262xp erratic run

[flyin'chips]

Played with tuning. Went all the way from rich bog to lean bog on L (1 1/4 - 3/4) and the idle behavior after wot is the same, down it goes to 2000-2300 to return to 2700 (or whatever it's adjusted to +- 150). The rpms, throttle response and power all react logically and well to all adjustments. So it's not the tuning.

Without the bar and chain and the idle behavior after wot is the same. So it's not the clutch.

Let it idle for some time and turned it upside down on two different tunes and all possible orientations. Upside down the idle goes a bit higher maybe but certainly does not slow down or stop. Thus the crank case does not get flooded. Otherwise the orientation has no effect on idle speed.

Pressure tested through the impulse hole in the cylinder to verify that the spark plug thread doesn't leak (before had the tester in plug hole) - no bubbles.
Pressure tested the carb through the fuel intake and press/vac the impulse hole, holds.
Checked for resistance in the fuel filter (I've had defective oem filters before) and fuel line, fuel flows nicely and easily.

Checked the intake boot again and aligned the gaskets, the boot and the carb according to the marks left on the gaskets and I can't see any retrictions in the impulse passage. All the intake holes grow larger when going from the carb towards the cylinder - no restrictions or lips allthough someone could probably enhance the flow dynamics inside the intake by chamfering all edges. I will enlarge the impulse holes in the gaskets a bit and chamfer the carb impulse hole like suggested.

Today I noted and it's probably been like this all the time that the saw was actually difficult to start when cold. 5-6 pulls on choke, then 3-5 without and died immediately. After many pulls later started on high idle but died when blipping the throttle. Then started on choke and high idle and stayed running. So it doesn't sound like it's flooding at least when cold.

The saw has a very rough sound on idle especially when the idle speed is 2500 or lower. Hard to explain but a bit like there was something hitting or playing another rough rhytm inside. Very far from the smooth idle sounds of 55 or ms261.

Thus I think this is most likely about ignition or about the worn piston skirt.
If the coil was bad, could the saw run well on WOT but not idle properly? I know nothing about ignition.
Anyway I will purchase a new used flywheel with a key and if that doesn't fix it then order a new piston.

Thanks a lot for all the shared experiences and suggestions!

 

[Bjorn]

Hallo. Check connecting rod Bearings, bad bearings can make rough rhytm like you explain. Regards Bjorn

 

[Stump Shot]

If the flywheel magnets are rusty, clean them off so as to give the best possible response.
Test the ignition by using a spark checker that makes the spark jump an air gap of 4-5mm. This will mimic how much is needed to fire under compression. You should get a nice thick blue spark with an audible snap, when rolled over fast with the spark plug out. If it's weak - (pale, white, yellow, orange, red) or non existent, replace the coil.
Modern ignition coils can fail in a multitude of ways so there's no real one way of how they fail unfortunately.

 

[smokey7]

I say to follow up on the flywheel timing. Then I would say follow up on your piston skirt clearance and possible worn piston and cylinder thoughts. I've had coils flywheel and worn piston cause every one of these. When you say the tune runs up to 12-12 5 and then runs away to 13.5 is that after idling for a while. ?

 

[flyin'chips]

I'm sorry I didn't notice the last posts before - thanks for the advice.

Anyway, I haven't had time to play much with this 262 for a while and this probably turns out to be a project for the next winter.


I tried to check the needle bearings for any misbehavior when the jug was off earlier but didn't notice any. I will check again. I will renew the crank bearings and the piston bearing if I get the saw to run properly.

The spark has not been checked yet.

Timing:
I filed a small mark in the crank axle and tried three different flywheel timings
-advance: the saw kicked back almost separating my hand from my shoulder on the first pull and then didn't even try to start. After some pulling the saw was totally flooded (never seen that before).
-retard: started and ran, didn't try in wood, didn't notice any effect on idle behavior
-as accurately as I could get it in original position (or what I think is the correct position): starts and runs like before

I'll get a new piston to put at least the worn piston theory to rest.

 

[flyin'chips]

I say to follow up on the flywheel timing. Then I would say follow up on your piston skirt clearance and possible worn piston and cylinder thoughts. I've had coils flywheel and worn piston cause every one of these. When you say the tune runs up to 12-12 5 and then runs away to 13.5 is that after idling for a while. ?

I'm not sure how often the top rpms change like that and really noticed it only once with a tach so far. Also I'm not sure whether it was after a longer idle. Can't remember. It must have while adjusting the carb because I had a tach in hand. But this is something to check more carefully.

This has not been the most straight forward case for the first "real" saw project. A nice nut crack if I ever crack it...

 

[drf256]

Reading this again now. Could be a defective check valve in the main nozzle of the carb.

The check valve is a flap that closes in the main nozzle when it’s not flowing (ie @ idle). If the valve doesn’t close, it will allow negative pressure created in the carb wet side to pull air in out of the Venturi through the main jet instead of pulling up on the diaphragm to lift the needle off the seat. Common issue when guys use compressed air to blow carbs out, the little disc pops out and the valve stops working.

I bet you have a defective carb. If you can try another, please do so. You can also try your old nozzle in the new carb-I am not certain that the 35 carb will have the same size though.

The only way to check the valve is by knocking the main nozzle out through the carb bore with a punch and then visually checking it. Scribe the nozzle end and the carb body before you punch out, in case your nozzle has a directional feed hole in it.

Sorry if I missed it, but did you replace the carb with a Beamer brand, or an AM China carb?

 

[flyin'chips]

Thaks for a thorough explanation on the check valve - this is all new stuff for me even though I've used chainsaws for 20 years.

The carb is a genuine Walbro HDA120, brand new, old stock from the 90's from a reputable dealer. It could still be defective.

I was all excited when I checked the diaphragms and noticed that one was a bit stiff because of age. But replacing the diaphragms and gaskets with fresh oem parts didn't have any effect on the run characteristics.
I got the carb for a good price but still it was not cheap. At the moment I don't have access to another similar known to be good part but I will look for one locally. Or maybe I'll stumble upon another 262 in a better shape and then be able to swap parts one by one to eliminate and isolate the problem.

Carburetor, coil, timing, piston, air leak that shows only when hot and running... We'll see. Actually the only really frustrating thing here is that I don't have enough time to investigate this.

 

[flyin'chips]

If the flywheel magnets are rusty ------

The magnets were very clean.
Might try another coil when I find one in decent shape.

 

[Creaky limb tree care]

Hi, are you getting fuel spitting back into the air filter when the saw is running, if so this could be the reason for an erratic tickover (caused by the scored bore and piston not sealing proper!) regards, Toby.

 

[flyin'chips]

Hi, are you getting fuel spitting back into the air filter when the saw is running, if so this could be the reason for an erratic tickover (caused by the scored bore and piston not sealing proper!) regards, Toby.

Actually I thought about that because there's some oily residue or staining (or what ever the correct term is) in the air filter. Not much but it's easy to see as the filter is new and I clean the filters very often anyway. I have not run the saw without the filter and top cover thus I can't say really how much gas is flying around. Somehow I have forgotten to mention this earlier.

 

[kneedeepinsaws]

Hi, are you getting fuel spitting back into the air filter when the saw is running, if so this could be the reason for an erratic tickover (caused by the scored bore and piston not sealing proper!) regards, Toby.

I always thought this is normal. I really dont want to add any confusion to a thread like this as the OP is looking for answers. But when the piston is at BDC and comes up the intake port is open in which until it is closed fuel/air mixture would naturally come back up the intake.
I just had to reply to that because i have seen intake horns on all of my saws, including when they were new slightly wet with fuel oil mixture.


To the OP i am having similar problems with the idle and flipping the saw on its back and side and like you i have pinpointed it to the fuel delivery.
Drf256 made a really really good point. And even though my HD12 is brand new, I am starting to suspect nozzle issues... its too bad it cannot be tested, and i understand this because the passages that feed it are in the middle of it recessed in the carb housing..
i have a feeling that its very possible that manufacturing particles could be affecting its seal. I have been told, like drf256 mentioned never to use compressed air on a carb. The walbro service videos mention to set your reg to 10psi and no more, but depending on your nozzle of your gun this could still damage the small little wafer disc that does the sealing.

If you have a small ultra sonic cleaner, it would be a good idea to remove the strainer disc, diaphragm and pump covers and give it a bath for 30 minutes with heat.
Dawn dish soap is the recommended method as it is a surfactant and will not affect the metal of the carb. If you want to love dangerously use diesel with it outside.

If one could build a Very soft rubber adapter to the end of a hose that somehow could seal the main jet nozzle face inside of the venturi and suck/blow, you would be able to find out real quick if it works without removal...

 

[kneedeepinsaws]

Reading this again now. Could be a defective check valve in the main nozzle of the carb.

The check valve is a flap that closes in the main nozzle when it’s not flowing (ie @ idle). If the valve doesn’t close, it will allow negative pressure created in the carb wet side to pull air in out of the Venturi through the main jet instead of pulling up on the diaphragm to lift the needle off the seat. Common issue when guys use compressed air to blow carbs out, the little disc pops out and the valve stops working.

I bet you have a defective carb. If you can try another, please do so. You can also try your old nozzle in the new carb-I am not certain that the 35 carb will have the same size though.

The only way to check the valve is by knocking the main nozzle out through the carb bore with a punch and then visually checking it. Scribe the nozzle end and the carb body before you punch out, in case your nozzle has a directional feed hole in it.

Sorry if I missed it, but did you replace the carb with a Beamer brand, or an AM China carb?

So i was doing some reading on the FSM on a 372 that shows a cut away of the how all three diffuser jets work.
If you wanted to test the main high speed difuser check and only that I think this might work.

remove the check valve screen on the fuel pump side of the carb, re assemble with just the gasket and tighten down.
Remove the diaphragm and needle spring lever assembly, re assemble but leave the sealing gasket and tighten down the 4 screws.

now tighten the low speed screw as far as it goes but very lightly not to ruin the inside seat for the tip. This, according to the diagram attached, should block off fuel flow to the low and mid diffusers. With only the high speed screw open hook up your vacuum gauge.
Using soapy water and wiring the throttle and choke butterfly fully open, soak the main nozzle diffuser and the other two holes.
Apply pressure with your vacuum gauge.
It of course will not hold, but it should only allow a bubble to form on the main nozzle and not the other two low speed holes.

if this is the case, the main nozzle is isolated for testing.

using just the smallest amount of vacuum on your pump, it should hold. This will indicate that the main nozzle check is holding without busting it.
I havent tried any of this, but im willing. if it works we would have a way of testing the main nozzle reliably without just replacing it.

What do you think?