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

[XP_Slinger]

My 371 is running the RWJ without issue. Takes patience to tune for sure though and it seems more sensitive to temp change than other saws.

Mine seems to always want more fuel. Somewhat stable tune is 3 turns on the H and near the same on the L. Pull muffler and the exhaust port is soaking wet. It’s bizarre, and I’m open to suggestions if that sounds like I’ve got other problems. It acted a little funny with the old XT top end also but nearly as bad as it does now.

 

[Lightning Performance]

Not dependably without making some sort of mount to hold the carburetor down and provisions for a V stack and a different type of air filter. The stock Dolmar air filter needs to be carved up and it still pinches the impulse and hits the high idle mechanism

Okay. How about moving everything toward the rear. A mounted new mid plate should do.
I was thinking about this 6401 Makita I have sitting in pieces. The bottom end is good. Have plenty of good open port 47mm oe jugs.

I can find better jugs later or go bigger but that's no fun.
The prettier Makita 6401 is getting a new 7910 or near new 7900 jug and leaving the carb stock for using with a 28" bar.
The other saw has an 8400 kit I'm still running in. It needs bigger transfers and more intake.

Be nice to have a fat carb on a small top end. The intake boot might need to move up bigger. What do you think?


I'm just learning about this type of carb and sizes on saws

 

[mdavlee]

Okay. How about moving everything toward the rear. A mounted new mid plate should do.
I was thinking abut this 6401 Makita I have sitting in pieces. The bottom end is good. Have plenty of good open port 47mm oe jugs.

I can find better jugs later or go bigger but that's no fun.
The prettier Makita 6401 is getting a new 7910 or near new 7900 jug and leaving the carb stock for using with a 28" bar.
The other saw has an 8400 kit I'm still running in. It needs bigger transfers and more intake.

Be nice to have a fat carb on a small top end. The intake boot might need to move up bigger. What do you think?

390 carb fits with a little carving on the filter box and air filter underneath. Choke doesn’t work but that never concerned me.

 

[mdavlee]

I never had much luck with the RWJ on any 7900 besides Matt’s. They would barely tune at 4-5 turns out.

 

[huskyboy]

I never had much luck with the RWJ on any 7900 besides Matt’s. They would barely tune at 4-5 turns out.

Drilled zama or stock HD6/12 runs great on them. I’m riding that line with my ported 7900 and stock jetted zama with mild mods to the carb... about 2 turns out on the jets. I was sucking the choke shut on jacks 7900/046 with the drilled zama, had to wind the choke spring tighter, that’s a nasty saw with a long bar.

 

[mdavlee]

Drilled zama or HD6/12 runs great on them. I’m riding that line with my ported 7900 and stock jetted zama with mild mods to the carb... about 2 turns out on the jets. I was sucking the choke shut on jacks 7900/046 with the drilled zama, had to wind the choke spring tighter, that’s a nasty saw with a long bar.

There’s been a few I did like that especially if you run the HD filter and restricted flow a little more.

 

[XP_Slinger]

FWIW...here’s a video of my saw from about a year ago. Listen to how it gradually leans out between cuts. Tune is a little on the fat side to start which is where I like them for work. This has been bugging me since the top end swap, with everything I’ve replaced with no improvement I can‘t think of anything left besides the carb that’s possibly to blame. Hence my question about a swap. Don’t mind the times, I used this video to bust my younger brothers chops about getting a faster saw after our shootout.

 

[beaglebriar]

Mine seems to always want more fuel. Somewhat stable tune is 3 turns on the H and near the same on the L. Pull muffler and the exhaust port is soaking wet. It’s bizarre, and I’m open to suggestions if that sounds like I’ve got other problems. It acted a little funny with the old XT top end also.

Experimenting with other carbs is certainly a good place to start. Have you kitted the RWJ? I assume you already switched to an unlimited coil?

 

[XP_Slinger]

Experimenting with other carbs is certainly a good place to start. Have you kitted the RWJ? I assume you already switched to an unlimited coil?

I haven’t put a kit in it, maybe it’s time. Bought the saw new in 2014...the years go by fast.

Switched to unlimited a few years ago when it had a ported XT top end, had to retune for the different advance but held pretty steady after that.

 

[beaglebriar]

I haven’t put a kit in it, maybe it’s time. Bought the saw new in 2014...the years go by fast.

Switched to unlimited a few years ago when it had a ported XT top end, had to retune for the different advance but held pretty steady after that.

Hard to say if a kit will make a lick of difference but it's worth a shot for 5-6 bucks.

 

[huskyboy]

Ported 2171 with great manners. Drilled rwj .

 

[XP_Slinger]

Hard to say if a kit will make a lick of difference but it's worth a shot for 5-6 bucks.

You’re right, just ordered one. Who knows, maybe some crap got into the fuel passages under the Welch plug.

 

[XP_Slinger]

I’m beginning to form a theory in my head regarding why the RWJ performed ok on the XT top end vs the XPW. Relates to why drilling is said to fix tuning issues. Still thinking on it, don’t know if it’s worth sharing yet lol

 

[beaglebriar]

@Terry Syd could be some help here.

 

[XP_Slinger]

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.

 

[dustinwilt68]

I’m no stickler on grammar, but I can’t follow your punctuation. Which do you despise and which work well?

I like them all except the RJW4.

 

[XP_Slinger]

I like them all except the RJW4.

I wasn’t sure either, thought you hated them all...lol! Was like “What’s left?” Lmao

 

[beaglebriar]

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. Or half of it’s total Venturi area which is somewhere slightly north of 19mm if I recall correctly.

Well you better get drilling! For science and stuff. Maybe start smaller than Kev did if possible.

 

[Lightning Performance]

What carb is cheapest? ...say 18-19mm?

 

[XP_Slinger]

What carb is cheapest? ...say 18-19mm?

Kinda want to figure this out. I know some guys have already to an extent but it still seems to be hit and miss. I’m under no delusion that I’m smarter on this carb crap than anyone else but I want to try now that I’ve gotten some info.