Vapor lock?

[CountryBoy19]

Ok, so I had a few small problems with my MS170 the other night.

It seems that it was having fuel problems, but only intermittent. It almost seemed like it was getting vapor locked because it was hot.

When cold it starts fine. When shut off and fired back up within a minute it starts fine. But if shut off and left to sit for a couple minutes it wouldn't start. I would have to feather between full/part choke and play with the throttle to get it stay running. It would often fire, and die within 2-3 seconds even at WOT if choked. I tried it by leaving the choke on full, and switching to half, and no choke and it still did it no matter what. After about 5-8 cycle of it firing and dying it would fire and run just fine until shut off and left for a few minutes again.

This saw had ran great previously, this problem just surfaced this week. The carb was covered in lots of gunk so a couple weeks ago I took it off and sprayed the carb and airbox off with carb cleaner, but I didn't touch anything; just sprayed the gunk off the outside and sprayed through the venturi to make sure I didn't knock any gunk inside it.

So my question is this, because it seems like it's a fuel delivery problem but it only happens when the saw it good and hot and is left to sit for a couple minutes, do you think it's somehow getting vapor locked? Or the fuel in the carb is evaporating off and it just has to get primed back up again?

Did I mention that the saw was good and hot? I was working it pretty hard.

 

[WoodRipper]

There was a posting on here a couple of weeks ago about this issue. Do a search and you should find it. There was a lot of good info. But, yes, sounds like you might have a vapor lock problem. The solution is richen the mixture so that the saw doesn't get so hot. You probably only got this problem because of the extra hot weather we've been having lately. If you waited unitl the weather got cooler your saw would probably run just fine and you would wonder why the problem ever happened to begin with. Anyway, richen up the Hi speed jet, maybe 1/8 to 1/4 turn and see if that solves the problem. Assuming it cold starts OK then the L speed jet doesn't need any adjustment. Depending upon the age of your saw, you may have to pull the orange plastic carb limiters in order to turn the screws more open. This is a simple, 15 minute procedure, and you can find details on how to do that on here as well.

Carbs need occasional adjustment depending upon the weather.

 

[Wolfcsm]

One of the things I was told to do, when I asked about the same sort of issue was to open the fuel cap and let off the presure in the tank. Then try and start it.

I try trading saws each tank and put the hot one in the shade while I use another.

Hal

 

[CountryBoy19]

Anyway, richen up the Hi speed jet, maybe 1/8 to 1/4 turn and see if that solves the problem.

There is no high or low speed adjustment on the 170. I've been looking for a replacement carb so I can do a muffler mod, but haven't found one for a reasonable price yet. Local dealer said he though he had an 025 for parts but he never called me back.

 

[stihlman32]

One of the things I was told to do, when I asked about the same sort of issue was to open the fuel cap and let off the presure in the tank. Then try and start it.

I try trading saws each tank and put the hot one in the shade while I use another.

Hal

Try cracking open the fuel cap as Wolf stated first. Could be a venting issue causing no fuel to be pulled into the carb. My 441 has done the same once or twice and it always fires when I open the cap.

 

[CountryBoy19]

Try cracking open the fuel cap as Wolf stated first. Could be a venting issue causing no fuel to be pulled into the carb. My 441 has done the same once or twice and it always fires when I open the cap.

I tried that the first time it did it and no luck. I'll be going to cut some more tonight and tomorrow so we'll see how it goes. Worst case scenario I just have to deal with it for a while. It's supposed to be quite a bit cooler tomorrow (in the 60's tomorrow morning) so hopefully I can see if it's the high temps contributing to vapor lock.

I also searched and found the other vapor lock thread that was referrenced but it doesn't help me much because I can't adjust the carb. I'm thinking maybe some 100ll aviation fuel would help it run a little richer/cooler. I may call the local airport and ask them if they would sell to the general public.

 

[CGC4200]

spraying carb cleaner thru venturi

That does not sound like a great idea,would go to P&C and remove
needed lubrication. I have had good luck removing jet screws and spraying
into fuel passages, usually have to remove carburetor, sometimes that
will flush gunk from H & L fuel circuits.

 

[CountryBoy19]

That does not sound like a great idea,would go to P&C and remove
needed lubrication. I have had good luck removing jet screws and spraying
into fuel passages, usually have to remove carburetor, sometimes that
will flush gunk from H & L fuel circuits.

You sure about that? It's just gas with a small amount of lead in it and different additives; a lot of people run 100ll in their saws when they can't find fuel without ethanol. It would boost the btu content of the fuel, basically making it run richer/cooler.

Am I mistaken on that?

 

[CGC4200]

anything other than fuel/oil mix in chainsaw

It would not be as bad as straight gassing & running chainsaw, but spraying
down the throat will do no good & might do harm. Carburetor cleaner is best
used for cleaning fuel passages of gunk. Starting fluid (ether) can cause
problems too. If I need to prime a saw, I use a squirt bottle of proper fuel/oil
mix down the carburetor throat, still get the needed lubrication.
MAKE SURE OF OIL

 

[mtngun]

The OP's description does sound exactly like vapor lock.

Some models are prone to vapor lock and others are not, I'm not smart enough to say why.

Richening the "L" does seem to help a little, but it doesn't always cure the problem.

Another thing that sometimes helps is to let it idle rather than shutting it off. If you must shut it off, let it idle for several minutes so it will cool down.

The opening-the-fuel-cap trick has never worked for me. In fact, I often experience vapor lock after stopping to refuel.

The textbook answer is to blame winter gas. Winter gas is more volatile than summer gas. However, the truth is that ethanol is more volatile than real gas and will be more prone to vapor lock.

But hey, let's not complain, we gotta keep buying ethanol so the millionaire farmers will get even richer.

 

[CountryBoy19]

It would not be as bad as straight gassing & running chainsaw, but spraying
down the throat will do no good & might do harm. Carburetor cleaner is best
used for cleaning fuel passages of gunk. Starting fluid (ether) can cause
problems too. If I need to prime a saw, I use a squirt bottle of proper fuel/oil
mix down the carburetor throat, still get the needed lubrication.
MAKE SURE OF OIL

Oh, I think we must have mistunderstood eachother. I thought you were talking about using 100LL av gas for fuel. But you were actually talking about cleaning the carb? I removed the carb to spray it. I didn't disconnect the fuel line, just pull the carb back and turned it to the side to spray through the venturi. No worries there.

 

[scottr]

Vapor Lock

A relative has a old book that has all kinds of home recipes. One that I copied down was for vapor lock, mix 3 parts kerosene and 1 part turpentine, use 1 ounce per gallon of gas. I have never had to use it but maybe on of the chemists could comment.

 

[demographic]

There is no high or low speed adjustment on the 170. I've been looking for a replacement carb so I can do a muffler mod, but haven't found one for a reasonable price yet. Local dealer said he though he had an 025 for parts but he never called me back.

Funny you mention this cos I read the thread title and thought about my Stihl 017 (as far as I know its the same as the 170) and that vapour locks when hot.

Seems to me that there should be a better heat shield between the carb and inlet.

 

[TreePointer]

One of the things I was told to do, when I asked about the same sort of issue was to open the fuel cap and let off the presure in the tank. Then try and start it.

I try trading saws each tank and put the hot one in the shade while I use another.

Hal

That works for my MS361. When I it's time to take a break in hot weather, I crack open the fuel cap for a few seconds to relieve the low pressure immediately after I stop cutting. If I don't do this, it can be a difficult restart after a 5-10 minute break.

 

[CountryBoy19]

Things went much better tonight. Cut for about 3 hours. Took a 5 minute break in the middle to go ask the landowner if I could cut up a couple other oaks further into the woods and came back and started just fine with normal starting ops (full-choke, half-choke, start). Still thinking about the AV gas. I tried to call the local airport but nobody answered the phone.

 

[Wolfcsm]

Things went much better tonight. Cut for about 3 hours. Took a 5 minute break in the middle to go ask the landowner if I could cut up a couple other oaks further into the woods and came back and started just fine with normal starting ops (full-choke, half-choke, start). Still thinking about the AV gas. I tried to call the local airport but nobody answered the phone.

There are threads about using AV gas and most how have tried it do not recommend it.

I have been using 93 octane gas in all of my 2 strokes. They all seem to run better. For a real kick, VP Fuels makes a 94 octane 2 stroke fuel - but it runs about $10 per gallon. http://www.vpracingfuels.com/sef-94.html

Hal

 

[GlenM]

....... came back and started just fine with normal starting ops (full-choke, half-choke, start)........

:agree2:
You've just found the secret to the mystery....some owner's manuals for some saws recommend 1/2 choke on warm start. I have saws that start one pull, no choke, others need 1/2 choke, one pull, etc..
and a weedeater that needs full choke, pull, no choke, pull and start.....