Cold Blooded MS200T?

[Paddy1000111]

So I rebuilt my 200T and I'm having an odd issue. I did a full rebuild using all genuine stihl parts. New piston, new carb (origional one had the hi jet issue where the one way valve lets air in so it always bogged on throttle), seals etc etc... The crankcase passes the pressure/vacuum tests in the service manual and compression is >150psi. When started for the first 4 seconds it bogs when the given any throttle, after it warms up it runs fine. So far I've tried a new spark plug just in case, I've tried giving it throttle with the L screw open 1.5 turns and closed to around 0.75 turns. same thing... Any ideas? 

[bmp01]

Used to be known as fuel preparation (or lack there of). For decent combustion in a petrol engine the fuel needs to be spread evenly through out the fuel/air mixture in near vaporised form. Carburettors do an approximate job at best - you rely on a bit of the fuel preparation to happen as the gas is drawn into the cylinder, the turbulence and heat help vaporise the fuel.  If you think about the low speed or idle circuit in the carb the fuel is drawn through deliberately tiny jets into a turbulent flow of air as the air comes past the butterfly valve. As soon as you open the throttle the fuel comes from the bigger main jet into a  less turbulent air flow. I suspect without the engine temperature to help atomise the fuel, preparation is poor enough that combustion struggles.

150 psi isn't that high either,  higher compression is more tolerant of poor mixtures. Compression might go up a tad as the engine beds in.

If it's only from a cold start (ie doesn't happen after a hot start) I'd say that's what it is. Alternatively could be something to do with the dreaded accelerator jet but on a brand new carb should be OK. Does the new carb have exactly the same part number as the old ?

Lastly,  if your on old fuel you might have lost some of the more volatile elements of the fuel - just the feature of the fuel you need under cold conditions.

HTH.

 

Edit - big fat spark helps with poor combustion issues too.

Edited October 28, 2020 by bmp01
See edit note in text

[Stubby]

5 minutes ago, bmp01 said:

Used to be known as fuel preparation (or lack there of). For decent combustion in a petrol engine the fuel needs to be spread evenly through out the fuel/air mixture in near vaporised form. Carburettors do an approximate job at best - you rely on a bit of the fuel preparation to happen as the gas is drawn into the cylinder, the turbulence and heat help vaporise the fuel.  If you think about the low speed or idle circuit in the carb the fuel is drawn through deliberately tiny jets into a turbulent flow of air as the air comes past the butterfly valve. As soon as you open the throttle the fuel comes from the bigger main jet into a  less turbulent air flow. I suspect without the engine temperature to help atomise the fuel, preparation is poor enough that combustion struggles.

150 psi isn't that high either,  higher compression is more tolerant of poor mixtures. Compression might go up a tad as the engine beds in.

If it's only from a cold start (ie doesn't happen after a hot start) I'd say that's what it is. Alternatively could be something to do with the dreaded accelerator jet but on a brand new carb should be OK. Does the new carb have exactly the same part number as the old ?

Lastly,  if your on old fuel you might have lost some of the more volatile elements of the fuel - just the feature of the fuel you need under cold conditions.

HTH.

 

Edit - big fat spark helps with poor combustion issues too.

Very comprehensive post ?

[spudulike]

Did you connect the impulse line up correctly? 

Sounds like lack of fuel, you could try opening the H&L screws up a bit more and see if anything changes. 

Did you give it a new fuel line and filter? 

Is the fuel tank breather OK?

If you have changed all the parts that need changing, it should run fine.

Did you lightly hone the bore when fitting the new piston? 

Did you reset the coil to flywheel gap?

 

Edited October 28, 2020 by spudulike

[Khriss]

Did you look at previous threads on this and think ' sod this,  I am going for a career in Estate Agency or Lion Taming' K

Edited October 28, 2020 by Khriss
( used to be- bought a saw an got couple thousand ton dropped or logged with it an thought it was a good un. Now its Mystic Meg yr needing ?

[Paddy1000111]

13 hours ago, Khriss said:

Did you look at previous threads on this and think ' sod this,  I am going for a career in Estate Agency or Lion Taming' K

I thought about a career in politics. I was thinking having experience with having the same old issues and still not being able to identify or remedy them could put me in good stead ?

 

That's one hell of a post bmp01! I can't say that it is 150psi for sure as I am only using a cheapo chinese compression tester that's not remotely reliable. All I know is that the crank bearings are new, the big end bearing has no play and the knuckle bearing is new. The cylinder and piston are a brand new pair from Stihl and the base gasket is also a new genuine one torqued down to 10Nm. Apart from deleting the base gasket and replacing it with something like hylomar I can't see how to improve the compression at all. 

 

The old carb was a C1Q-S61A and I replaced it with a C1Q-S126A again, genuine new one from Stihl.

 

I've checked the spark and it's very healthy!

 

Spudlike- 

 

• Impulse line is connected properly
• I've tried opening H&L screws but it doesn't change. Just makes it run rich, smoke a lot, spit oil out the exhaust and bog a lot when the saw is held nose down. 
• New fuel lines, filter and breather valve. Tested and checked, All okay.
• I didn't need to hone the cylinder, it was a new piston and cylinder pair from stihl
• coil flywheel gap set to .25mm as per manual

 

I'm having some slight tuning issues too. I can't seem to get it to idle correctly, when hung nose down the rpm drops slightly, same when it's on it's side. I think I'm going in circles with tuning though. I set the idle and ran 2 tanks though it yesterday cutting cookies and running in. I fully buried the bar in some big bits and it cuts well. Just has some odd quirks. I'm thinking of going to the stealership today and asking them to tune it for me as I feel like I am being my own worst enemy. If they can't tune it then it's either carb or compression. Hopefully they will have a decent compression tester. 

 

I think as bmp01 has said, it's compression, but I don't understand why a new piston, cylinder, base gasket combo would be low on compression? The saw is basically as it would come out of the factory so why is it "off"?

Edited October 29, 2020 by Paddy1000111

[bmp01]

If you have a question mark about the compression tester,  try it on some other good saws just to get a feel for where the 200T is in comparison.

150psi isn't outstanding, 200T's aren't high from factory either. I was trying to say if fueling is a bit wayward then high compression helps - in the context of fuel preparation. Plus it's a ball ache to pull it apart again to delete base gasket (did you check squish clearance when you reassembled ?)

 

Presumably your new carb is the latest and greatest version of original, Spud or someone with part number experience will know.

 

With the other running issues, ie different engine speeds depending on saw orientation I'd be looking at the metering arm setting in the carb, that can throw up some weird behaviour if it's a bit low. (I'm assuming you've checked there is some play in the throttle linkage and the engine mounts aren't shot etc).

 

 

 

[Khriss]

Never had luck with compression testers , experience gives you an idea as a rule of thumb. Was always some stupid split in a hose or dodgy coil , over the years . New designs seem to have more weak links sadly hence a 'Spud.   K

[Paddy1000111]

Well, I went to the stealership today. I'm going to stop calling them that now because they were very helpful and had a tech tune it with an EDT for free. It was all me being useless... I had the LA set too low and subsequently the idle set too low so the saw didn't have enough air on idle meaning that although the screws were in factory position it wasn't getting enough air hence it was flooding, giving the change in rpm when it was at an angle. The guy upped the LA and idle speed to where it should be and set the L jet then set the H screw which was too rich because I'm a wimp and didn't want to run it too lean. It now also doesn't bog when cold and starts faster too. 

 

He did say that the saw vibrated more than he liked on the handle which reminded me that the only non-genuine part on the whole saw was the AV rubbers as there weren't any in stock when I was re-assembling. I have now ordered some genuine ones. 

 

Hopefully this helps anyone with similar problems to me and stumbles across this thread! 

 

[Stubby]

5 hours ago, Paddy1000111 said:

Well, I went to the stealership today. I'm going to stop calling them that now because they were very helpful and had a tech tune it with an EDT for free. It was all me being useless... I had the LA set too low and subsequently the idle set too low so the saw didn't have enough air on idle meaning that although the screws were in factory position it wasn't getting enough air hence it was flooding, giving the change in rpm when it was at an angle. The guy upped the LA and idle speed to where it should be and set the L jet then set the H screw which was too rich because I'm a wimp and didn't want to run it too lean. It now also doesn't bog when cold and starts faster too. 

 

He did say that the saw vibrated more than he liked on the handle which reminded me that the only non-genuine part on the whole saw was the AV rubbers as there weren't any in stock when I was re-assembling. I have now ordered some genuine ones. 

 

Hopefully this helps anyone with similar problems to me and stumbles across this thread! 

 

Honest post , glad you got it sorted . ?