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

[XP_Slinger]

I need a pair of tweezers in the barn to pick these little drill bits out of their case.

 

[beaglebriar]

What cylinder are you going with?

 

[XP_Slinger]

What cylinder are you going with?

Since the XPW cylinders are NLA I’m going with an OE 50mm.

 

[MG2186]

I have 7 different saws with RWJ’s and none of the jets have been drilled. The dividers in the air filter are gone. They are a bit sensitive to keep adjusted in tune though, best be having a screw driver with you. My experience with them has been how Kevin explained it, in short cuts they do act rich until they get good and warm then come right out of 4 stroking under load

 

[XP_Slinger]

I have 7 different saws with RWJ’s and none of the jets have been drilled. The dividers in the air filter are gone. They are a bit sensitive to keep adjusted in tune though, best be having a screw driver with you. My experience with them has been how Kevin explained it, in short cuts they do act rich until they get good and warm then come right out of 4 stroking under load

I’ve done divider in and out, in seemed to be more stable with it in but it never settled safe enough for me to trust it. I might flood her out good after going rogue with those tiny drill bits...lol! Never know til you try

 

[XP_Slinger]

And I flooded it like a boss! High end was good but mid range bogs bad. Going to commence to filling the other jets with lead and re-drilling them to proper size. Have to wait for a new micro bit set to arrive, the set I have doesn’t go small enough.

Got the new cylinder ported today.

 

[huskyboy]

Nuffin like a good ole HD or C3m for a worksaw I guess. Less screwing around... literally. Lol

 

[XP_Slinger]

Nuffin like a good ole HD or C3m for a worksaw I guess. Less screwing around... literally. Lol

Meh, I shot myself in the foot. Drilled too many things lol.

 

[Steve]

Nuffin like a good ole HD or C3m for a worksaw I guess. Less screwing around... literally. Lol

+1 for c3m!!

 

[Steve]

Meh, I shot myself in the foot. Drilled too many things lol.

Ive got a brand new zama clone carb here if you want it.

 

[huskyboy]

+1 for c3m!!

They’ve always worked pretty good for the most part for me on 372/7900 series saws. The HD or a WJ/tilly does make a little more power/torque, but you don’t notice it unless you swap the carbs back to back or if your saw is modified. I like them all better than the rwj for sure though.

 

[XP_Slinger]

Ive got a brand new zama clone carb here if you want it.

Thanks, but I have to experiment with this.

 

[huskihl]

 

[beaglebriar]

Been running the 296 carb on my XPW the last couple weekends. Right now it's a little too much fuel on a cold start and blubbers for a bit until it catches up. Once warm it runs perfect. I think it'll be just right after port work. Haven't swapped over to the RWJ yet.

 

[XP_Slinger]

Ok, bear with me as I walk through what I did and what I learned.

Original problem - wandering tune with stock RWJ-4 carb on an OE 372 cylinder (non X-Torq). Tune would get leaner and leaner and could be tuned safe with both L and H screw out 3 to 5 turns depending on the time of year. With the jets way out, the next cold start would be pig rich and I would have to chase the tune all over again. Something just wasn’t right. What I needed was more fuel supplied to the jet screws.

So for science and stuff I did some experimenting based on advice received in here from some of the best carburetor whisperers. I also took things a step further in an effort to give it more fuel at every throttle position.

So here’s what I learned...

Blue arrow - Main Jet
Yellow arrow - Second Stage Transition Supply
Red arrow - Supply to Welch plug area (green)
Green arrow - Normally covered by Welch plug, contains L screw circuit supply and first stage transition (2 small holes in the middle)




Main jet was marked “51”, I drilled it to .6mm.
Welch plug feed jet marked “38”, I drilled it to .5mm
Second stage transition jet was marked “30”, I drilled to .5mm. Thought to myself “you want fuel? I’ll give you fuel you sombitch.” Lol!

It’s no surprise that when I put the carb back on that it was Pig and I mean PIG rich at half throttle. The first and second stage transition circuits feed directly into the bore of the carb under part throttle before the main jet takes over. First stage under Welch plug, second stage yellow arrow. Both first and second stage transition circuits are off shoots of the L jet circuit. So drilling those jets drastically bigger, flooded the saw out at anything other than idle or full throttle. Which makes sense because the transition circuits almost turn off at WOT because of the loss of vacuum. WOT is 100% main jet (H) and very little idle fuel (L) as far as I understand it.

I’ve gotten some ideas of how to salvage this from a friend, more to follow.

 

[mdavlee]

Ok, bear with me as I walk through what I did and what I learned.

Original problem - wandering tune with stock RWJ-4 carb on an OE 372 cylinder (non X-Torq). Tune would get leaner and leaner and could be tuned safe with both L and H screw out 3 to 5 turns depending on the time of year. With the jets way out, the next cold start would be pig rich and I would have to chase the tune all over again. Something just wasn’t right. What I needed was more fuel supplied to the jet screws.

So for the good of the community I did some experimenting based on advice received in here from some of the best carburetor whisperers. I also took things a step further in an effort to give it more fuel at every throttle position.

So here’s what I learned...

Blue arrow - Main Jet
Yellow arrow - Second Stage Transition Supply
Red arrow - Supply to Welch plug area (green)
Green arrow - Normally covered by Welch plug, contains L screw circuit supply and first stage transition (2 small holes in the middle)


View attachment 226031

Main jet was marked “51”, I drilled it to .6mm.
Welch plug feed jet marked “38”, I drilled it to .5mm
Second stage transition jet was marked “30”, I drilled to .5mm. Thought to myself “you want fuel? I’ll give you fuel you sombitch.” Lol!

It’s no surprise that when I put the carb back on that it was Pig and I mean PIG rich at half throttle. The first and second stage transition circuits feed directly into the bore of the carb under part throttle before the main jet takes over. First stage under Welch plug, second stage yellow arrow. Both first and second stage transition circuits are off shoots of the L jet circuit. So drilling those jets drastically bigger, flooded the saw out at anything other than idle or full throttle. Which makes sense because the transition circuits almost turn off at WOT because of the loss of vacuum. WOT is 100% main jet (H) and very little idle fuel (L) as far as I understand it.

I’ve gotten some ideas of how to salvage this from a friend, more to follow.

The transition jet and under the Welch plug you went over 82% in size gain is where the problem was. .51 to .6 wasn’t too bad. About 38% more fuel. The one was 167%. You might be ready for some methanol/nitro on that carb. There’s a few tricks to close them off and start over.

 

[XP_Slinger]

The transition jet and under the Welch plug you went over 82% in size gain is where the problem was. .51 to .6 wasn’t too bad. About 38% more fuel. The one was 167%. You might be ready for some methanol/nitro on that carb. There’s a few tricks to close them off and start over.

Yup you’re right on the money Mike. Explanation to follow with a pic for those of us that are not so carb savvy. Long story short I fixed it

 

[XP_Slinger]

As Mike stated above, I went way too far on the transition circuit jets.

So to dial it back from the extreme rich condition that I created, I plugged the second stage transition circuit jet with lead. Remember, this jet leads directly to the bore with no metering except for the size of the hole in the jet. This idea didn’t come to me on my own, @Stihlbro really helped me grasp how these circuits work and also suggested that blocking this jet might be a fix. Thanks Joey!

If you look closely you can see the small plug of lead.



So at this point, I have more fuel going to the Main and more fuel being supplied to the L circuit under the Welch plug. Time to try it out

Here is the result. VERY stable tune compared to before drilling things out. H and L adjustment screws are at 1.25 and 1.5 turns out. Initial tune was 13.8k and it only walked up to 14.1k after getting good and hot. HUGE improvement over its previous performance!

Only 1 question remains in my mind and that is...Did I create a lean condition at part throttle by removing that second stage of transition? My assumption is no because of the extra fuel going to the first stage transition. The saw certainly seems happy, holding tune better than it ever has.

In summary, I think the reliable fix for the RWJ-4 is drill the main to .6mm and drill the Welch plug supply jet to .5mm. Leave the secondary transition circuit jet alone! lol! Once my new smaller drill bits arrive I’m going to re-drill it to stock size (.3mm) and see how it likes it.

Many thanks owed to those with experience that offered help on this. Cheers gents

Side note...my new 50mm top end feels great. Don’t think I’ll be missing the 51.4 unless I’m cutting bigger stuff...and that’s what the 288 is for lol

 

[Lightning Performance]

Ok, bear with me as I walk through what I did and what I learned.

Original problem - wandering tune with stock RWJ-4 carb on an OE 372 cylinder (non X-Torq). Tune would get leaner and leaner and could be tuned safe with both L and H screw out 3 to 5 turns depending on the time of year. With the jets way out, the next cold start would be pig rich and I would have to chase the tune all over again. Something just wasn’t right. What I needed was more fuel supplied to the jet screws.

So for the good of the community I did some experimenting based on advice received in here from some of the best carburetor whisperers. I also took things a step further in an effort to give it more fuel at every throttle position.

So here’s what I learned...

Blue arrow - Main Jet
Yellow arrow - Second Stage Transition Supply
Red arrow - Supply to Welch plug area (green)
Green arrow - Normally covered by Welch plug, contains L screw circuit supply and first stage transition (2 small holes in the middle)


View attachment 226031

Main jet was marked “51”, I drilled it to .6mm.
Welch plug feed jet marked “38”, I drilled it to .5mm
Second stage transition jet was marked “30”, I drilled to .5mm. Thought to myself “you want fuel? I’ll give you fuel you sombitch.” Lol!

It’s no surprise that when I put the carb back on that it was Pig and I mean PIG rich at half throttle. The first and second stage transition circuits feed directly into the bore of the carb under part throttle before the main jet takes over. First stage under Welch plug, second stage yellow arrow. Both first and second stage transition circuits are off shoots of the L jet circuit. So drilling those jets drastically bigger, flooded the saw out at anything other than idle or full throttle. Which makes sense because the transition circuits almost turn off at WOT because of the loss of vacuum. WOT is 100% main jet (H) and very little idle fuel (L) as far as I understand it.

I’ve gotten some ideas of how to salvage this from a friend, more to follow.

That sounds really close to where this BB Dolkita needs to go. It is lean real bad anywhere off idol. Nothing has helped with carb swaps. It wants more. Gets hot quick even in cold weather and won't fourstroke. Starts out set all fat....runs good for three cuts and goes way lean. Stop for a minute....try again and it stumbles and then revs to the moon.

 

[XP_Slinger]

That sounds really close to where this BB Dolkita needs to go. It is lean real bad anywhere off idol. Nothing has helped with carb swaps. It wants more. Gets hot quick even in cold weather and won't fourstroke. Starts out set all fat....runs good for three cuts and goes way lean. Stop for a minute....try again and it stumbles and then revs to the moon.

Sounds EXACTLY like what my saw was doing except for the stumbling part. Frustrating as all hell