Stihl 028 won't rev up, running too rich.

[celliott]

Alright, so I got 3 (and a half picked parts saw) 028 wood bosses thrown in on a deal for a 385xp Husqvarna I bought.
I figured I would fix up the 028's and sell them, get my money back for the 385. I'm not really a Stihl guy, and the 028's, while good saws, don't really appeal to me.
I know Stihl made the 028 for YEARS. Lots of small variations and changes. So, not all my extra parts are compatible.
I got one (the newest I think) running 100%, after replacing a cracked fuel, impulse line, and torn intake boot and rebuilt the carb. One down.
#2, turns out it had bad bearings. OK now it's a parts saw too. No big deal.
#3, the problem child  >:(

What it's got so far- New fuel line + filter, impulse line, carb rebuild. Had no spark, so I replaced the coil with a known good one I tested on 028#1.
Here's what it's doing- It will start on choke, and run on the high idle position, but dies if you switch it to the run position or give it throttle and let off. The saw won't rev up when you give it throttle. It seems like it's loading up on fuel, running very rich. There is a fair amount of black oily residue coming out of the muffler. Running 32:1
Played with the low speed jet, and I adjusted the high speed out 3,4 turns, almost all the way out, didn't help.

What I've checked so far- Diaphragms in carb installed in wrong order (I've done it before) Nope. Metering lever set too high, nope, it's correct.
Timing maybe? Pulled flywheel off to see if the woodruff key had sheared, nope, it was fine. Regapped coil also.
Muffler too restricted? Spark screen wasn't plugged but I pulled it anyways. No help there.
I even tried running it without the B+C, thinking maybe something with the clutch? 
The compression is about 128-130 psi. Too low? The exhaust side of the piston is lightly scored, but the rings aren't stuck. I've seen saws run with worse than this.

Not sure where to go from here.
Any ideas on what else to check out? Or something to check out again?

Thanks guys 

[sablatnic]

Take a look at the air filter. You might have a slack return spring for the choke, has happened to me! Or start it without air filter for a brief look see.

[Jiles]

It seems like it's loading up on fuel, running very rich. There is a fair amount of black oily residue coming out of the muffler. Running 32:1
Played with the low speed jet, and I adjusted the high speed out 3,4 turns, almost all the way out, didn't help.

Yours may be different, but with all my chainsaws, turning H adjuster screw out makes it run richer. I usually start with One turn out and go from there.

[celliott]

Well I figured it out  8)
I tried running it without the air filter as sablantic suggested, no help, but it was spitting alot of fuel back out of the carb.
I then swapped the carburetor from the solid running 028 (a walbro HT, same as on the bad 028) and it ran correctly. Should have tried that a long time ago....  :
So, tear into the carb again...
Stripped it down, took the jets out, gave it a good cleaning and blew passages out with light air. Reinstall, same problem! What's going on?
I had a third carb from the parts saw (a tillotson this time) I put that carb on, and it ran, stayed idling, and I tuned it  ;D

What do you think was wrong with that walbro HT? It had all new gaskets+diaphragms, cleaned the screen, set the metering lever correctly. I had the good and bad carbs apart, side by side and couldn't see any differences.
Maybe the new carb kit was bad from the start? 

[Al_Smith]

On the carbs perhaps a couple of things .On some of the carbs in the butterfly is a very small slot .That slot has to perfectly align with the idle jet so it gets air flow over the jet .The idle jet remember is on the engine side of the of the butterfly and as such with the butterfly closed on idle without the slot it would otherwise be unable to get the pressure drop to suck the fuel in .

Another thing to check on that design is the intake boot .Often because of the way the anti vibes are mounted the boot can be stressed causing a tear on the under side which is nearly impossible to detect without removing the boot .This problem is common on the 028,038,042 and 048 saws which are the same basic design .

[celliott]

Hmm. Well I had the butterfly out, and reinstalled it the same as the good walbro. Maybe I had it the wrong way though.
Yup, know all about the intake boot. I pulled a good boot off from my parts saw and put on the first good 028 because that one was torn badly. This one was fine, I had the tank off and checked it out pretty well, for the reasons you mentioned.
Ah well, that carb will just remain a mystery. The tillotson I put on it runs good, so that walbro got put back in the parts box.

[Al_Smith]

Oh that was just general info on what could go wrong but get over looked .Believe me I've over looked a lot of stuff before and wondered how in the world I missed it .Were it a snake it would have bit me .

That slot in the butterfly Rocky J put me on to that one .I had never much payed attention before .

[Bilge Rat]

Look at the tip of the needle valve real close. Get out the reading glasses, optivisor, magnifying glass etc. My 56yr old eyes need this stuff to see up close.

The tip of the needle may have plastic/rubber on it. Ehtanol gas eats this tip allowing fuel to pass unregulated.
I have found a few bad HD carbs that i fixed with a new needle.

Saw carbs are simple but tiny!
Building a 4150 Holley for the race car is a breeze compared to the saw carb, you need to be a watchmaker for them.

[Tom King]

Clean the spark arrester screen.  One screw holds it in the muffler.  Take it out, hold it with an old pair of needle nose pliers, and heat it up to red hot with a propane torch.  When it cools, wearing an old glove, rub it between two fingers to get all the ash off.  Put it back in, and it should start breathing again.

Sorry, I was talking to someone while I typed that, and forgot you already checked the screen.

[celliott]

Well thanks for all the (after)tips guys. It runs good, I just swapped the troublesome walbro for the old tillotson that was on it. I'll never know what was the problem because that walbro carb is no longer in my possesion. The box of 028 bones I had picked from, and the two crankcases, everything else 028 I had, went to the chainsawr today. Gave me store credit on a 20" oregon powermatch bar and LGX chain for my new to me Jonsered 2171. I was happy, got a bar I needed and got a bunch of parts I'll never use again out of the shop.