MS390 / 039 pop up piston

[Terry Syd]

This explanation is incorrect. Where it says "It opens into the venturi about half way between the transition holes and the main nozzle". It actually enters the straight section of the carb body BETWEEN the two transition holes. You can confirm this by shining a light through the hole or spraying some WD 40 through the jet.

Note that the reason they give for the jet is that "to allow leaner running and maintain crisp throttle response" - bugger the 'allow leaner running' bit.

 

[stretch5881]

Nuthin' better to do on a crisp day like this, so I did the muffler mod today.
I believe tuning the carburetor is overly sensitive and the engine is acting like an air compressor unless the exhaust is relieved. I did the normal muffler mods, but that deflector????? It should deflect, not choke the engine.

 

[Terry Syd]

When you say the carb is 'overly sensitive', is that a reference to the idle?

 

[beaglebriar]

View attachment 103557 Nuthin' better to do on a crisp day like this, so I did the muffler mod today.
I believe tuning the carburetor is overly sensitive and the engine is acting like an air compressor unless the exhaust is relieved. I did the normal muffler mods, but that deflector????? It should deflect, not choke the engine.

Cut about a 1/2" notch in the deflector. Let's it flow a lot more.

 

[stretch5881]

When you say the carb is 'overly sensitive', is that a reference to the idle?

Not the idle, the mid to high rpm.

 

[Terry Syd]

I cut straight in just above where the bolt clamps down, then cut down to where I made the first cut. I opened up the entire side of the deflector.

 

[Terry Syd]

Not the idle, the mid to high rpm.

If it had been the idle, then I might have had a suggestion.

 

[stretch5881]

Cut about a 1/2" notch in the deflector. Let's it flow a lot more.

I don't believe that 1/2" is enough. I cut the deflector off and welded a 1/2" id pipe on it. I split the pipe and tapered it for a larger outlet. The finished opening is 1/2" wide and 3/4" tall. That shouldn't choke off the rest of the mods on the muffler.

 

[beaglebriar]

I don't believe that 1/2" is enough. I cut the deflector off and welded a 1/2" id pipe on it. I split the pipe and tapered it for a larger outlet. The finished opening is 1/2" wide and 3/4" tall. That shouldn't choke off the rest of the mods on the muffler.

I open up most of the rectangular indent underneath and notch the deflector. Flows enough to keep the saw happy and it's not overly loud.

But I'm doing them for guys that have never even heard of a muffler mod so I try to keep the decibels reasonable while making good power.

 

[beaglebriar]

I wish I could use a drill press on something like this. The drills are just so damn small that you need to use the pin vise to hold them. - The drills are 'fragile', perhaps in a factory with a large drill press where the feed can be rigidly controlled you could have them machined out.

I sometimes drill very fine holes in stainless tube using a sensitive drill chuck in a Bridgeport mill. It's a royal PITA...lol

 

[Terry Syd]

The above is a simplified drawing to show the concepts. The EPA circuit I mentioned is the circuit that runs from the high speed needle to the low speed needle. You can see the main discharge nozzle is halfway between the two needles.

Here's what I think happens in that circuit. When the throttle is opened and the venturi becomes operational, it puts a low pressure signal down the nozzle to pull fuel. Note that the low pressure signal goes to both the high speed needle and the low speed needle. In other words, when you are cutting the low speed circuit doesn't flow as much as it could. This makes the operator keep the revs up to make decent power.

They are trying to drop emissions. When the engine drops in RPM there is more time for the mixture to escape out the exhaust port. By making the operator keep the revs up, they can drop the emissions.

The first problem with this circuit is if you have a faulty valve on the discharge nozzle air will be pulled back through the low speed circuit at idle. The idle mixture can be all over the place.

By blocking off this circuit and drilling an access hole into the wet side for the low speed needle, you eliminate any influence of the nozzle or the high speed circuit. I noticed that my idle mixture became a little richer, the idle was smoother and for some reason I can't figure the high speed RPM was more stable. A more stable RPM indicates that the mixture is more consistent.

 

[Terry Syd]

Well, it is back together. I'm fairly confident my compression gauge is broken, it has the same 120 lbs registering.

I'll have to go find some wood...

 

[X 66 stang347 X]

Not sure if this is the Hd carb

 

[Terry Syd]

This is not the HD, it is however the Zama twin jet's (and Mikuni) way of setting up the low speed circuit.

If you look at the last picture you posted above it has the EPA circuit I referred to in the HD and you will see that the low speed needle limits/controls the amount of fuel in the low speed circuit.

The above configuration has a fixed jet to control the fuel to the low speed circuit. The low speed needle is used just to adjust the idle mixture and not the whole low speed circuit.

 

[Northwet]

The pop up piston is on sale
www.hlsproparts.com/Stihl-039-MS390-Pop-Up-Piston-Kit-p/cp31039.htm

 

[blsnelling]

The pop up piston is on sale
www.hlsproparts.com/Stihl-039-MS390-Pop-Up-Piston-Kit-p/cp31039.htm

Practically giving them away.

 

[stretch5881]

I got 1 on the way. Too cold to work on it now.

 

[fearofpavement]

I also just ordered one of the pop up pistons. I've got a cleaned up 49mm oem cylinder to put it in. I'm curious to see what (if any) improvement it is over the Farmertec kits I've been using for some years.

 

[Terry Syd]

I started having problems with the 029, it turned out to be the coil. I think the 4 degree retard on starting was also affected. I was going to put the old coil back on it to try it when I realized I had to virtually disassemble the saw to put the coil on. Bugger that, I ordered a new coil (A$17) and will wait until it arrives before I tear into the saw. Did I mention I hate working on these saws?

I figure Stihl made these saws the way they did as a marketing ploy. They are such a pain in the butt to work on that most people just buy a new saw.

The coil with the 4 degree of retard on starting is off the 066 and a few other saws. The number is #1122 400 1314.

 

[Lightning Performance]

I started having problems with the 029, it turned out to be the coil. I think the 4 degree retard on starting was also affected. I was going to put the old coil back on it to try it when I realized I had to virtually disassemble the saw to put the coil on. Bugger that, I ordered a new coil (A$17) and will wait until it arrives before I tear into the saw. Did I mention I hate working on these saws?

I figure Stihl made these saws the way they did as a marketing ploy. They are such a pain in the butt to work on that most people just buy a new saw.

The coil with the 4 degree of retard on starting is off the 066 and a few other saws. The number is #1122 400 1314.

You been drinkin?