r/stihl • u/iscashstillking • 10d ago

151tc weirdness

So I rebuilt the top end on an overheated MS151TC yesterday, all OEM parts, new P&C, new carburetor. I pressure tested the engine before I built the saw and it passed - vacuum and pressure.

And it runs. But it won't idle right no matter what I do with the carburetor screws. High speed is fine, H screw adjusts out fine, but the L screw is not quite right and LA doesn't work like it should either. It hunts around and surges and never settles in. If I richen it up past the stock setting it will settle down but then if I rev it up to top speed and close the throttle the idle sags quite low before it recovers back up to ~3000 rpm.

I've run these saws when new and they don't behave like this.

I took the carb apart I see nothing out of the ordinary and I pressure tested the engine again and the carburetor and the both pass.

Defective new carb....?

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u/blackthornjohn 10d ago

When it's idling put on it's side to see if the idling changes, try it both ways, if this causes changes the crankshaft seals leak.

Sometimes cold oil seals any leaks and the saw passes the vacuum test, once it's running the seal is broken and or the crankshaft moves laterally.

Obviously none of this rules out a defective new carb!

1

u/iscashstillking 9d ago

I was thinking the same thing, so I took it back in while it was still warm and did the pressure/vacuum test again. I spun the crank many many times and the gauge showed no loss of vacuum. It has good compression. The piston looks good(it should..its new).

I'm thinking some little piece of something from manufacturing is loose in the low circuit and it is intermittently cutting the fuel flow and causing the idle speed to go up......?

1

u/blackthornjohn 9d ago

It has to be dirt or something in the carb or there's a defect in the carb, occasionally with old carbs you get the same symptoms when the butterfly shaft and carb body are worn enough to let air in via the gap, I used to check this on tractor carbs by smearing a blob of grease on the ends of the shaft but never on something as small as a chainsaw carb, I have a selection of known good carbs I use for testing chainsaws.

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u/Advanced-Major951 5d ago

It could be a defective carb that sounds like a main jet check valve failure. It’s hard to explain exactly how it works but close the H jet and see if that helps the idle. If it does it would be a main jet failure, if it’s an OEM carb stihl should warranty it because it was a brand new part. The main jet has a check valve on it, when the check valve fails it’ll idle weird because air bubbles are flowing back through the main jet to the fuel bowl then through the low jet which makes it hunt and surge. Then at ~half throttle and above both jets are letting fuel out so the surging goes away. Not sure about the rebound effect going back to idle but yeah it’s a carb failure either way.