunk chain saws
1- Busy Beaver, 1- SpeeDemon Special
Stand Back, I Have A Very Extreme Case of CAD (Chain Saw Addiction Disorder).

Bill Becker

The 009, 011 are crankcase vacuum/pressure diaphragm pump oilers. These saws are discontinued. I have had 12 of them over the years, my climbers still use them for pruning and smaller take downs. Simple design and low price but not long lasting. I wish Stihl still made them.

Woody Weeder

Yep sorry I realised after I wrote it that I should have said that it was the oil pump diaphragm.

What he did say was something about petrol getting into the diaphragm which stuffs it. That is what I didn't get - is it possible that petrol could seep past the piston, into the crank case and into the oil pump diaphragm?

I did open up the plate over the housing of the o.p. diaphragm before taking it to the shop to see if anything was obviously wrong and I could have sworn there was petrol there. Or is this unlikely? I guess any petrol in there would be flushed out by the bar oil, so the petrol must get through to the other side of the diaphragm where the oil can't.

I also use one of these at work, they pack a fair bit of punch for their size and price but another down side is lack of anti vibration rubbers, tho i did see a 011AV the other day at the second hand shop.

Bill Becker

Fuel will be in the crankcase, that is how a 2 stroke gets its crankshaft and connecting rod bearings lubricated as well as the piston and rings. As the piston goes up and down it scavenges the crankcase through the side ports with fresh fuel and oil mix to lubricate the moving parts. On most saws the lubrication of the bar and chain is done with a mechanically driven pump which draws from the oil tank. On the 009/011 the pump is a diaphragm that pulses from the pressure/vacuum change in the crankcase. You may have seen fuel in the pump diaphragm, it's unlikely it will purge by running, you have to clean it out, change the diaphragm while you have it out. The diaphragm will wear out after awhile and stop pumping oil, I've changed several. I've never seen a failure that resulted in bar/chain oil getting into the crankcase.

Woody Weeder

Thanks for the great explanation Bill, I have a much better idea now of how it all works, cheers.

bobmac

Anyway, I can't exactly say I was keeping up with him, but the dealer said that choking it makes it run rich and and sends a pulse through the crank case and petrol ends up stuffing the diaphragm, thats about where he lost me.
Can anyone explain it a bit better so it actually makes sense?

On a somewhat related thought, but a bit different, many folks don't know to empty the fuel (by running the engine) when shutting down a 2 cycle engine they won't need for a spell. If you don't, the petrol/gasoline will evaporate and leave the oil behind to gum up the carby and anything else it gets into.
Cheers,
Bob M

ScottyAFoster

The amount of vacum can spike and drop when choking a saw. Choking starves the saw of fuel and cause it to sputter which causes the crank case impulse to rise ans and fall. On saws with vacum assisted oilers any fuel vapors that are in the crank case can sucked into the oiler vacum line.

olyman

you can go ahead and do that,,but it aint a very good thing to do.....when you do that,,the engine is running extremely lean on shutdown..and eventually,,you WILL score the piston/and /or the cyl wall...............

ScottyAFoster

A choke will not cause a saw to seize. Choking a saw to stop will cause the saw to build up carbon.