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

[Terry Syd]

My server has been down for more than three weeks, otherwise I would have posted earlier. I was writing this post in reference to Beaglebriar’s attempt to get my attention when the server went down. With me off-line for weeks, I had some extra time to expand on the post. -

I also dislike Walbros. I came to that conclusion when I decided to mod the HD-5 for use on my 64cc 029 – never modded a Walbro, it would be a good learning experience. Crikey, I ended up modding EVERY circuit, jet and air bleed hole in the carb, changed the metering spring and also blocked off two circuits and added another, it finally dialled in to be a strong running carb, but it still doesn't have the smooth idle like a Zama twin jet.

The Zama is a superior designed carb. Each function can be modded separately from the other functions. The idle screw simply changes the mixture of the idle, the twin jets can be drilled to change the low speed flow and tweaking the metering spring cleans up any throttle response issues.

The Walbro circuits affect each other. If you lean out the idle, you also lean out the transition/response and also the total low speed flow. Change one thing and you will be changing others, you can spend a lot of time and still end up with a compromise.

Further, the Zama has a superior designed idle circuit that ensures a more uniform mixture to the idle port.

It is worth noting the Husky factory engineers removed the HD-12 and put a modded Zama on their factory ported 372. Those engineers have access to dynos and a lot experience behind them to come to that decision. Significantly, they also incorporated a modded air filter on the saw, in other words, the Zama wasn’t the restriction in the intake tract, the air filter was the restriction. Most of these saws have inadequate air filters, if you want more top end flow then do a high speed check with the air filter on and one without the air filter. If you pick up some revs without the air filter, you may want to look into a different air filter.

I have a 365 with a ported 77cc BB kit on it, I run a modded Zama. Smooth idle, gobs of torque off the bottom, strong torque curve and it likes to cut around 12,000 - great work saw. It makes enough power that I didn’t bother with a high flow filter.

The biggest carb is not always the best carb. The function of the carb is to properly meter the fuel, if it has problems doing its basic function, then IMO it really isn't fit for that function.

I wish Mikuni made diaphragm carbs for saws, the Mikuni carbs for the boats and aircraft engines are designed far superior than the Zamas. However, a modded Zama twin jet is IMO the next best thing and that's what I would recommend. Perhaps Poleman is still modding Zamas that people send to him.

Which brings up another issue, the availability of the Zamas. The Chinese source has since dried up and the Zamas are getting more difficult to find. Perhaps the HD-12 can be modded for more performance. It appears that the air bleed/transition holes are in the ballpark, if so, then just a couple of extra mods should get the carb performing even stronger.

The first mod is to disable the EPA circuit from the main discharge tunnel to the low speed circuit. Several blokes on the forum are already doing this mod and could probably post some pictures. With the EPA circuit blocked off, then a 1mm hole needs to be drilled in the wet side to provide fuel to the low speed needle.

Then the next mod would be drilling the auxiliary jet to take it out from the stock size of .25mm to whatever works. If someone has already found the optimum jet size (say .35mm for a 71cc engine) that would save others from having to do some testing. If not, then if two HD-12s were modded in steps (.3mm test and compare, then .35mm in the next carb to compare to the .3mm, etc.) – Then finally shim the metering spring for throttle response and go cut wood.

RWJ
I’ve never worked on a RWJ, but it appears from the photo of the wet side that the carb doesn’t have the EPA circuit from the main discharge nozzle tunnel. Instead it has a separate ‘limiter’ jet on the idle/transition circuit. – To find out, if the low speed screw adjusts the flow from that jet, then it is a ‘limiter’ jet (ie: no matter how far out the screw moves, the maximum low speed flow is limited by the jet). In which case, I’d go big on the jet and just use the low speed screw to adjust the flow. (the ‘limiter’ jet is an EPA issue)

The extra jet feeding into the straight section of the carb appears to be an auxiliary jet, usually set at .25mm on Zamas and HDs. You can slowly open that jet up to increase the total low speed flow. However, it may not be necessary if you open up the ‘limiter’ jet on the idle/transition circuit and can get enough low speed flow.

Here’s where my lack of experience with the RWJ brings me unstuck, I don’t know how well the carb will idle. That’s the problem with the Walbro series that I referred to earlier – tweaking the low speed needle affects three things; idle, transition and total low speed flow.

It may be that opening the auxiliary jet a bit to get a bit more low speed flow and then not opening the low speed needle as far may be a preferred option – I don’t know.

Thanks to Ray Benson for finding this thread for me.

 

[XP_Slinger]

My server has been down for more than three weeks, otherwise I would have posted earlier. I was writing this post in reference to Beaglebriar’s attempt to get my attention when the server went down. With me off-line for weeks, I had some extra time to expand on the post. -

I also dislike Walbros. I came to that conclusion when I decided to mod the HD-5 for use on my 64cc 029 – never modded a Walbro, it would be a good learning experience. Crikey, I ended up modding EVERY circuit, jet and air bleed hole in the carb, changed the metering spring and also blocked off two circuits and added another, it finally dialled in to be a strong running carb, but it still doesn't have the smooth idle like a Zama twin jet.

The Zama is a superior designed carb. Each function can be modded separately from the other functions. The idle screw simply changes the mixture of the idle, the twin jets can be drilled to change the low speed flow and tweaking the metering spring cleans up any throttle response issues.

The Walbro circuits affect each other. If you lean out the idle, you also lean out the transition/response and also the total low speed flow. Change one thing and you will be changing others, you can spend a lot of time and still end up with a compromise.

Further, the Zama has a superior designed idle circuit that ensures a more uniform mixture to the idle port.

It is worth noting the Husky factory engineers removed the HD-12 and put a modded Zama on their factory ported 372. Those engineers have access to dynos and a lot experience behind them to come to that decision. Significantly, they also incorporated a modded air filter on the saw, in other words, the Zama wasn’t the restriction in the intake tract, the air filter was the restriction. Most of these saws have inadequate air filters, if you want more top end flow then do a high speed check with the air filter on and one without the air filter. If you pick up some revs without the air filter, you may want to look into a different air filter.

I have a 365 with a ported 77cc BB kit on it, I run a modded Zama. Smooth idle, gobs of torque off the bottom, strong torque curve and it likes to cut around 12,000 - great work saw. It makes enough power that I didn’t bother with a high flow filter.

The biggest carb is not always the best carb. The function of the carb is to properly meter the fuel, if it has problems doing its basic function, then IMO it really isn't fit for that function.

I wish Mikuni made diaphragm carbs for saws, the Mikuni carbs for the boats and aircraft engines are designed far superior than the Zamas. However, a modded Zama twin jet is IMO the next best thing and that's what I would recommend. Perhaps Poleman is still modding Zamas that people send to him.

Which brings up another issue, the availability of the Zamas. The Chinese source has since dried up and the Zamas are getting more difficult to find. Perhaps the HD-12 can be modded for more performance. It appears that the air bleed/transition holes are in the ballpark, if so, then just a couple of extra mods should get the carb performing even stronger.

The first mod is to disable the EPA circuit from the main discharge tunnel to the low speed circuit. Several blokes on the forum are already doing this mod and could probably post some pictures. With the EPA circuit blocked off, then a 1mm hole needs to be drilled in the wet side to provide fuel to the low speed needle.

Then the next mod would be drilling the auxiliary jet to take it out from the stock size of .25mm to whatever works. If someone has already found the optimum jet size (say .35mm for a 71cc engine) that would save others from having to do some testing. If not, then if two HD-12s were modded in steps (.3mm test and compare, then .35mm in the next carb to compare to the .3mm, etc.) – Then finally shim the metering spring for throttle response and go cut wood.

RWJ
I’ve never worked on a RWJ, but it appears from the photo of the wet side that the carb doesn’t have the EPA circuit from the main discharge nozzle tunnel. Instead it has a separate ‘limiter’ jet on the idle/transition circuit. – To find out, if the low speed screw adjusts the flow from that jet, then it is a ‘limiter’ jet (ie: no matter how far out the screw moves, the maximum low speed flow is limited by the jet). In which case, I’d go big on the jet and just use the low speed screw to adjust the flow. (the ‘limiter’ jet is an EPA issue)

The extra jet feeding into the straight section of the carb appears to be an auxiliary jet, usually set at .25mm on Zamas and HDs. You can slowly open that jet up to increase the total low speed flow. However, it may not be necessary if you open up the ‘limiter’ jet on the idle/transition circuit and can get enough low speed flow.

Here’s where my lack of experience with the RWJ brings me unstuck, I don’t know how well the carb will idle. That’s the problem with the Walbro series that I referred to earlier – tweaking the low speed needle affects three things; idle, transition and total low speed flow.

It may be that opening the auxiliary jet a bit to get a bit more low speed flow and then not opening the low speed needle as far may be a preferred option – I don’t know.

Thanks to Ray Benson for finding this thread for me.

Wow lots of info in your post, thanks for taking the time. So far my carb is working much better with the mods I made, high end tune is staying where it should and it comes back to a stable idle cut after cut. I still haven’t redrilled the secondary transition circuit to its factory size of .3mm, been running really good with that circuit plugged and extra fuel going through the Low end /primary circuit. Still going to drill and test, it might run even better

 

[Terry Syd]

With the extra fuel flowing through the low speed circuit the throttle response may become a bit slower. You can tweak the metering spring with a washer/shim or two to clean it up. I think the washers are 2.5mm and a pack of 50 isn't expensive.

Shimming the spring will also slightly lean out the idle. You may find that as an alternative to drilling a hole in the throttle plate to lean the idle.

 

[XP_Slinger]

With the extra fuel flowing through the low speed circuit the throttle response may become a bit slower. You can tweak the metering spring with a washer/shim or two to clean it up. I think the washers are 2.5mm and a pack of 50 isn't expensive.

Shimming the spring will also slightly lean out the idle. You may find that as an alternative to drilling a hole in the throttle plate to lean the idle.

Interesting, thanks for the tip. I just started really understanding these little carbs, a week ago this would’ve been Greek too me but it makes sense.

 

[Terry Syd]

The metering valve/spring system in these diaphragm carbs is a clever way to 'ramp' in the fuel in the low speed circuit. Without the metering valve the carb would flood the engine with fuel. - The fuel would flow faster than the air flow increased as the throttle opened, thus we have to slow down the fuel flow to keep the engine from flooding.

The angle of the 'ramp' (or the rate of increase in fuel flow) is determined by two things, the size of the metering orifice and the spring rate on the metering spring. The thicker the wire and the greater the pitch, the slower the fuel comes in. Likewise, a smaller metering orifice requires the valve to have to lift higher to allow more fuel flow.

The pre-load on the spring determines when the valve begins to lift or the 'start' of the ramp. For chainsaws that don't run at part throttle, the fine tuning of the ramp isn't important and we can do most our tuning with just the pre-load on the spring.

The term 'pop-off' doesn't really mean anything worthwhile for our tuning. A metering orifice that is twice the area of another orifice will have HALF the pop-off pressure of the smaller orifice. (there is twice the area for the pressure to act on, so it pops off sooner). - I wouldn't have a clue what the 'pop-off' pressures of any of my saws were.

If you want to get into the fine tuning of diaphragm carbs then this manual by Mikuni on their carbs goes into that kind of detail.

https://mikuni.com/pdf/sbn_manual.pdf

 

[XP_Slinger]

The metering valve/spring system in these diaphragm carbs is a clever way to 'ramp' in the fuel in the low speed circuit. Without the metering valve the carb would flood the engine with fuel. - The fuel would flow faster than the air flow increased as the throttle opened, thus we have to slow down the fuel flow to keep the engine from flooding.

The angle of the 'ramp' (or the rate of increase in fuel flow) is determined by two things, the size of the metering orifice and the spring rate on the metering spring. The thicker the wire and the greater the pitch, the slower the fuel comes in. Likewise, a smaller metering orifice requires the valve to have to lift higher to allow more fuel flow.

The pre-load on the spring determines when the valve begins to lift or the 'start' of the ramp. For chainsaws that don't run at part throttle, the fine tuning of the ramp isn't important and we can do most our tuning with just the pre-load on the spring.

The term 'pop-off' doesn't really mean anything worthwhile for our tuning. A metering orifice that is twice the area of another orifice will have HALF the pop-off pressure of the smaller orifice. (there is twice the area for the pressure to act on, so it pops off sooner). - I wouldn't have a clue what the 'pop-off' pressures of any of my saws were.

If you want to get into the fine tuning of diaphragm carbs then this manual by Mikuni on their carbs goes into that kind of detail.

https://mikuni.com/pdf/sbn_manual.pdf

Thanks!

 

[XP_Slinger]

I makeded some more cookies, still haven’t re-drilled the secondary transition circuit. Holding tune really well.

Also switched to an 8 pin. Fattened the tune just a little from previous video

 

[Sawrain]

I have a 365 with a ported 77cc BB kit on it, I run a modded Zama. Smooth idle, gobs of torque off the bottom, strong torque curve and it likes to cut around 12,000 - great work saw. It makes enough power that I didn’t bother with a high flow filter.

Hi Terry,
I was wondering what cylinder you have on your 365?

I have a Zama fed Jonsered 2165 that I have been thinking of turning into what it should have always been.

Reports of the hyway cylinders have been ok, at least as a starting point.

 

[XP_Slinger]

Hi Terry,
I was wondering what cylinder you have on your 365?

I have a Zama fed Jonsered 2165 that I have been thinking of turning into what it should have always been.

Reports in the hyway cylinders have been ok, at least as a starting point.

Thanks,
Jake.

Don’t know if you’ve looked into it or not but OE 50mm topends are pretty cheap and they run great. That’s what I put on mine when I found out my BB cylinder was toast.

 

[Sawrain]

I had not, will do.

I was also thinking of doing some porting on the cylinder as well, and liked the idea of possibly messing up the cheaper one more.

That and I just used my chainsaw funds finding an oem 262xp cylinder.

 

[Terry Syd]

Heck I don't know who made the cylinder. It was a 'white box' kit based upon a bored out 372 cylinder (bigger transfer ports than the 365).

The cylinder and piston needed some work to get the transfers to flow properly. The top of the exhaust port was also wonky and needed some work. Not the best AM kit available, but after some work it was fine.

I understand that since then there is another kit available that is not only better quality, but contains a pop-up piston. If I ever toast this cylinder, I'll get that kit as a replacement.

 

[Sawrain]

That sounds like the Hyway 52mm Pop-Up GX Cylinder Kit.

The extra 2mm is worth ~6cc/~8% volume, pretty good, if a little port work and cleaning up can actually make use of the extra volume, it should be a good thing.

 

[Terry Syd]

Sawrain, mod the Zama while you're at it and you won't believe the difference - it may weigh and handle the same, but not the same saw, not even close.

 

[XP_Slinger]

Still working on improving this carb fellers, don’t start drilling to my previous findings yet. Had the saw in the woods for a day felling and bucking and the rich mid range was pretty annoying. Showed up big time when cutting slow to avoid a pinch. Just made a change, about to try it out again

 

[huskihl]

Still working on improving this carb fellers, don’t start drilling to my previous findings yet. Had the saw in the woods for a day felling and bucking and the rich mid range was pretty annoying. Showed up big time when cutting slow to avoid a pinch. Just made a change, about to try it out again

That’s been my biggest beef. If I stuff my 365, sometimes it’s hard to get it to clear out even. If it doesn’t die, the best way is to just shut it off and hold it wide open while you restart it. Definitely curious about your findings

 

[XP_Slinger]

Tried some stuff.

Filled the Welch plug supply jet and drilled to .4mm (down from .5mm) and the saw got more doggy in the mid range. Not smokin, felt like a lean bog initially, dead throttle...baw baw.



Here’s how it ran, part throttle cuts for testing, pardon the mid video tuning intermission lol

So I reopened the secondary transition jet, drilled to .3mm and it got WAY worse. Refilled it with lead.



Next I tried filling one of the primary transition outlets in the bore of the carb. It’s better, but still not clean in mid range. Didn’t get a video of cutting because of the rain. Just piss revving in the barn and holding part throttle.

Supposed to be 2 holes inside the red circle. The shiny spot to the right is the one I filled. These are supplied by the Welch plug area.



Next I might try filling and drilling the main jet a touch smaller. Max tune is very stable but maybe the main comes into play sooner than I thought and it’s over supplying fuel in the mid range. More to follow. Long shot being the main is regulated by the H needle jet. Perhaps the more logical place to go next would be under the Welch plug again...got some ideas for under there.

 

[XP_Slinger]

That’s been my biggest beef. If I stuff my 365, sometimes it’s hard to get it to clear out even. If it doesn’t die, the best way is to just shut it off and hold it wide open while you restart it. Definitely curious about your findings

Man I hear ya Kev, it got on my nerves and even stalled a couple times from loading up. Couldn’t believe it after how well it ran on the test cuts.

Would be easy to buy an HD12 or Zama. But I’m following through on this because I’m sure I won’t be the last dude to try to save a buck by making the XT carb work after converting to an OE top end

 

[huskihl]

Man I hear ya Kev, it got on my nerves and even stalled a couple times from loading up. Couldn’t believe it after how well it ran on the test cuts.

Would be easy to buy an HD12 or Zama. But I’m following through on this because I’m sure I won’t be the last dude to try to save a buck by making the XT carb work after converting to an OE top end

There are knockoffs available for the 390 now. Might be worth a try for $20 or whatever they are.
I’m not a fan, I get it. But for playing around, $20 vs $90...

 

[jacob j.]

There are knockoffs available for the 390 now. Might be worth a try for $20 or whatever they are.
I’m not a fan, I get it. But for playing around, $20 vs $90...

That's why you check out those ladies in east Detroit when you're low on cash...

 

[Terry Syd]

Supposed to be 2 holes inside the red circle. The shiny spot to the right is the one I filled. These are supplied by the Welch plug area.

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.