Crank leak or carb?

[Pioneer]

Just put together an ms250 with a new long block. It starts easy and idles ok with the idle needle reasonably close to what it should be, but it will not run at high speed even with the high speed turned out twice as far as it should be. I rebuilt the carb but that's no guarantee its not plugged up somewhere.
Would a crank leak effect the low side more? What psi should I use if I go the pressure test route?
I dread the prospect of taking apart this little monster again.

 

[motorhead99999]

Sounds more like carb to me or i would think it would not idle nice. Surging or reving up on its own I would look more into a air leak

 

[PV Hiker]

Needle jet and diaphragm, high side jet passageways blocked? Butterfly not opening. Fuel filter not so good, along with fuel hose. Air cleaner? Coil gap incorrect. You could expand possibilities further.

 

[gary courtney]

high speed turned out twice as far as it should be.

the more you turn ccw the fatter it is

 

[Deleted member 117362]

Doesn't your manual state 3/4 turn out on high needle?

 

[dmb2613]

carburetor is my bet, they are cheap from Stihl around $30

 

[MacAttack]

i also think carb, starving fuel once you get more into the main fuel circuit.

 

[SteveSr]

Just put together an ms250 with a new long block. It starts easy and idles ok with the idle needle reasonably close to what it should be, but it will not run at high speed even with the high speed turned out twice as far as it should be. I rebuilt the carb but that's no guarantee its not plugged up somewhere.
Would a crank leak effect the low side more? What psi should I use if I go the pressure test route?
I dread the prospect of taking apart this little monster again.

This sounds exactly like what you would get with a carb main nozzle check valve that is stuck closed. I doubt that you have an air leak if it idles o.k. Turning the "H" screw won't help as it never gets to the "H" circuit. If the valve is intermittent you might be able to feather it to WOT by partially applying the choke. If it all of a sudden wants to run right after this you found the issue. Replace the carb. it WILL come back!

 

[Tom B.]

Interesting way to diagnose things.

 

[Orangeinthebarn]

Just put together an ms250 with a new long block. It starts easy and idles ok with the idle needle reasonably close to what it should be, but it will not run at high speed even with the high speed turned out twice as far as it should be. I rebuilt the carb but that's no guarantee its not plugged up somewhere.
Would a crank leak effect the low side more? What psi should I use if I go the pressure test route?
I dread the prospect of taking apart this little monster again.

Check impulse

 

[Pioneer]

I wound up soaking the carb in laquer thinner (after removing diaphragms etc) and it seemed to help. Starts and idles well, but still needs too much of both needles turned out to run well. Does the low speed circuit feed off of the high or vica-verca? It seems to act like it has a low vacuum signal, will larger than normal cylinder transfers and/or a muffler mod cause this?

 

[HarleyT]

I agree on checking the impulse line.
Especially since you just rebuilt the saw.

That impulse line can be tricky!!

 

[Pioneer]

The saw will start 7th pull from a bone dry carb. Is that about right, or should it do better? If the impulse line was leaking wouldn't initial startup be a problem?
Does the low side feed off of the high side? I'm thinking the high side still may be a little plugged. I have another carb I can try but I have to pull it off of another trash saw and I don't know if it is any better.
If it doesn't smarten up after having fresh gas sitting in the carb for a while and dissolving any leftover deposits of gunk, I will pull the impulse line, check it and the fittings for proper sealing.

 

[SteveSr]

I wound up soaking the carb in laquer thinner (after removing diaphragms etc) and it seemed to help.

Lacquer thinner is a bit harsh for carb cleaning as it will also dissolve/soften other rubber components Have you checked the operation of the main nozzle check valve? That can mimic an air leak at low rpm or cause other issues at WOT. I believe that the Walbro manual show you how to test one.

Starts and idles well, but still needs too much of both needles turned out to run well. Does the low speed circuit feed off of the high or vica-verca?

Both L & H feed off of the same metering chamber so the adjustments interact somewhat. at low rpm it is L only at WOT it is L +H flow. BTW, have you checked the metering lever height, especially if you installed a new kit with a new metering arm? THe old metering arm is was set at the factory while the new one wasn't and will need to be adjusted.

It seems to act like it has a low vacuum signal, will larger than normal cylinder transfers and/or a muffler mod cause this?

Muffler mod... No. Has the transfers/porting been changed from stock?

 

[Pioneer]

The transfer ports have been changed as it's a new aftermarket cylinder, the new ones looked a bit wider than stock. That aside I think I found the problem, assuming the ms 230 and 250 use the same carb, Zama in this case.
I pulled the carb off of the other trash saw, a 230, and opened it up in prep for cleaning. There was a HUGE discrepancy in the meter lever height, the 230 was quite a bit over the carb body edge, the 250 that I just rebuilt was noticeably under. I re-used the stock metering lever and spring and changed the seat needle when I did the carb rebuild. I did a Google search on Zama metering lever setting for the model of carb (C10) but was unsuccessful. Anyone know where these suckers are actually supposed to be set? Or is this going to be a case of trial and error because of the mix of old meter lever and new meter needle?

 

[HarleyT]

It is a Zama C1Q.


Here is the gauge.......

 

[Pioneer]

Thanks, looks like it is flush with the surface. Had to take the whole saw apart again, the rtv silicone on the crankcase didn't set up properly. At least now I know for sure the carb boot and impulse tube are in good shape.

 

[SteveSr]

Thanks, looks like it is flush with the surface. Had to take the whole saw apart again, the rtv silicone on the crankcase didn't set up properly. At least now I know for sure the carb boot and impulse tube are in good shape.

What did you use for sealant on the crankcase pan? Dirko HT (what Sthil uses) is readily available.

If you used the old metering arm the height should be set properly assuming that you used an OEM carb kit. If not all bets are off.

BTW, the Stihl carbs are calibrated to run at standard needle settings only on a stock engine. Your AM engine may have significantly different fuel requirements which throws Stihl's needle calibration out the window.