Help Request on Stihl 028 AV Super Wood Boss - Bogging

[Vintage Engine Repairs]

Two things to consider regarding pressure and vacuum. First, if you’re using a Mityvac, it measures vacuum in inHg, with roughly 2 inHg equaling 1 psi. So make sure the vacuum scale is around 15 and the pressure scale around 7. Second, turn the crank over multiple times under vacuum.

My initial suggestion is to lean out the L screw on the saw slightly. While it may not be the main issue, it’s definitely too rich and not quite right.

The fluctuation at full throttle suggests some restriction, which could be due to the tank vent, a carb/fuel blockage, or an intermittent air leak.

Amazon carbs can be hit or miss—your success rate is likely below 50%. I always advise people to hold onto their original parts unless they’re beyond repair.

Also, whenever you’re explaining something like this, it’s helpful to include a short video clip to illustrate your points, for the reasons I’ve mentioned.

In your case, I’d recommend retesting the vacuum at 15-20 inHg and turning the crank at the same time. If everything checks out (and you haven’t used rubber to block the intake), make sure the tank vent is functioning, the filter is clean, the fuel line is in good condition, flexible, and properly sealed. Then, try putting the knockoff carb kit into the OEM carb and check that all passages flow properly with brake cleaner under pressure.

Diagnosing faults can be a long and frustrating process, especially if you’re not used to it. Take a step back, breathe, and approach it with a clear head—problems get solved faster that way.

Apologies if I’ve repeated anything you’ve already tried. The thread’s gotten quite long, so I may not remember everything.

 

[muddy42]

Just a few additional points:
- I agree that the amazon carb could be a problem. Id keep going with the original carb, but rather than just cleaning it, use a new carb kit as well , the flexible rubber parts will have aged and become brittle. Sorry I don't know your experience level but make 100% sure the diaphragms etc are reassembled in the right order.
- All old rubber should be a focus for leaks. Clean Fuel tank breather or run the saw with the fuel cap slithtly lose. examine fuel line closely, bend it around for cracks or use your new pressure tester on it. Replace / remove fuel filter temporarily.
- you don't describe your procedure for pressure testing, but I prefer to block the exhaust port, block behind the carb and send pressure through the impulse line. try performing the pressure test under water, the bubbles are better than a guage at revealing a leak. Don't do a vacuum test under water!

 

[irhunter]

I suggest watching this vid...anything the Walbro Guys suggest which you have not yet done?

Roy

 

[SteveSr]

...Maybe I knocked some crud loose under that welch plug that came back down and is now completely blocking something.

Unlikely. The issue with removing welch plugs is to get them to reliably seal again. Even the factory can't do it reliably and the have the proper tooling! They resort to using a proprietary sealant.

I only remove a welch plug as a last resort and if I replace it I I clean the area under (idle passages and metering chamber good with automotive brake cleaner and let ti completely dry. I thn apply a bead of water thin superglue around the edge of the plug. Now set the carb metering chamber up in a humid place for several days until you can verify that the superglue has completely cured.

BTW, what is the model of the OEM carb that is on the saw?

 

[cbrTodd]

I got a few minutes to work on the saw tonight, so here are the updates.

I checked out the fuel tank vent - it was free. I took out the fuel filter and the fuel tank vent 'grub screw' and tried running without either of them - no change. So I put both back in. Fuel delivery upstream of the carb seems fine.

I took the original (Walbro) carb apart and soaked the hard parts in my ~20 year old bucket of chem-dip carb cleaner for about 45 minutes, followed by rodding out all the passages again and then spraying about half a can of brake cleaner through everything. I'm confident it doesn't have anything plugged. I put the diaphragm and flapper check valve gasket from the Amazon carb in it and then started the saw with that. It likes the needle valve positions at drastically different positions than the Amazon carb, but it gets back to a point where it operates just the same. It starts fine, idles ok, and bogs out the instant it gets load put on it. Here's a video of it at that point.



I'll look at the linked video when I get a minute, but I've cleaned more than a few of these over the years, so I don't think I've missed anything. Usually when I get one that won't behave after a good cleaning, I assume it's one of the rubber components, at which point it's usually cheaper to buy a whole carb than it is to buy a carb rebuild kit. I've done that several times and this is the first time that it's not fixed things, but to be fair on this saw it's performing no different than what came off it (which had been OK for ~5 years). Others may have a <50% success rate, but mine's been at or near 100%.

I'll do another pressure / vacuum check later to see if anything is different. I'm familiar with the units conversion and have been using the values in the shop manual now that I have it. I'd done my tests so far with the piston at TDC as the manual says but I can try rotating the crank while doing it to see if it aggravates something. I like the idea of disconnecting the impulse hose and attaching the tester there, so I'll try that next time I work on it. I've made block off gaskets out of EPDM sheet that I put under the muffler base and under the carb, if it matters how I've done that. Being able to attach to the impulse tube will eliminate me having to use the compression tester adapter to the spark plug, so it's less places for the test equipment to leak.

I did try running the saw without the outer piece of the muffler right before I had to stop. It did rev a little more freely, but it still bogged instantly and had no power under load. I tried richening it up by 1/8 turn on the high screw from there, and it wouldn't even rev up smoothly. Leaner than that was even worse.

I know the comment was said that I need to step back and look at it with a clear head. I just keep coming back to, if it has fuel, air, compression, and spark, it will run. Fuel at the carb seems to be responding to the adjustment screws like it should be. I can make it either too lean or too rich, on both the low and high circuits. On the air side, it isn't restricted any way that I can see, and the pressure / vacuum tests so far have showed it to be fine. Spark, it didn't change with the different coil or the new NGK plug I put in today, and the flywheel has not sheared the key. Compression is starting to seem more suspicious to me. The numbers are lower when hot than they are when cold, which is backwards from anything else I've worked on. Now it's ~65 psi on the first pull, ~95 on the next, and then it takes several more to get up to ~135 when hot. The manual has good instructions for taking off the cylinder and piston - pending the leak checks coming out OK next time I work on it, I'm probably going to pull the cylinder off just so I can see exactly what I'm dealing with.

FYI I've got other stuff piling up because I've spent so much time working on this saw. So it may be a while before I post again. Please don't take that as a sign of ingratitude, I'm very thankful for the attempts to help me.