carbeez

mweba

Well-Known Member
Jan 10, 2012
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Experience, stories? Everyone talks port numbers. All comes down to atomization...

In and out flow seems like a simple situation short of a 200+ cycle per second function. My experience is a carb size and flow is just as strategic as a tuned pipe.
 

mweba

Well-Known Member
Jan 10, 2012
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SS, I've read and played enough to know that tuned pipe "acoustics" can be a make or break game. Same goes for carb venturi and manifold length. Straw theory...small straw gives a strong low signal in the end...



Have read some dang good info on the subject in the past, hardly an expert beyond muh own experience. Unfortunately most has been lost.
 

stihlbro

Active Member
Jan 10, 2012
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So what you are saying is a bigger carb is not always better? Just curious... I think it has to do a lot with the application. Depends on what you are trying to achieve. I've made gains say on top end, but lose idle characteristics or throttle response as a example.



Then comes the fit....you know throttle linkage, choke lever, air filter.....getting the covers back on........we are talking chainsaws right? Lol
 
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WoodChuck'r

HNIC
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Mitch you talking about woods saws....???


I do know that that bigger carbs don't always mean more power - gotta make them burn it before yer able to dump more in them.


But won't the 346 run a lot better with just a 359 carb and nothing else done to it....??
 

mweba

Well-Known Member
Jan 10, 2012
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So what you are saying is a bigger carb is not always better? Just curious... I think it has to do a lot with the application. Depends on what you are trying to achieve. I've made gains say on top end, but lose idle characteristics or throttle response as a example.



Then comes the fit....you know throttle linkage, choke lever, air filter.....getting the covers back on........we are talking chainsaws right? Lol

Exactly. The application needs to be taken into consideration. I had this topic on my mind after the last two saws I rebuilt. One was a stock 350 while the other was modified. It seams to me guys spend so much time working out timing numbers but do not find the right carb "fit" for the saw. Take a 350, mount a zama and the saw will run just fine for the average user. Take that same saw mounted with a walbro carb and the fuel curve differs. In my experience they run better with a walbro. Same goes for the 359/357.

Now I have a 350 that I tried a 372 carb on and it ran very well at top end but was a total pain to tune and like you mentioned, idled rough. Thinking it would be nice to have a thread dedicated to carb characteristics.
 

mweba

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Jan 10, 2012
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Mitch you talking about woods saws....???


I do know that that bigger carbs don't always mean more power - gotta make them burn it before yer able to dump more in them.


But won't the 346 run a lot better with just a 359 carb and nothing else done to it....??

Yes they do in my experience. the 350 and 346 like the walbro 199.
 
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Hedgerow

Mizzurruh Saw Gangsta
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Exactly. The application needs to be taken into consideration. I had this topic on my mind after the last two saws I rebuilt. One was a stock 350 while the other was modified. It seams to me guys spend so much time working out timing numbers but do not find the right carb "fit" for the saw. Take a 350, mount a zama and the saw will run just fine for the average user. Take that same saw mounted with a walbro carb and the fuel curve differs. In my experience they run better with a walbro. Same goes for the 359/357.

Now I have a 350 that I tried a 372 carb on and it ran very well at top end but was a total pain to tune and like you mentioned, idled rough. Thinking it would be nice to have a thread dedicated to carb characteristics.

On a true race saw, they run full throttle always I think... As in warm saw after priming, then lock throttle open, rip the cord, and make 3 cuts and kill... Yes???
A "work saw" has to be started conveniently, Idle properly, and limb without waiting a second to spool up...
Thus a smaller carb may be in order... Also cc and case capacity makes a difference...
FWIW, I have a 540 Dolmar with a 372 carb on it...
It's probably too big, but useable... The 365 I ran this weekend has a 390 carb on it, and owns it...
 
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Wigglesworth

autotune gives me a pop-up
Jan 9, 2012
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Carbeez?

Dems dose round hole thingies on the intake side, right?

I hate carbs here lately. I've done several swaps, done some machining to em, a little bit of drilling, some polishing, and made some runners. I've also wiped out, and given up on a handful of em too.



Some good calculations out there on sizing and such.

I've used this one before with good results.

http://www.rcuniverse.com/forum/m_839599/anchors_3671949/mpage_1/key_/anchor/tm.htm#3671949
 
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BloodOnTheIce

~Ice Gangsta~
Jan 10, 2012
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On my 250cc bikesaw it had "too small" of a carb Mikuni TMX 38cc
but we got the thing to pump enough fuel through jetting and it runs strong.

I had a Husky 350 with the upgraded intake and exhaust from the 357/359.
I ground the lower transfers, and polished the exhaust, and the carb pumped
so much fuel I had to muffler mod it or would constantly four stroke and bog.

Once I opened the muffler it let it flow better and ran strong.

My 3120 runs on straight Methanol, and runs a bored out polished 3120 carb.
didn't have to run a Tillotson HL.
 

srcarr52

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Feb 11, 2013
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Same thing applies to any engine. For example most people put a Holley 750 on their warmed over small block but really they'll run/drive better and have more throttle response with a 650, but you'll loose about 5 peak HP on the dyno.

With too big of carb you won't have the velocity needed at low RPM to make a stable vacuum in the Venturi so your A/F ratio will not be consistent.
 
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komatsuvarna

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Same thing applies to any engine. For example most people put a Holley 750 on their warmed over small block but really they'll run/drive better and have more throttle response with a 650, but you'll loose about 5 peak HP on the dyno.

With too big of carb you won't have the velocity needed at low RPM to make a stable vacuum in the Venturi so your A/F ratio will not be consistent.

I agree. A chainsaw doesnt have much vacuum either, around about 5in.
 

mweba

Well-Known Member
Jan 10, 2012
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Spot on, its a balancing game.

The proper carb needs to be matched. Port time area is not just at the cylinder wall. Years ago I was reading about intake length and pulse timing. Seems some guys were getting positive results playing with length and shape.
 

mweba

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Jan 10, 2012
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Yep

I never understood why guys would grind the intake 10 miles wide, so wide it would nearly touch the lower transfers, then put a 17mm carb on it??

Well like many things in life, people only know what they are shown. Heck I would argue many of the at home "builders" even understand what time chainges do. Most build threads spend no time on carbs and the like. This was partially my point with this thread. SOOO much credit is given to port size/shape/timing at the cylinder wall.

Oh yes and big muffle holes....right Shaun
 
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Hedgerow

Mizzurruh Saw Gangsta
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Well like many things in life, people only know what they are shown. Heck I would argue many of the at home "builders" even understand what time chainges do. Most build threads spend no time on carbs and the like. This was partially my point with this thread. SOOO much credit is given to port size/shape/timing at the cylinder wall.

Oh yes and big muffle holes....right Shaun

I like mine to have 2...
Cause 1 ain't enough...
:ahah:
 
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srcarr52

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Well like many things in life, people only know what they are shown. Heck I would argue many of the at home "builders" even understand what time chainges do. Most build threads spend no time on carbs and the like. This was partially my point with this thread. SOOO much credit is given to port size/shape/timing at the cylinder wall.

Oh yes and big muffle holes....right Shaun

I always pay attention to the carb... I just don't show it. Although I don't go as far as old BS and bore the whole venturi out. :jerkoff:

Muffler... what muffler. I thought your 350 spit it's muffler off.