I can't follow this "suck back" thing

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rupedoggy

rupedoggy

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I have seen a lot of threads that relate to a two strokes sucking foreign particles back into the cylinder while running. I just can't quite buy it. Crofter seems to be a believer. I would challenge him (or anyone else) to put some colored, but non-harmful powder into a two stroke muffler and show how it gets back into the cylinder. I am not talking about piped saws but the normal muffler or even modded saws. No cheating by tipping the saw and having gravity do it (which is why I think this "suck back" thing got started).
I have been wrong before but like to see some real test before jumping on the band wagon. Now having said this, I am truly a believer in "spit back". I can observe the clowd of vapor behind a non-filtered carburetor. Some of the big old 125 Mccullochs even had a type of filter to catch it. The 066 Stihls do it also. Mike
 
Well I see Tom is reading this thread; so I'll be interested in his comments; but, I have to agree with you, based upon my limited experience. Never happened. Is it a myth or a fact? :cheers:
 
Just stuff your muffler full of 3M scotchbite pads. That should take care of any suckback issues.
 
A lot of explanations and hype on Ebay sure are a joke. however, if you dont think there can be reverse flow thru the exhaust port try removing your muffler entirely and see how it affects your saw. See how much you have to richen the carb to get the idle back down from a chain dragging 4 grand idle. Is that because clean air instead of escaping charge is coming back in to mix with the next charge. Now imagine another scenario. Run your saw up to 14 thou and take your finger off the throttle. The remaining intake opening will be approx 1/100th of a square inch so there will be virtually no crankcase compression or flow of charge up the transfers. As the momentum of the crank and flywheel forces the piston up and down it will displace air in and out the exhaust port and up and down the transfers till the rpm drops back down to where the idle circuit is supplying enough mixture to recommence firing. A similar but much longer duration motoring of a two stroke engine occurs in ultralight two stroke aircraft on long power off descents and has caused failures from lack of lube and running lean combined with cold squeeze in the winter. (You know what they recommend? blipping the throttle! Lol, but that is another issue)
I think that under sustained load there is darned little if any flow reversals through the exhaust, but at idle or coast conditions .................?
 
Come on, I would like to hear the explanation of why you dont believe the suckback or the connected problem of how you can seize a saw with a plugged filter.

Close the intake off with the throttle plate and move the piston up and down (by the momentum of the flywheel, this is the coast down mode from full throttle) The exhaust is wide open to atmospheric pressure and the piston is displacing air. Since the intake is shut what will prevent the piston from displacing air in and out the exhaust port and up and down the transfers.
 
Crofter said:
Come on, I would like to hear the explanation of why you dont believe the suckback or the connected problem of how you can seize a saw with a plugged filter.

Close the intake off with the throttle plate and move the piston up and down (by the momentum of the flywheel, this is the coast down mode from full throttle) The exhaust is wide open to atmospheric pressure and the piston is displacing air. Since the intake is shut what will prevent the piston from displacing air in and out the exhaust port and up and down the transfers.
Click to expand...

The piston skirt.

Gary
 
Thanks for responding Frank. This was discussed long before the "joke from Ebay" entered the scene. Some guys had failures after modifying their mufflers and blamed it on the exhaust sucking back some of the metal left from the modification. I could not envision that happening. I could see tilting the saw and having gravity help with the contaminants.
So Frank, can you obtain some bright colored powder of such fine matrix that no damage could occur to a saw, and do a little "show and tell" for us? If we lived close I would donate a craftsman to experiment on. Mike
 
West Texas said:
Well I see Tom is reading this thread; so I'll be interested in his comments; but, I have to agree with you, based upon my limited experience. Never happened. Is it a myth or a fact? :cheers:
Click to expand...

Sorry Tex, I had to run abit ago. Bout the only thing the saw will suck in is air. Prime example is to remove the muffler all together and watch how the saw reacts, its idle goes way up. Same holds true for cars. Remember the good ole days of hot rods and if you cracked a exhaust mainfold you would burn a valve if you didn't get it fixed ASAP. Same with a saw. There is some sucking going on though many won't believe it. Whether its srong enuff to actually suck in some kind of object is beyond me.
 
I don't buy it either.

But I think a set of sweet ceramic exhaust bearings will never allow that to happen.

Oh and now that its getting to be summer time in North America, don't forget to switch to synthetic blinker fluid in your cars to help with the homolgation of the molecular globules, possibly creating a flux capacitance across the end line of the rotary girder.

I'm retarded.:jester:
 
I dont buy it either.
FWIW its easy to see if it happens as debris ingestion shows tell tale signs on the piston skirt. I have never seen this on my saws and I toss the park screens ASAP after when I buy a new saw.
 
jp233 said:
I don't buy it either.

But I think a set of sweet ceramic exhaust bearings will never allow that to happen.

Oh and now that its getting to be summer time in North America, don't forget to switch to synthetic blinker fluid in your cars to help with the homolgation of the molecular globules, possibly creating a flux capacitance across the end line of the rotary girder.

I'm retarded.:jester:
Click to expand...


Where ya get those ceramic exhaust bearings, sounds like just what I need for my new sky hook,:D :D :D
 
Ceramic exhaust bearings not only prevent suck back but the also provide for a 7% increase in cut speed, sound "cooler" and make chicks swoon at the mere sight of you.
Now, send me your money.....:greenchainsaw:
 

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