Porting the dolmar 421...

[drf256]

Intake. What's the carb Venturi size Joe?

Here we go again, lol....

 

[drf256]

You may have piston to spark plug clearance issues Joe. I had that on the 490, also on most Huskys I do.

A popup piston would be a lot less work here. Would be offset on the piston obviously. You could maintain the factory transfer openings and just widen the exhaust and intake.

Many options here.

 

[drf256]

What size is the wrist pin Joe?

 

[Red97]

Looking at this little jug makes me think about a lot of things.

I'm gonna put posts separately to try to keep things straight here.

So you have .023 squish WITH gasket. Are you planning on running one Red13.5?

I see why it runs so well. They left little on the table.

So you take 20 off the base, leaves you with -17 sans gasket. Take 37 out of the band. Am I following here?

Yep, cut the base. 025. Then cut the band to get my .020. No gasket.

Exhaust height.

You've got a limited coil again Joe.

I'm not sure if raising the roof is gonna do that much. The reason this little sucker pulls so well may be that exhaust height. I think I'd raise it back to 113 and leave it there. If the compression is too high, I guess you can raise it.

That flywheel is a work of art joe. As this is a small displacement saw with a rather large overall size, I bet compression around 230 would be a nice compromise.

See, this is where the eCho porting comes in, that saw was anemic stock, and pulls like a mofo now, by raising the ex. I plan on raising this to 108 ish.

Is this gonna be a one shot try or you gonna tear it down a few times? I only ask as I may add a couple degrees to the blowdown for initial testing! Looks good so far my man. I'm watching and liking!

Thank you,
I have had great luck with one shot wonders. Lol
Now if I don't like the resuls, I will go back in.
I was thinking about blowdown #'s last night. Stock is 18*
I'm thinking it will like 19-21*.

Transfers.

Do the entry angles differ from exhaust/mid/intake roofs?

You should have LESS case pressure from lowering the intake floor. You'd need a higher (physically) than stock upper to compensate. But you also are removing the muffler cat and venting exhaust better. That would make me want a lower transfer opening.

I'd personally go with openings of 129/130/131-opening the exhaust first, then the mid, then the intake. I'd open the exhaust one more squarely to the chamber and increase the angle upwards as you get to the intake one.

Depending on just how crazy you wanna go, you could fill the exhaust transfer with JB weld and make it thinner in width. Kinda make it into an exhaust finger.

You have a lot of relative cross sectional transfer area there Joe.

I'm just thinking out loud. I love the puzzle of porting theory.

I don't see a lot left on the table here. They probably created a saw on the edge of max performance.

The transfers are a lot smaller than they apper. All open at the same time stock.
I will take a better look at the case, but it didn't look to have a lot of capacity.
The cat was already removed.
I will get a lot better idea where I want to go, after I cut the base, and lay out the timing number I am thinking.

Intake. What's the carb Venturi size Joe?

Here we go again, lol....

I want to say it is in the 11-12mm range....

You may have piston to spark plug clearance issues Joe. I had that on the 490, also on most Huskys I do.

A popup piston would be a lot less work here. Would be offset on the piston obviously. You could maintain the factory transfer openings and just widen the exhaust and intake.

Many options here.

Don't think I want this saw to have my first popup. It uses a small 10mm plug, and the chamber seems plenty deep. We will see.

What size is the wrist pin Joe?

That I'm not sure, looks like 10mm, but don't plan on pulling it.

This is great stuff fellas, gotta stop and think about it a bit.

Keep it coming. I am enjoy ting to understand others approach to porting.

 

[fordf150]

Just remember on these that screwing up the jug is an expensive mistake on these. $190 list.

 

[Red97]

Just remember on these that screwing up the jug is an expensive mistake on these. $190 list.

Ouch...

Guess if it is that bad, better off just buying a new saw lol

 

[David Young]

What the plan for the coil. if you lower the jug that much the coil will be hitting.

 

[David Young]

how about a poulan coil? Looks like a wood shark coil.

 

[Backtroller]

I haven't seen how the cool is attached but could you slot the mounting tabs to get the movement out of the coil you need?

 

[Red97]

What the plan for the coil. if you lower the jug that much the coil will be hitting.

Slot the holes a lil deeper, I will put the jug/ coil back on before I cut the base.

how about a poulan coil? Looks like a wood shark coil.

Ya think? I have a couple lil poulans here, guess I could check the coils...

Thank you

 

[Red97]

I haven't seen how the cool is attached but could you slot the mounting tabs to get the movement out of the coil you need?

Pg 3,

Bolted to the top of the jug.

Might have to swap to std socket head cap screws to get the right amount of adjusment. But should be do able.

I'll go check.

 

[David Young]

on a jonsered 535 i did I remove some of the threads near the head of the bolt it was enough to move the coil what I needed.

another cat skinning method

 

[exSW]

Ouch...

Guess if it is that bad, better off just buying a new saw lol

Let me know..

 

[Backtroller]

Pg 3,

Bolted to the top of the jug.

Might have to swap to std socket head cap screws to get the right amount of adjusment. But should be do able.

I'll go check.

I meant what those plastic insulators looked like. I know you have it covered. I'm not worried in the least.

 

[exSW]

Pg 3,

Bolted to the top of the jug.

Beer can piece and a leather punch?

 

[Red97]

Jug bolted down, no base gasket. Most I can get is .010 air gap.

But the fat head coil bolt, is almost hitting the flywheel.

Think I'm just gonna drop the jug .015. Replace the coil bolts with normal Allen head ones. And file to fit. Plenty of meat to get the extra .015

The poulan coil is a no go, the legs are opposite of this.
Don't think it will be worth the trouble...

This saw was turning 12,600-12,800 with a nice plug color. Hopefully get it up round 13,300.

That is an awfully large flywheel, wonder if that is why it seems to "lug" really well.

 

[Red97]

Coil mod done
Swapped the bolts for a couple stainless m5 capscrews.
Filed the slots.

That card is about .030 so that should give me the room I need to drop the jug another .015.

Much more than that the coil starts getting into the jug.

 

[Red97]

Roundy roundy round we go....



.015 off the base .035 off the extensions.

Hopefully tomorrow, I can get my other mandrel's made to se the squish, and layout some numbers.

 

[Red97]

I think I have a pretty solid plan this time.

Still have to double check a few variables.

So total drop on the jug was .035. .015 off base +.020 gasket
Will be taking about .015-17 out of the band.

Set the ex at 110
19* blowdown on the small exhaust transfer
21* on the intake pair

Where ever the intake lands, hoping 76/77 raise it back up the .015 for more time area. I think this increased intake time is very important to this saw with such a limited carb.

Intake is close to 60% if I remember correctly. Might try an widen it a bit after I take some more measurements.

The main reason I want to raise the exhaust is again, to increase the area. It is already approx 63% width. So I think the added height will really help flow.

Plan on opening the baffle like f150 posted and adding another outlet on the other side if I have room with a matching hole in the baffle.

In the tests redbull did on the 661 made me think the baffle can be used as a tuning device. It is already in, might as well use it.

My take on what the factory was trying to achieve with this saw is.

Ex set at 113, to increase the power stroke. (Torque)
18* blowdown to keep the top end up. Shown with the 9500 max hp spec. (The beloved 242xp is rated at 9900rpm max power)

Intake is low for max hp, but keeps good fuel economy.

Just the way I read the numbers.

Feel free to chime in to the discussion. We all here to learn, I am open to opinions

 

[mdavlee]

I messed with a 401 and a I can't remember the numbers used in it but it was torquey