MS390 / 039 pop up piston

[beaglebriar]

I just measred, it' about .045

Thank you sir.

 

[Northwet]

It started out as a 290 so it is alot stronger, I never tried it with the flat top 390 piston. I'm bummed I missed the cross cylinder on sale after thanksgiving.

 

[Terry Syd]

It has a nylon mesh filter

That may be an easier solution to the restrictive felt filter than building a bigger filter. Have you tried a high RPM test to see if there is a difference in max RPM?

The original 029 was 54cc, the 390 is 64cc - and they both run the same size filter. Add a muffler mod and a timing advance and the engine is trying to pull a lot more air in through that little filter.

 

[beaglebriar]

What carb do you like on these Terry?

 

[beaglebriar]

Ordered up a Hyway kit as kind of a compromise. Plus it'll be good info to add to this thread.

I'll be sure to give updates when the parts arrive...

 

[94BULLITT]

That may be an easier solution to the restrictive felt filter than building a bigger filter. Have you tried a high RPM test to see if there is a difference in max RPM?

The original 029 was 54cc, the 390 is 64cc - and they both run the same size filter. Add a muffler mod and a timing advance and the engine is trying to pull a lot more air in through that little filter.

I've never put a tach on the nylon mesh filter saw. I'll get a pic of it tomorrow. It is very open.

Have you tested a timing advance on these saws? I tried my 039 and it did not seem to pick up. I'm sure there will some difference between coils.

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[Adirondackstihl]

What carb do you like on these Terry?

HD11 or 16
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I know, my name is not Terry

 

[Terry Syd]

I'm sure there will some difference between coils.

It is more that coils. The Stihl manual indicated that the 23.5 to 28.5 degree difference was because of 'manufacturing tolerances' - that is shocking, a 5 degree range of advance from the factory. So much for superior German engineering. I expect that much of the difference is in the placement of the magnets in the flywheel, but I don't know.

I do know that I got one of the saws with timing closer to 23.5 degrees. There was a big difference when I got the timing in the ballpark. I fine tune it with a coil that has the mounting holes notched for about 2 degrees of movement.

 

[beaglebriar]

HD11 or 16
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I know, my name is not Terry

Fairy is close enough I guess...[emoji12]

 

[Terry Syd]

What carb do you like on these Terry?

I made up my own carb using a Zama twin jet body and the shafts out of a 044 carb. I run 30/35 jetting.

The HD-5 that came on it isn't a bad carb if you are willing to do some mods - like block off the circuit from the main discharge nozzle to the low speed circuit, drill a hole from the wet side to just in front of the low speed needle seat to feed the low speed circuit and then open up the auxiliary jet to .35mm (I think that is what I used).

 

[beaglebriar]

I made up my own carb using a Zama twin jet body and the shafts out of a 044 carb. I run 30/35 jetting.

The HD-5 that came on it isn't a bad carb if you are willing to do some mods - like block off the circuit from the main discharge nozzle to the low speed circuit, drill a hole from the wet side to just in front of the low speed needle seat to feed the low speed circuit and then open up the auxiliary jet to .35mm (I think that is what I used).

I have zero Stihl parts laying around. If I get brave maybe I'll try those mods. Otherwise I'll try one of the carbs that Jeremy suggested. I know Dallas had good luck with a 044 carb on his 290.

 

[Adirondackstihl]

Fairy is close enough I guess...[emoji12]

WOW

 

[94BULLITT]

@Terry Syd

I took a pic of my nylon mesh filter but this picture is better.

https://www.hlsproparts.com/Stihl-029-MS290-MS310-039-MS390-air-filter-p/h50029.htm

 

[Terry Syd]

Since it fits on an 029 it likely doesn't have the 'intelli carb' nozzle. Therefore it probably has a bit more area than a later filter. Is that that case? That it doesn't have the 'intelli carb' nozzle to compensate for a clogged filter?

If so, it might be a cheaper and easier fix for the filter than making up your own.

 

[Adirondackstihl]

Just put a piece of foam in the snorkel or swap out carb cover

 

[Northwet]

@Czed
What aftermarket 044 carb did you use on the 029 you fixed a few weeks back?

 

[94BULLITT]

I did some testing yesterday and little this morning. Mine 390 is around 35% faster than stock without porting. I wish I knew how much the pop up was helping.

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[blsnelling]

IMHO, you can't tell much about the benefits of a higher compression piston if using them in an AM cylinder with inconsistent and greater than OEM squish. You're essentially defeating the purpose. All official testing should be performed on OEM cylinders so as to have some resemblance of a standard.

 

[Fruecrue]

IMHO, you can't tell much about the benefits of a higher compression piston if using them in an AM cylinder with inconsistent and greater than OEM squish. You're essentially defeating the purpose. All official testing should be performed on OEM cylinders so as to have some resemblance of a standard.

Modding clamshell farm & ranch saws isn’t exactly a mainstream, regulated sport.
All official testing.... I must have missed that thread.

I don’t want to replace a piston in a clamshell anymore than I have to. The original saw in this thread, that was my second time in, and one time too many IMO.
I wouldn’t open one up and only change one variable again, I would port it, advance it, mod the muffler and put in the pop up all in one shot.

 

[94BULLITT]

IMHO, you can't tell much about the benefits of a higher compression piston if using them in an AM cylinder with inconsistent and greater than OEM squish. You're essentially defeating the purpose. All official testing should be performed on OEM cylinders so as to have some resemblance of a standard.

I'd just like to know for myself how much better it performs with the pop up piston. I don't think there are that many 039 and 390s around with OEM cylinders on them. Most people stepped up to a prograde saw at that price point. You could have got a prograde 60cc for a few bucks more or for a little less you could get a prograde 50cc. I think there are probably far more 029s and 290s with aftermarket 390 cylinders on them. I bet the popup piston would be with in a couple percent on AM or OEM cylinder. I bet most of these Cross pistons will be going in Cross or Hyway cylinders. I have an OEM 039 I may order a piston for and see what it does. I know with this Cross cylinder saw bone stock the throttle response is better than any 039/390 I have ran.