036 possible vacuum leak? Advice needed...

[72thing]

Hi all. I have an 036 that drops down to a lower than normal idle speed after it comes out of the cut. It'll drop down to ~1900-2000 and I think it's going to die, but hasn't so far. I have idle set ~2700-2800. It takes a few pulls to get her started when cold, but it'll start first pull every time after its warmed up. It'll idle all day long and has great off-idle throttle response all the way to the top end, which is set at about 12,700. This has it four stroking under no load but clearing up great in the cut. The only problem is the dropping to a low idle when exiting the cut. I guess it doesn't affect much of anything, but its very annoying to me.

So, does this sound like a possible vacuum leak? Its suspect to me, so I blocked off intake and exhaust ports with bicycle innertube behind carb and exhaust heat shield. I regulated shop air down to about 4 psi hooked to the impulse line. Then I turned off the supply and it held pressure for 45 minutes with no leaks (didn't have a gauge to watch, but the innertube stayed bulged out the whole time). I soaped every joint except flywheel oil seal (awaiting arrival of puller from treatland scooter place) and not a single leak I could see. So now that I knew my setup was sealed up tight, I hooked my Mityvac up and tried to vacuum test it. Results didn't seem good to me, but I'm not sure. I pumped it to 15 in Hg and it took 25 seconds to drop down to 10", then 40 seconds to drop from 10" down to 5". I did try to dribble some light machine oil around the clutch side oil seal and I didn't see it get sucked in thru the seal or see it slow the rate of leak. Oh yeah, I turned the crank several times throughout both tests and I didn't see any change.

Saw laying on its side with bulging innertube visible behind exhaust heat shield

My pressure testing setup with regulator and various adapters

Thanks ya'll.
Will

 

[roostersgt]

Sounds like it needs the LA screw turned clockwise a bit. Air leaks usually create wild high idles. How does the saw respond when you turn it on its sides, rocking it back and forth, while running?

 

[72thing]

Saw seems to run just fine when its tilted or on its side. I idled it up a little higher to ~3000 RPM and it does the same thing, acts like its gonna die coming out of the cut, but everything else running great. For L screw adjustment, I found the peak RPM (~3300) by turning screw in, then backed off about 1/4 turn, then adjusted idle to ~2700. I tilted front to back, side to side, and it runs great.

Any other suggestions? Should I even worry with it? I've also been meaning to put in a carb kit, but it runs so good I'm tempted to leave it be for now.

Thanks,
Will

 

[roostersgt]

Carb kit can never hurt. Might be the problem. Don't believe your issue is an air leak. Try the carb kit. Tillotson carb? Or newer Zama replacement?

 

[pgg]

looks to me it's just tuning, if it's leaned out on the lo it'll hang and slowly drop to normal, when it's too rich on the lo it'll stall when you let off the trigger from high revs, but blip the throttle lightly as it drops down in revs and it'll likely not stall, and hi and lo are intertwined and affect each other. Nothing to do with air-leaks or seals, better to tweak the tune by ear and feel rather than with a "needle in a gauge"

 

[72thing]

I do tune the saw by ear and then use a tach for reference. This way I have some numbers to refer back to and a way to "convey" my tune to ya'll. But hey, I'm still learning. Will it hurt anything if the L screw is too lean? At the least, I now know how to pres/vac test a saw. (first time doing this)

But, since I have it down, does the vacuum loss sound acceptable? It just seems like it should hold vacuum better than that, but I don't have the experience to back that judgement up. I've searched and am still unsure. So, what is acceptable vacuum loss?
This is all the manual says:

Operate lever (2) until pressure
gauge (3) indicates a vacuum of
0.5 bar.

Note:
If the vacuum reading remains
constant, or rises to no more than
0.3 bar within 20 seconds, it can be
assumed that the oil seals are in
good condition. However, if the
pressure continues to rise (reduced
vacuum in the crankcase), the oil
seals must be replaced, even if no
leaks were detected in the pressure
test.

Am I wrong, or does this seem a little loose? It takes mine about 25 seconds to do this, so it's OK by the book. But man, that needle is going down fast!

Will

Edit: its an older 036 with the Zama C3A carb, (non compensated)

 

[albert]

I think your main trouble is that your are way rich on the high mixture, spec is 13,500 rpm. Being too rich on the H needle will cause your trouble coming out of the cut, and if alittle too rich on the L needle will compond it. Stihl revised the max rpm on the 036 a long time age from 13,000 to 13,500 rpms. I have a 036 like yours Zama non compensated carb, and the extra 500rpms makes noticable difference in performace.

 

[72thing]

Thanks, Albert. I'm just paranoid about running it too lean. I'll try that and see what difference it makes.

Will

 

[mtngun]

Operate lever (2) until pressure
gauge (3) indicates a vacuum of
0.5 bar.

Note:
If the vacuum reading remains
constant, or rises to no more than
0.3 bar within 20 seconds, it can be
assumed that the oil seals are in
good condition. However, if the
pressure continues to rise (reduced
vacuum in the crankcase), the oil
seals must be replaced, even if no
leaks were detected in the pressure
test.

Am I wrong, or does this seem a little loose? It takes mine about 25 seconds to do this, so it's OK by the book. But man, that needle is going down fast!

It's funny how Stihl publishes different leak testing specs for different saws. Shouldn't it be the same for all saws ?

Ya, I think their spec is way loose.

That said, your issue does sound more like a carb problem.

Re: your positive pressure setup. You really need a low-pressure regulator, as a standard shop regulator may not be accurate at that psi. I gave up on shop air and bought a mightyvac radiator testing pump for about $50. It has a gage so you can watch the pressure drop. Besides, it comes in handy for working on radiators. :msp_rolleyes:

 

[72thing]

I went out while ago and looked over everything again after paying local Stihl dealer $4 to pull the flywheel. I soaped all around both oil seals, intake/exh ports, base gasket, impulse, and all my rigged up pipe fittings. Didn't find any leaks. Then it suddenly occurred to me that I didn't soap the spark plug last night. I sprayed it down and it was leaking like crazy!! I took it out and cleaned the cylinder around the hole and cleaned the washer real good and it quit leaking. Isn't it amazing when you overlook something so simple? I also realized that when I had shut off the shop air to test it last night, I didn't even think about my ball valve being at the tank. So, even if there was a leak, it would've taken forever for the 130 psi or so in the 40 feet of 1/2" air line to leak out at only 4 psi. :bang: After this amazing and excellent discovery, I hooked the mityvac back to the impulse line and pulled it down to 18" Hg. 40 minutes later, it's only dropped 1" Hg. So, I'm satisfied with my testing and learned a great deal. Now I know the ins/outs of press/vac testing and feel confident in it. Now to get this sucker put back together and tweak on the carb a little more. Thanks for everybody's help, I do appreciate it:msp_thumbsup:

Will

 

[72thing]

I played with the carb a little this evening while cutting some cookies. All I have is a ~16" pine log, but I can almost fully bury the bar so it loads it pretty well. I leaned it out just a bit so now it turns ~13000-13100 RPM no load. It definitely screams more now and I can tell it cuts faster. With the muffler mod and earplugs its harder to tell right where it needs to be on the topend. I erred on the side of caution and had it set rich. I guess I'm just paranoid about running it too lean. I also leaned out the L screw some and it seemed to help reduce the "almost dying" situation I spoke of earlier. It still transitions from low to high exceptionally well and still starts with one pull after I shut it off while warm. So, that seems pretty good to me.

My question is, as long as I keep it under 13,500 (max spec) it should be safe, right? I realize there's a little more to it than that, but I just want to treat this saw good. Its in great shape and I only have a couple hundred bucks in it so I don't wanna have to buy a new topend any time soon.

mtngun, I'll have to look into the radiator pressure tester setup. Like you said, it might even work on radiators, too.

Will

 

[albert]

If you muffler modded it, it should run higher than 13,500, as long as it's 4 stroking after throughly warmed up. I wouldn't take it past 14K, see where it cut the strongest without loosing torque. Then go a touch richer for a safety margin.

 

[72thing]

I'll slowly get there, albert. Like I said, when its screaming on top end, I have a hard time telling when its four stroking. It sounds so different from stock its hard for me to tell. Right now, my H screw is only 7/8 of a turn out and that has it turning ~13000 or so. But I thought with a muffler mod it should be richened up a little over stock. So, does the position of the screw matter at all? Am I being overly worried?

Will

 

[albert]

Leave it at 13,500 and make some larger cuts, to ge it fully warmed up. While your in a cut ease off the presure, or slightly lift up on the saw,while still wide open throttle, and listen to the sound, it should 4 stroke ( sound rough, burrble) then it should smooth out as soon as you reapply presure. Use it like this awhile and the you can fine tune it further. 7/8th out is ok on the H needle, you probaly will only need maybe 1/32 of a turn more, it does usually take much when you get close. Good performace and torque when you lean on it a bit is what your looking for, not how high the no load rpm is. With that said, stock 036's run best aound 13,500 but with a opened muffler, it will like more. Be warned, the closer you set it to the edge, the greater the risk of being too lean due to changes in temperature, fuel, ect. Leave your self 2-400 rpms on the rich side of peak, in the cut you won't see any difference. And also check you tune when cutting by listening for the burbble when slightly lifting the saw in a cut.

 

[72thing]

Thanks man, I appreciate the detailed advice. :msp_thumbup:

Will

Edit: Rep sent your way albert.