Hillbilly Carb Mods: what I do may work for other folks

[hillwilliam]

Holy smokes! I've been pretty distracted and looking over my shoulder at the West Fork Complex Fire - 83,000 acres so far - but I am getting a little work done.
View attachment 302270

Now the carb stuff:

There isn't going to be anything new here; it's all bound to be in various places on AS. A couple of the tricks I learned here, and I don't know where the credit goes. It may help someone to find this stuff in one spot, though.

I can't call myself a saw builder, but I've done up quite a few carbs, mostly bigger ones, pretty much this way, going back about 25 yrs. I've never been scientific about it, other than timed test cuts. I haven't run a saw at low altitude for over 30 yrs., so I can't say. But these mods work well here (my house is at 8,000') and at well over 10,000' altitude.

This carb is off a 372 that I'm modding for someone else. The plan was to trim the limiting tabs to allow proper tuning. You may be able to see in the photo that both jets are as rich as they can go. Stock saw actually ran pretty nicely this way. The guy doesn't want 'nicely' - he wants 'gnarly'. Okey doke.

Is this picture fuzzy, or is it just my eyes?
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I ended up cutting the limiters completely off, because they seemed to want to slip on the little splines.

Next is to grind some metal off the shafts. Plenty of meat is left where it needs to be, and I've never had a problem (yet) from lost strength.

I cut the ends of the screws off and I grind the heads down, leaving just enough to get a good bite with a screw driver. I'll use Loctite on final assembly.
View attachment 302274

Precise boring of the carb venturi would be good, but careful grinding works. You could just remove any casting flash, but I go farther. The last one I did was on a beater 372 with a big barrel top end. I took the venturi to 19mm on that one, and it idles kind of funky. (Not an air leak, but that old saw idled funky anyway.)

I'm going to 18.5mm on this one. (It ended up working fine on this nearly new saw.)

Grind, check, repeat, going for the best uniformity I can get. I know for sure that it's possible to go to 19mm on these little carbs without going into any passages. I also know that I have ground into a passage at 18.5mm. It sucks to buy an new carb or get out the JB Weld.

Then I polish up the venturi, give everything a good cleaning and reassemble. I set the metering lever flush with the top of one gasket. Then I add another gasket to enlarge the metering chamber a little.
View attachment 302276

Then I button everything up and tune to the idiosyncrasies of the saw.

It really doesn't take me long to do these mods, and I'm so slow that it takes me an hour and a half to watch 60 Minutes.

On my next saw, I need to try the carb mods and the muffler mods separately to attempt to document gains. I did both on this one before I tried it. To me, it made a huge gain in power/ speed in timed cuts with muff mods and carb mods. I even called the owner to check again if he still wanted it ported. Yup, I started porting the next day. Now it runs like it oughta.

Of course, you could just see about retrofitting a bigger carb. That works too.

I hope to get some positive critique on my redneck methods. It seems to work pretty well without getting differential pressures too out of balance. Still enough air velocity in the venturi to suck plenty of fuel in.

Hope this does somebody some good.

 

[Perry pioneer]

Going through old posts, I too belive in flatter throttle shafts and ground screws seems every bit helps. Could not bring up your photos but I understand the concept good idea.

 

[hillwilliam]

Going through old posts, I too belive in flatter throttle shafts and ground screws seems every bit helps. Could not bring up your photos but I understand the concept good idea.

Thanks. Not sure I'll ream the venturi on any more Tilly carbs that way, but the Walbro carbs on my old Pioneers work just dandy. Maybe it has to do with reed valves.
I guess the pictures don't come up because the original post is so old.

 

[Perry pioneer]

Rebuilding an 224 for my Husqvarna 266 I got a couple carbs one has the throttle shafts milled flat on one side I'll use that one seems to open up a lot of space and I locktited and ground down the screw should work good.

 

[hillwilliam]

Rebuilding an 224 for my Husqvarna 266 I got a couple carbs one has the throttle shafts milled flat on one side I'll use that one seems to open up a lot of space and I locktited and ground down the screw should work good.

Sounds good! On some of my own saws, I've also eliminated the whole choke shaft. Then I need to use a true 'hand choke' or just squirt in a little fuel with a pipette and put the air filter back on.

 

[Perry pioneer]

Sometimes I'm dumb. I was trying to tune the rebuilt carb kept turning the low speed needle nothing was happening, pistoff took the carb off and apart looked good put it back on, saw started to make strange noise and quit running in thought I blew a piston no compression I'm ready to cry, something was ratthing inside the sidecover so I took it off and the flywheel was loose I sheared the keyway put a new one on and figured out I was adjusting the high speed instead of the low ,dumb ass, now it runs great. Husqvarna says adjust to highest speed than turn back 10 minutes if the needle was a clock. That saw screems before you turn it back wish I had a tach. Allswell . What a day.

 

[Jimbo209]

Thanks. Not sure I'll ream the venturi on any more Tilly carbs that way, but the Walbro carbs on my old Pioneers work just dandy. Maybe it has to do with reed valves.
I guess the pictures don't come up because the original post is so old.

can you repost them for me/us please

 

[hillwilliam]

Sometimes I'm dumb. I was trying to tune the rebuilt carb kept turning the low speed needle nothing was happening, pistoff took the carb off and apart looked good put it back on, saw started to make strange noise and quit running in thought I blew a piston no compression I'm ready to cry, something was ratthing inside the sidecover so I took it off and the flywheel was loose I sheared the keyway put a new one on and figured out I was adjusting the high speed instead of the low ,dumb ass, now it runs great. Husqvarna says adjust to highest speed than turn back 10 minutes if the needle was a clock. That saw screems before you turn it back wish I had a tach. Allswell . What a day.

Whew! You got it figgerd out anyway.

 

[hillwilliam]

can you repost them for me/us please

Here's what I could find:


I'm still not sure how valid reaming the venturi is. Other saw I've done, I haven't run for a while, but my Pioneers have run very well like this for decades.

 

[Big_Wood]

your metering chamber size increase is interesting although i can't see what could be a benefit of this. i've had many HD12's not feed consistently at high speed (rich, lean, rich, lean if ya know what i mean). i'm wondering if that mod would help keep fuel feed to the jets consistant and get rid of that inconsistent feeding. i've played with the metering lever height and it does help a little but doesn't make it perfect. the 372 seems to drain the metering chamber quicker then it can be filled. lowering the intake of a 372 increasing duration makes for a longer intake stroke which pulls less fuel but longer. it corrects most saws i have done. i figure this may be because i am relieving the demand put on the carb by giving it more time to do the same work. i'm gonna toy around with that idea when i have time next.

 

[hillwilliam]

your metering chamber size increase is interesting although i can't see what could be a benefit of this. i've had many HD12's not feed consistently at high speed (rich, lean, rich, lean if ya know what i mean). i'm wondering if that mod would help keep fuel feed to the jets consistant and get rid of that inconsistent feeding. i've played with the metering lever height and it does help a little but doesn't make it perfect. the 372 seems to drain the metering chamber quicker then it can be filled. lowering the intake of a 372 increasing duration makes for a longer intake stroke which pulls less fuel but longer. it corrects most saws i have done. i figure this may be because i am relieving the demand put on the carb by giving it more time to do the same work. i'm gonna toy around with that idea when i have time next.

You likely know more about the whole business than I do. I think you figured out the benefit of enlarging the metering chamber, answering your own question. I didn't invent the idea, but it's easy to do and doesn't seem to have any adverse effects.

The way you explain the longer intake duration allowing more time for the carb to do its job makes sense. I know for sure I've given a couple of saws too much intake duration, but they run ok. I think I'm beginning to understand compression duration - that's something I don't need to worry about with my old reed-valve saws.