028 troubleshooting...tearing hair out.....

[Molecule]

on the Tilly HU there is a very fine filtering screen under the smaller welsh plug - it protects the main jet. If you had grunge clogging up the main filter on the high pressure side of the fuel pump (which is fine, but coarse compared to the filter under the welsh plug, then the filter covering the main jet is likely gummed up too.

If you go to the trouble of pulling a carb and rebuilding it, there are probably more than a few experienced saw builders who will tell you, *definitely check underneath the welsh plugs. For example, the Tillotson rebuild kit for the HU carbs includes both welsh plugs--for a reason.

If you have a drill press, carefully start drilling a small hole at the center of each welsh plug, and stop drilling when the drill bit is just about to penetrate the interior-lower surface of the plug. It's just a plate of very soft aluminum, so it drills with no effort at all, which is why the drill press is necessary, for depth control. Don't let any drill tailings get into the carb, so keep the drill bit clean, and do *not drill past the welsh plug as you may destroy the body of the carb. Go to your dentist and get a strong dentil pic, and use the pic to poke thru the rest of the way, and lift the plug up and out. (The Tilly manual says which size drills to use, but I use 1/16" for both.)

You can now clean both sides of the jets (the main jet) and the two or three low speed jets, with carb cleaner and air, and a thin strand of soft copper wire if you want.

When replacing the welsh plugs, I press them in using the drill press again, but I put a short piece of round stock in the chuck. Important: the round stock shoudl be just slightly less in diameter than the welsh plug socket itself, or close thereto. Whatever diameter bar you use to "press" the new welsh plug flat, spend the time to make one that will fit that welsh socket exactly. For the large plug on a HU, a flat arrowhead is exactly the right size.

Carefully air clean the welsh plug sockets, and press the dome of the plugs down flat. This will press the sides of the soft aluminum outward into the sides of the welsh plug sockets, and seal off the jets from the high pressure metering chamber below. I didn't see where the Tilly manual said use Loctite or anything, so I just pressed them in, with a precisely fitting tool, until they are exactly flat.

I'm sure that I won't be alone in confirming that grunge does get in underneath the welsh plugs. They're not that hard to work with.

====

The Tillotson service manual says to "Drill a 1/8" diameter hole through the [larger] 11/32" diameter welsh plug. This hole should just break through the welsh plug. Deeper drilling will seriously damage the body casting and render the carburetor non-serviceable. Carefully pry out the welsh plug [taking care not to damage the well seat] and clean ... To reinstall, place welsh plug in well, convex side up, and flatten with a flat end tool slightly larger than the welsh plug. ... Nozzle [smaller] welsh plug can be removed ... using a 1/16" drill."

 

[Ax-man]

Molecule,

That has got to be my problem is that smaller Welsh plug, I hate doing Welsh plugs because they are so sentitive, it is last resort if after putting in new diaphrams and the carb still won't test out.

I haven't pulled that smaller Welsh plug because it is not in the kit I have, the bigger one is.

The number on my kit is 18114, RK 14 HU Tillotson, this cannot be the correct one if it only has one welsh plug, or they just don't include it, I didn't think there was that big of a difference for a HU Tilly

I have all the tools you describe to do this and not afraid to try, it is frustrating with these Tiily and Walbro carbs for 020, it gets to be a matter of principal to get them to work.

Larry

 

[Molecule]

Larry,

On the HU carbs, the gold colored "plug" is not a welsh plug! If you drill it, you may be venturing into ... new AS territory ?.

It's called a "Nozzle Check Valve" which apparently also functions as the main fuel jet. It probably has a little plastic ball inside it, which oscillates if its free. For the HU40 the check valve (part no 363-559) is about $4 and shipping from Tillotson (VE Petersen) is 4 to $9. I've never replaced one, but I may be about to if yours turns out to be a problem.

If that valve is sticking, I can imagine the saw will definitely not run steady at WOT.

On sealing the welsh plug, which covers the low speed jets (I can't tell if there are 2 LS jets or 3), I don't imagine a supertight seal to be a hypercritical thing. On the high side, the metering cavity is maybe at +12 psi, on the low, the jet outlet and venturi at maybe -3pis, so if the metering adjustment valve takes half the drop, there's only 7-8 psi across the plug -- with the higher pressure fuel area tending to seat the plug, by pushing it inward toward the low pressure at the jet. Seems to me it would take a real hatchet job when replacing the plug to screw it up so bad that a significant quantity of fuel would bypass the adj valve and go straight to the jet. what little bypass may happen, would tend to "deaden" the effect of the valve toward a rich condition (screwing it down to zero would not stop flow to the jet-opening it up will add fuel to the jet), so if you're getting leaned out, the problem is somewhere else.

If you decide to drill out and pull the check valve, please let us know how it goes ... what does it look like, can it be replaced easily etc. Otherwise maybe it means a new carb ... ($53 from Tillotson)

 

[Ax-man]

Molecule,

As of this post I haven't drilled out the gold plug, I'm glad you mentioned this about the check valve. I have another kit and will redo this carb from scratch with the advise you gave earlier.

Even though this saw was bought as a non runner, I'm going to swap a different Tilly from another AVP that runs to see if this saw will come to life.
The non running saw shows evidence of running very rich.

If the second attempt goes good on the carb when I test it, but still won't get the saw running, I'll have to venture into the unknown, it is the only thing left that hasn't been looked at. I'll let you know how it goes, even if I screw it up.

Larry

 

[stihltech]

basics

You need to start over, you missed some very important steps in the diagnosis process.
Pick the saw up by the starter handle. Does the rope come out very slowly or quickly? Slowly, good compression, quickly, bad compression.
Crank it over on choke. Does it spin nicely or act like it is binding? Binding? Pull the muffler, look for scoring.
Pull the plug, check for fuel. Dry? No fuel. Ground it to metal (a jumper wire works well). You want a nice, blue spark. Dull or none? Problem.
You say it does not run off choke. If you put fuel in the carb, does it run longer?
At this point I would check all hoses and filters. Replace bad ones.Also the intake boot.
Check the carb screen on the inlet. It must be clean.
This carb does not use a high side screen, that is a nozzle jet. It will not usually cause this problem. It will cause a idle stall condition.
The inlet needle must be free, the inlet lever must spin freely on its axle.
I would now consider an air test. I have seen the seal under the clutch worn through.
Hope this helps.

 

[Molecule]

I know this is a little out of left field, but I might also check the quality of gasoline used. The carbs in older saws, more specifically, the fuel pumps and metering systesm in the older carbs, were not set up to function against the high --in some cases, extremely high, vapor pressures of some brands of fuel.

IMHO for an older saw, look for the highest octane number you can find, with the *lowest vapor pressure. In the mid-atlantic area, that puts Shell hi-test gasolines out of the running. The Amoco straight white gas, "Ultimate" brand name, seems to have the lowest vapor pressure per octane number. (Without adding the cost of spending $40 in car gas to drive to an airport, and getting past security at the fueling station, to purchase $10 in "av" gas. For a lucky few, av-gas might be an economic option. For the rest of us, it's not. Race gas is also hard to find.)

My theory is based on the observation that the liquid side of the fuel system, from the gas tank to the top of the metering valve, is sealed system. When that closed system is filled with gas, it becomes "pressurized" by the natural vapor pressure of the gas in the tank at the operating temperatures. Thus the liquid fuel is pressurized by the sealed vapor gases in the tank. So, the fuel side of the fuel pump diaphram, the bottom side of the diaphram (the fuel pump is on top of the carb), is now pressurized. Let's also observe that the ratio, of (a) the cross sectional area of the high-side orifice of the fuel pump to (b) the cross sectional area of the fuel pump well, has to be such, that by the time the net positive pressure pulses from the crankcase are routed up to the other side of the fuel pump diaphram, there is enough *relative* positive pressure available across the diaphram, to push it down into the pump well and thereby do its job and squeeze out the fuel, e.g. lift the flapper valve over the metering needle which is maintaining constant pressure on the needles and jets. That is to say, the pump will increase liquid pressure of the fuel according to the ratio of the areas of the pump well and outlet orifice. For example purposes only, let's say that ratio is 15-to-1, and the crankcase is delivering +2 psi to the diaphram, then absent any pressure resistance from the fuel itself, the pump can potentially put 30 psi on top of the metering needle. Now, let's add a positive pressure on the fuel side fo the diaphram, which resists the positive pulses from the crankcase, by using a fuel which as a vapor pressure of +1. The net pulse from the crank is now only +1psi, and the metering system can be charged up to a max of 15 psi. Go up to a high vapor pressure fuel, with a vapor pressure of +2, and the crank pulses on one side of the diaphram just equalize with the vapor pressures on the closed system on the fuel side, and the diaphram doesn't move. The flow of fuel to the metering system, needles and jets comes to a halt. Lean seizure follows -- from a high vapor pressure gasoline being used in a carb whose fuel pump and pulse delivery system was designed to operate against a low vapor pressure. Some try to blame this on "vapor lock" arising from "increase in temperature" of the saw. Saws operate at high temperatures ... the problem is finding a fuel with a high enough octane to not destroy the piston top by detonation when idling, and low enough vapor pressure for the pulse delivery-fuel pump-jet metering system to work correctly...

 

[Ax-man]

Please don't take this post as me being some kind of smart ass, know it all, I have been a good student and doing my homework out in the shop using all this good info from you guys who repair saws for a living. I have gotten deeper into engines and carbs and have sharpened my saw repair skills ten fold. My days are numbered as a climber and doing fulltime tree work, repairing saws might be a handy skill to fall back on for a guy like me.

Back to the AVP that won't start. I did the basic checklist you outlined Sthiltech, handy trick that hold it by the handle to check compression, gauge verified it too, 150 lbs. I bypassed the pressure test for the crankcase after the saw started briefly after squiting some mix down the venturi of the carb. Pressure testing crank could wait if I got the saw to at least run, even if erratic.

This is where the trouble starts, pulled carb, air test on carb shows no leaks, but H - L don't check out when putting wire through vent on metering side to lift inlet needle off seat. Plugged up right??

Next step take carb apart, inlet screen plugged like Fish said, debris from fuel filter, not uncommon, remove inlet needle and lever, soak and clean body of carb, reinstall needle and lever set to specs, visually see that it works good. put the carb back together, air test again, still nada on the H&L.
Didn't mess with Welsh plugs, has to be the problem, pop welsh plug and clean those passages, reassemble carb, only now we have an air leak, finger nail polish over Welsh plug, didn't work. I know this fingernail polish does work, I tried it on another carb that was leaking but didn't have a kit for it.

I'm going to stop here as it is getting long winded, if someone else wants to comment feel free to jump in. There is more to this little story.

Molecules way of getting welsh plugs out and reinstalled works slick, much better than the crude methods I was using.

Larry

 

[stihltech]

Jets

Just a dumb question, you are opening each jet a turn before you depress the metering diapragm, right? And you are hooking the pressure guage to the fuel inlet?
Many a person has hooked to the pulse side, ain't hard to do.

 

[Ax-man]

Yes, one jet closed the other open one turn to test the individual fuel circuits.

Air tester is connected to the fuel inlet side of the carb. I have tried many a different ways to get the inlet needle off it's seat to check this Tilly and other 020 carbs , mostly Walbro carbs. The results are the same, pass the air test but nothing on the H - L circuit. I have done the same test to other carbs and they test good the first time out.

I must be doing some thing wrong, some how, I can't believe the 4 to 5 carbs I have left for 020 could all be bad. There has to something common to these carbs that I am missing for these carbs not to work. Before I start to tear into all of them it would be nice to look for a specfic problem that would be common to these carbs.

I bought a carb off e-bay for an 020, it was a Walbro, it tested good right out of the box. That one is on another working AVP I have.

Our local Sthil dealer has had the same problem with these carbs for 020, they just quit trying to rebuild them and would got new ones, but now they say they don't have a source any more for new 020 carbs, the way I understand this is, Sthil doesn't carry carbs for 020 anymore.

Any more thoughts on this would be appreciated, I just don't understand why something so simple like these carbs won't work after putting in new parts and set it according to the specs.

Larry

 

[Fish]

Not to spank a dead monkey, but when you had the needle out, did you
blow the passage out with compressed air, and visually verify that the
passage was clear? That sounds like your problem, you must look through the
channel, and have no distortion, especially after you admitted that the
screen was choked up with debris, as the rest of it is as well. The carb
spray can does not always cut it.
It just sounds like the area you have overlooked.

 

[Fish]

Now where is that dam monkey?

 

[Ax-man]

Sthiltech and maybe a few others, I owe you guys a an apology or a case of beer, because you really got me to thinking.

It hit me like a ton of bricks on what I'm doing wrong. I was hooking the air testor to the impulse side. The 020 carb doesn't have an inlet fuel elbow on the carb body like most carbs, it has that funky flange and boot for the fuel inlet. I was taking the carb off this flange and testing it using the pulse side DRRRRR.

This is why the ebay carb tested good, it was a complete assembly.

I put together a carb, flange and boot from the parts I have, sure enough one carb tests good. H-L and all.

Hindsight is a wonderful thing I guess, but no excuse for not thinking something through.

The Tilly got sacrificed last night, thinking the problem might be that valve, yeah I drilled it out, it is a little hard platic ball in there. If for some reason someone suspects this to be a problem with a carb, don't drill it, try to press it out into the venturi of the carb from what I could see it should come out that way with a little room to spare. I honestly don't see how it could ever be a problem, but it can happen.

I know I may look like some kind of bonehead to you guys, but for me I learned a little more than I did before when it comes to these carbs, especially on these Welsh plugs that I have never really messed with before.

Larry

 

[Lawn Masters]

Well hey, you learned something you didnt know before, thats what matters. I guess I did too, subconsiously.

 

[Molecule]

Ax-man --- That's really helpful info on the HU mainjet/valve thing - so it presses straight out and I guess also presses in. No matter how it might be removed, replacement would probably be with a new part.

Any chance you could post a step by step summary of what you did for the carb testing ...

What value did you use to test popoff pressure of the metering spring? The only thing I could find was 10-12 psi from the site glens has pointed to several times, but that is for airplane carbs, which fly at 10,000 feet. Anyone know, is 10-12 psi also ok for saws?

When testing the L and H side jets, did you have the carb/fuel line filled with a fluid (WD40 or ?) before bringing it up to pressure.

When looking down the throat of the carb to test the jets, when you tickle the metering diaphram with a tag wire thru the vent opening at the bottom of the carb, what do you look for -- I guess it should just squirt straight up across the venturi, as per the opening of the jet opening?

 

[chestand]

First of all, thanks to each one of you that replied. It has helped. The saw doesn't run yet, but information is never a bad thing.
Being a Stihl neophyte, I knew that my 028WB was a points/condenser ignition system. I pulled the flywheel on Friday and discovered the points to be fairly pitted on one side. I'm leaning toward replacing them with the electronic aftermarket unit. One brand I've seen is the MagnaFire made by a company called Stent. Opinions?
Also, a couple of questions in general.
I've never seen an eccentric as part of a flywheel hub before. On old lawn mowers, the eccentric is part of the crankshaft. How on a Stihl saw would you set the gap? I've read a procedure using a powered continuity light connected from ground to one side of points. Something like that?
How is it possible to keep a set of points from floating at the RPM levels a typical chainsaw operates at? As I understand it, you're looking at upwards of 12K RPM at WOT. I know the spring on the points is fairly stout, but it still seems incredible that it operated at all.
Anyway, thanks again and I look forward to hearing from you.
Charlie

 

[Fish]

Stihl makes a nifty little point setting tool for that purpose, drop by the local Stihl
dealer with the flywheel off, with the points cleaned off, and ask him how much, if
any he would charge you to set them. He might be nice and do it for free.

 

[Ax-man]

We have strayed a little from the 028 problem, my fault, derailing a thread is nothing new on this site.

Molecule, I know what your talking about when it comes to this pop off pressure, I have been to that site too. Unfortunately I can't help you on any numbers for a chain saw. There is a thread here in the archives, but you might have found that too.

On the carb testing, I don't put any fluid in the carb, I use the method that is in the " Chain Saw Service Manuel " under carb testing with an air pump and gauge.

Sthiltech had a post in one of the threads here about two months back that would say the same thing as I would on how to test a carb. I can put up a short version of how to test a carb.

I'll see if I can get it up tommorow, maybe. If someone wants to jump in on this,, go for it ,, I'm all typed out for now.

Larry

 

[lostone]

Generally speaking,bad seals will become more evident as the saw warms up,not usually at start up.

Al, I am running into this problem with an 028WB that I have, Is there a way to get the flywheel off without buying the special puller? (1110 890 4500) my local Stihl dealer isn't exactly warm to selling stuff to people so they can fix their own saws, I have had to order stuff through the mail because they (local dealer) just make it so difficult to get stuff. I am wanting to replace both of the seals at the same time, I figure if I have to replace one I better do both at the same time. I have rebuilt the carb, the hoses seem to be in good shape but I will replace the pulse hose as well as the fuel hose when I do the seals to be done with it all at one time. It has a new fuel filter and air filter, the engine starts, idles and revs great, then after about 5 min of running it, it warms up and then acts like it is leaning out so I have to adjust the L screw richer then take the and back out on the idle screw to slow the rpm's for idle, I have noticed when I put the saw on its side (bar side down) it acts up the worst. I haven't checked it for leaks on the seals as I need to remove the fly wheel in order to check the seal on that side. OK got the flywheel off, I made a removal tool if anyone needs to make one here is a link to making one removal tool

 

[BDA]

Wow, what a wealth of information is spread around from you guys. What a great place to be and get breakdown tips. thanks for the reading material guys

 

[Fish]

Most importantly, is to use a nail polish of a non-gay color.