Stihl MS192 TC Bogs/Stalls when Cold

[BSS]

Hey all,

I was "fortunate" to find a Stihl 192TC at a garage sale for $20 recently. Catch was the carb was bad, but the fellow selling it threw in an additional carb (C1Q-S135) and a few other misc parts from another 192TC he had gutted for free. I figured "why not?" For $20 seemed like it might be a fun project/learning experience if nothing else. Fast-forward, I cleaned the "good" carb, replaced its gaskets/diaphragms, put in a new spark plug and reassembled. The good news? The saw runs, sort of. It starts, will idle, but is pretty grumpy about revving up, at least when cold. If I fully depress the throttle, the saw will bog down, but I can get it to rev up if I feather the throttle a few times. Once it revs, it's slow to hit full RPMs and doesn't have a lot of power (will easily bog down again when cutting). Once you let off the throttle, it will also die about 50% of the time. Another 'odd' behavior, the saw will also bog if I point the nose down, but it revs up no problem if I tilt the bar upwards to the sky.

Interestingly, if I keep the throttle high for a good minute or two and let the saw warm up more, these issues more or less go away. I was able to do a few test cuts without much power loss and the tilting power loss issues also seemed to mysteriously vanish. I Initially set the carb to the factory defaults (3/4" H, 1 full for L) but I've noodled with them a little to no avail. Is this carb also toast or is there something else I'm missing here? Any feedback/suggestions are greatly appreciated.

Side-note, I found an OEM Zama C1Q-S257 carburetor on Ebay for $30, but I'm unable to find any authentic Zama C1Q-S258's (which is what Stihl's technical documents say to use). Do any carb-heads out there know if an S257 will work in lieu of an S258? They both seem about the same as the S257 is used on the 192T. I'm just not super familiar with the carb specifications, so again, any help there would be much appreciated!

 

[Woodslasher]

Sounds like an air leak. I fixed a buddy's saw that would bog, run like ****, and just give me nothing but grief. Long story short, I re-sealed the clamshell joint and voila! It runs like a top and holds a tune perfectly!

 

[BSS]

Hey, thanks for the reply! The clamshell, that's the joint for the crankcase/piston? (sorry, still a bit of a novice here) I assume that would be something a compression test would verify, right? I don't currently have a compression testing kit, but maybe I should grab one. . .What did you use to reseal the joint?

 

[Woodslasher]

Hey, thanks for the reply! The clamshell, that's the joint for the crankcase/piston? (sorry, still a bit of a novice here) I assume that would be something a compression test would verify, right? I don't currently have a compression testing kit, but maybe I should grab one. . .What did you use to reseal the joint?

The 192 is a clamshell, and yes, that is the joint between the cylinder and crankcase halves. You'd need to do a pressure/vacuum test to figure out where it's leaking from, either you can do one or you can pay a shop to do that. I used red hi-temp Permatex RTV to reseal the joint, I've used it in several saws without any issues.

 

[BSS]

Awesome, just went down the rabbit hole watching someone rebuild a 192TC and reseal with the same stuff! It sounds like a pressure/vacuum test is probably the next step then. . .so, just to pick your brain a bit more, if you don't mind, is the thinking that the symptom (running poorly cold, at odd angles) due to the seals, etc being "looser" when cold, but then after warming up they expand a bit and "reform" their bonds, giving better compression/power when warm?

 

[Woodslasher]

Awesome, just went down the rabbit hole watching someone rebuild a 192TC and reseal with the same stuff! It sounds like a pressure/vacuum test is probably the next step then. . .so, just to pick your brain a bit more, if you don't mind, is the thinking that the symptom (running poorly cold, at odd angles) due to the seals, etc being "looser" when cold, but then after warming up they expand a bit and "reform" their bonds, giving better compression/power when warm?

The running poorly at odd angles is because at any given point your saw will have "excess" oil floating around in the crankcase, so when you tilt it to one side or another the excess oil will temporarily slow or stop the air leak, tilt it the other way and the leak becomes greater again.

As for the temperature stuff, I'm not 100% sure on the principles behind that, but I'll explain what I believe and if somebody else on here proves me wrong then disregard all this. Running poorly cold can be because the metal has pulled away slightly, it's also because saws are more sensitive to stuff until they warm up. Cold air is denser than warm air, so you get more air when the saw is cold but when it warms up it fattens up the tune a bit.

 

[BSS]

Gotcha, that makes sense! I found another post of a saw with similar symptoms and it wound up being the fuel lines/manifold that were bad. I have a full set of fuel lines from the other saw that was gutted, so I might try to swap those out "just in case" before I take the plunge on a vac test since I don't currently have a testing kit.

 

[mountainlake]

Is the muffler clogged up? Steve

 

[BSS]

Is the muffler clogged up? Steve

Not sure, I haven't checked it yet, but it does not have a spark arrester on it. I did notice some white smoke coming off the muffler the first few times I fired it up (it seems to have stopped doing that though) so I figured it was just a build up of crap on the outer shell. I'll take a look in there though, can't hurt!

 

[mountainlake]

The spark arrestor is what clogs up on those saws so that shouldn't be a problem. Steve

 

[stihltech]

Basics as posted above. Vacuum and pressure test. If there is either leaking, you can never get the motor adjusted.

 

[BSS]

A bit of an update for anyone who's interested:
I had a couple of hours today, so I tore into the saw a little more. Started by taking the pull cover off to inspect the impulse line and quickly realized I didn't really have the correct tools to remove the fly-wheel (did I mention I'm a bit of a novice?) I did, however, notice that the kill switch's lead had slipped off the ignition coil (did I mention that the saw suddenly would not turn off last time I ran it?) While I was reattaching it, I noticed that the ignition coil itself had some odd behavior: The black plastic cover that is wrapped around the metal layers slid back and forth about 1/8" of an inch without much force. It did not seem to be affecting the metal sheet position as torque screws held those in place, but I decided to swap it out anyway because the man who sold me the saw had apparently put a spare OEM ignition coil in the parts bin as well (the ignition coil with the squirrely cover was apparently an aftermarket part). Once I got the ignition coil off, I was able to see the impulse line a bit better, and while I didn't fully remove it, I did clean it a bit and inspect it for leaks. Everything seemed sound, so I moved on.
I found another technical manual online that had a bit more in-depth information on the carburetor, so I decided to take it apart again (must be the 3rd time now). This time I pried off the H jet's cap and removed/sprayed carb cleaner in there. I also removed and cleaned the accelerator pump/cavity, which was unhappy about coming out and turned out to be rather sticky and gunked up (smoking gun?) Finally, I replaced the one fuel line I had a spare for and the fuel vent which I had a spare of as well. Put everything back together and crossed my fingers.
This time, the saw fired right up and revved like a champ, clean and smooth with no bogging! When I released the throttle, the saw immediately died, haha. After a few more retries and noodling with the idle jet, I was able to keep it from dying and attempt a retune on the carb. I'm not an expert, but I think I got it close enough. After that, the saw seemed to work great; responsive throttle and smooth idle. Only thing I noticed is it still doesn't seem to have a ton of power cutting through anything larger than an 8" diameter, but I'm used to my 372 XP, so I'm not sure if I'm just pushing it too hard or if there is still a problem. I did notice the idle crept up a bit on me after running the saw for a few minutes, so I'm thinking I will try to figure out a vacuum/pressure test next, but since I don't have a kit, I'm not sure how much it's worth it to spend on this saw at this point. I suppose I could just get new oil seals and reseat the clamshell without a pressure test as a catch-all? I'll see how much the dealer chargers for a pressure test first. . .
Thanks to everyone who responded with suggestions, I'll update again if I make any more progress on this!

 

[BSS]

Oh yeah, one other thing I noticed after running the saw a bit while testing: the white smoke coming off the muffler seems to maybe be coming from burning chain oil? There's a liquid that I think is coming from underneath the bar sprocket and landing on the muffler (could this mean the oil pump is leaking?) I didn't get a chance to look at it today, but if anyone has seen this before and has any insight, I'd appreciate it!

 

[Captain Bruce]

Hey, thanks for the reply! The clamshell, that's the joint for the crankcase/piston? (sorry, still a bit of a novice here) I assume that would be something a compression test would verify, right? I don't currently have a compression testing kit, but maybe I should grab one. . .What did you use to reseal the joint?

Don't tear the engine out before checking the intake boot....which is the #1 spare part for these top handle Stihls. I have rebuilt 34 to date..........not 1 came in with a good boot.
They split just under the clamp. The engine will tease you with wanting to run, and cut, but its sucking air and leaning the fuel mix provided by the carb. While you are at it, install an OEM NIB carb.....sounds like you have a handful of chinesium parts. When running as designed these little TH's make even a 1/2 day project, quick easy work out of the gate.

 

[BSS]

Don't tear the engine out before checking the intake boot....which is the #1 spare part for these top handle Stihls. I have rebuilt 34 to date..........not 1 came in with a good boot.
They split just under the clamp. The engine will tease you with wanting to run, and cut, but its sucking air and leaning the fuel mix provided by the carb. While you are at it, install an OEM NIB carb.....sounds like you have a handful of chinesium parts. When running as designed these little TH's make even a 1/2 day project, quick easy work out of the gate.

Thanks for the tip! I did spray some carb cleaner in the boot when I was first testing it because my main saw (husky) had a similar issue when it was brand new! However, I don't think I got the carb cleaner on those areas you mentioned, so I'll give it another shot and see if I can find a leak. I haven't seen any other aftermarket parts on this thing, other than the ignition coil, but I obviously don't know its entire history. The carb on it is an original OEM to the model (Zama C1Q-S135) but I'm not very experienced with rebuilding carbs, so it's possible either I didn't clean it very well or it is on it's way out as well. The accelerator pump cylinder head had a bit of a groove worn into it from the throttle bar, so I'm wondering if I need to just replace that as well. I have a spare from the saw's old carb, but I'm not sure if it's in any better shape.

Thanks again!

 

[BSS]

EDIT: This update did not fix the problem.

Ok, have a bit of an update. . .pending more testing next chance I get, I think I might have found the problem? I'll post my findings here in case anyone else has a similar problem:
Based on the recommendation of a Stihl dealer I spoke with, I decided to try the poor man's vac/pressure test: took off the clutch cover and ran the saw, sprayed carb cleaner around the clutch drive, sprayed (tried to anyway) around the flywheel, the fuel lines and all over the boot. . .no engine stalls whatsoever. I wasn't sure if I just didn't get the carb cleaner in there well enough, but I decided to quit for the afternoon and follow the other lead of the accelerator pump again later. I'm not very familiar with these carbs, or any carbs for that matter, so when I cleaned out the accelerator piston shaft/gasket earlier, I didn't really think anything was off, but had noticed wear, so figured it would warrant a closer look.

Flash forward to tonight, fortunately I had another C1Q-S135 carb that was supposedly "bad" and decided to take a look at that one's accelerator pump area. Right away I realized something was off with the "good" carb. The "bad" carb had a brass cap covering the hole to the throttle shaft. . .the one I was using did not. Of course, I took the throttle shaft out before prying the cap off, which gave me a hell of a time figuring out how to get it off without breaking it, but I eventually managed. When the piston came out, it was also different, stainless (I think) instead of brass. Spring was longer too. Both carbs were s135s, so guessing the previous owner had already replaced it once, perhaps. Well, I put the "new" piston/spring back in the "good" carb and re-secured the cap/throttle shaft/clamp. Put everything back together and fired it up. Saw started much faster cold than I've ever seen it start (about 2-3 pulls for a rev on choke, then one more pull with choke off). Saw revved right up, no bogs, no stalls, felt much more powerful. I can't for sure say it's "fixed" since I haven't done any cut tests, etc, but I'm feeling pretty positive about it this time! I'd love to hear from any experienced repairers if this fix makes sense, or if I'm still barking up the wrong tree. If it does, why would the accelerator pump act so similarly to an air leak? Maybe the lack of the brass cap was causing the carb to allow air into the engine? Would love to hear thoughts on this if anyone is so inclined. . .

I'll update again once I get a bit more test data!

 

[BSS]

Ran the saw this afternoon. Started right up again and seemed to work pretty good from the start, but idle was way too low (2400-2600 RPM) so I tried tuning it again. After some noodling to try to get the idle in a good spot, I couldn't get it to rev at all, haha (I'm not very good at tuning carbs, apparently). Decided I probably screwed it up and reset to factory defaults, which got the saw working pretty well again. After that, it seemed to hold pretty stable H/L RPMs, though the response on the throttle up felt just a tiny bit sluggish to me. Additionally, the cut still felt a little underpowered; saw would slow down a bit faster than I'd expect sometimes when cutting 12-14" test rounds, though I never was able to stall it out, which was good. I can't say for sure if it's underpowered, just that it felt a little weak to me. Maybe I'll try to post a video of a cut to see if anyone thinks it seems off. After about 10 minutes of test cuts, the idle was still holding mostly stable and the highs didn't seem to be creeping up, so that was good also. I did notice after throttling down, the idle would sometimes stay a little high (3,400-ish RPM) before coming back down to ~3000 again after 15 seconds or so. Does anyone think I might still have an air leak here or could this just be the symptoms of a possibly tired carb that probably didn't get cleaned very well and/or has some damage around the accelerator pump shaft? I'm considering buying another OEM carb just to rule it out, since it's a pretty quick swap, but figured I'd see what you all thought first before I throw more money at this thing.

 

[BSS]

Another update:
Saw is NOT fixed, haha. This saw keeps faking me out. One day it seems to run 'fine', another day it's like I'm right back to square one. I realize I could probably save myself a lot of headache if I just bite the bullet and get a pressure/vac tester, but I've chosen to torture myself instead, apparently. Up until now, I've been assuming that the bogging when tipped forward was a symptom of the main problems (low power, slow revs, etc) but I decided to take another look at the fuel lines last night because a lot posts on the internet seemed to point in that direction. The problem is, the issue is sporatic, so I can think I've solved it, only to run into it again next time I test it, which makes it very difficult to determine if I've fixed it or not. That being said, last night I had the bog when tipped forward issue right off the bat, so I turned it off, changed the position of the fuel filter/line and when I fired it up, the bogging was gone! This made me think the fuel filter might have been sucking air depending on the angle, so I swapped it with another spare I had (which also had a larger nipple despite being the same part number - what?) At any rate, I could not get the issue to re-appear, but I'm still unsure if it's gone or just "gone - until the next time I start it cold again - gone".
After that, the revving was still a bit sluggish like before, and while I didn't try any cuts, I assume underpowered as well. This leads me back to the carb's accelerator pump/stuck valve perhaps? I did replace the piston, but I've read that if there is wear on the old piston (there was) that replacing it won't necessarily fix anything. Another observation: When I had the carb exposed while testing, I noticed that the primer bulb was very slow to refill and could hear a distinct "gurgling" sound coming from the back of the carb while the bulb inflated. Is this normal, or an indication of a stuck valve/bad accelerator pump chamber? At any rate, I finally decided to swap the carb for a new one just so I can officially rule it out one way or another. I will update again once that shows up and I've tested it a bit.

 

[Woodslasher]

Another update:
Saw is NOT fixed, haha. This saw keeps faking me out. One day it seems to run 'fine', another day it's like I'm right back to square one. I realize I could probably save myself a lot of headache if I just bite the bullet and get a pressure/vac tester, but I've chosen to torture myself instead, apparently. Up until now, I've been assuming that the bogging when tipped forward was a symptom of the main problems (low power, slow revs, etc) but I decided to take another look at the fuel lines last night because a lot posts on the internet seemed to point in that direction. The problem is, the issue is sporatic, so I can think I've solved it, only to run into it again next time I test it, which makes it very difficult to determine if I've fixed it or not. That being said, last night I had the bog when tipped forward issue right off the bat, so I turned it off, changed the position of the fuel filter/line and when I fired it up, the bogging was gone! This made me think the fuel filter might have been sucking air depending on the angle, so I swapped it with another spare I had (which also had a larger nipple despite being the same part number - what?) At any rate, I could not get the issue to re-appear, but I'm still unsure if it's gone or just "gone - until the next time I start it cold again - gone".
After that, the revving was still a bit sluggish like before, and while I didn't try any cuts, I assume underpowered as well. This leads me back to the carb's accelerator pump/stuck valve perhaps? I did replace the piston, but I've read that if there is wear on the old piston (there was) that replacing it won't necessarily fix anything. Another observation: When I had the carb exposed while testing, I noticed that the primer bulb was very slow to refill and could hear a distinct "gurgling" sound coming from the back of the carb while the bulb inflated. Is this normal, or an indication of a stuck valve/bad accelerator pump chamber? At any rate, I finally decide to swap the carb for a new one just so I can officially rule it out one way or another. I will update again once that shows up and I've tested it a bit.

A slow bulb means a plug/blockage somewhere, bad fuel filter maybe? Plugged screen in the carb?

 

[BSS]

A slow bulb means a plug/blockage somewhere, bad fuel filter maybe? Plugged screen in the carb?

This is what I'm wondering, the screen was clean though, last time I checked it. I did replace the fuel filter last night and when I tested it today it did seem to inflate a bit faster. I saw another post that suggested a slow bulb could also be caused by a clogged carb check valve. I wonder if this carb is just too gunked up for my skill-set/tool-set to restore it properly. I know an air leak is still a strong possibility based on the rest of the replies, but there are a lot of unknowns for me about this saw (its repair history/etc). Just going off of what the previous owner told me - "carb is bad" - is an enticing lead though. I know he had it diagnosed by someone at Ace hardware (there was a sticker on the saw) but who knows if the diagnosis was accurate and/or if there were other problems as well since he never had it repaired. He also gave me the 2 carbs, so I could have put the "bad" carb on it by mistake, haha! But the other carb was missing its welch plug, so I figured that was the "bad" one. . .I guess at this point, a new carb replacement is a lead I'd like to rule out and it's a pretty easy swap compared to some of the other repairs suggested.
Also, started the saw cold today and it still bogs when tilted downwards, but only for the first 2 mins of running it or so - back to square 1 again!