Part Seven: Carburation

[grizz55chev]

Spare time exists? *feints*

Faints?

 

[Wonkydonkey]

Wel, no6 blowdown. An't anything to do with carbs. This is port timings

The rest is in other threads if you can find and read them.

Also there’s no simple answer as to whatsize to drill a jet, it’s just go a bit bigger if it seems to be lacking fuel and you have unscrewed your needle more than x and once gone this far it will not deliver any more.
And what carbs fit other saws ?.. that’s a game of finding a thread if someone’s done it and if was easy or hard and your skill level or do you have a carb and will it fit, oh and maybe work abit.etc.

 

[I Know Something]

Faints?

Old gammer joke from my WoW days. Lol

 

[Nutball]

Should you drill out where the needle goes, or only drill the little brass insert thing?

 

[Ketchup]

How about trimming down butterfly shafts and screws? Anybody think it’s worth it?

 

[huskyboy]

How about trimming down butterfly shafts and screws? Anybody think it’s worth it?

Yes and no. If it’s a everyday worksaw, no not really. It’s also very easy to screw up the carb doing this if you don’t know what your doing. If your trying to get every bit of gain, then yes it does help a tad. Not a big gain, but everything adds up.

 

[Terry Syd]

The biggest restriction to flow on most saws is the air filter. You will likely pick up more flow by re-working the air filter than by trimming the end of the screw on the butterfly.

As far as carburetors, I try to get them to do best what the job description is about - calibrate the fuel correctly. I'm often surprised at how much extra power is available just by getting the fuel mixture correct.

 

[MustangMike]

Low restriction air filters make a huge difference. If you have a good P+C, a muff mod, timing advance and low restriction air filter can sometimes result in near ported saw performance.

Some saws seem to run real strong a little rich, and others will not run strong unless they are right on the verge of lean, but the tune can make a big difference.

 

[Terry Syd]

Here's a fairly good article with some charts that give a good indication of what is happening with power and fuel flow.

http://www.mummbrothers.com/SRF_Stuff/Secrets/Driveline/Air_Fuel.htm

Notice in the second chart how power is not as adversely affected by a richer mixture, but when you get into the lean side of the chart a bit leaner makes you loose power much quicker.

In other words, if you are willing to burn a bit more fuel, you can ensure that you are getting as much power out of the engine as you can get.

 

[Fruecrue]

Here's a fairly good article with some charts that give a good indication of what is happening with power and fuel flow.

http://www.mummbrothers.com/SRF_Stuff/Secrets/Driveline/Air_Fuel.htm

Notice in the second chart how power is not as adversely affected by a richer mixture, but when you get into the lean side of the chart a bit leaner makes you loose power much quicker.

In other words, if you are willing to burn a bit more fuel, you can ensure that you are getting as much power out of the engine as you can get.

Good read, thanks for the link.

 

[rogue60]

The RC engine crowd is the place to look for information on modifying carbs.
You won't find much on modding carbs on chainsaw forums put it thus way definitely no step by step this is how to do it with pics.
Lots of (why and how) to modifying carbs on RC forums the thing being these guy's are using the same types of carburetors as used in chainsaws.

 

[MustangMike]

Good chart, but what it does not show is that with a chainsaw, a leaner mix can result in higher RPMs, which almost always results in more power.

 

[huskyboy]

Too lean and you loose torque/power. Gotta find the balance.

 

[MustangMike]

Won't debate that at all. My point was just that some saws seem to like it a bit lean, other's don't. There is always a sweet spot.

FYI, most hybrids don't like to be rich, they thrive at higher RPMs. I had a ported 044 that was the same.

 

[RI Chevy]

Not always Mike. You gotta experiment and time your cuts. Saw may sound and feel faster, but it really isn't. There definitely is a point of diminished returns.

 

[RI Chevy]

Mason and I experimented. My CFB Hybred was stroking at 16200. But made more power tuned back at around 15200.
Same for Eric's MOFO Hybrid...

 

[huskyboy]

Yep anything over 15 ish with hybrids lost power and cut slower.

 

[huskyboy]

Vise versa anything under 14.3k ish was a bit too rich and looses power on the hybrids we tested. Got to find the sweet spot, depending on the saw.

 

[NSEric]

My ported big carbed (372 carb on a 54cc saw) joncutter 5800 china saw hates being a tiny bit rich.
I tried to turn it down a to 14000rpm so it wouldn't be lean if I was cutting on a cold day but it was slow as hell and wouldn't stop 4 stroking in the cut unless you stood on it, so I turned it back up to 15000 where its happy. It still cuts good leaned out a little more and running close to 16000 rpm but I don't know how long it will last at that rpm lol.

 

[RI Chevy]

We tested 3 Hybrids. Each saw had its own sweetspot. Couple hundred rpms here or there.