Still bogs down
- Thread starter Echoboy
- Start date
Could be a million and one things.
But, gasses getting in (blocked air filter) and gasses getting out (mud dauber nests in the muffler) are two immediate thoughts.
What model saw?
Is it new to you and this is your first experience trying to get it going?
Or, have you owned it for years and this just started happening after years of running well?
But, gasses getting in (blocked air filter) and gasses getting out (mud dauber nests in the muffler) are two immediate thoughts.
What model saw?
Is it new to you and this is your first experience trying to get it going?
Or, have you owned it for years and this just started happening after years of running well?
20 year old 290- take the muffler off to check it- take a photo of the piston through the exhaust port.
I am willing to bet the piston is scored up and the saw has a pretty major air leak.
Plastic clamshells are well known to do this- I have several boxes of 029-390 saw wrecks here as they are a popular farmer type saw here.
Some form of pressure and vacuum test needs to be done at likely air leak spots- intake manifold, clamshell joint and main oil seals.
If the saw runs and idles- the Bubba test of spray can while running might narrow it down.
Is it a hard starter? Many pulls and then eventually fires up, idles a bit, runs out of puff and stalls?
Scored up and excessive ring wear = low compression, lots of rope pills add lots of mixed fuel to the cylinder and eventually add enough oil in there to take up the slack and it starts- then the excess burns off and it dies.
You could remove the spark plug, tip a teaspoon or so of oil down the plug hole, swill it around and put the plug back in. Try to start the saw as per normal. If it starts fairly easily and runs okay ish but smokes a lot- low compression is the problem.
I am willing to bet the piston is scored up and the saw has a pretty major air leak.
Plastic clamshells are well known to do this- I have several boxes of 029-390 saw wrecks here as they are a popular farmer type saw here.
Some form of pressure and vacuum test needs to be done at likely air leak spots- intake manifold, clamshell joint and main oil seals.
If the saw runs and idles- the Bubba test of spray can while running might narrow it down.
Is it a hard starter? Many pulls and then eventually fires up, idles a bit, runs out of puff and stalls?
Scored up and excessive ring wear = low compression, lots of rope pills add lots of mixed fuel to the cylinder and eventually add enough oil in there to take up the slack and it starts- then the excess burns off and it dies.
You could remove the spark plug, tip a teaspoon or so of oil down the plug hole, swill it around and put the plug back in. Try to start the saw as per normal. If it starts fairly easily and runs okay ish but smokes a lot- low compression is the problem.
Piston rings look okay through the port, may or may not be a trick of light showing a red/brown streak on the piston like it has been hot. Over heated piston usually scores up the clutch side front quarter of the piston you cannot see through the port- but thats only a possibility.
Seemed to be a fair bit of carbon build up in the exhaust port as well- could be a fuel mix/oil choice issue, or a tuning issue.
You could always try running it while the muffler is off- if it reaches wide open throttle well, it will be loud and obnoxious- but shows there may be obstruction within the muffler.
Google pressure testing a chainsaw engine- often easier to follow videos and such rather than a typed out explanation.
Basically you block/seal off the intake and exhaust ports of the cylinder, fit a pump to the impulse line in this case and apply pressure to the inside of the engine.
You then use soapy water spray at the likely leak points of main crank seals, clamshell joint, spark plug threads, intake boot, decompression valve- anywhere air can enter or escape and look for bubbles being blown.
There are other ways- like starting the saw and spraying engine start, carb cleaner or something out of a can on to those same areas- if the engine pitch changes or it stalls- it is sucking in the vapour around that point.
Seemed to be a fair bit of carbon build up in the exhaust port as well- could be a fuel mix/oil choice issue, or a tuning issue.
You could always try running it while the muffler is off- if it reaches wide open throttle well, it will be loud and obnoxious- but shows there may be obstruction within the muffler.
Google pressure testing a chainsaw engine- often easier to follow videos and such rather than a typed out explanation.
Basically you block/seal off the intake and exhaust ports of the cylinder, fit a pump to the impulse line in this case and apply pressure to the inside of the engine.
You then use soapy water spray at the likely leak points of main crank seals, clamshell joint, spark plug threads, intake boot, decompression valve- anywhere air can enter or escape and look for bubbles being blown.
There are other ways- like starting the saw and spraying engine start, carb cleaner or something out of a can on to those same areas- if the engine pitch changes or it stalls- it is sucking in the vapour around that point.
chips_n_dust
New member
There's one thing between the tank and the carb that usually gets skipped in these threads: the fuel line and the impulse line. At 20 years old they're the same rubber, and both go hard and start passing air long before they look bad.
A cracked impulse line is the sneaky one. It doesn't leak fuel, it leaks the pressure pulse that runs the pump - so the saw idles fine on what's already in the metering chamber and then can't keep up the moment you open the throttle. That fits "idles well, struggles as I throttle up" exactly, and it won't show up on a compression test.
Before chasing seals, pull the filter out of the tank and look at the line where it bends at the tank fitting - that's where they crack. Check the tank vent as well; a plugged vent gives the same symptom and costs ten seconds to rule out.
On the smoke: a year-old aftermarket carb isn't automatically a good carb. The metering lever height is set wrong on plenty of them, and that runs rich at part throttle. If the H and L screws behave normally otherwise, I'd look there before blaming the piston.
A cracked impulse line is the sneaky one. It doesn't leak fuel, it leaks the pressure pulse that runs the pump - so the saw idles fine on what's already in the metering chamber and then can't keep up the moment you open the throttle. That fits "idles well, struggles as I throttle up" exactly, and it won't show up on a compression test.
Before chasing seals, pull the filter out of the tank and look at the line where it bends at the tank fitting - that's where they crack. Check the tank vent as well; a plugged vent gives the same symptom and costs ten seconds to rule out.
On the smoke: a year-old aftermarket carb isn't automatically a good carb. The metering lever height is set wrong on plenty of them, and that runs rich at part throttle. If the H and L screws behave normally otherwise, I'd look there before blaming the piston.
Don't know that it gets skipped- pressure testing the saw through that very line will show if it is good- or bad.