Saw bogs need help tuning.

[starsailor]

Hi,

I've got an old Homelite XL-850 that I've just recently identified on this thread http://www.arboristsite.com/showthread.php?p=580012

Here's my problem. The saw starts and seems to run fine, but when I bury the 18" bar in a 16" log of white ash the saw will bog down about 1/2 way thru. It will run back up to WOT if I back off and pump the throttle a few times. I can't seem to get it run smoothly by adjusting the carb hi screw. I needs some help tuning here.

Here's what I've done so far. As part of identifying and cleaning up the newly aquired saw, I've torn it down as far as removing the clutch and flywheel and thoroughly cleaned at least a pound of gunk out of every nook and cranny inside the housings. That was and adventure in itself as I had to make both flywheel and clutch removal tools out a some scrap steel bar stock, but that's expected when working on old engines. The tear down was motivated by figureing out what exactly what model saw I have as the data plate and model decals/stencils are long gone. As part of that I found a disintegrated fuel line in the tank, replaced with yellow tygon, a plugged fuel filter, cleanded and blown out with compressed air, and a blocked up HS-56A carb, which I rebuilt with full rebuild kit, including both filter screens and removal of the welch plugs. I've checked all the seals and bolts behind the flywheel and the cylinder nuts. Everything is tight there so I don't think there are any air leaks. The compression is 135 PSI cold and 120-125 PSI hot.

As part of the carb rebuild started the lo screw at 1 turn and hi screw at 1 1/4 turn according to the Tillotson HS series service manual I found online. I've tried making it richer all the way up to 2 1/2 turns on the high screw. More on the hi screw improves it some, i.e. it will cut a few seconds longer before bogging down, but not enough. I'm not sure opening up the high screw further will do much good nor is right way to go here.

Can anyone with more experience with one of these saw please offer some advise or suggestions on what to check or where to go from here?

 

[dtnodya]

I'm no expert by any stretch but one thing that might help is opening up the exhaust ('muffler mod') ... Did you make sure that everything looks good on that end? Like everything is relatively clean and free-flowing?

Air filter ... ??

Just throwing stuff out there.

 

[1953greg]

its bogging down because its running out of gas. could be cracked or crimped fuel line outside the tank, or impulse line. or maybe tank vent plugged. those are the obvious.

 

[starsailor]

No mods for me...

Hi dtnodya,

No mods for me. I've pulled the muffler and cleaned it up really well. No obstructions. Just the baffle plate and spark arrestor screen. So that can't be the problem.

Thanks.

 

[starsailor]

I'll try that...

Hi 1953greg,

You may be right. Fuel starvation could easily cause this. It's clearly getting enough to idle fine and even run at WOT with no load though. Fuel line is brand new yellow tygon from the carb to the outside tank fitting and from the inside tank fitting to the fuel filter. I've checked it out good and there are no kinks or cracks and doesn't leak end to end with a vac. pump. I don't think either of the new fuel lines are collapsing either. So, for now, I don't think fuel line is the problem. There is no impulse line per se on this saw as it's built into the solid fitting between the carb and reed valve body.

Along the same lines as your thoughts though, I didn't replace the fuel filter. I just cleaned with some carb cleaner and compressed air. The tank and carb were heavily varnished and needed some work to clean up. Maybe I didn't clean the fuel filter out enough. Maybe I'll try a new one as they're only a few buck and I can get one locally this afternoon. Any suggestions on what filter to get? I'm sure the original Homelite filter would be hard to find. Would a Sthil or Husky filter be OK?

 

[lesorubcheek]

Any suggestions on what filter to get? I'm sure the original Homelite filter would be hard to find. Would a Sthil or Husky filter be OK?

Actually, its very easy to find replacement aftermarket elements that fit the original Homelite metal bobs. Most online stores are a dollar or so, or you can get complete filters with the bob for 2 or 3 dollars. Walbro has their own flter that will work fine. Please don't contaminate your saw with a part for a Stihl or a Husky. It just wouldn't be right, especially after you've now finally determined what model your saw really is.

And yes, it does sound like you're starved for fuel when the saw's under load. When you cleaned the filter with carb cleaner and air, did you seperate the element from the metal bob or leave it together? If it was left together, there may have been some gunk inside that never was removed. In addition to restricting flow at the filter, there may also be small particles that have travelled through your lines and partially blocked the screen on the carb, so it may mean taking a look inside the carb once again also.

Dan

 

[starsailor]

Somewhat better now.

Hi lesorubcheek,

I promise not to wierd out my saw with Stihl or Husky parts!

You were right though. Before you posted I went and took the top (fuel pump side) off the carb and as you suspected the primary screen was full of crap, actually little black flakes. Oh, and there was a bit of fuel pooled in the carb box that was evaporated down to a heavy oil mix. I presume the flakes could only have come from the inside of the old fuel filter because everything else between there and the carb was brand new. Also, I suspect some of the flakes may have gotten stuck in the diaphram flap of the pump on the tank side of the screen and causing the pump to be forceably pump some fuel out of the carb jets into the carb box.

Just this afternoon, I got a Walbro filter locally, looks like a metal top with a fiber filter element held on the bottom with a clip. After cleaning the primary screen and popping in the new filter I set the carb back to a base setting of lo 1 turn open and hi 1 1/4 turns open as in the Tillotson service manual. I'm pretty sure no crap got past the primary screen because it was well seated in the recess. So I don't think anything else got newly plugged up. I did a real good job before cleaning it out and inspecting all the passages during the rebuild too, so I'm relatively sure it's still nice and clean inside the carb.

I then reassembled and warmed it up at idle. BTW, it fired up at idle on the first pull. After warm up it seemed no better at first. At more than 1/4 throttle under no load it would bog. In fact from idle any throttle punch would bog. So I started to open the hi screw up. Past 2 turns I could get it run at WOT under no load but it would bog in any wood over 6" and I couldn't get it 4 cycle under no load at all. I couldn't get it to not bog in wood at WOT with the hi screw in any position up to all the way out with the hi screw almost falling out

I fiddled with partial throttle for a while and found the best I could get. The best is at 1/2 to 2/3 throttle, running at a medium RPM (I'd estimate about 4K RPM), it won't bog and will cut hard white ash up to the 18" bar quite well, albeit a bit slow. It seems like it could run there all day at 1/2 throttle.

Any other suggestions?

BTW, this carb is a Tillotson HS-56A. Mr. Acres site shows the XL-850 came with a HS-26A or HS-73A. Are they basically the same or do I have a different sized carb?

 

[thomas72]

I do not know if I missed anything said earler, but you said you removed the muffler. Did you look at the exaust port while the muffler was off, as it could be clogged with carbon. If it is use a piece of wood or anything that is soft, and clean the port. Also later on if needed call Hasting Ring Manufacturing and order a set of rings. The cost should not be no more than 5 dollars a ring.

 

[starsailor]

Exhaust side is clean.

Hi thomas72,

I know the exhaust side is squeeky clean. It was pretty good when I took the muffler off, but I even cleaned it up a bit more. As for rings, if it has compresion of 135 PSI cold and 120-125 PSI hot do you think it needs rings or more? Am I missing something?

 

[thomas72]

If you can make it run good with 125 psi then I would not do anything, but the signs of low compression are it will not idel, oil build up on plug, and overall loss of power. If it runs okay I would not put rings in it, but if you are going to put alot of hours on it you may look to replacing them later. I have a poulan super 68 with 120 psi compression and it does not give me any trouble. Also, I think that model has reed valves. After you run it a bit take the air cover off and pull it over with the igniton switch off, and see if any gas is blowing back out of the carb. If gas is coming out of the carb then your reed valves are not sealing.

 

[lesorubcheek]

Also, I think that model has reed valves. After you run it a bit take the air cover off and pull it over with the igniton switch off, and see if any gas is blowing back out of the carb. If gas is coming out of the carb then your reed valves are not sealing.

Your beat me to this point. I had a 925 with bad reeds and the chamber would fill with gas as it idled. Definitely check those reeds if you see gas blowing back from the carb as settling in the chamber.

Dan

 

[starsailor]

Finally checked reeds and more too...

I finally had a chance to spend a few hours on the saw. After another partial tear down I checked the reeds as thomas72 and lesorubcheek both suggested. The reeds and reed carrier look OK, a little carbon on the face of the reeds, but they were sealing just fine and the carbon cleaned up with a touch of carb cleaner on a cloth.

So seeing as the reed valves aren't the problem and given that the saw was half apart anyway I decided to do a vacuum and pressure test. It took a little ingenuity and a trip to the local hardware store, but I fabricated a vacuum and pressure testing kit out of an old bicycle inner tube I got from the bike shop across the street from the hardware store, some aluminum bar stock, and various small brass fittings. I sealed off the exhaust and put the fittings on the plate fitted to the intake, where the reed valve goes. I installed the vacuum fittings first.

Sadly to say the vacuum test gauge immediately showed a moderate air leak under vacuum, from -5 in HG to 0 in about 20 seconds. Swapped out to the pressure fitting and that test wasn't so bad, but it was still leaking from 5 PSI to 0 in about a minute. Not enough to hear where it is, nor could I find it with a little bit of spray cleaner (aka., soapy water). So I pumped it up to 5PSI again and stuck in a 5 gal. pail of water, that is after removing all the electrical ignition components. Found a leak in side plate gasket that's behind the flywheel. Now that air I'm presuming will definitely explain part of if not all of the problems getting the saw to run without bogging down at WOT. So, I'm not really losing my mind over simple carb adjustments.

Well at that point I figured it's time to at least open the crankcase and replace that gasket and have a look around inside the crankcase. It looked pretty messy inside with lots of carbon gunk especially on the exhaust side. So I said what the heck, might as well pull the cylinder and really see what shape this old saw is in. Unfortunately I found more unhappiness inside too. From the looks of the cylinder gasket that may have been leaking too. The exhaust side of that gasket looked very oily and swelled up (from oil) compared to the opposite side. The cylinder studs weren't loose, but the exhaust side stud nut were definitely not a tight as the opposite side. Time for a check of the studs and a new cylinder gasket.

Now for inside the cylinder. Well the piston definitely has some scoring as do the rings. Most of the scoring seems aligned with the cylinder wall intake vanes (right name?) down low in the cylinder. From the scoring patterns on the piston skirts and up to above the rings it looks like there may even be some piston slap. The cylinder wall itself isn't as scored, probably due to the hardness of the chrome, but I need some other opinions on that. Most of the cylinder wall is nice and shinny with even a few spots of the original honing marks. There is some more wear corresponding to the scoring on the piston, but nothing that I can feel with a finger nail or running a sharp edged tool across it. However, there is a definite high spot, at least 3-4 1/1000ths in. high where the gap in the piston rings is and it seems to be somewhere just shy of 1/32 in. wide up and down the whole cylinder. I don't know how much of a problem that is but it is certainly a surprise especially being that wide and high.

I understand that ring gap in 2 strokes are almost always fixed in one place for each ring with a pin in the piston groove to keep the ends of the rings from hanging up on the ports. But, for the life of me, I can't understand why any engineer in his right mind design the piston so the gaps in both rings would line up in the same spot causing this wear pattern and not offset them so there would be less blow by and more even cylinder wall wear.

Anyway, bottom line here. I'm into this tear down for a side plate gasket, a cylinder gasket, and probably piston rings too. Now I need some other opinions. From the attached pics in this post and the next can any of you homelite experts tell me if, one the piston is toast from the scoring (worst scores seem to near 5/1000ths in. deep), two is the cylinder also toast or is the wear and ring ridge there OK enough to just run new set of rings.

Thanks for reading this far (sorry for the long post), and any opinions too. BTW, I have more pics too if the attached ones aren't clear enough.

 

[starsailor]

Rest of the pics.

The rest of the pics from the previous post...

Also, all these pics are 640x480. I have the original from the camers in 2560x1920 (5 mega pixel) buy they are 2.68 MB in size apiece. Let me know if more detailed pics are need and I can email them if needed.

 

[habanero]

I'm by no means an expert, but I've seen worse scoring in saws that were running just fine. I think if it were me I'd throw a set of rings in it, replace the gaskets, and cut wood. I just got done rebuilding a Super EZ a while back that the cylinder didn't look much better than that and Dad ran a light hone down it a few times. Thing runs like a champ now.

One thing I would suggest while you are in it this far is to replace the crank bearings and seals. A busted cage on the ball bearing is why I was rebuilding my Super EZ. It's cheap insurance while you have the thing torn apart anyway.

 

[starsailor]

Sounds like some sage advice...

Hey Habanero,

Thanks for the advise. I'm still a little unsettled about the cylinder. However, the added insurance of doing the crank bearings and seals too while I'm staring right at them will add piece of mind later.

On a similar note I've always been a fan of replacing every low cost part that took a lot of high cost labor to get another part nearby. For example I always replace the throwout bearing and rear main seal on a car when replacing a clutch. I'd hate to spend $500 in labor to get to $5 seal 6 mos. later if you know what I mean.

Anyway I digress, but thanks for good advise.