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

[XP_Slinger]

The air bleed/transition holes are an area of dual function in the carb. Not only do they provide the 'transition' of the fuel as the throttle opens, they also provide the air bleed for the idle port - that is, they combine air with the fuel flow to create the idle mixture.

This is the hardest part of tuning a Walbro type carb, trying to get the idle and transition to work.

If you need to lean out the idle, you can drill one or both of the transition holes. If you drill the hole furthest from the idle port it will lean out the initial transition. If you drill out the hole closest to the idle port it will tend to make the initial opening a bit richer.

It doesn't take much to make a big change in the transition holes. Air will flow through a hole is a lot easier than fuel going through it. Going up one size on the drill size can make a big difference - in other words, go slowly and have a bunch of welch plugs handy to keep popping them back in.

So it made sense to plug the hole closest to the idle port in an attempt to lean the transition mix. Just making sure I’m following correctly.

 

[Terry Syd]

Yeah, that would lean it out as the throttle plate moved forward. However, I tend to think there is something else in play. I wish I had some experience with that kind of carb, all I can refer to is concepts of tuning a carb.

I'd be more inclined to keep the air bleed/transition holes stock and go from there.

When you finally get the bugger running good, it would be informative to pop a modded Zama on the saw and see what works better on a work saw.

I've run around in circles sometimes, most of my 'learning' has been copping the mistakes and figuring out what I did wrong.

 

[XP_Slinger]

Yeah, that would lean it out as the throttle plate moved forward. However, I tend to think there is something else in play. I wish I had some experience with that kind of carb, all I can refer to is concepts of tuning a carb.

I'd be more inclined to keep the air bleed/transition holes stock and go from there.

When you finally get the bugger running good, it would be informative to pop a modded Zama on the saw and see what works better on a work saw.

I've run around in circles sometimes, most of my 'learning' has been copping the mistakes and figuring out what I did wrong.

I agree that something else is going on that hasn’t become apparent to me yet. With all the fuel I’ve taken away I can’t believe it’s still rich in the mid range. Kinda why I had the thought to go back to the main jet...I’m missing something. Still trying to find a source for bulk Welch plugs.

What model Zama should I be looking for if it comes down to it?

 

[Terry Syd]

Just the 3CM with the 'twin jets'. Some of the early Zamas didn't have the twin jets next to each other, but if you open up the 'wet side' it is obvious when there are two jets, one on each side of the welch plug.

Poleman has the most experience modding Zamas, he's modded Zama carbs for 45cc-84cc. If you can find a Zama twin jet, just tell him what you've got and he can dial it in.

 

[XP_Slinger]

Just the 3CM with the 'twin jets'. Some of the early Zamas didn't have the twin jets next to each other, but if you open up the 'wet side' it is obvious when there are two jets, one on each side of the nozzle.

Poleman has the most experience modding Zamas, he's modded Zama carbs for 45cc-84cc. If you can find a Zama twin jet, just tell him what you've got and he can dial it in.

Thanks brother

I’m finding a Tillotson also, perhaps another viable option

 

[Terry Syd]

Never even had a Tillotson in my hands. Perhaps it could be better carb, but I can't comment on it.

I do wish that Walbro had gone to 'best practices' and adopted the 'idle/transition' circuit of the Zama and Mikuni on the idle/transition circuit. Crikey, carburetion technology has been around for more than 100 years, you would think that Walbro could have adopted it.

Perhaps they might do that, OR, Zama decides to make bit larger carb for the bigger saws.

 

[XP_Slinger]

Never even had a Tillotson in my hands. Perhaps it could be better carb, but I can't comment on it.

I do wish that Walbro had gone to 'best practices' and adopted the 'idle/transition' circuit of the Zama and Mikuni on the idle/transition circuit. Crikey, carburetion technology has been around for more than 100 years, you would think that Walbro could have adopted it.

Perhaps they might do that, OR, Zama decides to make bit larger carb for the bigger saws.

Yeah this carb has me a bit frustrated at the moment. I’m going to try try blocking all the transition circuits and see how it runs with just idle fuel and main jet fuel. If it still doesn’t run right I tend to think it’s a venturi velocity issue, not atomizing properly. Just a theory based on how the carb was designed to function with a divided bore. I’ll report back over the weekend.

 

[Terry Syd]

Welcome to the 'carb modding' fraternity, it will drive you nuts at times. When you get it right, it feels like the light of God has shown down on your pathetic efforts and blessed you with a bit of comfort - just to keep you trying another round.

Good luck....

 

[XP_Slinger]

Welcome to the 'carb modding' fraternity, it will drive you nuts at times. When you get it right, it feels like the light of God has shown down on your pathetic efforts and blessed you with a bit of comfort - just to keep you trying another round.

Good luck....

Lol! Yep, sounds about right

 

[XP_Slinger]

Reviving this thread for an update.

Had my saw out working a couple days ago. All seemed normal until it suddenly started running pig rich. I’m thinking a lead plug fell out somewhere. I’m done with the RWJ4, just ordered a Zama 3CM based on the recommendations in this thread. Will post updates once I receive it and get it dialed in.

 

[Steve]

Reviving this thread for an update.

Had my saw out working a couple days ago. All seemed normal until it suddenly started running pig rich. I’m thinking a lead plug fell out somewhere. I’m done with the RWJ4, just ordered a Zama 3CM based on the recommendations in this thread. Will post updates once I receive it and get it dialed in.

You will be glad you did!

 

[dustinwilt68]

Hd12 FTW

 

[Wonkydonkey]

I'.m not about to read all the previous pages, I will later thou.
but while watching yt I saw that iron mule was fitting one to buckings sons saw 266\272 .. he mentioned something about the plating coming off as well

I found it boring af... I think I skipped most of it, I never saw it run or heard if it was all fixed...

I,m not saying go and see it, it’s that I was wondering if someone has been reading this thread

 

[XP_Slinger]

Hd12 FTW

I believe you Dustin, Joes XPW has always run great with that carb. My curiosity about the Zama won...we’ll see how it goes.

 

[XP_Slinger]

I'.m not about to read all the previous pages, I will later thou.
but while watching yt I saw that iron mule was fitting one to buckings sons saw 266\272 .. he mentioned something about the plating coming off as well

I found it boring af... I think I skipped most of it, I never saw it run or heard if it was all fixed...

I,m not saying go and see it, it’s that I was wondering if someone has been reading this thread

Using a Zama or the RWJ4? I won’t watch his video, dude makes me mentally vomit lol

 

[Lightning Performance]

Hi guys. Someday I might read this thread but the last page here says a lot.
These divided carbs I've never played with but the transition problems are created by the velocity issues of not dividing the carbs. High pressure and flow on the outside turn. The short side is damn near dead so they must run pig rich to have a decent delivery. Working backwards from there is great but you need to double the flow through the the carb undivided to overcome the pig rich condition. Going down a carb size, removing the divider and porting the power head should be much closer to something that has a good fuel curve. Drilling the hell out of the opposite side of the the butterfly from the transition circuits should get you there. I think it could be like a TS400 carb with that big dumb hole in the butterfly. I think it was that carb... I know one of the Stihls was smaller and richer with a bypass hole in the throttle plate.
The opposite is normally my problem, lean after mods on Stihls Cutting a slot between the bleed port faces sometimes helps me to smooth out transition stumbles as you throttle up. Seems to add a smooth rich curve or it goes in the parts pile to live another life.
It must be pia to test with all that raw fuel choking you out, yuck.

 

[huskyboy]

I had to put a bypass hole in the throttle plate on a 044 zama in order to get it to work and tune right on the L side for a ported 460. On the flip side I had to port a 440 to get it to run right with a HD16 off a 460 on it. Sometimes it is best to run what was designed for the saw in the first place, and that is an HD12 for the 372’s. Leave the RWJ on the Xtorq’s. They are too fussy for a worksaw on other stuff. If I was gonna swap a carb onto a 372 it would be a 385, 390 or 395 carb not the rwj. They tune way better than the rwj.

 

[XP_Slinger]

I had to put a bypass hole in the throttle plate on a 044 zama in order to get it to work and tune right on the L side for a ported 460. On the flip side I had to port a 440 to get it to run right with a HD16 off a 460 on it. Sometimes it is best to run what was designed for the saw in the first place, and that is an HD12 for the 372’s. Leave the RWJ on the Xtorq’s. They are too fussy for a worksaw on other stuff. If I was gonna swap a carb onto a 372 it would be a 385, 390 or 395 carb not the rwj. They tune way better than the rwj.

That’s the point I‘ve come to. I converted my XT saw to OE and at the time it was common practice to run the XT carb. Having never been happy with it I decided to try to fix it for science and stuff. Can’t learn if you don’t try kind of thing. A positive Outcome for me is gaining a much better understanding of how these little carbs work. Had lots of smart dudes chime in and help me grasp the where and why of things. I’m still no expert but I’m a lot better than I was.

 

[XP_Slinger]

Long overdue update on this thread. Installed the Zama C3M and my saw finally acts like it should. No problems whatsoever with wandering tune or fat mix in the mid range. I do however feel like the saw lost a little bit of power so I’m going to do a little modding for more fuel. I don’t think it will take much. More to follow...

 

[Steve]

Long overdue update on this thread. Installed the Zama C3M and my saw finally acts like it should. No problems whatsoever with wandering tune or fat mix in the mid range. I do however feel like the saw lost a little bit of power so I’m going to do a little modding for more fuel. I don’t think it will take much. More to follow...

They will due to the smaller venturi over the walbro HD12. For me being a firewood hack ill take reliability over max performance. But in production, time is money.

Thinning or half-shafting the throttle and choke helps the c3m breathe better.