High Quality Chainsaw Bars Husqvarna Toys Hockfire Saws

Part Seven: Carburation

Mastermind

Chief Cat Herder
Staff member
GoldMember
Local time
5:40 AM
User ID
4
Joined
Dec 3, 2015
Messages
48,638
Reaction score
316,788
Location
Banner Springs Tennessee
Country flag
So far we've delved into six different topics on chainsaw performance.

We've covered the exhaust port.

http://opeforum.com/threads/part-one-the-exhaust-port.7279/

The intake port.

http://opeforum.com/threads/part-two-the-intake-port.7290/

And the transfer ports.

http://opeforum.com/threads/part-three-the-transfer-ports.7306/

And compression.

http://opeforum.com/threads/part-four-compression.7318/

And ignition timing.

http://opeforum.com/threads/part-five-ignition-timing.7331/
http://opeforum.com/threads/part-five-ignition-timing.7331/
The exhaust system.

http://opeforum.com/threads/part-six-the-exhaust-system.7372/

Now let's spend a little time on carburation.

Is a bigger carb better? What if I'm running out of fuel? What about intake reeds?

Will a smaller venturi supply more fuel than a small one?

You guys kick this around. :)
 

dall

Mastermind Approved!
Local time
6:40 AM
User ID
795
Joined
Feb 5, 2016
Messages
46,455
Reaction score
185,862
Location
grafton wv
Country flag
So far we've delved into six different topics on chainsaw performance.

We've covered the exhaust port.

http://opeforum.com/threads/part-one-the-exhaust-port.7279/

The intake port.

http://opeforum.com/threads/part-two-the-intake-port.7290/

And the transfer ports.

http://opeforum.com/threads/part-three-the-transfer-ports.7306/

And compression.

http://opeforum.com/threads/part-four-compression.7318/

And ignition timing.

http://opeforum.com/threads/part-five-ignition-timing.7331/
The exhaust system.

http://opeforum.com/threads/part-six-the-exhaust-system.7372/

Now let's spend a little time on carburation.

Is a bigger carb better? What if I'm running out of fuel? What about intake reeds?

Will a smaller venturi supply more fuel than a small one?

You guys kick this around. :)
explain please
 

Mastermind

Chief Cat Herder
Staff member
GoldMember
Local time
5:40 AM
User ID
4
Joined
Dec 3, 2015
Messages
48,638
Reaction score
316,788
Location
Banner Springs Tennessee
Country flag
Will a smaller venturi supply more fuel than a small one? here randy lol

From what I've seen.......yes.

The low pressure signal is higher with a small venturi.......so it will pull harder on the main nozzle.

That is a good way to explain it.......but not really accurate.

You will have a lower than atmospheric pressure........so the pressure of the atmosphere will push the fuel thru the carb.

14.7 PSI at sea level.
 

Stump Shot

Disciple of Monkey's
GoldMember
Local time
5:40 AM
User ID
1377
Joined
Jun 5, 2016
Messages
31,187
Reaction score
194,804
Location
Northwoods of Wisconsin
Country flag
From what I've seen.......yes.

The low pressure signal is higher with a small venturi.......so it will pull harder on the main nozzle.

That is a good way to explain it.......but not really accurate.

You will have a lower than atmospheric pressure........so the pressure of the atmosphere will push the fuel thru the carb.

14.7 PSI at sea level.

I think what Dallas was referring to was smaller than a small one. Did you really mean to say smaller than a large one?
 

Mastermind

Chief Cat Herder
Staff member
GoldMember
Local time
5:40 AM
User ID
4
Joined
Dec 3, 2015
Messages
48,638
Reaction score
316,788
Location
Banner Springs Tennessee
Country flag
I think what Dallas was referring to was smaller than a small one. Did you really mean to say smaller than a large one?

I don't know. After I eat I'm gonna lay down.

My wife had to help me up outta bed this morning.......

i understand what randy is saying now i thought bigger venturi would draw more air through but the smaller moves it faster making more vacuum

Exactly.
 

Chainsaw Jim

Con Artist LLC
Local time
3:40 AM
User ID
836
Joined
Feb 8, 2016
Messages
4,014
Reaction score
80
Location
Springfield Oregon
It depends on how much you're getting out of the other end of the motor. If you're pulling so much volume that the entire carb is restricting flow then there will be a loss. If you're increasing the performance but not out pulling the carbs cfm capabilities then you'll potentially run into fuel starvation under load. That's when the metering passage and nozzle need enlarged. The Poulan pro 325 is a good example of a high performance carb that has a tiny venruri and intake but has an exceptionally large metering seat passage... that which compares to an HS carb on older, larger saws.
I think a good example of the other end of the spectrum is a McCulloch 101 with the four petal intake block and BDC carb.
 

Chainsaw Jim

Con Artist LLC
Local time
3:40 AM
User ID
836
Joined
Feb 8, 2016
Messages
4,014
Reaction score
80
Location
Springfield Oregon
I don't know. After I eat I'm gonna lay down.

My wife had to help me up outta bed this morning.......



Exactly.
Yikes! I hope that goes away very soon.
I ended up taking 1000mg's of ibuprofen last time that happened to me. It worked well enough to forget about it for a few minutes. Lol
 

dustinwilt68

Wilt Built Work Saws
Local time
6:40 AM
User ID
1007
Joined
Mar 8, 2016
Messages
5,931
Reaction score
26,225
Location
Southwest PA
Take care brother, hope you feel better soon. I don't understand the science of the carbs just that if I can get 1 from the next size model on the saw with little effort I try and do it, ie 357 wahlbro hd199a on a 346xp I also cjland the filter set up to the larger as well, it helps them a lot, not sure of the science behind it, just that it works.
 
Top