Tuning two-stroke chainsaw carburetter

[rmjbn1]

I have a couple of problems:
1. Chainsaw wont rev and dies on idle. I have to keep squeezing the throttle to keep it running. It'll rev for a while but then die and struggles to start.
2. When it does idle, the chain is turning.

I have a couple of ideas why:
1. Low jet needs adjusting? Running too lean?
2. Idle speed too fast?

It's a 40cc petrol chainsaw. It's also a cheapo, bought from B&Q in the UK, I suppose their own brand, manufacturer's label is Kingfisher from Netherlands (model fpcsp40-2)

I've had it 3 years, didn't use it for a year (drained fuel tank), fired it up to clear recent storm damage and it won't run properly.

To my surprise, nobody around here knows anything about tuning the carbs. My neighbours all take them in, spoke to the shop and they don't adjust them, they send them away. Which they won't with mine because it's an unfamiliar make.

They don't sell any tools for adjusting the carbs. Mine needs a 'PacMan' tool.

I tried a new plug and new fuel, but no improvement.

I'm at the point of ordering the tool I need and adjusting it myself. I have a bit of experience adjusting jets on carbs on old cars, but no experience of two strokes.

Am I on the right track? Any advice would be very gratefully received.

@Pugglt Auld Jock @bugsymike @porta @mowermender70 sorry for calling out to you yet again, but you're the first folk that come to mind on here knowledgeable about tuning two-strokes

I'll try and upload a video of the symptoms for all to enjoy

 

[bugsymike]

I have a couple of problems:
1. Chainsaw wont rev and dies on idle. I have to keep squeezing the throttle to keep it running. It'll rev for a while but then die and struggles to start.
2. When it does idle, the chain is turning.

I have a couple of ideas why:
1. Low jet needs adjusting? Running too lean?
2. Idle speed too fast?

It's a 40cc petrol chainsaw. It's also a cheapo, bought from B&Q in the UK, I suppose their own brand, manufacturer's label is Kingfisher from Netherlands (model fpcsp40-2)
View attachment 460878
I've had it 3 years, didn't use it for a year (drained fuel tank), fired it up to clear recent storm damage and it won't run properly.

To my surprise, nobody around here knows anything about tuning the carbs. My neighbours all take them in, spoke to the shop and they don't adjust them, they send them away. Which they won't with mine because it's an unfamiliar make.

They don't sell any tools for adjusting the carbs. Mine needs a 'PacMan' tool.
View attachment 460877
I tried a new plug and new fuel, but no improvement.

I'm at the point of ordering the tool I need and adjusting it myself. I have a bit of experience adjusting jets on carbs on old cars, but no experience of two strokes.

Am I on the right track? Any advice would be very gratefully received.

@Pugglt Auld Jock @bugsymike @porta @mowermender70 sorry for calling out to you yet again, but you're the first folk that come to mind on here knowledgeable about tuning two-strokes

I'll try and upload a video of the symptoms for all to enjoy

I recently had a play with two of my mates chainsaws, one a modern Stihl or copy, piece of shi*, spent hours on it and came to the conclusion poor bottom end compression so not "sucking" fuel in, had carb apart but nothing obviously wrong there and good spark. General compression on pull start felt OK, maybe crank seals don't like modern E5 fuel or very low oil mix ration?
The other much older and abused, had no spark, so flywheel off cleaned the points fixed the spark, then had to sort the carb, tank was full of rubbery muck, cleaned that, the pick up and the diaphragm flap valves as full of years of muck. It then started first pull and revved cleanly then idles happily, so result.
Years ago I had a Danarm 110cc, a beast but ran great, I bought a then S/H modern Danarm 55 what a load of rubbish, came to the conclusion lack of compression and was going to strip and rebuild with new seals and rings etc. until told they only supplied a complete engine!
Told mate in pub who was looking for one and he happily swapped it for a Honda 125XL, queried the engine side, his answer was , it's a current model I will hire one and swap the bits over!!!!
That stopped my thoughts of starting a small hire company!
On yours if carb is clean and fuel primes up OK, try and see if good suction at inlet.

 

[rmjbn1]

Thanks Mike! Aye, it's always hard to know how far to go chasing a problem on a cheap piece of kit.

One of those cases where it's only for occasional, not very heavy use on trees in the garden so spent probably less than I should have. Used kit out here seems clapped out and overpriced.

I'll have a go at cleaning and adjusting the carb and see what's what.

 

[irc]

Sounds like a blocked jet in the carb. Had the same on my chainsaw last time I used it (probably 3 years ago now), took it round to a local landscaper i know and he cleaned it out, fixed the chain oil pump and sharpened the chain while I fixed his computer and WiFi problems.
Apparently its the new normal with E10 fuel in garden equipment.

Had the same with my (4-stroke) strimmer and lawn mower since then, both gummed up with stuff that looks like varnish (and feels like snot!). Both had been drained of fuel before winter, but still got gummed up.

I guess that's progress.

 

[bugsymike]

Sounds like a blocked jet in the carb. Had the same on my chainsaw last time I used it (probably 3 years ago now), took it round to a local landscaper i know and he cleaned it out, fixed the chain oil pump and sharpened the chain while I fixed his computer and WiFi problems.
Apparently its the new normal with E10 fuel in garden equipment.

Had the same with my (4-stroke) strimmer and lawn mower since then, both gummed up with stuff that looks like varnish (and feels like snot!). Both had been drained of fuel before winter, but still got gummed up.

I guess that's progress.

Just reminded me of a joke, man comes over to the owner of a broken down car and says "crap in the carb?" The guy replies "might as well I have tried everything else!"

 

[The Panda Nut]

These carbs often have a diaphragm inside and if its been unused its likely gone hard. They only need to be slightlyUniversally normal symptoms of bad carb are what you describe. Carburettors are often found on eBay and for peanusts. I totally regard them as service items. If you can get a match just buy a new one. I have never had a situation that was not cured by doing this. There is so little inside these things it mostly has to be seals gaskets and diaphragm failure. Its amazed me how many machines each carb type actually fits.. The last one I did cost just £4.98 and sorted the problem. Gasket / seal kits are hard to get hold of and more expensive but will do the job. The diaphragm needs to be nicely lfexible or the needle valve sticks and its unlikely you will get around it. There are few avialable adjsutments idle srew is normally i and 1/4 slackend from fully in. The main screw needs adjusting so the engine picks up cleanly when you open the throttle. There may be an idele screw that has a wedged end which allows fine adjustment. Idle needs to be as set just nicely below the speed where the chain is stationary. This should be sufficient to ensure it will not stall. Also check the fuel pickup filter is in reasonable clean condition too.

 

[bugsymike]

Thanks Mike! Aye, it's always hard to know how far to go chasing a problem on a cheap piece of kit.

One of those cases where it's only for occasional, not very heavy use on trees in the garden so spent probably less than I should have. Used kit out here seems clapped out and overpriced.

I'll have a go at cleaning and adjusting the carb and see what's what.

They usually have a "tillotson " type of carb that can be run upside down as it uses a diaphragm and valve setup to draw fuel to run rather than a float chamber and needle valve, so if you care fully undo the four screws holding the top on (make sure you note position as it only goes on one way) you will be able to see if dirt in there or not. Also follow the fuel pipe in the fuel tank it is usually on a flexible rubber pipe so it can draw fuel at odd angles, it has filter on it and if careful again you can hook it out of the fuel filler enough to check the filter and clean it , what I also did was blow carb cleaner spray through that hose and whilst the diaphragm was off it would push any dirt out of the valve flap, then reassemble and test.
I assume it has a good spark and plug not fouled etc. like old two stroke motorbikes in the past.
Apart from that you may be able to feel if a reasonable suction on the inlet /carb when pulling starter cord with the plug out.

 

[The Panda Nut]

Sounds like a blocked jet in the carb. Had the same on my chainsaw last time I used it (probably 3 years ago now), took it round to a local landscaper i know and he cleaned it out, fixed the chain oil pump and sharpened the chain while I fixed his computer and WiFi problems.
Apparently its the new normal with E10 fuel in garden equipment.

Had the same with my (4-stroke) strimmer and lawn mower since then, both gummed up with stuff that looks like varnish (and feels like snot!). Both had been drained of fuel before winter, but still got gummed up.

I guess that's progress.

E5 is bad enough but E10 is unsuitable for most horticultral machinery and you need additive. It helps extend the perishable aprts life span. Older fuel pipe will literally dissolve and go sticky. I use Briggs & Stratton fuel additive which seems touch wood to have stopped problems. My normal practice is to tip the petrol back into the can and then run the engine till it stops. You can also get synthetic petrol which is a sound bet for winter periods. Ive never had issues since I started synthetic fuel for that last use before seasonal layups. Its a bit pricey but can be had in single litre bottles.

 

[rmjbn1]

Thank you all

I used to run it till it conked out of fuel at the end of a cutting session to get it dry.

But then I read my manual stated never to do that :/

So yes I drained the tank but there was fuel in the lines for a good year (well, an OK year).

Maybe there's something to this modern fuel gumming up over time :/

I'd been running 40:1 oil to petrol as stated on the saw rather than 50:1 as the oil can recommends. Maybe too much oil?

I cleaned the air and fuel filter.

Once my tool arrives I'll give the carb a good clean.

 

[rmjbn1]

Tool arrived!

I'm sure in the old days a screwdriver would have sufficed, but now of course each carb needs its dedicated tool which they don't give you. Grumble grumble.

Removed, stripped and cleaned the carb, bit of faffing about with the adjustment screws, now she's purring like a kitten again Quite a sickly kitten with a sore throat, but still a kitten. So thank you all for your help

I read factory settings for the high and low jets are typically fully in, then back out 1 to 1 1/2 turns.

I had to back both jets out a fair bit more than that to stop it dying on idle and bogging down on throttle. Diaphragm past its best?

 

[The Panda Nut]

Tool arrived!

I'm sure in the old days a screwdriver would have sufficed, but now of course each carb needs its dedicated tool which they don't give you. Grumble grumble.

Removed, stripped and cleaned the carb, bit of faffing about with the adjustment screws, now she's purring like a kitten again Quite a sickly kitten with a sore throat, but still a kitten. So thank you all for your help

I read factory settings for the high and low jets are typically fully in, then back out 1 to 1 1/2 turns.

I had to back both jets out a fair bit more than that to stop it dying on idle and bogging down on throttle. Diaphragm past its best?

There are two actions withe the two screws If it runs, you need to get it idling with the low speed screw. There may be a third screw which has a tapered end allowing this to act mechanically on the throttle plate lever as a variable slow speed stop. When it idles, the high speed screw adjustment is correct when you get a clean pick up when hitting full thottle. I have to say I hate these carbs. Usually they either work with little fettling, or are bin jobs. I never worked out why they are so tempremental. They just are. SHOUT AT IT. It always makes me feel better. If you want hassel free life get a Makita. Totally bomb proof!

 

[rmjbn1]

Thank you TPN I tried to get the idle right by balancing the idle speed screw and low jet.
Then when I had it idling not too fast and not conking out I backed out the high jet to get clean pick-up on throttle.
I suspect I'm just pushing more fuel through it than ideal, but I cleaned out the filters, lines and carb as well as I could and there wasn't any obvious debris in it.

Carburettors are often found on eBay and for peanusts. I totally regard them as service items.

That's a good idea. I'll try and pick up a carb next time I'm in the UK.