Stihl 250 takes 4 turns out new carb or old carb , WHY ?

[kingofcoins]

The only way I can get my Stihl 250 up and running is to use the choke EVERYTIME to start warm or cold, and turn the LOW screw out
about 4 turns and turn the HIGH screw out about 3 1/2 turns or it will not rev up or idle...and its Now it has the same problem when I installed
a brand NEW Zama carb.....why is it doing this when all other Stihl saws seem to be running fine with about 1 turn out of the L or H screws and
start without the choke when warm ?

 

[stihl86]

I’ve run into this on Stihl handheld equipment and especially the bac pacs. Also a few saws, but not a 1123 family. But that doesn’t mean too much.
The recommended settings aren‘t even close. Again I don’t ever remember it on a 250. If it’s a NEW OEM carb, the only other thing it might be is an air leak. But that almost always shows as overspeeding and high/no idle.
Can you or have someone vac/press test it? In that order, first vac test then pressure. It’s important to do the vac first and rock the crank when crankcase is under vac. That checks the seals. But in 40 plus years working on saws in a dealer environment, I’ve never seen leaking seal on a 025/250 or 251.
Havingto restart on choke is a non-issue

 

[pioneerguy600]

Pull a vac and pressure test on it, that will tell you a lot.

 

[KASH]

My 250 floods very easy a pain to start if the slightest bit flooded.Another case of cost cutting measures the one switch off on choke lever and my worst hatred no throttle lock.
Kash

 

[Pioneer]

Both my 250's did exactly the same, it's the choked muffler. After the mufflers were modded, the screws had to be turned in to half their original value. It was so bad originally, the air filter was constantly wet from spitback. Nice and dry now.

 

[pioneerguy600]

That just don`t seem right, a choked muffler requiring a richer mix to run. Now mind you I only have owned 7 of these 250 & 230 saws , two came to me with torn crank seals, two had leaks around the pan and seals, three were tuned too lean and scored so I fixed them all and gave four away to young guys just getting started out cutting firewood. I have never modified the mufflers on any of them or seen where they required rich tuning to run well.

 

[jltrent]

The difference is the leveling meter and or diaphragm inside the carb if everything else is equal is a cats hair different. I have tinkered with that carb and put a new leveling meter in and had to totally reset the mixtures. Forgot to mention if the saw is air tight and the coil is firing like it should.

 

[Pioneer]

The main reason you need such a rich setting is because the choked muffler holds back a massive amount of exhaust gas in the cylinder and prevents complete scavenging. This leans out the charge and you have to compensate with richer screw settings.
I have seen increased flow and leaner settings with each step I take in getting these mufflers to flow better, as you pull more air the vacuum signal gets stronger and the mix gets richer, so you have to lean them out. The amount of residual exhaust gases also decrease which cuts down on dilution/lean charge.

 

[rupedoggy]

Interesting theory

 

[SEAM]

Check the impulse hose (hose connection on right side front part of filter housing from below - difficult to see) and/or the fuel line just above the tank (tends to crack at the bend). My bet would be on the impulse hose...

 

[vhmtach38]

There is definitely extra air entering somewhere to be opening both H/L that much...

 

[Bob Hedgecutter]

So, if I understand this fully- you had to tune the original carb way rich and use the choke to start the saw at every turn- so you replaced the carb with an OEM Zama new one and have had to do the exact same thing to get the saw to run?

Then it AINT the carb that is your problem.
Pull the muffler off and give us a photo of what the piston looks like through the exhaust port.

 

[rd35]

Pressure/Vac test needed for sure. Gotta be leaking air somewhere. Last one of these I wrenched on had loose pan bolts and was leaking air around the pan and seals (outsides of seals...not at the crank).....oh....and the piston was toast as a result.

 

[kingofcoins]

Update on this problem....I modified the muffler, the piston looked normal, and compression was good ....but the saw still
does the same... I started with clean plug, would not fire unless on choke, then stalls at idle or as soon as throttle is opened
..by turning open the low and high screws eventually it will run for a bit but still stall if throttle is touched, AND then if screws are opened
the 3+ turns or so to get it started , then the plug fouls wet and it will not start until I dry it and start the sequence all over again...
So now its either too lean and won't run at all, or too rich and dies anyway ....though it runs I can't see much any spark ...does anyone
think a bad ignition module might be involved ? Saw is 12 years old with little use, and never used at all by the prior owner the last 8-9 years ...

 

[brandonstc6]

Got an air leak for sure. Inspect your intake boot and impulse hose. If both of those are good. You need to remove the pan and replace the seals and re-seal the pan.


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[Bob Hedgecutter]

So 90% of the replies tell you to "pressure & vac" test the saw and tell you they suspect a severe air leak........ but you decide to modify the muffler in an attempt to cure things?

 

[rupedoggy]

Lot of people don't put much stock in free advise. Makes you wounder why they ask.

 

[stihl86]

Lot of people don't put much stock in free advise. Makes you wounder why they ask.

They are waiting for the answer they want. Not need.
Having to screw the adjustments out 4 turns has absolutely nothing to do with ignition.

 

[TheTone]

When I get an old clamshell saw, the engine gets rebuilt as a matter of course. Almost always there is an air leak, and it is at the pan-cylinder junction. I don't even bother to check anymore. Also, it's a great opportunity to check piston, cylinder, bearings, and replace rings and seals. I think the instructions to "run a bead" of sealant is not optimal. I clean both surfaces meticulously and use my finger to spread an even coat of sealant on both surfaces, including the oil seal recesses. Then the pan is put on and short bolts are used and torqued to 70 in-lbs. A thin bead of sealant (yamabond 4 or equivalent) is spread around the case at the pan-cylinder junction, where the leaks are most prevalent. The engine sits for two days and then the short bolts are removed (carefully, criss-cross) and the engine mounted in the case and torqued to 95 in-lbs. I don't worry that the seal will crack after the short bolts are removed. This is because I once had to go back into a case after the sealant had set up and had to use a hammer and screwdriver to get them to pop apart. It's a good idea to retorque the engine bolts after break in, since these are subject to a great deal of vibration and flexion. This should save you from having to go back into that "dreaded" clamshell.

 

[Wood Doctor]

Reminds me of an 025 I fixed four years ago. Good compression but would not run until I changed the seals. They were shot. New seals fixed it and it runs today. Yes, it's a PITA, but it's what must be done to save the saw.