262xp R&D

[awol]

Now I'm confused. Lol. You said to keep the port opening to the cylinder the smallest part, no? Normally they are already larger than the intake tract. But now you said you widen.
I must be missing something...

Yeah me to......!What I'm trying to say, if you're gonna make the port bigger, it's much better to widen it than to raise it, as there is more area when the port first opens and pressure signal is at its highest.

Oh, and I do like this conversation, it helps make me think!

 

[jmssaws]

Yeah me to......!What I'm trying to say, if you're gonna make the port bigger, it's much better to widen it than to raise it, as there is more area when the port first opens and pressure signal is at its highest.

Agreed 1000000%

 

[drf256]

I square the bottoms and the tops, but leave the tops more rounded nowadays. A square top can catch ring gaps on some models as well.

 

[Chainsaw Jim]

That's the ticket right there. the top of the intake can match the same shape as the exhaust roof and rings will not hang unless an end is exposed. that's why I remove that nub in the older stihls entirely.
One major thing easily overlooked is the need for symmetry on each side. the offset intake to the center of the cylinder like an 044/046 will easily trick someone into exposing the upper ring end side. I'm not yet sure but I'm starting to think an intake floor too flat can cause premature wear on the bottom of the skirt. I think @jmssaws recently mentioned this along with a few other reasons.

 

[jmssaws]

All I've ever noticed a flat intake floor do is wear the bottom of the skirt and make it use more fuel and more intake noise.

 

[awol]

And depending on intake tract length and port timing, a wide port tends to spit back more agressively.

 

[paragonbuilder]

I'm enjoying the differing views. Jim, you build some radical stuff! What you do works. Alan, you build some crazy strong saws and in a way that inspired myself and Al to try some things contrary to the standard advice, and it kicks ass! Jason knows the big stihls like no one else, and Al builds some super strong saws too.
I started this thread because I wanted to expand my learning. Please keep up the banter without taking others opinions as an attack on yours. Carry on men!

 

[Chainsaw Jim]

Now for the carb!
I was farting around in my carb drawer and noticed you can use a bing 48 carb as a direct bolt up if you swap out the throttle shaft. Though it'll have no choke unless you drill two holes through the carb to install one.
But I tell you what....I don't think it'll be lacking at the carb end after that. And the way the venturi is shaped should compliment the intake as it is. I'd try the carb upgrade before changing the intake though.
I hope this wasn't mentioned already and I'm just being a parrot. lol

 

[drf256]

Now for the carb!
I was farting around in my carb drawer and noticed you can use a bing 48 carb as a direct bolt up if you swap out the throttle shaft. Though it'll have no choke unless you drill two holes through the carb to install one.
But I tell you what....I don't think it'll be lacking at the carb end after that. And the way the venturi is shaped should compliment the intake as it is. I'd try the carb upgrade before changing the intake though.
I hope this wasn't mentioned already and I'm just being a parrot. lol

Not mentioned. That's a great find Jim.

Anything that could replace the coveted 87 would be a major breakthrough.

Is the Venturi larger than the 87? What's it measure?

If that's the 038 carb, I think it's smaller than the 87 and may not be worth the work. May even be smaller than the 120/144. I can measure bores in a bit.

 

[Landmark]

Not mentioned. That's a great find Jim.

Anything that could replace the coveted 87 would be a major breakthrough.

.

Heres a great thread @Tor R did on alternative carbs for hda87.

 

[drf256]

Here's some GREAT stuff guys. Jim did stumble upon something pretty great here.

Throttle plate opening:

HDA 87/199 = 19mm
Bing 48/Zama for 036 = 19mm
HDA 144/120 = 17.4mm

Venturi size:

HDA = 16.4mm
036 Zama = 15mm
120/144 not measured.

The 87/199 have a much larger choke plate opening than the 036/Bing. The bing has an even smaller venturi with one flat side where the nozzle comes out of than the Zama 036 carb.

This is some serious food for thought here.

Visually, it appears that the walbro 87/199 can be easily fitted to the Stihl 036. It also appears that the OEM Zama for Stihl 036 can be made to fit the 262 with not much work. The choke plate and the Venturi should be simple to open to 87 specs. Choke plates are available for around $2 each for the 87 carb.

I'll measure the choke openings in a bit.

Wow. Something to wonder about here.

Anyone have some spare 199 carbs they wanna donate to science?

 

[drf256]

120 Venturi 15.2mm

Choke openings

120 17.4
036 Zama 18.8
87/199 19mm

 

[drf256]

So opening a Zama 036 Venturi 1mm and swapping shafts may be the ticket here.

 

[Chainsaw Jim]

I guess it gets even better because I'm pretty sure the bing 48 is the 038 carb. The 038 mag might also run that carb but they went to a Tillotson HE after bing.
Here's a bing 48 on the left and a Tillotson HE on the right. it should give a good idea of venturi size difference beetween a two 038 carbs that can be fitted to the 262.

 

[drf256]

They should measure 19mm Jim?

 

[Chainsaw Jim]

They both measure 19mm in and out. The tilly actually has a 17.85 venturi where the bing just has the two humps.

 

[paragonbuilder]

This is great news guys!

 

[drf256]

@David Young

Do you have the measurements for max venture size for CC of saw?

Larger may not necessarily be better for every saw.

The 191 replaces the 199. Can anyone say whether the venture sizes are the same? Accelerator pumps on both?

 

[awol]

Now for the carb!
I was farting around in my carb drawer and noticed you can use a bing 48 carb as a direct bolt up if you swap out the throttle shaft. Though it'll have no choke unless you drill two holes through the carb to install one.
But I tell you what....I don't think it'll be lacking at the carb end after that. And the way the venturi is shaped should compliment the intake as it is. I'd try the carb upgrade before changing the intake though.
I hope this wasn't mentioned already and I'm just being a parrot. lol

The Solo 662 also uses a Bing 48, but it has the choke shaft installed in it. Ive been looking for a better replacement for the 48, so this topic is kinda interesting!

 

[David Young]

@David Young

Do you have the measurements for max venture size for CC of saw?

Larger may not necessarily be better for every saw.

The 191 replaces the 199. Can anyone say whether the venture sizes are the same? Accelerator pumps on both?

I can dig up the info but off the cuff what I remember is that most of the time chainsaws are on the low end of the spectrum the saws that had higher than average max power rpm typically had a larger venturi.
I talked to the tech manager at walbro one time regarding how is a carburetor selected for a motor. He said a engine will come to them with an output spec from the manufacturer. They have a good idea what size they will need from past experience. they put the smallest carb they can on the motor to create the desired power output. Next question was why the smallest carb? For manners and fuel economy. If I wanted more power put a bigge carb on.
I suspect that there is a whole dark side to carbs we are throwing darts in the dark at. There is a ratio between the throttle bore and venturi that I am sure affects the fuel delivery. We know that a bigger carb will flow more air and make more power. However, look at the experiment with the bing carb on the 036. If I was going to pick a carb for performance based solely on appearance it would not be the bing with a non concentric venturi.

I'll go look for the link. brb