Thread: Stihl 028WB Chain Saw – Starts but won’t run

Stihl 028WB Chain Saw – Starts but won’t run - “Nozzle Check Valve“?

Hello,

Can you please help diagnose this problem?

First the problem history:

Been cutting fine for 2 weekends. Then, after 30 minutes throw the chain off. The engine revs high for just a second. I shut down, re-fuel, re-oil, put the chain on. (Same fuel and oil as I’ve been running for weeks.) Saw fires, but won’t run.

I pulled the air filter and it’s a mess. I brush it off and reinstall - same performance. I try to run it without the filter – same performance. I splash fuel in the carburetor throat – runs a few second longer. Remove the plug and check spark – looks strong and blue.

I ordered a Tillotson HU40B carb kit. Replaced meter needle, pivot mechanism, diaphragms, gaskets. Removed/replaced Welch plug and filter screen after a thorough jet cleaning with carb cleaner, but not compressed air, oops. I re-set the H/L screws to factory 1 ¼ settings (I hope I did not crash these into their seats.) I did not mess with brass plug.

I removed and inspected impulse tube and intake manifold. I can not find any defects, but I have not inspected with specialized leak finding techniques. No giant holes here.

Compression seems OK – same as ever, but I have not measured. Some old dried oil spun radially outward from crank bearings. I’m sure these need attention, but I don’t want to go there if I can help it. Running a new OEM spark plug now.

Reassembled the saw except bar and chain and basically have the same performance. With lots of pulling, fires and runs very slow and rough for a few seconds @ WOT only. A fine mist of fuel sprays out from carburetor throat. Oil/Fuel is coming out of muffler. I tweak the L screw closed some, then open some, also close H screw completely and can’t perceive a change in performance.

I think that covers most everything. Do I need to press-out the brass carburetor plug “Nozzle Check Valve“? Apparently that may influence idle.

Help?

CaptainLarry

Pull the muffler off and have a look at the piston.Sounds like an air leak,or a lean mixture of some type may have cooked your piston.Just curious, does your gas mix contain ethanol? Ken

Check/change in tank fuel filter, carb is not getting enough fuel.
Pioneerguy600

Ignition......

Originally Posted by Captainlarry

Some old dried oil spun radially outward from crank bearings. I’m sure these need attention, but I don’t want to go there if I can help it.

Please explain.

+1 on taking the muffler off to check piston for scoring.

Thanks for the feedback guys. I’ve read so much of “your” stuff now, I feel like I’m talking with famous folks.

Ken, I’ve wanted to pull the muffler wondering if it was clogged and preventing good flow. I’ll pull it. Not sure I’ll be able to positively ID a “cooked piston”. I thought the piston looked OK on the intake manifold side. Yes, our fuel is said to be “up to 10% ethanol.” I did buy fresh “mid-grade” octane fuel for the re-build test.

Pioneerguy, I know I changed the fuel line not too long ago. I thought I would have changed the filter then too. I’ll investigate. Pretty sure I can suck good flow through the line…yuck! But you think it sounds lean and I’ve got fuel/oil coming out the muffler? BTW I run 40:1 with aftermarket 2 stroke oil.

Fish, Yeah, I’ve questioned ignition too. Ignition timing in particular. It seems to have good blue spark every time I check it. I could not make sense of the timing marks. Looks like the lines match up just ahead of TDC which makes sense. But the marks could have also been set to line-up right at TDC. My fear was the sudden acceleration after throwing the chain sheared the key partly. Seems like a far fetched theory. I’ll just remove the nut and see if the groves line-up. Otherwise I have not pulled the F/W or changed the points to electronic.

Jonathan, What I mean by “old dried oil” is that I figure I have some leaking of the old crank seals, don’t we all? I figured oil would migrate out from the seals and move radially with the air off the flywheel, no? I just did not want to do the work of changing these seals if I can avoid it.

I’m really surprised none of you guys chimed in on the “nozzle check valve” theory. What is that crazy thing supposed to do?

I did more research on AS last night and I realized I put the gasket on the wrong side of the metering diaphragm. I did not expect it to be different from the pump side. I also need to verify the height of the meter pivot mechanism (“inlet control lever”) tonight. These could influence fuel flow, which I’ve been suspecting is too rich right now.

Larry

Yes, by all means rule out the fuel system. But don't screw up your carb in the process. Make sure there is no debris under the needle and it is sealing good,
and make sure that passage is clear.

But otherwise, I think the trouble is ignition related.

You say that if you splash fuel down the carb it will run for a second longer than under normal starting conditions? This to me would imply it is not flooding/way rich.
I am assuming you have tried starting again having corrected the diaphragm/gasket order.

I would still be pulling the muffler, trust me you will know if it is fried or not. I have experienced your described symptoms before on saws that have scored pistons, often rings are held in by aluminium meaning charge can escape back down cylinder.They will fire but not run, if you dump fuel in carb they run for a few seconds on fast idle then die.
Is the saw hard to get to fire or does it take roughly the same number of pulls as normal?

As for the crank seals, are you sure oil is leaking from them and not elsewhere?

Jonathan,

Yes, I understand your thinking that it is not rich because of my splashing fuel trick. I'll fix the gasket order tonight and try it again.

I'll pull the muffler too, if the bolts come free easy enough.

No, I'm not certain about the oil leaks. Maybe bar oil is finding it's way around. Right now I'm just looking to see the saw run.

Larry

Larry, I also own a 028, and it is a great running saw. Oh buy the way,Welcome to the site!Many individuals here that will help you.They love pictures! Ken

Ok, I ruled out some things, I think.

(1) I switched the gasket/main diaphragm order and (2) set the inlet control lever so it is flush with the diaphragm floor. It was pretty high, so I bent it down a tad. A high lever would mean a rich setting, right? So it should be more lean now. I think that I see less (none) raw fuel coming out of the muffler now too.

(3) I pulled the muffler and had a 2-stroke dirt biker friend look at the piston. He thinks it looks fine. The rings move freely up/down. (4) He also confirmed I was getting good compression. (5) The muffler was not restricting flow.

Fish, I re-checked spark. (6) I really am getting a good spark with every piston cycle. I know spark changes under comression, but the saw will start pretty reliably - just won't run.

I re-checked fuel flow through the filter...Yuck. There has to be a better way. (7) It flows!

So I ruled out what, 7 things? I could not see if the key was sheared. Almost had the flywheel pulled off using a bicycle crank remover tool. It was the wrong thread pitch. I'll see if my dirt biker friend has a puller that fits.

So the saw will now only start with throttle closed. Runs for a few seconds - dies. If I feather the throttle it "baw-baws" and dies. The H/L screws are at 1 1/4 TO per OEM. Minor tweaks seem to have no influence. Little or no raw fuel at muffler. Fuel still sprays back at me when I open the throttle while pulling.

WHAT NEXT?

Captain Larry

Larry, I am afraid that you are beating a dead horse.When the saw raced at the beginning of your post, I have good reason to suspect that you may have melted your piston.As an owner of a 028 I find it hard to see the piston from the intake side. Not saying it cannot be done but the muffler side is easier. Now pull the muffler off and put a few drops of oil on the piston. Now slowly pull the rope and watch for bubbles.Your friend with the bikes may have not had a really good look.To cut through the lard and get to the ham get a compression tester or have it tested.I suspect that it will be in the 90-115 lb range.Buy the way, I hope that I am wrong.Ken

I agree with Fish,just had same trouble with mine, electronic conversion, will most likely clear you up for $20 bucks, Atom module at Napa part number 7-01749, switch neg. and positive leads and you don't have to mess with timing, did 2 this week !!!!

Check the piston to make sure it's not toast. Take a picture and post the rings on the saw. Then check the flywheel key while your at it. If it's shot it throws off the timing of the saw.

Yes, We're beating a dead horse.

The piston is fine. I'm not pulling the muffler again. We saw bubbles the first time with the rich mixture oozing down the exaust port. And yes, I did see the piston through a larger window on the intake side after removing the intake mainfold. The rings are floating freely. I'm really sorry I did not take pictures to prove it. I'm not accustomed to photo documenting.

So, now I'm sounding like a know-it-all snot. Sorry. Don't mean to. I did what was suggested and my opinion and my very experienced biker friend both think the piston/compression looks fine/typical.

Won't get a chance to work on this until 2-21-11. I need to get this thing running before next weekend's planned cutting day. I'm planning to re-adjust the inlet control lever height because I'm afraid the setting height may need to account for back lash, though Tillotson did not explicitly mention it. The current setting would account for the new lean condition.

Motoman, thanks for the Napa lead.

Talk to you Monday.

LArry