372XP/OE - Fix RWJ4 or Swap for 390XP Carb?

[huskihl]

Well here goes. The RWJ has a large venturi but in its original application on the XT saw only half of that Venturi was used to deliver atomized fuel to the engine through a VERY small intake port/tract.

Now, put the RWJ in an intake tract that is not divided for fresh air strato stuff and you’ve essentially doubled the volume of air that is mixing with the same amount of fuel as it passes through the Venturi at lower velocity...which is another aspect I’m thinking about. Being lower velocity, it’s likely not the same amount of fuel. Yes, my divider is in place in the carb, but after the divider is where I think the fuel/air ratio is getting screwed up and causing the saw to eventually go lean as it gets hotter and hotter.

I’m thinking the fuel jets in the RWJ are designed to deliver fuel to half the volume of air that my saw is pulling through the Venturi because there is no separation between the fuel side and the strato side as it is on a stock XT saw.

I’m sure there are some details I’m overlooking, but that’s where my brain is at right now. Still thinking through some velocity vs volume stuff in the intake tract but I think I’m understanding why fuel mods need to be made to make this carb play nice with non-strato saws.

This thinking is leading me to one verdict...the large Venturi of the RWJ is a deceitful SOB and might not lead to “gains” as easy many of us think it does. It is essentially designed to deliver fuel using a 9.75mm venturi with higher velocity. Or half of it’s total Venturi area which is somewhere slightly north of 19mm if I recall correctly.

Your saw doesn’t continually keep leaning out though once you’ve made a half dozen big cuts like that though, does it? If it did, then I would think you had a problem. Anytime you add heat, it requires more fuel. No different than tuning for a short bar versus turning for a long bar

 

[huskihl]

I am in no way saying that the RWJ is better than the others. If I had my choice, I would use a 390 or 395 carb for a long bar saw. If it was a short bar saw then I might opt for the RWJ. If I had one or the other, that’s what I would use and not buy the other. It just never really made that much difference to me

 

[XP_Slinger]

Your saw doesn’t continually keep leaning out though once you’ve made a half dozen big cuts like that though, does it? If it did, then I would think you had a problem. Anytime you add heat, it requires more fuel. No different than tuning for a short bar versus turning for a long bar

No it seems to settle at what I consider a play tune. Kinda difficult to put to words. Did you watch the video I posted? You can hear it progressively lose the healthy 4 stroke between each cut.

 

[XP_Slinger]

I am in no way saying that the RWJ is better than the others. If I had my choice, I would use a 390 or 395 carb for a long bar saw. If it was a short bar saw then I might opt for the RWJ. If I had one or the other, that’s what I would use and not buy the other. It just never really made that much difference to me

I hear ya Kev, didn’t take any of your posts that way. I’m in agreement with you, I’d like to get this thing working rather than buy another carb...for now. Lol

 

[huskihl]

No it seems to settle at what I consider a play tune. Kinda difficult to put to words. Did you watch the video I posted? You can hear it progressively lose the healthy 4 stroke between each cut.

Yeah I watched it. Like I said though, I think you’re just adding heat. After a half dozen cuts like that the heat should stabilize and so should the tune.

 

[XP_Slinger]

Yeah I watched it. Like I said though, I think you’re just adding heat. After a half dozen cuts like that the heat should stabilize and so should the tune.

I’ll pull the carb apart and do some drilling. Hopefully I’ll have some positive results to report back. Thanks for the help.

 

[mdavlee]

I think the jet is a .49 or .52mm. I think a .55 is a good start if I remember right. If you go too big smash some lead back in it and go smaller.

 

[Fruecrue]

I put an RWJ on the 7900 stroker.
Too lean at first, even with the screws backed out as far as possible.
I ended up drilling the jets with success. Ended up tuning in the 2 turns out range.
May or may not have notes on what I did in the shop.
If I recall, much like many other carbs, there are parallel circuits (2 each) for both high and low feeds, one each fixed, one each adjustable. I believe I just used the next size up micro drill on each of the fixed jets. This allows more flow through those circuits, therefore less is needed through the adjustable circuits.

 

[XP_Slinger]

I put an RWJ on the 7900 stroker.
Too lean at first, even with the screws backed out as far as possible.
I ended up drilling the jets with success. Ended up tuning in the 2 turns out range.
May or may not have notes on what I did in the shop.
If I recall, much like many other carbs, there are parallel circuits (2 each) for both high and low feeds, one each fixed, one each adjustable. I believe I just used the next size up micro drill on each of the fixed jets. This allows more flow through those circuits, therefore less is needed through the adjustable circuits.

Sounds just like The problem I’m having. Thanks Dave

 

[XP_Slinger]

I think the jet is a .49 or .52mm. I think a .55 is a good start if I remember right. If you go too big smash some lead back in it and go smaller.

Thanks Mike. Good to know a rough starting point.

 

[Lightning Performance]

You need Toon Dip with Benzene...the good stuff

I’ll pull the carb apart and do some drilling. Hopefully I’ll have some positive results to report back. Thanks for the help.

 

[RIDE-RED 350r]

Ummmm....wanna try my carb?? For scienses n stuffs?

 

[XP_Slinger]

Ummmm....wanna try my carb?? For scienses n stuffs?

If I can’t get mine to improve I will absolutely take you up on that offer. Thanks Joe

 

[RP81]

How does one tell which is on your saw?

 

[XP_Slinger]

How does one tell which is on your saw?

Most immediately obvious thing too me is the jet adjustment screws. On the RWJ4 they are flush with the carb body with spring loaded metal rings around them. Limiting of the screw adjustment range is also an internal mechanism. On the HD12B the jet limiters are plastic and the jets screws stick out of the carb body. I’m sure there are more visible differences but this is what comes to my mind as a simple way to tell.

Also, if your saw is an X-Torq model it will have the RWJ4 unless someone has swapped it out.

 

[Stump Shot]

Well here goes. The RWJ has a large venturi but in its original application on the XT saw only half of that Venturi was used to deliver atomized fuel to the engine through a VERY small intake port/tract.

Now, put the RWJ in an intake tract that is not divided for fresh air strato stuff and you’ve essentially doubled the volume of air that is mixing with the same amount of fuel as it passes through the Venturi at lower velocity...which is another aspect I’m thinking about. Being lower velocity, it’s likely not the same amount of fuel. Yes, my divider is in place in the carb, but after the divider is where I think the fuel/air ratio is getting screwed up and causing the saw to eventually go lean as it gets hotter and hotter. Also explains why the jets are open so many turns...can’t supply enough fuel unless they’re WTFO. And even so they still can’t keep up.

I’m thinking the fuel jets in the RWJ are designed to deliver fuel to half the volume of air that my saw is pulling through the Venturi because there is no separation between the fuel side and the strato side as it is on a stock XT saw.

I’m sure there are some details I’m overlooking, but that’s where my brain is at right now. Still thinking through some velocity vs volume stuff in the intake tract but I think I’m understanding why fuel mods need to be made to make this carb play nice with non-strato saws.

This thinking is leading me to one verdict...the large Venturi of the RWJ is a deceitful SOB and might not lead to “gains” as easy many of us think it does. It is essentially designed to deliver fuel using a 9.75mm venturi with higher velocity. Or half of it’s total Venturi area which is somewhere slightly north of 19mm if I recall correctly.

The top half has no venturi at all, just straight through. So leaving the divider in is most likely the way to go and the only way I've tried it. Left the elbow divider in as well. It's all going to get mixed up in the crankcase anyways. About a turn and a half out seems to work for me set up in this way, if it's any help.
Here's a vidja set up in this manor.
Edit: It's not been drilled either.

 

[XP_Slinger]

The top half has no venturi at all, just straight through. So leaving the divider in is most likely the way to go and the only way I've tried it. Left the elbow divider in as well. It's all going to get mixed up in the crankcase anyways. About a turn and a half out seems to work for me set up in this way, if it's any help.
Here's a vidja set up in this manor.
Edit: It's not been drilled either.

Great info Steve. It seems there’s lots of ways to skin this cat. Nice saw

 

[Stump Shot]

Great info Steve. It seems there’s lots of ways to skin this cat. Nice saw

It might be useless information to you, but I thought I would throw it out there just in case you could get something out of it.

 

[XP_Slinger]

It might be useless information to you, but I thought I would throw it out there just in case you could get something out of it.

I don’t know what is useful/useless yet. Everything helps at this point

 

[XP_Slinger]

Well I drilled some stuff. Just waiting for my new top end to arrive so I can put it back together.

Drilled out the main jet and the two low speed jets. At least thats what I think the other two brass plugs are lol.