Saw cutting out on me question

[tbrickner]

Hi folks,

I have a cheap homelite 33cc saw with a 16" blade.  The saw works fine in winter.  The summer is a different story.  In the summer I will run it till it gets low on gas and turn it off.  If I let it sit for about 15 minutes I can't restart it.  On the other hand if I immediately refule it before 15 minutes and start it back up it will turn on immediately and I just let it idle till I need it.

Unfortunately this is a cheap home owner saw that has only and Idle adjustment.  The low and high adjustments need a special tool to adjust that I don't have.  I was thinkin of trying to replace the high and low adjustment screws with something else.

Any Idea what may be causing this symptom.  By the way I don't let the saw run out of gas before I shut it down and I do have a tachometer so I am not over running it on high.

Any thoughts would be helpful.

Thanks,
Tom

[Rocky_J]

Vapor lock. It's not in the carb adjustments, it's the fuel boiling in the tank/ line. The carb is designed to meter liquid fuel. If the fuel vaporizes before it gets to the carb then it won't run.

[John Mc]

Would cleaning cooling fins help avoid the vapor lock? Would letting it idle a bit before shutting it down help (hopefully keeping the fuel flowing in the line so it doesn't boil while things cool off a bit)?

If the saw is running hot, perhaps opening up the muffler a bit?

[tbrickner]

Rocky J,

Do you feel I should just take off the muffler or open it up. 

I do keep the fins clean so it is probably a bad design on the cooling system.

Would you recommend running the saw till I get to the 1/2 tank level and waiting 2 minutes before turning off the saw?  Typically I cut off the saw at about 1/4 full tank after letting it idle for 2 minutes to let the engine cool down but it doesn't help if I leave it for more than 15 minutes.

What are your thoughts for a solution.

Thanks for your help Rocky,
Tom

[Rocky_J]

On many of those cheaper saws, the fuel tank isn't isolated far enough from the exhaust/ motor heat. When the tank is full, the liquid fuel acts as a cooling agent. But when the tank is near empty, it gets hot. Especially in the summer.

A long time ago I got frustrated with vapor lock and started trying to take small steps to help prevent it. Keeping your fuel can in the shade, keeping unused saws in the shade, etc. I've worked in the Florida sun all my life and so it's second nature for me to keep myself and my equipment as cool as possible.

You're on the right track. Once you know what's happening, it becomes second nature to take little steps to avoid it.

[Brucer]

A few years back I had that problem. An "old-timer" advised me to try shutting down the saw in hot weather by pulling the choke. That did the trick (but only if I hadn't run out of fuel ;D ).

[joe_indi]

Sorry, I hit the enter key before I was finished.Read the next one.
Joe

[joe_indi]

Tom,
Vapor lock may not be the cause because you say the saw starts up once you re-fuel it.
Though I dont get to see many Craftsman saws in my country, the ones I have seen had very flimsy tubes, more suited for IV sets, as fuel hoses.
These hoses tend to become brittle and crack, developing leaks.
If the fuel level drops below one of the cracks, nothing but air is pulled into the carb.
This could be the reason why the saw runs when you fill up the tank.
Changing the tubing should have your saw up and running .
If I am not mistaken this saw also has a primer bulb.Tubes to this primer too have the same 'quality'.
It might be a good idea to change these tubes also.

Joe

[chucker]

 a trick i learned from a big woods faller from sw. wash. was to use a cap full of tranny fulid to each tank to prevent vapor lock in saws that were run hard and long . worked on the older saws ? so give it a try .....

[John Mc]

I'm not familiar with the fuel delivery system, but if it has a primer, would hitting that a few times force liquid fuel into the offending gas line and flush out the vaporized stuff? Or does the primer have a separate path to the carb?

John

[Ianab]

I'm not familiar with the fuel delivery system, but if it has a primer, would hitting that a few times force liquid fuel into the offending gas line and flush out the vaporized stuff? Or does the primer have a separate path to the carb?

John

Not sure... One theory of mine is that the modern saws have pressurised fuel caps. When you set them down hot the fuel in the tank vaporises and forces extra fuel into the carb, as there is no other vent. Pick them up 5min later, and they flood.

I know my ms310 has some issues on a warm start. Cold start is fine, hot starts after 30sec is fine. Leave for 10 mins, no worries. But let is set for a minute or 2 hot, and it can be tempermental.  If I then treat it like a flooded saw (wide open throttle) it will generally  start OK. An actual vapor lock would suggest you need to choke it to draw in more fuel, but that just makes the flooding worse.

Zero scientific evidence, just personel experience on the starter cord.

Ian

[joe_indi]

I'm not familiar with the fuel delivery system, but if it has a primer, would hitting that a few times force liquid fuel into the offending gas line and flush out the vaporized stuff? Or does the primer have a separate path to the carb?

Though we call it the  primer, the proper term for it is 'purger'.
A primer, like John mentions, would force fuel from the fuel tank into the carburetor.
But the purger (primer) does just the opposite.The fuel tube runs from the tank to the carburetor.From the carburetor it goes into the inlet of the purger.A tube from the outlet of the purger goes back into the fuel tank.
So, when you press that bulb and release it, it draws fuel from the fuel tank, through the carburetor.When this happens, any air or vapor in the tubing or in the carburator are sucked into the purger bulb.When you press the bulb, the contents of the bulb(fuel & air) are sent back into the fuel tank.
This removes any trapped vapor in the carburetor.

Not sure... One theory of mine is that the modern saws have pressurized fuel caps. When you set them down hot the fuel in the tank vaporizes and forces extra fuel into the carb, as there is no other vent.

Regarding the pressurized caps, a saw with its engine running does not have a positive pressure in its tank, rather it has negative pressure because fuel is being sucked out.Too much of negative pressure would result in the saw stopping because the fuel pump might not be able to draw fuel against too much of negative pressure.
To prevent this from happening there are vents in some form on the fuel tanks.
The common arrangement is a bit of tubing with a headless screw in the middle.
A bit more sophistication is a small non-return valve which lets air into the tank but does not allow tank fumes to flow out.
The most sophisticated are the non-return valves with a  filtration setup to prevent sawdust from entering the fuel tank.
If hot saws were to to vaporize the fuel in their tanks, that should be happening here where the temperature hits 35 -37deg C, maybe even 40 once in a way.
Blocked pickup bodies, carb strainers, leaks in impulse lines and fuel hoses have caused problems here, but vaporizing fuel and vapor locks I am yet to come across.

Joe

[tbrickner]

Guys,

My saw is ok the first 15 minutes I let it sit.  But left longer than 15 minuts after running it hard and it won't start up right. Priming doesn't help even with a full tank of fuel

If I fuel it up within the 15 minutes and start it up it is fine but I have to let it idle otherwise it will cut out.

Tom

[John Mc]

That sure does sound like vapor lock. It doesn't happen when the saw is running (either in-use or at idle), because there is constantly new fuel being drawn into the fuel line an carb. It doesn't sit there long enough to get heated up and vaporize. When it sits for a while, the heat from the engine soaks into the fuel line and can start vaporizing the fuel. If you start it back up quick enough, the fuel has not vaporized.

John Mc

[tbrickner]

John Mc

You just described what is going on with the saw to a "T".  Any suggestions on what to do to help the situation.

Thanks for your input,
Tom

[John Mc]

John Mc

You just described what is going on with the saw to a "T".  Any suggestions on what to do to help the situation.

There are others on here with a LOT more expertise than I have in this area.

I think letting the saw idle for a bit after heavy use might help. My thinking here is that you continue to get fresh fuel flowing through the fuel line as the saw cools down. After it cools enough, shutting it off might not boil the fuel in the line. Idle should generate less heat than working it hard... on the other hand, there will be less cooling air flowing as well. At any rate, this is an easy experiment to do. Try idling for varying amounts of time and see what it does. Let us know what you find.

The other tack is to figure out if you can make the saw run cooler. A few thoughts:

• keep the chain sharp... the saw has to work harder to cut with a dull chain than a sharp one
• Check your mixture setting. If things are getting really baked, maybe richen it up a bit to see if that helps
• The muffler may be too restrictive. I had a catalytic muffler on my Jonsered 2152. I had some problems with it getting too hot and not wanting to run. Switching to a non-catalytic muffler made it run noticeably cooler, and added more power. (You can also just gut a catalytic muffler to turn into a non-cat, but I've heard there are some nasty things inside -- you don't want to inhale or ingest any of the dust from working on this.) If you are already running a non-cat muffler, you may want to open it up a bit. Search for threads on Muffler mods for ideas on that. Basically, it involves drilling a hole in the muffler to allow the exhaust to flow out more freely.
• Is there any chance of re-routing or sheilding the fuel line to keep it away from the heat? I don't hold out much hope for this option. If it were that simple, they'd have made the saw that way.

I hope this helps, I hope others will chime in if I'm way off base here. I'm mostly thinking out loud here, and I'm far from an expert on this stuff.

John Mc

[LeeB]

On many of those cheaper saws, the fuel tank isn't isolated far enough from the exhaust/ motor heat. When the tank is full, the liquid fuel acts as a cooling agent. But when the tank is near empty, it gets hot. Especially in the summer.

It don't just happen on the cheaper saws. I had it happen to bit my huskies the other day. On is a 372 and the other is a 346. I thought maybe they were out of gas and when I opened the lid you could see the bubbles coming back into the tank. Never had this happen before but it acted just the same as what tbrickner is describing.

[Al_Smith]

I had a 2800 Poulan that did the same exact thing . Come to find out for reasons unknown it was pressurizing the fuel tank .

Aha says I ,a carb rebuild ,most likely faulty check valves --wrong . I fixed that bad boy, installed a tank vent similar to those used on Stihls .

[LeeB]

One saw, the 372 is 8 years old. The 346 is brand new. It was a hot day and they were both sitting in the sun in the bucket of my backhoe.

[tbrickner]

AL Smith,

Do you have a link to what a tank vent looks like.  Is it basically a pressure relief valve.  Could you link me to the one used on Stihls as a reference.

Thanks,
Tom Brickner

[tbrickner]

Johny Mc,

I'll try some of your methods to see what is going on and to see if that hellps the situation.  I am pretty finiky about keeping the chain sharp so that isn't the issue.  It may be what LeeB and Al Smith are describing.  Opening up the carb maybe will help with the cooling.  Thanks for the tips.

Tom Brickner

[Al_Smith]

Well,I can't post a picture to show a Stihl tank vent so about the only thing I can do to help is describe one as best I can .

All it is is a small piece of tubing with a little screw in it to act as a kind of a check valve . This is old style .Newer designs like the 200T have a little breather on the end which evidently is a check valve of sorts .

On the 2800 Poulan I mentioned I found a small barb connection that used 10-32 threads for connection to the hose . I drilled the top of the tank forward as far as I could go and threaded in the barb .To this I connected a small piece of 3/16" line and installed a small set screw in it .This allowed air in but not gasoline out . It seems to me I had to kind of experiment with set screws to get it to work properly but I perserverved and it works well .

[timber tramp]

  I'm not hip enough to post pics either, but the part number for the tank vent on a stihl 024 (my smallest saw) is 0000 350 5800. Hope this helps, good luck.              TT

[tbrickner]

Thanks guys for the help on the pressure relief valve.

I am going to do a few tests on the saw to see how it reacts to the modification and as a last resort try the relief valve.  I won't be cutting over the next two weeks but when I do I'll let you all know what I found works.

Thanks again for the insite,
Tom

[johnjbc]

I've had that problem on my saw and came up with a workaround. As soon as the saw runs out of fuel or I stop to refuel I loosen the cap so pressure cant build up, but don't refuel till I'm ready to start the saw. If it works then you know what the problem is.