Part Seven: Carburation

[Mastermind]

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]

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]

explain please

Which part brother Dallas?

I should warn you.......I'm not feeling very well. I really overdid it yesterday.....in a lot of pain.

Nothing I say today may make any sense. lol

 

[tree monkey]

I can't keep up

 

[dall]

Will a smaller venturi supply more fuel than a small one? here randy lol

 

[dall]

scroll up

 

[Mastermind]

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.

 

[Motorhead]

 

[Stump Shot]

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?

 

[dall]

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

 

[Mastermind]

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]

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]

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

 

[Lee H]

I run 780 Holley's on my saws.

 

[dall]

Randy you may need to go to the dr

 

[mdavlee]

You can go too big and not get a good signal and idle sucks. They run fine wide open.

 

[Chainsaw Jim]

You can go too big and not get a good signal and idle sucks. They run fine wide open.

Sometimes a longer velocity stack can correct that.

 

[mdavlee]

Sometimes a longer velocity stack can correct that.

That is true.

 

[Dub11]

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

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



Exactly.

GWS

 

[dustinwilt68]

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.