Part Seven: Carburation

[T.Roller]

But what's the tell tale sign ? If a saw tunes and cuts within a reasonable adjustment on the high side, what are you looking for that says hey, I need a bigger jet.

Saw running out of fuel in a cut

 

[MustangMike]

In extreme cases, but much more likely that it will just be a bit anemic.

 

[huskihl]

Usually the H screw will only allow more fuel in up to about 1.5-2 turns out. Beyond that, there isn't much more fuel it can let through. The jet that supplies fuel to the needle is probably sized proportionately to the venturi. So if you're out 2.5 turns, and going lean in a long cut, you would need a larger carburetor. Boring the venturi and using a bigger H jet might allow enough extra air and fuel without needing to upgrade the carb.

I think that's what I'm seeing in my mind's eye

 

[T.Roller]

In extreme cases, but much more likely that it will just be a bit anemic.

doesn't have to be extreme

 

[mdavlee]

I've seen big cut time cha he's in carb swaps on the 7900. Up to 30%. The Solo was 5 turns out on an Rwj4 and might have been down around 15.5k-16k. Just not enough fuel. 385 carb fixed that.

 

[MustangMike]

Usually the H screw will only allow more fuel in up to about 1.5-2 turns out. Beyond that, there isn't much more fuel it can let through. The jet that supplies fuel to the needle is probably sized proportionately to the venturi. So if you're out 2.5 turns, and going lean in a long cut, you would need a larger carburetor. Boring the venturi and using a bigger H jet might allow enough extra air and fuel without needing to upgrade the carb.

I think that's what I'm seeing in my mind's eye

Like everything else, the A/F ratio the carb provides may be based more on emissions than on performance (my guess anyway). So making it a tad richer is likely to increase performance, just like the Holley carbs did on the cars (even if you did not increase the size).

 

[Deets066]

I've seen big cut time cha he's in carb swaps on the 7900. Up to 30%. The Solo was 5 turns out on an Rwj4 and might have been down around 15.5k-16k. Just not enough fuel. 385 carb fixed that.

Exactly, my solo had a noticeable gain from swapping to a bigger carb

 

[CR888]

I am looming for a Zamma 3cm twin jet carb for my Solo 681 that has a Walbro. I can mod the Zamma easily to perform. Maybe a 385 carb is worth a try too.

 

[mdavlee]

I am looming for a Zamma 3cm twin jet carb for my Solo 681 that has a Walbro. I can mod the Zamma easily to perform. Maybe a 385 carb is worth a try too.

The 385 carbs have the best manners. I had a modded Zama and it wouldn't feed it.

 

[Deets066]

Hard to beat the Tilly

 

[bikemike]

When you suck the choke shut you need more carb

Sounds like 026 talk

 

[mdavlee]

Sounds like 026 talk

7900 can

 

[astnmacgto]

7900 can

I'd like to see that

 

[mdavlee]

I'd like to see that

It was fun. Griffonks owns one that did.

 

[astnmacgto]

It was fun. Griffonks owns one that did.

Was it heavily modded or just ported

 

[NCFarmboy]

EHP said pulling choke shut on 7900 lets you know port work is right.
Shep

 

[mdavlee]

Was it heavily modded or just ported

Hot woods port.

 

[MattG]

Ok, I've got a carburation question. See if any of you are gonna bite. My MS341, which is stock engine-wise, except for a muffler mod, runs beautifully....starts well, has ace response, idle, performance etc.

But the one annoying characteristic it has is that it is VERY sensitive to H screw adjustments. It will go from 12,900-13,500 in about 1/16 of a turn (if not less). So that's from running marginally fat to starting to sing at no load WOT. I know this since yesterday I was giving it a last tune before selling it on to my neighbour, and I wanted to be sure that he'll have good performance, and I'd have peace of mind that he won't burnt it out. I did get it settled somewhere between so that it still fourstroked enough for deaf old me to tell, but I wondered it you guys have any ideas what causes this feature, and if there are easy workarounds.

For more info, the saw is air-tight, has v. good piston and jug etc. and I recently serviced (diaphrams, needle, spring etc.) the stock carb (except for it's trimmed limiter caps). And we live at sea level.

Any ideas?

 

[exSW]

Gas

 

[MattG]

Gas

?
Kindly elaborate please dear sir....