Non GS, non mo'cycle. 2 cycle engine

[Redman]

2 cycle chain saw engine question.

Dont want to post in off topic because I dont want to know if if I am too conservative or if I am too liberal. And I already know I am somewhat ignorant of this subject.

Basic question: I can take a compression test on a 2 cycle engine, right? I have a compression tester that has a couple different adapters for different sizes of spark plugs.

Situation: Have had chainsaw new from 1982, Craftsman/Poulan.
Sat unused for 15 years (in attatched garage). Seemed to have have good compression based on my observation of it going fump-fump-fump when pulling the cord.
And the ignition made spark.
I cleaned out the tank, replaced the fuel filter in tank. Took apart the carb and the diaprahams looked good. Squired carb cleaner in various orifaces. Replaced the air filter. Replaced the bar and chain (were rusty). And got it to run, and run well. Used it for about 20-30 minutes on one occassion.
Is older design, so is 16:1 mixture, and that is what I used, blueish smoke exhaust and all.
And Then went to use it again, and cant get it to start.
Will start and run for a bit if I drip gas in carb. Still seems to have the same fump-fump-fump when pull the cord. Carb diaphrams still looked good.
Took it to an indepandant small engine shop.
They said cyclinder looked good at first glance (service manager; when I first took it there took off muffler and looked, said it looked real good) but now on closer inspection (technician) says it has one deep score in it that makes it so will not pull gas up to carb. And deemed it not worth repairing.
Huh?
That make sence?
Has good enough compression to run, but not eneough suction to pull gas to carb...? I dont fully understand what developes the suction anyway.

I suspect maybe they just didnt want to bother with trying to get the older parts.

Any comments?

[Agemax]

have you checked the plug? have you still got a spark and is it wet with fuel? 16:1 sounds like a very rich fuel/oil mixture to me, especially if you are using modern 2 stroke oil

[tkent02]

Yes you can check compression
two strokes get scores in the cylinders quite often, may or may not be a problem.
He may know he can't get parts

Does it use suction? I thought they has a little fuel pump that is powered by engine vibrations?

[Redman]

It will run for a bit if I drip fuel in carb, so I do not suspect ignition problem.

16:1 is what is listed in original owners manaul (from 1982). I suppose that recomendation is based on the metallurgy of the time, but maybe is based on the oil of the time. But anyway, that is what I would contunue to use. I do notice that more recent 2 cycle engines use 32:1 or 50:1 (but that is a different discussion).

I can find lots of parts for this model on a Sears parts website for not too terrible price. DOnt know what specific Poulan model it is to look for parts that way. But, yah, maybe that shop cant get the parts thru thier normal supplier.


.

[tkent02]

If it still goes thump thump thump there is enough compression, maybe not enough to idle well but it should start and run.

Can you tell how the fuel gets into the engine? maybe something in there is clogged?
A kinked fuel line is very possible.
I'm always getting sawdust in my gas tank, could this have caused blockage somewhere?
Is the fuel really old? Fuel could have evaporated out, leaving too much oil in the mix.

And, the fuel should be mixed at the ratio the engine was designed for, to mix 50-1 in an engine set up for 16-1 would make the fuel mixture very rich, and to go 16-1 on an engine designed for 50-1 would make it run very lean.

It is opposite of what you would think, but the oil does not burn as fuel, just takes the place of some of the fuel, leaving less of it. So too much oil in the fuel equals too lean, and can burn things up even with all that oil.

[Redman]

...
.....
Does it use suction? I thought they has a little fuel pump that is powered by engine vibrations?

Gas line goes from tank to side of carb, and that part of the carb has some dipahram and the diaphram has a little flap that seems to cover an oriffice. I would think that diaphram is operated by some pressure during part of the cycle and some suction somehow at an other part of the cycle.

Seems like niether of us really knows. But then again, I think we are friends enough that I can say that I often cant tell if you are joking or serious.

And this is all in addition to the bigger reed valve under the carb between the carb and the crankcase

..

[hampshirehog]

Like tkent says, check that there is petrol in the carb and not just oil - a common problem on 2 strokes that stand a while. Plus if your plug is getting wet it's an ignition problem, if it stays dry it's either a carb or induction fault (could be crank seals which go hard on 2 stroke left standing).

Ditto what he says on the petroil mix as well. Don't waste money on synthetic oil but at that ratio it'll pay to get low ash oil if you use it a lot as otherwise you'll be decoking the thing all the time.

[tkent02]

Gas line goes from tank to side of carb, and that part of the carb has some dipahram and the diaphram has a little flap that seems to cover an oriffice. I would think that diaphram is operated by some pressure during part of the cycle and some suction somehow at an other part of the cycle.

There must be air passages to one side and fuel passages to the other?
Are they all clear? Is the little flap in good shape? Does it seal? Is the diaphragm installed correctly?
Is there perhaps a spring missing there?

But then again, I often cant tell if you are joking or serious.

There is that.

And this is all in addition to the bigger reed valve under the carb between the carb and the crankcase

..

This is in the intake port, just like on a dirt bike? Just increases engine torque, nothing to do with fuel...

I also have a garage sale leaf blower, two stroke, won't start, fuel issue.
I might get inspired now and dig into it today.
Off to do some chain sawing first, if my saw starts... Good luck.

[chiphead]

I believe the fuel is drawn into the lower crankcase as the piston moves upwards then is forced through transfer ports up into the combustion chamber as the piston moves downwards. When the piston again moves upward it again draws fuel into the crankcase as it's compressing and igniting the fuel in the combustion chamber. Rinse and repeat.
Any leakage (like crank seals for instance) would result in the crankcase sucking in air instead of fuel. So when you dribble gas into the carb throat you're manually loading the crankcase. This assumes that the carb is actually capable of feeding fuel.
If the score is deep enough in the cylinder wall (and what caused that anyway?) it is possible that the negative pressure needed in the lower end no longer exists so it won't pull fuel.

[Steve]

Did you dip the carbs?
Have you synched them?
Did you use all new o-rings from chainsaworings.com?
Have you checked the stator output?
Are all your electrical connections clean?
How is the battery?

Oh, sorry, wrong topic. We just get used to tossing out all these questions.

.

[Dave8338]

Though the mixture is stated at 16:1 by any and all current standards...that is too rich. I run all my 2-cycles on 32:1 no matter how old they are and have done so for up to 10hrs at a time, without ANY problems. Actually the older 2-cycles were over oiled even by standards back then. Additionally...spark is VERY important on a two stroke. I would start by pulling the carb again (complete tear down) and there is usually a screen (very fine) in there some where. Often times IT gets coated in sludge from old pre-mix and is the main culprit.

On the saws, Lawn Boys, wet saws, ect...the exhaust stroke pulls vacuum from below the piston to draw fuel, via the "fuel pump" to the carb(s).

The reeds are just another way to close the holes in a cumbustion chamber. Same job as the valves in a four stroke engine.

[tkent02]

I believe the fuel is drawn into the lower crankcase as the piston moves upwards then is forced through transfer ports up into the combustion chamber as the piston moves downwards. When the piston again moves upward it again draws fuel into the crankcase as it's compressing and igniting the fuel in the combustion chamber. Rinse and repeat.
Any leakage (like crank seals for instance) would result in the crankcase sucking in air instead of fuel. So when you dribble gas into the carb throat you're manually loading the crankcase. This assumes that the carb is actually capable of feeding fuel.
If the score is deep enough in the cylinder wall (and what caused that anyway?) it is possible that the negative pressure needed in the lower end no longer exists so it won't pull fuel.

This is how air is drawn through a two stroke, the fuel is added by a venturi in the carburetor, also basically the same as any carb. What we are missing here is the method of moving fuel from the tank into the carb so it can be sucked into the engine. Gravity won't cut it, saws need to run at all kinds of odd angles, they have clunks on the fuel pickup in the tank.

[flyingace]

I thought gas was 'pulled' into the carb via a vacuum diaphragm operated on engine vacuum from the crankcase.

[bwringer]

I didn't see a mention of replacing the fuel line(s)... these seem to go bad and split every couple of years on more modern stuff, so I can't imagine they're any good on an older machine.

[Dave8338]

I didn't see a mention of replacing the fuel line(s)... these seem to go bad and split every couple of years on more modern stuff, so I can't imagine they're any good on an older machine.

Good point. They seem to harden and split overnight.