High Quality Chainsaw Bars Husqvarna Toys

Numbers for porting a 390 or J Red 2188

wcorey

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Since no one has responded...
I haven't done one of these yet so you can take this for what it's worth but I have taken lots of notes for the day when I do find one.

The big determining factor with the 390 is that the exhaust opens too high, like 97.
Squish and base need to be cut to get it down to around 100.
That puts the intake a bit past 80 so there you go with that unless it gets epoxied.
Transfers are nothing like the opposite extremes of say, an 046 or 7900, so blowdown of around 20 is probably fine.
 

Mastermind

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OK, what are good numbers for a 390.

A serious question needs a proper answer.......

I was taking .040 out of the squish band to get the exhaust down as far as possible.....had a few starter pawl failures out of some loggers in MI. So.......I dialed it back to .030 out of the squish.

I set the clearance to .020, and widen the intake and exhaust a little. I set the uppers to 120, and don't widen them at all.

On the ignition timing I advance it about 1/4" at the outside edge of the flywheel by filing the key. It's cast in on the 390, but it works that same way......

Lots of muffler opening helps that saw too.
 

mdavlee

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A serious question needs a proper answer.......

I was taking .040 out of the squish band to get the exhaust down as far as possible.....had a few starter pawl failures out of some loggers in MI. So.......I dialed it back to .030 out of the squish.

I set the clearance to .020, and widen the intake and exhaust a little. I set the uppers to 120, and don't widen them at all.

On the ignition timing I advance it about 1/4" at the outside edge of the flywheel by filing the key. It's cast in on the 390, but it works that same way......

Lots of muffler opening helps that saw too.
I've done them with .030-050" from squish. They all ran close but one had higher compression. Depends on the bar length as well. If you want to run short bars you wouldn't need as much out of the chamber to get the exhaust lower. I've not seen any gains from lowering the intake to 85 on any saw. Epoxy it back up to 78 is better to me.
 
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Mastermind

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I've done them with .030-050" from squish. They all ran close but one had higher compression. Depends on the bar length as well. If you want to run short bars you wouldn't need as much out of the chamber to get the exhaust lower. I've not seen any gains from lowering the exhaust to 85 on any saw. Epoxy it back up to 78 is better to me.

I think you meant intake........and I agree.

I'd rather leave to exhaust a little higher, then to take more out and end up with the intake real long.
 

mdavlee

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I think you meant intake........and I agree.

I'd rather leave to exhaust a little higher, then to take more out and end up with the intake real long.
Yep. I should have proofread it twice.
 

metallic

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Is it standard to refer to exhaust and transfer timing numbers as degrees after TDC and intake timing is referred to as degrees before TDC?
 

David Young

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yes,
it is when the port opens thats why there is that convention.

or you can assume symmetry and just list the duration.
 

jmssaws

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Thanks David. I'm more used to measuring duration so as to avoid having to deal with TDC.
I'm trying to use duration now but I still gotta figure it in my head everytime but I'm getting there. Makes more sense to talk duration instead of degrees ftdc.
 
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