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2 cycle, how much visible smoke is "right"?

2973 Views 19 Replies 10 Participants Last post by  jerry_nj
I am a bit worried when adjusting the mixture (carburetor) on a 2 cycle engine that it will be too lean and that can damage the engine due to a insufficient lubrication. On the other hand, too rich means less power and more visible smoke.

Is there any useful information that can be gathered from looking at the exhaust smoke out of a small 2 cycle (e.g., on chainsaws)? This question assumes a fairly modern engine designed to work on 50:1 or 40:1 mixtures, not 32:1. That noted, I'd suppose the 40:1 would smoke more than an equivalent engine running on 50:1. Is there any real difference, i.e., what is the risk/damage of running 50:1 in an engine that calls for 40:1? I may be willing to accept faster wear for less smoke/pollution. That is on a low cost unit that get light use may take 10 years to accumulate 100 hours. So if such an application/use is the case running a 40:1 engine at 50:1 may mean it will exhibit troublesome wear at 200 hours of use. I'd say that's good enough for me to run the engine will less smoke and smell.
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Well, this is all beyond where I am at the moment, but looks like good training/advice...maybe it should be a "sticky"...

My recurring problem is just keeping the saw running...even poorly. This saw has seen other post threads in the last month, one on "deteriorated gas line" which was a core problem The ongoing problem is I fix something in the gas line area and the saw runs well enough to cut wood/trees for 15 minutes, then something else goes wrong. This go around I found that the new gas line I installed wasn't fitting tightly to the carburetor, it even has a tear. I repaired that, but think it still needs help, like a very small hose clamp. I order a few 1/8" clamps from Amazon. In the meantime I put a larger diameter piece of hose over the area of the carburetor nipple and used a plastic tie to snug it up. The saw then started and ran, even got it to idle reliably, if not perfectly. This got me to start this thread as I was making a serious attempt to tune the L and H mixture adjustments.

Back for a moment to smoking, I took the saw out to take down a dead Easter Ceder with a double trunk about 18" across and 10" wide. The tree was about 25' tall.
This cutting, warmed the saw up and there was no longer any noticeable smoke. I was lucky and laid the tree down right where I wanted it and so I started trimming off limbs to get to the core trunk to cut into firewood rounds. I wasn't working long before the saw started dying. I finally raised the "hood" and looked at the gas line and could see the engine was dying because of the lack of gas. I don't think this was due to any leaks in the gas line as the gas would flow for a few seconds, then go empty. I think the diaphragm pump is leaking, intermittently. Is that possible? I can't get a rebuild kit for the Zama carburetor (sears offer a new carburetor for $20, so why rebuild anyway?). I had opened the carburetor in my try to get the last running series going. It has very few hours on it and the insides look very clean... and when I reassembled and fixed the leaky gas line the saw started a number of times during testing over two days, and as said above ran for a take down of a tree, then quit running... noting loss of gas flow being visible in the new gas line.

On the primer, I had asked elsewhere and didn't get a lot of help. This series of trouble shooting has lead me to believe the primer bulb works as an air pump between (one side of) carburetor and the top of the gas tank. This is why the primer line just terminates in the top of the tank, does not go into the tank to gain access to the fuel. What I noticed whas when I pump the primer I can see gas pump up through the gas feed line and into the carburetor. Thus, the primer must be pumping air into the gas tank, putting air pressure on the fuel, while drawing this air from the carburetor venture area.. or thereabouts.

As said, I may need to make a specific post on the question: can a carburetor fuel pump diaphragm exhibit the failure sequence I see... engine runs for a while then stops:dunno: and inspection of the gas line shows the engine is dying because of a loss of fuel flow in the gas feed line? If this was a hard fault I'd not be asking, but on this engine the thing runs for a while and then quits. I'll try to start again today as the engine (diaphragm) fully cools off. If it starts this would suggest the failure in the ability to pump gas is temperature depended, wouldn't it? Again, I'd expect a bad diaphragm to not pump gas not to pump sometimes and not other times.
Thanks, the same is true for the chainsaw I'm working on. It is a $150 saw at the most. But, I hate to throw things away, expecially if they look in good physical condition. This saw also leaks bar oil. So, after use, it is best to empty the remaining bar oil, otherwise it will end up on whatever the saw is setting on.

I think I have learned something else about using the gas line to trouble shoot. On a low cost Homelite 14" saw that also will no longer start I found after pumping up the primer circuit and then pulling the gas line off of the carburetor that gas gushed out, at least 2 oz of gas/oil. As the gas tank is below the level of the gas line when removed from the carburetor, there had to be pressure in the tank. Thus, as I have speculated elsewhere, the primer pumps air from the carburetor (thus the hose to the carb) to the gas tank, and thus back up the gas feed line into the carburetor.. and raw gas into the carburetor. As this second saw was holding pressure I think the carburetor is plugged up (dirty). I may take another look at that. I may try putting some vacuum on the carburetor gas input nipple and see if I can draw air back out, I think I should. As it turns out I have just purchased a Mityvac to help me do brake flush work by myself on autos and pickup.:thThumbsU
Thanks, I decided to check the carburetor further, in part because I have another saw (a low end Homelite) that will not run either. When I put that saw on my bench last night and pulled the fuel line off the carburetor gas pumped out in at least a couple of ounces. Thus, the attempts to start had accumulated some back pressure in the tank. This made me decide to check the input feed on that saw with my Mityvac hand pump and I found I could not draw air back from that carb even with 25 in hg. Simple side track here as I think the problem with the Homelite saw is the gas line is plugged, not sure how to clean it, soak in carb cleaner?

Anyway I then went to the subject saw for a comparison and when I put the vacuum gauge on that carb's fuel input I couldn't develop any back pressure, confirming the other saw is plugged up. Anyway, this caused me to look again at the carb (a Zama in this case) fuel line connection. This carb has an very small diameter nipple and even though the new line I put in was picked up from a small engine parts place were I took a piece of the original line, the new line doesn't have the real snug feel when it is pushed on. This time after putting the fuel line on I pumped the primer to see if I could see gas coming into the fuel input line. I couldn't but decided to pull on the starter rope anyway, and the saw started. Looking again at the new and easy to see into fuel line I decided it was full and there were none of the past observed bubbles... so it is clear this original problem is ongoing. I have ordered some 1/8" hose clamps (spring wires) and hope that will snug the line on the nipple. In any case, the saw is running fairly well and I took it out and cut branches and a few rounds over about 15 minutes and the saw ran well. So, as you suspected, the problem is air leak not bad carb. This would fit with my past history since replacing the fuel line of the saw running for a while, then it stops running.

Do you have any suggestion on how to snug up the fuel line on the nipple? I hope the yet to arrive hose clamps will help. I can again try wrapping the hose at the nipple with something to make it thicker there so the clamps will put more pressure on that fitting.
Thanks for the confirmation on the primer circuit. I was initially troubled when I didn't see the primer line going into the tank, I at first thought it to had disseminated. My last followup post tells how after reattaching the fuel in line to the carburetor that the saw again ran, and rather well (the two cycling verses 4 cycling sound is beyond my skill and hearing - so if the rpm is steady I feel it is idling well). I put the vacuum pump on this carburetor more for reference and as noted the very small nipple left me without a hose/adapter that could seal on the nipple. However I have one cone shaped adapter that fits well enough, I estimate, to establish some vacuum if the line was closed. In the case of this Craftsman/Poulan saw I couldn't see any vacuum build..again not a great seal at the nipple.

But, on another troubles saw, a Homelite, which has a larger nipple I found that a vacuum measurement there showed a complete block...thus I am prepared to guess the problem with this saw is the carburetor is dirty/plugged. The two saws have very similar carburetors physically, but different manufacture: Zama on the Craftsman and Walbro on the Homelite.
On the Craftsman saw, the initial subject of this thread, I can't see any vacuum on at the fuel input nipple. But this is the saw I've been having trouble getting a good air tight gas connections... by the way, I started that saw this morning and did about 20 minutes of tree cutting. When I was done I had to switch the ignition off to stop it...running great. The only reason I tired to test the Craftsman saw was to confirm the good or bad result I got on at the same point on the Homelite carburetor. This one would hold a full 25" HG. I figured this might be an indication that the Homelite will not start because no gas can get in. But, you say the needle should seal tight enough to hold some vacuum pressure. The reason I started looked at this was because when I pulled the gas line on the Homelite gas came out to troublesome extent, I'd guess 2 ounces. Enough it took a full 3 to 5 seconds before all the pressure was gone. Given the discussion on the way the primer works, I assumed pumping the primer pulled/pushed (I will re-read you explanation on the primer circuit) gas into the venturi (cylinder) as well as put gas at the mixture needles. Thus, I assumed the reason the gas came out when I pulled the hose was because there was pressure in the gas tank that couldn't be released via fuel transfer into the carburetor.

Again, the Craftsman is running about as well as I need (not perfect I am sure). The fact it appears the reason I couldn't "pull a vacuum" is due to a poor seal at the nipple, my history problem area - since I noticed the gas line was completely deteriorated in the tank, the filter was just laying loose in the tank.

Speaking of the history, ethanol damage assumed, does putting gas stabilizer in the gas mitigate the ethanol effects? My autos which have been subjected to ethanol in the gas since whenever NJ started doing that (just a few years I think, and my oldest is a 2004 Forster) do not suffer (yet?) from the ethanol eating gas hoses and gaskets.
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