Carb tuning/mods for modded saw?

[Rusty-880]

If I remember correctly photo 032 should have a light black line on intake port surface, looks a bit like a shadow line- just looked at it. This is roughly where the slot finishes in cyl

 

[Rusty-880]

How do you machine 32 thou off the base without machining a pop-up piston, too ? Unless it had 52 thou squish to start with, not likely for a Stihl.

Took gasket off approx .020" and took extra .012" from cyl base. Left .018". Put it back on with gasket silicon

 

[little possum]

Yikes. Never seen anything like that..

Well yea, McCulloch had something like that I believe, but it was to help with starting. I think it reduced compression. But I cant remember if it went up towards the combustion chamber, or down towards the crank..

I think a simple woods ports would have been best for a milling saw. Maybe advice on gettin more torque outta the porting work.

Hope it will work out for ya

 

[mtngun]

The pic inside the jug is too blurry to tell much, but looks like the intake port goes all the way to the edge of the piston skirt ?

Not sure about the little divot at the top of the intake port ? You might try filling it with epoxy.

An 880 is not a good saw to make mistakes on. Too expensive.

 

[mtngun]

Took gasket off approx .020" and took extra .012" from cyl base. Left .018". Put it back on with gasket silicon

That implies that the engine had 0.050" squish stock. I'm skeptical.

 

[tdi-rick]

You can try filling that with epoxy as the boys above have suggested and start again.
It'll never work well as is.

Curious, are the ring ends in the same position on the 088/880 as the 084 ?

If so, I'd be worried about snagging a ring too, especially when you consider Stihl were so concerned they used that small extension into the intake port to support the ring ends on the 084 cylinder. (although a lot of porters do away with it)

 

[Rusty-880]

That implies that the engine had 0.050" squish stock. I'm skeptical.

Two 880s here both had .050"+ clearance. 2nd saw needs bottom end rebuild and new piston. Picked it up at scrap yard.

 

[Rusty-880]

You can try filling that with epoxy as the boys above have suggested and start again.
It'll never work well as is.

Curious, are the ring ends in the same position on the 088/880 as the 084 ?

If so, I'd be worried about snagging a ring too, especially when you consider Stihl were so concerned they used that small extension into the intake port to support the ring ends on the 084 cylinder. (although a lot of porters do away with it)

Think ring ends run on either side of intake on 880

 

[Rusty-880]

Anyone want to look at my original thought for this thread?

Found this in old two stroke book;
"The single difficulty with pressure pulse caburetors is that they are extremely sensitive to both cylinder displacement and crankcase compression ratios, working well only on engines for which they are specifically designed"

If mods make a saw flow and scavenge better wouldn't the carb need recalibrating to work with differences in pulse pressure?

I don't mean this saw, earlier I ported it as per the recommendations on this site and ever since then always had trouble tuning properly, fuel needed for idle caused it to be too rich at top revs with H screw just off the seat

 

[AUSSIE1]

Think ring ends run on either side of intake on 880

I suppose you have nothing to loose trying to fill that void. Use the best filler available. You can use a ring to see if there are any high spots.

I'd work on this area before worrying about any tuning issues.

A carby usually only needs fine adjustments after some port work.

 

[Rusty-880]

Put a stock cylinder back on it now so not worrying about that one at this stage.
Not much sense in porting at ANY level tho if I can't get it to tune after its done. Don't get much gains if running to rich.

 

[wigglesworth]

I don't mean this saw, earlier I ported it as per the recommendations on this site and ever since then always had trouble tuning properly, fuel needed for idle caused it to be too rich at top revs with H screw just off the seat

I've read a lot about porting on this site, but have never seen anybody recommend doing that to the intake port.

Does it spit back fuel pretty bad like that? I would think it would. I would think the way that port was added in, would/could induce positive pressure to the intake tract.

 

[Rusty-880]

Had saw on mill all yesterday. Will pull all the way down to 4000rpm, very broad power range but not that high hp peak.

 

[tdi-rick]

Anyone want to look at my original thought for this thread?

Found this in old two stroke book;
"The single difficulty with pressure pulse caburetors is that they are extremely sensitive to both cylinder displacement and crankcase compression ratios, working well only on engines for which they are specifically designed"

If mods make a saw flow and scavenge better wouldn't the carb need recalibrating to work with differences in pulse pressure?

I don't mean this saw, earlier I ported it as per the recommendations on this site and ever since then always had trouble tuning properly, fuel needed for idle caused it to be too rich at top revs with H screw just off the seat

Doing a simple port job of widening the intake and exhaust ports and a bit of case blending you aren't changing things that much from stock, and the changes are well within the realms of your basic HS and LS screws on your basic saw carby.

 

[Rusty-880]

Well, for what ever reason mine needed 1/2 turn on L to idle and closed or just off seat for H and still could have been leaner at revs.
Someone who raced karts mentioned that spring pressure for needle and seat can be very touchy, they had to tweak it depending on barometric pressures ect.

 

[Terry Syd]

Speculation on my part, but here goes. The maximum delivery ratio of the transfer ports occurs at peak torque. That is when the most mixture from the crankcase is delivered into the cylinder and the piston closes the transfer ports.

Above that max torque point, there is less time for the mixture in the crankcase to move into the cylinder so the torque starts dropping off.

The opposite occurs below max torque, all the mixture goes into the cylinder at the lower rpm, but since the transfer port is still open and the piston is moving up - some of the mixture gets sucked back down in the crankcase - again the torque drops off.

OK, you put a direct passage into the intake manifold during the transfer period. At high rpms you may be getting some of your transfer port mixture back down through your modified port. At the lower rpm, below max torque, you will start sucking some of the mixture back down in the crankcase - this usually means you also suck some of the exhaust gas back in through the exhaust port. However, with open passage to the intake, you may be sucking some mixture from the intake directly into the cylinder.

I don't know how the powerband feels, but there may also be a contributing factor of intake resonance. It could be that a resonance wave is coming back down the intake as a positive pulse. Normally, it would bounce off the closed piston and head back out towards the carb. In your case, it may be that the positive wave is pushing extra mixture into the cylinder.

 

[tdi-rick]

Speculation on my part, but here goes. The maximum delivery ratio of the transfer ports occurs at peak torque. That is when the most mixture from the crankcase is delivered into the cylinder and the piston closes the transfer ports.

Above that max torque point, there is less time for the mixture in the crankcase to move into the cylinder so the torque starts dropping off.

The opposite occurs below max torque, all the mixture goes into the cylinder at the lower rpm, but since the transfer port is still open and the piston is moving up - some of the mixture gets sucked back down in the crankcase - again the torque drops off.

OK, you put a direct passage into the intake manifold during the transfer period. At high rpms you may be getting some of your transfer port mixture back down through your modified port. At the lower rpm, below max torque, you will start sucking some of the mixture back down in the crankcase - this usually means you also suck some of the exhaust gas back in through the exhaust port. However, with open passage to the intake, you may be sucking some mixture from the intake directly into the cylinder.

I don't know how the powerband feels, but there may also be a contributing factor of intake resonance. It could be that a resonance wave is coming back down the intake as a positive pulse. Normally, it would bounce off the closed piston and head back out towards the carb. In your case, it may be that the positive wave is pushing extra mixture into the cylinder.

I doubt it Terry, but you never know....

That sort of boost port (only much larger and with the roof angled up towards the chamber) works well on a (barrel) reed valve engine as the inlet port is totally closed during the transfer period, that gouge in the intake will really knock top end. (case reed and rotary valve engines use boost ports too, but the intake goes directly into the crankcase)

It could really be dog rich as you are having a double dose of air (and charge) going backwards and forwards through the carby.
First up during a 'normal' (sort of) induction period and then backwards up the intake during the transfer period.

It'd be much like if you shorten the intake skirt (too much) or raise the intake port roof, either way you increase the intake period in the wrong direction and get a fat fuel hog.

 

[Terry Syd]

Rick, I agree with your high rpm analysis. A backflow into the intake would cause a richer mixture. Thus, this may explain his drop off in high rpm power. However, the opposite occurs below max torque, so that may explain his increase in torque.

Rusty, we have been discussing pop off pressures (POP) in the Aussie thread. One of the researched threads was this one - http://www.arboristsite.com/arborist-101/67662.htm

EDIT: There might also be the issue of blowback down the intake at high rpm by excessive exhaust gas still left in the cylinder (another way of saying inadequate blowdown). If it was blowing back down the intake, it would not be pushed out like it would at the transfers and so would contributing to a dilution of combustion mixture on the next intake cycle.

 

[drkptt]

The opposite occurs below max torque, all the mixture goes into the cylinder at the lower rpm, but since the transfer port is still open and the piston is moving up - some of the mixture gets sucked back down in the crankcase - again the torque drops off.

When does a chainsaw operate at WOT at a speed below peak torque?

 

[splitpost]

Two 880s here both had .050"+ clearance. 2nd saw needs bottom end rebuild and new piston. Picked it up at scrap yard.

Holy sh!t Rusty ,you certainly picked an expensive saw to experiment on,that takes some balls,wish i had a scrap yard like yours around here