Stihl HS80 hedge trimmer won't throttle up

[mowmow]

Hi Guys,

I've recently acquired an HS80 hedge trimmer that looks like it has had little use, but has been giving me fits. Here's a description of the problem:

The unit starts fine, and once it warms up a bit, it will transition ok - not stellar, but maybe a little hesitation - but it won't stall.

As the unit runs longer, it starts to misbehave. Hesitation (bogging) increases, and if I don't baby it, it will stall. I am familiar with engine tuning, and it definitely acts lean - also, running partially choked will consistently make it run GREAT. Sometimes, tipping the unit on its side when idling will immediately stall the engine.The unit seems to need to cool back down before I can get it to run semi-normally again. The idle is always fine, and if I can get it to full throttle, it sounds great there as well. The carb is a Zama and it has no high speed mixture adjustment.

The behavior pattern I've described here is the closest I've been able to determine, but sometimes it just seems to defy the pattern - just acting flaky and unpredictable. Just when I think maybe I fixed it, it goes bad again. Here is what I've checked/done so far:

* Compression "feels" good, and it never feels seized
* I've loosened the gas cap in case a vacuum was building up - no help
* The primer works fine
* Rebuilt the carb with Zama RB-66 kit. I believe that every port passed cleaner normally (except possibly the main jet - see paragraph below). No change.
* tried a wide range of idle mixture settings. Best operation (idle) is close to 1-2 turns out per the manuals. This had little impact on transition or high throttle performance.
* New intake gaskets
* Air filter elements are great
* New Fuel pick-up/filter
* New spark plug
* Cleaned and inspected fuel lines - they are in great condition.
* Cleaned spark arrestor - also removed it altogether - no help
* Tried several different two-stroke mixes - it currently has 3/4 tank of 50:1 Stihl oil to good-quality (no ethanol) 89 octane per the manual - no difference.
* Cutter bar is clean and moves freely. Slight idle-up will put them in motion.
* Spark is strong - even when the engine is hot.
* Everything is clean as a whistle on this unit.
* During testing, the unit is consistently held stable on a level padded surface on a concrete floor.
* Failure only takes about 5 minutes when started cold.

When rebuilding the carb I noticed that the main jet is not consistent when I try to shoot cleaner through it like most jets that i've cleaned. It's not a straight through hole (there are crimp marks on the inside of it like it is a multi-piece assembly), and it seems to scatter the carb cleaning fluid differently each time I spray. Does this jet have a check-valve or something in it, and does this behavior make sense? The jet is pressed in and a replacement doesn't come with the kit.

My fingers are tired - I think I have covered everything here. Any ideas would be greatly appreciated!

 

[Saw Dr.]

Have you pressure/vac tested it? Sounds like a set of crank seals may be in your future.

 

[mowmow]

Thanks for the reply Saw Dr.
No, I haven't pressure tested it, and I see in the service manual that this process is described, but I don't have the Stihl tools for this. I wonder If I can rig up a fixture to do that from misc stuff in my barn. A crankcase leak of some sort does make sense if the intermittent part can be explained somehow. I wonder If I can wet the seals with a spray lube etc as a quick test to see if the problem temporarily goes away.

 

[Saw Dr.]

Thanks for the reply Saw Dr.
No, I haven't pressure tested it, and I see in the service manual that this process is described, but I don't have the Stihl tools for this. I wonder If I can rig up a fixture to do that from misc stuff in my barn. A crankcase leak of some sort does make sense if the intermittent part can be explained somehow. I wonder If I can wet the seals with a spray lube etc as a quick test to see if the problem temporarily goes away.

You can spray some WD-40 in there and see what she does. It can leak under pressure, vacuum, or both. Quite often with chainsaws, if it quits immediately when you tip it on the side, the cranks seals need attention.

 

[madhatte]

Only thing I can add is this.

I have been using an HS80 for the last couple days -- a loaner from work -- and it had been exhibiting the same symptoms. Attempting to adjust the carb, I noted that the (lo-only) adjustment was set to no-place in particular. I spun it shut, then back open 3/4 turn, and it idled better. Without a hi adjust, that's about all I can do. I have been running it at 1/2 choke and it seems to run OK. If it were mine, I'd slap a fully-adjustable carb on it.

 

[BloodOnTheIce]

Since Stihl bought Zama carbs have gotten much cheaper, the whole carb is most likely around 30-40$ retail.

 

[mowmow]

Ok - I tried the spray trick. With the shroud off and the pull-starter unbolted, I was able to wet (spray with WD-40) the top seal, quickly put the recoil in place, start it, remove the recoil (while running) and then gun it. I also sprayed the crank case gaskets while the engine was running. None of this changed anything - even for a moment, so it doesn't appear to be the top seal anyway, unless WD-40 is just too thin for this type of test.

However, spraying the throttle shaft of the carburetor had an immediate effect which lasted for a minute or so. I wonder if there is too much air getting past the shaft, or is the WD-40 getting burned as it seeps along the shaft and enriching the mixture that way.. It does not appear that the shaft is grossly loose. Maybe I'm getting the oil somewhere else near the shaft. I'll look a bit further tomorrow.

Also, the main jet behavior that I described still puzzles me. Spraying carb cleaner through it results in an inconsistent output spray in the inside (draw side) of the jet. Does anyone have details on how this main jet is constructed? I have the Zama C1Q-S42B carb. I wish I had a Walbro WT-412A (also used on this model) to try - it also has the high speed mixture control that the Zama carb lacks. Didn't know that Stihl bought Zama.

Interesting info on the tipping/stalling being common with chainsaw seals. Maybe the WD-40 thing is leading me astray. I can hear the calls to take this into the service center. What I am glad NOT to hear is that it a scored cylinder is the likely culprit.

 

[nmurph]

are you sure that you reassembled the diaphragms and gaskets in the correct order? i picked up a ht 75 polesaw that exhibited some of the same symptoms. when i pulled the carb apart the PO had put the metering side gasket and diaphragm in the wrong order. i had to use a new kit bc he also put liquid gasket bw the body and covers, ruining the parts. but a new kit and it runs great.

 

[RAMROD48]

BEFORE you go any further....

Remove the ENTIRE muffler and inspect the exaust port for build up of carbon....

I have found a bunch that the screen was fine....but the port was almost completely clogged...

 

[mowmow]

RAMROD48: I forgot to list the muffler checkout earlier. When I cleaned the spark arrestor, I also removed the muffler. There was a little buildup at the exhaust port, but not much. I blew the muffler out with the air compressor and nothing really came out. It is actually very clean - you can read some stamped numbers in the metal inside. I also blew through it (bleh!) to get a sense for any back pressure and it is wide open. I also ran the engine without the muffler altogether, and it ran slightly worse (without retuning) if anything. Thanks.

nmurph: Just to be sure (and with my just-received parts diagram in hand) I checked the carb assembly and all is correct. Thanks for the thought.

Saw Dr: Well I rigged up a pressure/vacuum tester (my honeydo list was short this weekend!) using an old automotive vacuum/fuel pump pressure gauge, my air compressor (with the regulator set LOW), and a vacuum pump that was given to me long ago (and apparently works!), a small valve and some misc plumbing. I also fabricated an intake fitting and exhaust plug. The result: The unit holds the +/- 0.5 bar (about 15 inches mercury vac, 7 psi pressure on the gauge) spec infinitely (at least 20 minutes with NO needle movement) where only 20 seconds is required in the service manual. Moving the piston (from TDC) wiggled the pressure gauge for a sanity check. I was not able to get it to leak down by rotating the crank or tipping the unit. I've attached images of the fixturing in case anyone is curious. So it sounds like the seals are good. I think the test was valid - the fixture has very small volume compared to the crankcase so any leaks should show. Also, I was surprised how slow the pressure drop was when I didn't plug the exhaust port (piston blocking it) - it almost made spec that way.

BTW: Regarding the tipping/stalling thing. I think I've found a pattern there. When tipped clockwise (carb is on top - downdraft), the unit bogs more severely. When tipped CCW (carb is underneath - updraft), it accelerates much better. In normal position, it is somewhere in between. I don't know what to say about this, but the carb just keeps coming back in my mind.

At least the thing is behaving consistently. I lost count of how many times this thing has been apart and back together now with no change in behavior.

madhatte: I'm with you on the new carb if I can find a Walbro. No real hits for a WT-412A on the web. I'll call the local Stihl guys and some carb vendors tomorrow.

Any other ideas? The upshot of all this I guess is that I will be VERY familiar with it when it finally works!

 

[SkippyKtm]

I've got the same trimmer with the same issue, I've been too lazy(busy?) to fiddle with it and have been running it with the choke partway on.
The amount of hedges I have doesn't justify my needing it to run perfect, although I'll admit that it is quite annoying. I'm interested to see what the cure is though.

 

[nmurph]

mow, i re-read your initial post and something stuck out; you say this problem starts after 5 mins from a cold start. so, for the first few minutes it is ok from cold??? if so, i think you need to look at your coil as a culprit as they are prone to fail on this line of motors.

 

[mowmow]

Yes, it seems to go through a short period of time where it runs "OK" - not great, but ok - as far as the low-high transition is concerned. And yes, it also seems that the hotter, the worse it gets - after initial warm-up. Lately I am just attributing this to the fact that engines generally don't run well cold, and they also tend to lean out as they heat up. Can the ignition module be tested for this failure beyond basic spark? I have found that it still has a strong spark when it's misbehaving. Also, I would think that poor ignition would not tolerate running under partial choke, where this unit actually runs great when the choke is partially applied.

 

[nmurph]

it is easy to check the coil; take $30 to you local stihl dealer and swap it for a new coil. install and gap it with a business card!!!!! if it runs like it should it was the coil......


i don't know.....so, you are saying it never runs correctly???? i have two stihls that run this model of engine and they both run fine from cold. crank and give them about 10 seconds to clear up and off i go......no, i don't go WAO until they are warm, but they don't stutter after a few seconds......

 

[mowmow]

Yes, I'm saying it never really runs well. Definitely not as well as my to chainsaws or even as well as my old Weedeater brand hedger or weed trimmer. The Stihls never hesitate like this thing does - cold or warm.

 

[Fish]

Take the carb apart again. Remove the spring/lever/needle and hold it up
to a light and look through to the screen on the other side.
Blow it out, then look again, should be totally clear.
Then during reassembly, make sure the lever height is correct.

 

[belgian]

Also, I would think that poor ignition would not tolerate running under partial choke, where this unit actually runs great when the choke is partially applied.

While we can exclude an air leak, the carb is definately your problem. I think Fish is on the money with the needle lever not being rightly positioned or a clogged fuel channel inside the carb. Choking increases the pressure differential between venturi and metering chamber, so the needle lever position maybe a tad too low.

 

[mowmow]

OK. I think I will call this success.

The passages were clean but the needle lever was not at its optimum bend. Thanks belgian and Fish for the suggestion to revisit that. The reason for the original adjustment is that per the attachment, the lever shape resembles Figure b MUCH more than Figure a. The "extra" bend in Figure a is so small on the lever (both the original and the replacement part) that it looks incidental - more of an imperfection. Thus, it was (per the tech manual) adjusted per Figure b. This resulted in a valve setting that would close much sooner than if adjusted per Figure a. So this time around, I adjusted it to a zero gap per Figure a in hopes to put it on the other end of the scale relative to where it was. This leaves the valve open longer and enrichens the mixture. Though still not as good as I would hope, it no longer stalls.

It is still sensitive to rotation per my earlier description, and I have to wonder if all HS80's are this way at least the ones with my carb model. I've noticed that in spark-plug-up position, the unit runs extremely well, and maybe other equipment that uses this engine/carb use the engine in a different orientation vs. the HS80.

I suppose it's also possibile is that this carb model (Zama C1Q-S42B) is more sensitive to position than others. This (according to the parts list) is the simplest (least parts) of the three carbs that were used in the HS80. The other two (Zama C1Q-S29C and Walbro WT-412A (C)) have control valves in the carb body, and the Walbro has a high-speed mixture control. I believe that my carb has a the control valve integral to the main jet, which may have caused my concern when cleaning the carb. So, I am curious as to which carb the others have, who are having the same problem I was - at least two others posted as having the same problem.

Anyway, I may play around and try to adjust the lever even farther to see if I get additional improvement, but will probably also check into getting a Walbro. But for now, it is running, and it seems that it may be operating more typically now for this unit. It did stall on me once - when it was running out of gas. I'm thinking it is overly sensitive to running out of gas...

That was a joke.

Thanks everyone for your time an patience!

 

[mowmow]

A quick follow-up:

With the improvement that the adjustments of the previous post made, I decided to take the adjustment a bit further. With reference to Figure a in the lever.jpg image above. I bent the lever to more closely match that shape and then adjusted it so that when released, the tab now rises ABOVE the reference surface by 20 to 30 mils. So again per Figure a, I adjusted it to about NEGATIVE 25 MILS, where 0 to 12mils is called for. This was the maximum that I felt I could get away with safely, as a significant further increase could start to force the diaphragm beyond its travel limits, which would result in the valve remaining permanently open. It now runs BEAUTIFULLY!

I don't know what to say regarding the necessity to do this. I just have to wonder how much people have put up with on their HS80's (there were several with the exact same problem). Again, this may may only exist with 1 of the 3 carb types used on this equipment. The only other possibility I can imagine is that the main jet (which I have been suspecting to some degree all along) may be compromised in some way. The problem I have with that is it appears to have an integrated check valve, and that would likely cause much bigger problems if the check valve were unable to operate due to something lodged etc. Also, I was able to get a pulsating behavior when spraying cleaner through it, and I must have put an entire can of cleaner through it, so the check valve was definitely not stuck.

In any case, I have put 2 solid hours on it since this last adjustment, operating in ALL orientations (yes, that was a bit of a workout) and the thing performed great. I am now the samurai of shrubbery!

I also bet that the engine will last longer now due to the fact that it is probably running a little richer (and cooler) than the EPA-driven factory setting would allow.

Should something change (I am really pretty confident that it won't though), I will be sure to give you guys an update.

Again, thanks everyone for your great input and guidance.

 

[belgian]

Glad you got your problem solved, congrats ! The metering lever height indicated in the manual is theoretical and based upon everything to full specs.

Once someone has fiddled with the system, there are some variables (lever bending, diaphragm stiffnes, etc. which may call for further adjustments of the lever. I have this problem quite often on oldie saws.

Once you get to understand the symptoms of an engine, it's easier to diagnose the remedy.

Fish expects a beer for the hint though