Primer bulb not filling

[gwa000]

been using my old Craftsman 42cc / 18" (~2003 model) a lot the past few days. was sitting idle a long time (no gas stored in tank). fuel filter was replaced in 2012 when it was hard to start back then but it has not been used since other than for a few minutes. air filter was cleaned out before starting my sawing a few days ago.

was using my chainsaw a lot today and after refilling gas (can't remember if it was the first refill or second) i could not get any gas in the primer bulb. i was doing some 18"-20" logs (for firewood) so the engine was fairly hot.

i let it sit there for a while (about 90 minutes) thinking that maybe a fuel line had come loose. the bulb seemed fine. just before i was going to take off the cover i decided too try priming again and voila! the bulb filled. and the saw started on the 1st or 2nd pull.

started googling around and general feeling i got for what i experienced is called vapor lock which happens when the engine is hot and seems to be common with Poulan and Craftsman brands. this may also explain why on previous days that the saw always seemed to be harder to start then when it was cold. also on those days i was alternating between log sizes so the saw was able to cool off or not run as continuous.

1. does this seem like what i experienced, vapor lock?
2. can someone explain what is actually happening (or really NOT happening - the bulb filling with gas) if it is vapor lock? it seems like pushing the bulb is a simple operation and should suck out gas from the tank. how does vapor lock prevent the bulb from priming?
3. some other posts mentioned that a hole in the gas cap is plugged. should there be a vent hole? not sure if this is dependent on the manufacturer.

thanks!

 

[Andyshine77]

The primer can only pump liquid not the pressurized fuel vapor. Vapor lock also messes with the check valve. It does sound like vapor lock may be the issue. Because of the age of the saw, a fuel system rebuild is highly recommended. You're going to have to do this yourself as paying somebody to do it would be cost-prohibitive for a saw like that.

If the fuel tank vent gets clogged, the saw will start to starve for fuel while running, and will resolve if you remove the fuel cap.

 

[Efisher26]

Probably a clogged breather hole or stiff breather in the cap.


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[Andyshine77]

Probably a clogged breather hole or stiff breather in the cap.


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I'm not so sure, that causes the saw to go lean while cutting. What he's describing has happen to myself and many others due to vapor lock.[emoji111]

 

[Brent Nowell]

The primer can only pump liquid not the pressurized fuel vapor. Vapor lock also messes with the check valve. It does sound like vapor lock may be the issue. Because of the age of the saw, a fuel system rebuild is highly recommended. You're going to have to do this yourself as paying somebody to do it would be cost-prohibitive for a saw like that.

If the fuel tank vent gets clogged, the saw will start to starve for fuel while running, and will resolve if you remove the fuel cap.

I agree he is experiencing vapour lock.

To the OP, when you are trying to prime for fuel when the saw is hot, you cannot because the fuel literally boiling. At 90 degrees at atmospheric pressure fuel will boil. The bulb is pulling fuel, the problem is that the fuel is turning to vapour before it gets to the bulb due to the heat.

A tip was told to me to treat a hot start as a cold start in this condition. So run full choke until she sputters then goto half choke until it runs. A cure for hot starts... I never did try it tho, haven’t had hot enough weather since

 

[gwa000]

The primer can only pump liquid not the pressurized fuel vapor. Vapor lock also messes with the check valve. It does sound like vapor lock may be the issue. Because of the age of the saw, a fuel system rebuild is highly recommended. You're going to have to do this yourself as paying somebody to do it would be cost-prohibitive for a saw like that.

If the fuel tank vent gets clogged, the saw will start to starve for fuel while running, and will resolve if you remove the fuel cap.

the rebuild will probably not happen since i use it so infrequently. once i am done with this wood the saw will probably sit idle for years.

as far as your comment regarding if the vent gets clogged i do not think it is this as the saw has never starved for fuel while running. revs up fine and idles fine. and starts pretty easy except when it has been used a lot. starting to make sense that the more i used it (without idling to move logs, etc.) the hotter it got and the harder it was to restart after filling the tank.

thanks for your input!

 

[Andyshine77]

the rebuild will probably not happen since i use it so infrequently. once i am done with this wood the saw will probably sit idle for years.

as far as your comment regarding if the vent gets clogged i do not think it is this as the saw has never starved for fuel while running. revs up fine and idles fine. and starts pretty easy except when it has been used a lot. starting to make sense that the more i used it (without idling to move logs, etc.) the hotter it got and the harder it was to restart after filling the tank.

thanks for your input!

Could also be losing compression when it gets hot. Anyway take care.

 

[gwa000]

I agree he is experiencing vapour lock.

To the OP, when you are trying to prime for fuel when the saw is hot, you cannot because the fuel literally boiling. At 90 degrees at atmospheric pressure fuel will boil. The bulb is pulling fuel, the problem is that the fuel is turning to vapour before it gets to the bulb due to the heat.

A tip was told to me to treat a hot start as a cold start in this condition. So run full choke until she sputters then goto half choke until it runs. A cure for hot starts... I never did try it tho, haven’t had hot enough weather since

thanks for the excellent explanation of what is happening (as far as the fuel literally boiling). when i was able to prime the bulb after the engine had cooled off i figured it was not the bulb itself or any lines. luckily we now have search engines to research this stuff as well as forums like this to get answers. why, back in the old days...

i guess for the remainder of the wood i'll give the saw a rest after refilling the gas.

any idea if this is common for the better brands (e.g. Stihl) or is it just a bad design on the cheaper brands? i just need the saw to hold out for maybe 2 more days!!

 

[Andyshine77]

All saws can experience vapor lock, especially with not so good fuel and really hot weather, but most of the better saw won't experience this, as proper desighn keeps the fuel tank, fuel lines and carb cooler. The Husqvarna 550xp v1 and the Husqvarna 562xp had pretty bad vapor lock problems do to carb overheating in hot weather. The Stihl 461 had some issues as well.

 

[CR888]

Replace the primer bulb, then test if if works with a cold not running saw. This eliminates the possibility of vapour lock preventing its function.

 

[hotshot]

Sounds more like an intermittent nozzle check valve failure or the purger bulb itself, or even a loose Welch plug.

What’s the Walbro carb number?

Purger equipped carbs all require a fuel tight seal to pull liquid from the metering chamber via the inlet needle valve.

+1 on a new OEM carb rebuild kit, then you can check these other things out while you’re inside there. A hand held pressure tester is a must for working on these midget sized carbs, or else you’re just pizzing in the wind.

 

[SteveSr]

been using my old Craftsman 42cc / 18" (~2003 model) a lot the past few days. was sitting idle a long time (no gas stored in tank). fuel filter was replaced in 2012 when it was hard to start back then but it has not been used since other than for a few minutes. air filter was cleaned out before starting my sawing a few days ago.

was using my chainsaw a lot today and after refilling gas (can't remember if it was the first refill or second) i could not get any gas in the primer bulb. i was doing some 18"-20" logs (for firewood) so the engine was fairly hot.

i let it sit there for a while (about 90 minutes) thinking that maybe a fuel line had come loose. the bulb seemed fine. just before i was going to take off the cover i decided too try priming again and voila! the bulb filled. and the saw started on the 1st or 2nd pull.

started googling around and general feeling i got for what i experienced is called vapor lock which happens when the engine is hot and seems to be common with Poulan and Craftsman brands. this may also explain why on previous days that the saw always seemed to be harder to start then when it was cold. also on those days i was alternating between log sizes so the saw was able to cool off or not run as continuous.

1. does this seem like what i experienced, vapor lock?
2. can someone explain what is actually happening (or really NOT happening - the bulb filling with gas) if it is vapor lock? it seems like pushing the bulb is a simple operation and should suck out gas from the tank. how does vapor lock prevent the bulb from priming?
3. some other posts mentioned that a hole in the gas cap is plugged. should there be a vent hole? not sure if this is dependent on the manufacturer.

thanks!

Primer bulbs are just a PITA! As you have already found out the saw will run fine WITHOUT it! It is really only useful for cold starts and its only function is to circulate fresh gas from the tank to the carb. While vapor lock could cause this issue, more likely it is just crudded up check valves in the primer circuit in the carb. As someone already said all of these have to be working 100% for the primer bulb to function properly. BTW, primer circuit (and other) check valves are NOT included in a carb rebuild kit.

 

[gwa000]

Primer bulbs are just a PITA! As you have already found out the saw will run fine WITHOUT it! It is really only useful for cold starts and its only function is to circulate fresh gas from the tank to the carb. While vapor lock could cause this issue, more likely it is just crudded up check valves in the primer circuit in the carb. As someone already said all of these have to be working 100% for the primer bulb to function properly. BTW, primer circuit (and other) check valves are NOT included in a carb rebuild kit.

would this crudded up check valve only cause a problem (i.e. not being able to prime) if the engine is hot? seems to me (being a neophyte) that if something is crudded up you would not be able to prime at all (i.e. with the engine cold or hot).

i only have the difficulty of priming if the engine is hot. and from my gut feel it appeared that if the engine was really hot i could not prime at all. but if the engine was moderately hot i could prime somewhat but the saw was harder to start (i could eventually get it going after doing some more priming and pulling).

yesterday when i used it for the first time for the day the primer bulb filled immediately with a few presses and the saw started up after a couple of pulls.

 

[SteveSr]

would this crudded up check valve only cause a problem (i.e. not being able to prime) if the engine is hot? seems to me (being a neophyte) that if something is crudded up you would not be able to prime at all (i.e. with the engine cold or hot).

i only have the difficulty of priming if the engine is hot. and from my gut feel it appeared that if the engine was really hot i could not prime at all. but if the engine was moderately hot i could prime somewhat but the saw was harder to start (i could eventually get it going after doing some more priming and pulling).

yesterday when i used it for the first time for the day the primer bulb filled immediately with a few presses and the saw started up after a couple of pulls.

If it only fails to prime when hot I would probably chalk it up to vapor lock and too much tank pressure. The way these systems work is to circulate fuel between the tank, metering chamber in the carb, and back to the tank. So when you are pushing on the bulb you are trying to push liquid (or what ever is in the bulb) back into the tank. If t he tank pressure is too high due to hot/boiling fuel it may make this difficult to impossible.

BTW, Just like your car these tanks are designed to be sealed/pressurized to keep unburned hydrocarbons out of the atmosphere. They are supposed to let air into but no fuel vapors out of the tank.

 

[gwa000]

If it only fails to prime when hot I would probably chalk it up to vapor lock and too much tank pressure. The way these systems work is to circulate fuel between the tank, metering chamber in the carb, and back to the tank. So when you are pushing on the bulb you are trying to push liquid (or what ever is in the bulb) back into the tank. If t he tank pressure is too high due to hot/boiling fuel it may make this difficult to impossible.

BTW, Just like your car these tanks are designed to be sealed/pressurized to keep unburned hydrocarbons out of the atmosphere. They are supposed to let air into but no fuel vapors out of the tank.

yes, it only fails to prime when it is hot.
i've never smelled any vapors so i think the tank is pretty well sealed.

thanks for you input!

 

[CR888]

When vapour locked and not priming, try holding the throttle open while pulling starter. Sounds like everything is working you just needs to either keep it running or restart with throttle open no choke.

 

[gwa000]

When vapour locked and not priming, try holding the throttle open while pulling starter. Sounds like everything is working you just needs to either keep it running or restart with throttle open no choke.

problem is is that chainsaw eventually runs out of gas.

don't you have to prime it after you run out of gas?

 

[scottr]

gwa000 if you give us the model number of your saw it would help .

Have you ever cleaned the spark arrestor screen ?

 

[scottr]

would this crudded up check valve only cause a problem (i.e. not being able to prime) if the engine is hot? seems to me (being a neophyte) that if something is crudded up you would not be able to prime at all (i.e. with the engine cold or hot).

i only have the difficulty of priming if the engine is hot. and from my gut feel it appeared that if the engine was really hot i could not prime at all. but if the engine was moderately hot i could prime somewhat but the saw was harder to start (i could eventually get it going after doing some more priming and pulling).

yesterday when i used it for the first time for the day the primer bulb filled immediately with a few presses and the saw started up after a couple of pulls.

gwa000 , there is a hot start procedure for your saw in the owners manual .

 

[hotshot]

Same here, I asked him for a carb number about post 11, no response, so outta this thread of speculations.

gwa000, pls stop calling that a primer!, it’s a purger, and does nothing to effect a prime.