262xp erratic run

[flyin'chips]

Hi all,

This is my first post on this great forum. English is not my native language - sorry for all the misspelling and a long post.

I acquired a 1990 262xpg in a bad condition recently. It didn't even try to start. I took it as my first chainsaw project. It has the original KS cylinder and piston.

The saw had water and debris in the gas tank, rust and corrosion inside carburetor and actually turned out it had a wrong carburetor (HDA35). It had several air leaks. Both cylinder and piston have vertical scratches but no real scoring. Ring was very worn. Throttle linkage was slightly bent not giving max throttle. Cylinder has had one muffler bolt thread stripped and repaired but there is some exhaust leak as the heat shield does not seal perfectly to the cylinder. As a bonus it has a sheared flywheel key and some small scrathes on it - probably touched ignition.

The following parts have been exchanged (all parts OEM):

-crank seal (clutch)
-crank seal (flywheel)
-O-ring (flywheel)
-piston ring
-both intake gaskets
-spark plug
-carburetor (brand new HDA120)
-air filter
-fuel filter
-fuel line

Also
-piston has been very lightly sanded where scratced
-cylinder has been lightly sanded (one scratch can just be felt with fingernail)
-flywheel hole has been ground with valve grinding paste
-throttle linkage repaired (no binding)

The situation:
-HOLDS BOTH VACUUM AND PRESSURE 7 psi / 0.5bar at least 5 minutes while turning the crank by hand
-the saw starts, runs and has relatively good power (clearly more than Husky 55 or Stihl ms261)
-accelerates well
-about 3 tanks has been gone through when tuning and testing

The issue:
I'm unable to tune it properly. Will run on factory carb settings (1/1) but I have tried a variety of settings with roughly the same end results. I can tune the WOT rpm easily anywhere (with a tach) but the idle is unstable. I can't find any airleaks. The saw has stopped on it's own maybe one or two times per tank of fuel when on idle.

Max rpm on WOT is now set at about 13500 (factory spec). Idle varies between 2100 and 2800. Sometimes when going WOT it will run on 12800-13000 for about 1-2 seconds and then suddenly rise to 13500. Always when I release the throttle after full rev the rpm drops immediately but it drops too low to 2100-2300 and then slowly rises to 2700-2800 and stays there. If I blip the throttle the idle drops to 2500-2600 (factory spec) and slowly starts to rise to 2700-2800.

Questions:
What are the possible causes for unstable idle and sometimes erratic max rpm?

-improper tuning?
-minor airleak that shows only when hot and running?
-worn piston skirt?
-scrathed cylinder and bad compression when hot?
-misaligned flywheel/ignition?
-problem with flywheel magnet polarity?
-dead av-elements causing excessive vibration and possibly cinching the fuel line?
-bad bearings?
-exhaust leak?

I do not have the means to measure the compression. Also I would not like to throw money and parts blindly at this machine as there is no guarantee it will ever work properly with the old jug.

I'm grateful for any suggestions and help.

-J

 

[drf256]

Check the flywheel key.

I’d also pull carb apart and check it. Just cause it’s new doesn’t mean it’s right. There are some bad pump side diaphragms on some that don’t pump very well.

 

[cus_deluxe]

if that were mine i would start w metering lever height.

 

[Johnmn]

What these guys said ^^^^^ plus maybe a new clutch or at very least clutch spring.

 

[dustinwilt68]

Check the intake block also, if it has a crack it could do that

 

[flyin'chips]

Thanks for all the suggestions!

Today I pulled the cylinder off to check again the piston wear. As far as I can tell the clearance between piston and cylinder is less than 0.1 mm (0.004") although the lowest 5 mm (1/5") of the skirt on intake side is clearly worn/polished shiny. A narrow strip of 0.1 mm thick paper is very difficult to get in as a gauge, two strips of 0.05 mm paper jams the piston. It's possible to "rock" the piston slightly inside the cylinder but I have no idea what is excessive and when it's piston slap territory...

Checked the tank vent and also opened the fuel cap when running the saw - no difference in rpm.

Checked both vacuum and pressure after reinstalling the jug, held both for 5 minutes. I heated the cylinder with a heatgun and the pressure rose steadily with the temperature and held.

I checked the intake block and it seems solid, couldn't find any cracks wieving against light.

I will soon change the clutch spring and sprocket as it seems quite worn.

Carburetor:
-The carburetor may be 10-20 years old even though "brand new". I changed all the diaphragms (oem) and although the pump side felt aged it didn't help.
-Metering lever was as accurately level with the base as I could test with a steel ruler

Could there be something else going on with the carburetor because of age?

Flywheel: Is there a way to check timing/key position without special tools? The problem is that the flywheel had spun on the crank grinding the key and almost all of the remains of it. It's impossible to see exactly the original position. There could very well be an error of +- half a key width in flywheel position. So would there be any other solution except buying another flywheel? In what position should the piston be when the magnets are aligned with the coil? Or is there anywhere available accurate measurements that would allow grinding a groove for a new key to a correct position? I checked and realigned the coil to factory spec (0.3 mm). No help.


So the problem persists and the idle drops too low after reving and I noticed today that it goes the lower the longer the wot rev (is that a sentence???). After a short blip/rev the idle returns straight to 2500. It takes maybe 5-15 seconds to rise to 2800.

Thanks again!

 

[dustinwilt68]

Definitely seems carb related to me.

 

[Stump Shot]

@flyin'chips sounds to me like the relationship between the idle speed and the idle mixture may be off. I would try re-tuning first, before any other diagnostics.

 

[trooney]

I would get a known good carb if you can, 89,129,144. Then try it. Carb is old and sat a long time by the sound of it.

 

[trooney]

120

 

[dustinwilt68]

87,120,144 I believe

 

[trooney]

You are right, off two numbers... its been a long time since I looked at my 262 chit...

 

[flyin'chips]

Thanks again for all the suggestions.

Today ran almost 2 tanks of fuel (always husq premix) through the saw. Did not seize (yet).
I tried a variety of carburetor settings and ran the saw in different positions even quite hard. The different carb settings have an effect on power as supposed and at the moment it feels pretty good except for the idle drop allways after wot.

The clutch (still old spring) starts to lightly engage between 3300-3400 rpm and the chain is fully moving at 3500-3600 rpm. Factory spec for engagement is 3600 rpm. Probably a bit worn spring?

Think I need to go through the carburetor and everything fuel related carefully again. Maybe also try to obtain a new flywheel with a key.

 

[huskyboy]

Is there any lip in the intake port?

 

[flyin'chips]

Thanks, I will check the intake along with the carb.

According to the IPLs the intake block was allways the same part regardles of the carb and cylinder combination. I first thought that the difference in size between HDA87 (the original carb in 1990 262) and HDA120 (now installed) could have caused fuel pooling inside the intake block but it should not be the case. Basically the construction should align the carb, intake block and both of the gaskets to the cylinder relatively accurately because the mounting screws go through all the components.

Unfortunately I do not have access to another carb to try at the moment.

 

[huskyboy]

Sometimes there can be a small lip on the intake port right before the cylinder wall where fuel will puddle slightly and cause it to have erratic idling problems. I had a ms440 here with that problem and once I ground that smooth it idled great all day long. Not saying it could be your problem but it’s something to consider for sure.

 

[trooney]

Between the 87 and 120 stock there is little difference. They both run good.

 

[flyin'chips]

Good info and great suggestions, thanks to all the contributors so far!

I will check the whole intake for possible causes for puddling.

Also it's possible that the tuning is simply off even though I've gone through a variety of combinations.


If the idle mixture is set lean and H rich, does this load the crank case with extra fuel during wot?

If the piston skirt is worn and doesn't seal the intake properly, could this cause fuel puddling in the intake? What actually happens in the motor when it's running with a worn piston compared to a good one?
Questions...

 

[Bjorn]

Hallo. I would check the inpulsline in the intake block. Hade problem With my 272 and the inpulsline was partly blocked and it Also had erratic running.

 

[drf256]

Check your intake block gaskets again. Make certain they are on properly in correct orientation.

Also, view where they sit in relation to the impulse ports on the intake block/cylinder/carb.

I have found them to be slightly off and sometimes cover a bit too much of the impulse opening on the face of the carb. I almost always chamfer the carb face impulse opening so that more area of the port is open to the gasket and block.

As for the flywheel, see if you can locate the old key witness marks. Use a punch to mark the center of it on the flywheel face. Then align that imprint with the left side of the keyway slot, or just a hair before it (turning flywheel counter clockwise). That should give you 1/2 key of advance, which is generally where I set them when I port them. Without the increased compression of machine work, the advance shouldn’t adversely affect the saw performance.

I have an extra flywheel here, but I’m not sure a pic will get you close to what you need. I’ve sheared a key on a 262 before and did it ever make me nuts-I swore it was the carb. And those intake gaskets are super easy to install wrong.