Two colored spark plug

[glens]

I'd learned it was called "chop" because the <i>ignition</i> was being chopped.&nbsp; The throttle is to be held open during the chop for exactly the reasons sedanman gave.&nbsp; Once the engine has stopped turning the throttle position is immaterial (but it probably should not be re-opened if there's an accelerator pump).&nbsp; The engine is to remain disengaged from the transmission afterward, though.

It might be worthwhile to perform a plug chop on a racing saw, but I agree with Ben otherwise as to their necessity for chainsaws.

 

[sedanman]

Here's is my final thought on the subject ( I promise!). Ask yourself how the choke works, does it somehow manage to add fuel by opening the throttle? NO. At cranking speed 3 conditions are possible. Choke open throttle closed, a hot saw will start in this position. Choke closed throttle position doesn't matter as the choke is the limiting factor to air flow, this is where we start a cold saw. Choke open throttle open, we use this position to clear a flooded engine as this provides the leanest possible mixture to the engine. What do we learn from this? At a given engine speed the widest throttle openinng delivers the least fuel. When we close the throttle on a running engine the engine speed drop, we all know this. When we kill the ignition on a running engine the speed drops to zero over time. With the throttle open on an engine with no ignition a full intake charge is avialable to be comressed, with no power pulse to keep things going the engine will QUICKLY come to a stop. Remember the throttle does not control fuel flow rather it meters air. Only vaccuum draws fuel.

 

[glens]

Hi Paul.

It's maybe a little more complicated than that.&nbsp; In general, however, you're right.&nbsp; A good seat-of-pants method to get jetting into the ballpark is to cruise long enough at a given throttle setting and note what happens immediately as the throttle is quickly let off a tad.&nbsp; If you gain power momentarily, you're lean.&nbsp; If it falls too hard on it's face, you're fat.&nbsp; (Or do I have them backwards?&nbsp; It's been a while.)

However, with normal air flow through the venturi, all applicable jets are involved.&nbsp; The vacuum increase resulting from a closed throttle on a spinning engine will pull the higher-speed jet(s) out of the loop, but will greatly exaggerate the contribution of the low-speed fuel source(s).&nbsp; Overall there may (or may not) be more total fuel being pulled, but instead of a well-mixed mist, we're now dealing with raw globs which will puddle and cause poorer combustion.&nbsp; The balance is wrong, but it may still be less fuel than "a moment ago".

The choke causes the same effect via <i>all</i> jet orifices, doesn't it?

Glen

 

[glens]

I should state that I'm talking (above) about engines with carburetors designed for use at more than fully open or close positions.&nbsp; It has little to do with current chainsaws.

Glen

 

[Crofter]

Originally posted by bwalker
More fuel would be sucked into the motor with a open throttle butterfly verse a closed one. .

I wont get into plug chops because I have never used the procedure, but Ben is right in the above statement. Tho the vacuum goes momentarily high when the throttle is snapped closed with engine revs high, the air flow is minimum and only the low speed jet is exposed to this vacuum.(Unlike in choke circuit operation) Whether as Glen says that very small charge per stroke is actually richer than the open throttle would be I dont know because of the great backwash from the exhaust due to the engine being windmilled with closed throttle.

Frank

 

[Crofter]

Originally posted by glens
but instead of a well-mixed mist, we're now dealing with raw globs which will puddle and cause poorer combustion.&nbsp; The balance is wrong, but it may still be less fuel than "a moment ago".



Glen [/B]

Why the gobbs of fuel. would it be any different from idle condition? Vacuum may not be much higher so flow no greater than idle.Glen you know vacuum is not proportional to rpm in a piston pump and is limited by the volume differentials and ultimately by atmospheric pressure.

Frank

 

[glens]

Frank,

We're talking about the few cycles incurred while the engine comes to a stop while neither driving nor being driven.

I don't know if the total amount of fuel would be more from the low-speed orifice under such a condition than would be admitted from all the jets as when the throttle is open.&nbsp; I believe I'd stated that clearly enough.&nbsp; What I see I'd said is that the amount of fuel being passed from the low-speed circuit would be greater than normal for the moment.

I guess I'm mixing responses of running engines with the topic under consideration.&nbsp; Placing dash-pots on throttles to prevent them snapping closed was common practice on the later carbureted auto engines (and was done to prevent the condition Paul and myself allude to) (constant velocity carbs remove direct throttle control in both directions, to also avoid too great a sudden loss of vacuum).&nbsp; There may be (marginally) no more vacuum produced per intake stroke on a faster-spinning engine than at idle, but the limited area through which that vacuum is relieved over time is proportionately much less, resulting in a greater store, and subjecting the LS jet to abnormal conditions as it acts as more of a vacuum bleed than usual.

Nevertheless, it's proper to leave the throttle open when performing a plug chop.&nbsp; From a purely technical aspect it probably matters much less than missing the proper timing of ignition shutoff and load removal (to prevent back-driving) does.

Glen

 

[jsikkema1]

I always remove my test plugs at WFO while the saw is running (thats wide F'ing Open) that way they are untarnished from anything but WFO operation. Sure is tricky though- the hair on my nearly bald head stands straight up every time I do that and they are hard to loosen too...

Next time I do that (I hope not too soon either) I'll take a .mpg and post it so that you can see it-

Jeff

 

[bwalker]

Glens, What about the inertia of the crank, chain and spocket?

 

[glens]

You're in the cut, aren't you Ben?&nbsp; I mean, we're talking "under load"; at least I am.

Glen

 

[bwalker]

Brain fart on my part, Glens. I keep thinking of the bikes and selds I have did them on. Of course a saw is totaly differant due to the load, etc.

 

[glens]

Yeah, the sleds would be something without a manual clutch.&nbsp; What is it, a centrifugal setup?&nbsp; If so, how long will the track over-drive the engine before the clutch drops out?

Glen

 

[tony marks]

d---,amazin post.. and to think this ole hayseed actually understood some o what yall said..i keep reading this stuff ,,
im liable to open a chainsaw shop myself..nah

 

[Crofter]

Tony I Know What Ya Mean

I have used plug color on car engines in diagnosing problems but figured the mix in 2 strokes would cloud the issues. will have to give that another serious look. The modified 372 I have sounds so different fom what I was used to hearing, I keep second guessing about the mixture being rich enough and I dont like holding the throttle open no load to hear if it is breaking into 4 stroking.

Frank

 

[bwalker]

If so, how long will the track over-drive the engine before the clutch drops out?

A long time when coming down from a 110 mile per hour lake run. Off course you can just lock the track with the brake if you have studs, but thats a little ruff on the driveline.