Husky 435 erratic idle

[Dengel]

Hey folks,
A buddy dropped his 435 off on my bench for a carb rebuild, and after the rebuild it idles erratically. Sounds like an engine missing a cylinder. I've vac checked fuel lines, pulled the carb (Zama) to make sure the impulse was clear, ordered and installed a whole new carb(OEM), and put in a new plug. Fuel filter is fine. And when I try to accelerate, it spits and sputters and never gets above where the high idle setting is. Any ideas?

 

[Dengel]

I plan on making a blanking plate with a hose barb on it for the exhaust port this weekend so I can press/vac test it.

 

[rupedoggy]

They won't accelerate with the low speed screw to lean.

 

[Ozhoo]

435's are one of the most possessed saws on the planet.

Base carb setting are 2 1/2 on the L and 2 on the high. The AV is so soft that flexing between the tank and engine will cause the engine to rev/slow/wobble/other.

 

[backhoelover]

did you pull muffler to check piston?

 

[Dengel]

They won't accelerate with the low speed screw to lean.

I have the low screw set at the recommended 2.5 turns out. And fine adjustments to that make no difference in performance.

 

[Dengel]

435's are one of the most possessed saws on the planet.

Base carb setting are 2 1/2 on the L and 2 on the high. The AV is so soft that flexing between the tank and engine will cause the engine to rev/slow/wobble/other.

Well, that's encouraging... I have the new carb set to recommended settings, and fine tuning produces no change. I did try to hold the body so it did't "vibe" so much, and still zero change.

 

[Dengel]

did you pull muffler to check piston?

Yep, pulled the muffler. Can still see cross-hatching in the bore. The ring moves when I poke at it. Weird thing is, I ran it with no muffler with a new plug in it, and it cleared right out. I haven't been able to replicate that condition since.

 

[scottr]

Yep, pulled the muffler. Can still see cross-hatching in the bore. The ring moves when I poke at it. Weird thing is, I ran it with no muffler with a new plug in it, and it cleared right out. I haven't been able to replicate that condition since.

By saying that it cleared right out are you saying that it reved to full rpm ?

 

[dougand3]

Is muffler screen clear?

 

[Dengel]

Is muffler screen clear?

Yep.

 

[Dengel]

By saying that it cleared right out are you saying that it reved to full rpm ?

Precisely. It started rough, spit and sputtered for a few seconds like a four stroke with a dead cylinder, then came out of it and ran fine. I put the muffler back on and it ran like crap again.

 

[scottr]

Have you tried to fine tune it without the spark arrestor screen ?

 

[smokey7]

I would also replace the fuel filter as part of a carb rebuild. I factor it into the repair cost or itemize it depending on If the customer acts like i replacing stuff that is fine. I say you brought it to me to "fix" right? Well this is how you get a lo g ladting repair i tell them.

 

[backhoelover]

does the muffler have a cat?

 

[Sam R]

I'd also add it's a good idea to replace intake (maybe exhaust too) gaskets when doing carb work. Don't know specifically if this saw has those, some just have that rubber grommet fitting - but if it does, and they get a little wet especially with fuel, they crap out.
Also from your original post; why did you vac test the fuel lines? Was it just to check the tank vent? Because otherwise a pressure test will show you leaks a lot better. And when you said you checked the impulse line did you actually verify it's getting that impulse?
If it's running better without the muffler it sounds like you might have a blockage in there; might not be just the arrester screen - although I always just torch those for a moment when inspecting a tool. It's surprising how cruddy they can be and still be see-thru. Also, torch the muffler with a butane torch, there may be a lot of carbonization or some little critter has set up shop in there.

 

[Dengel]

Hey folks, sorry for the delay. I was sawing out on an island these past few days Sooo, when it ran that one time with the muffler off, I think it was some kind of fluke. i haven't been able to replicate it since. The muffler itself is damn clean. The saw does not have a lot of hours on it. Just sat around mostly. @Sam R , I vacuum tested the fuel lines primarily because my pressure tester was at work. And since the fuel lines are under vacuum when the saw is running, I figure that's a pretty good way to test them. This saw does not have an impulse line. It has a port built into intake bracket. The intake bracket is one of these cast aluminum affairs that bolts to the cylinder with molded gaskets on both mating faces. The carb then bolts to the bracket. It's so weird and frustrating. It acts like the timing is advanced, or it has just barely too much fuel going to it. Where it's not quite flooding, but close. The only way I can describe it is it can't get out of it's way. I also pulled the flywheel to check the key, and that's in fine shape.

 

[Huskitoter]

Ignore - misread

 

[Sam R]

I'd definitely check your fuel lines with pressure rather than vacuum. Vacuum operates the fuel pickup, true, but the tank itself and the fuel lines are pressurized when it's running - and cracks could be hidden by just vacuuming them. Also forgive my ignorance of Husqvarna saws, I'm a Stihl mechanic first, very limited experience on Husqvarnas.
I suspect something in your fuel delivery system. Have you verified your tank vent is opening under vacuum? While you do that also make sure the tank will also take & hold pressure. It may not have a hose for impulse but it has to have impulse from the crankcase to the fuel pump in the carburetor somewhere, that's just how they work. The easiest thing to do is put a small amount of grease over that hole and rotate the flywheel back & forth - the grease should blow out or suck in, either means impulse is good.

 

[Dengel]

Ok, I'll rig up my pressure tester and give it a shot. Grease in the impulse port is a great idea. Thanks!