stihl 034 not idling, dies when running

[Dominique]

Hey arborists, this is my first post so thank you for letting me into the forum. I have a stihl 034 saw that isn't performing after having sat for a few months.
It ran for a few minutes and then died. We started diagnosing L&H settings but these are ok. The spark seemed weak and plug was fouled so we replaced the plug and ignition coil. We cleaned the tank vent too. Unscrewing the gas cap doesn't help either so I don't think it's vapor lock. The current symptoms are as follows:
If the saw has been sitting for a while it will start up and idle ok for a few minutes but then when warmed up, it won't idle and it will die suddenly after a bit, even with the throttle open a bit to keep it running. When warm it won't start up at all unless it's on the fast idle setting.

I currently suspect the impulse line heats up, a crack in the line expands and the pumping action fails, starving the saw of fuel. However, when I pull the carburettor, plenty of vaporized gas exits the engine block.

The saw has seen a fair amount of use and we're okay with replacing it, but I really want to know what's wrong with it. I'm not a mechanic so I'm seeing this as a learning opportunity. I'd love to hear what you guys think is wrong with it.

 

[drf255]

Check the piston out through the exhaust port. Make sure it’s not scored.

Vacuum test the saw, see what you find.

Pressure test the carb.

Not running while hot is usually ignition related, but you already swapped in a new coil and plug.

Unless you start with the basics wann a saw starts doing strange things, you’ll be chasing your tail.

 

[Dominique]

From reading the other threads here, I expected these suggestions. The exhaust torx screw is stuck, any advice on getting it out? Probably hitting it with the torch for a bit? I don't have the tools for a vacuum test / pressure test but willing to purchase, though it will take a while with the current circumstances. The compression feels okay from pulling the cord but it could be better, and from the little I could see from the intake side, I think the piston could be in a better condition. If the piston was shot, how can it run fine at high rpm and why does it die suddenly? That does not make sense to me.

 

[drf255]

Heat the screw with a torch, then let it cool.
I believe heat can make compression losses even worse, but cold starts are usually worse if comp low.

Just going from experience, doing the basics first is never a bad idea. It solidifies a few basic principles of the saw in question.

 

[Vintage Engine Repairs]

I agree with Doc, I have heard many times warm run problems tend to be caused by ignition coil. I think he’s spot on with his advice and a great way to go forward.

 

[Dominique]

Not great. Advice is appreciated.

Is it normal for the diaphragm to be curved naturally? I have a replacement kit on the way anyway since it hasn't been changed ever I think.

 

[cus_deluxe]

seen worse but thats definitely scored and needs to be torn down, but not before some diagnostics, press/vac test mainly. options range from keep it as a parts saw, to tear down and rebuild. just depends on what you wanna do.

 

[Old2stroke]

If it starts and runs but dies suddenly, it's not likely a compression issue even though the compression may be on the low side. It sounds like it is just running long enough to use up the fuel in the carb's fuel chamber so if the above suggested tests don't show anything obvious, it's probably just a carb that needs careful attention to rebuilding from a kit. That fuel pump diaphragm looks like it's been in use for a long time. What do you mean by "verifying" the setting of the L&H screws?

 

[Dominique]

What do you mean by "verifying" the setting of the L&H screws?

Verifying they are set to standard settings, one turn out on this saw as indicated by manual/cover plate. We tried adjusting it in every reasonable way but it didn't help.

The carb is as old as the saw. It has never been rebuilt, I ordered a kit so I will try that next. When it gets here I'll take pics of the carb.

What's strange to me is that it will idle okay cold for a bit, but not when warm. But it will rev up fine. Also it's not like it idles poorly and sputters out, if you let off the gas it cuts out immediately. If it was the carb, I'd expect it to not idle when cold. If it didn't idle at all I'd expect some blocked passage.

 

[Old2stroke]

When you are starting cold with the choke on, the intake vacuum will pull fuel up the line and fill the fuel chamber even if the fuel pump is not working, there will then be enough fuel in the chamber to let the saw run briefly and then stop because the pump isn't supplying more fuel. When it is warm, are you trying to restart with the choke on or off.

 

[Dominique]

On fast idle setting, it won't run on normal on setting.

 

[BuckthornBonnie]

Scored... you’ll need to go through the saw. I can’t blow up the pics right now but that piston looks shot.
1125 leak/problem points-
Impulse line
Fuel line, filter
Carb gaskets
Intake boot
Seals

Probably in that order.

 

[BuckthornBonnie]

And check that gas...

 

[TRTermite]

When you are starting cold with the choke on, the intake vacuum will pull fuel up the line and fill the fuel chamber even if the fuel pump is not working, there will then be enough fuel in the chamber to let the saw run briefly and then stop because the pump isn't supplying more fuel. When it is warm, are you trying to restart with the choke on or off.

"Even if the fuel pump is NOT working"
Two Stroke is trying to reset your way of thinking..
"When it is warm are you trying to restart with the choke ON or OFF?"
There is a lot of info for you to absorb in that post.
The sooner you get that piston replaced / cylinder cleaned up and underlying problems corrected the Happier you will be with your saw..
I had a rough time with an old fuel line cracking from flex after I checked it and must have happened when putting the carb back on. I overlooked the line because I was sure it was ok .. Always something to keep one thinking wit a saw.

 

[Okie]

Couple things:

Make sure the spark arrestor is not clogged. (saw will operate cold and get worse as it runs)
That carb diaphragm is toast. Sometimes a new stihl carb can be had for the price of a kit on some models. Sometimes you can save a few bucks and throw a clone carb on it for a test, but I only use clone carbs for testing then replace or rebuild with the real deal carb. I've seen Stihls with your described symptoms with a soft pulse hose or split pulse hose. some of the pulse hoses are in behind the carb and not readily visible for inspection. I WOULD NOT flog it and start taking the saw apart just because the piston is scratched. You say it has good felt compression when pulling the rope. You also say you replaced the ignition coil and new spark plug. If carb work/pulse hose/spark arrestor does not fix the saw you need to at least check the existing compression with a reliable know small engine tester (not automotive type) and if the compression is less than 125 the piston/cylinder is sick. The saw will normally run on 125, some at 100 but the lower the compression the harder it will be to start. Anything below a 100 on a stihl is really bad. 140 and up is ok. Might be time to get yourself another saw and try to repair this one as a backup if you really need a good reliable saw. A man always needs two chainsaws saws.
And on getting the torx screw loosened if not already. Clean the bottom of the torx screw hole good and then insert a torx driver and tap on the driver or take a smaller drift punch that fits loose inside the torx hole and tap so as to vibrate the bolt. (heat/cool is also your friend) Heavy muscle in the brain and arms is not your friend with seized exhaust bolts.

 

[Dominique]

Hey Okie, regarding the carb, I ordered both a gasket/diaphragm kit and a chinese replacement. This way I can use the replacement for testing like you do, and refurb the OEM one. I've checked the impulse line and it feels/looks like new really, but I don't have a good tool to pull it out/put it back on, I'm waiting on that in the mail as well. The saw has, and right up until it didn't, started very well. Even now it starts fine on fast idle. I suspect we may get some life out of it with a new carb for this year and then look to replace, or I might repair it/take it apart for fun.
I also own a stihl 066, so we will live, but the 034 is a lot lighter and more forgiving on the body. We have a few hours of firewood sawing left, after that I have all the time in the world to work on it.
There's no spark arrestor so that's not it.
I managed to get the torx screw out ok with some leverage on the driver.

Thanks for the informative post. I'll update this thread when I know more.

 

[Ax-man]

If that saw ran ok and now won't run because it sat for a long time . I would get into that carb before assuming the worst . That carb diaphragm is shot . If saws sit around for awhile the carb dries out leaving behind fuel residues in the carb body and the inlet screen. When the saw is refueled these residues come loose and end up in the screen . That screen may look clean but is actually clogged not letting fuel into the metering side of the carb.

 

[Okie]

For working on your own saws you need to consider eventually collecting the following tools/testers in the future.
SMALL engine compression tester and make sure it's reads small engine compression correct. The special low pressure schrader valve needs to be in the spark plug hole end of the hose. Several threads on here about such and making your own. Several expensive compression testers will lie to you when doing a comp test on a small cc engine. I built my own and I think the new one I have that I mainly bought just for the adapters is a Actron, but I had to replace the Actron valve with the special low pressure type schrader for it to read the same as my correct reading homemade tester. I always use a backup comp tester (2nd opinion) when a engine checks low compression. I've seen results of some auto mechanics overhauling a chainsaw because their comp tester indicate low and they said comp was lower after the overhaul and the saw only had minor carb issues in first place, so keep a heads up when doing comp tests on a small cc engine.

Also eventually get a Mityvac pressure/vac tester, one that does both pressure and vac and the metal type that can be re-kitted if necessary. I think the model number of the good ones is like maybe 8500 and around $65. Get a big plastic tool box for the mityvac because you will collect several adapter, etc, after awhile.
The Mityvac vac/pressure tester is really useful for doing small engine carb tests (too see if needle/seat is holding pressure) and for testing automotive vacuum.
operated parts. You can also do your own brake bleeding by yourself and not have to bother the wiffee with doing the brake pumping. Just don't get Brake fluid inside the vac tool, it will ruin the check valves.
The mityvac is built to last several years with little bit of TLC. I use a Briggs fuel filter on the output of mine when doing vac tests to keep small pieces from getting drawn into the check valves of the tool.
HF has a China one but II have no experience with it, therefore cannot recommend such. Man needs reliable tools when working on his Stihl saw.

 

[BuckthornBonnie]

Has anyone looked at his piston pictures??

 

[BuckthornBonnie]

It’s scored— time to find the reason why and fix it back up. I’d argue that 1125s are a great saw to start with. They’re good saws, easy to work on, and have aftermarket support for some non-essential items.
Good AM for this series- Meteor piston, linkages, some plastics.