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

[Spike60]

Don't think so. They use the rev limited green coil to control it.

 

[XP_Slinger]

There's the HD30 from the 375K power cut. Bolts right in with zero changes.

part number 503 281 811

Thanks bob, I assumed it ran an HD12. Never even gave it a second thought.

 

[Spike60]

Shhhh!!!! Can’t be giving out secrets. Lol

Yeah, and there's all kinds of secrets.

 

[huskyboy]

Bump

 

[XP_Slinger]

Bump

I know, I’m a slacker lol. I have an HD12 to test against the stock Zama, then I plan on modding the Zama. Just for gits and shiggles. Also have an HD30, not sure it’s worth throwing that in the mix. Mostly identical to HD12 except for a very small difference in main jet pick up size.

 

[Terry Syd]

Now that would be a great comparison, especially if it could be run up on a dyno. When Poleman did a comparison of a stock HD-12 to a modded Zama it was night and day. First he did a 30-35 on the 7900 then he did a 30-40 on the Zama - he was all for the 30-40 on the Zama for a 79cc engine.

I never knew if he cleaned up the venturi on the Zama for the tests. I don't think he did, if that is true, then there is that more than can be added to the equation.

 

[sixonetonoffun]

Gets complicated. Tunability seems like the best benchmark for a "work saw".

It would be interesting to know when the 390/395 carbs actually put something on the table. Besides being heck of a lot easier to tune than a RWJ4.

 

[huskyboy]

Gets complicated. Tunability seems like the best benchmark for a "work saw".

It would be interesting to know when the 390/395 carbs actually put something on the table. Besides being heck of a lot easier to tune than a RWJ4.

The 390/394/395 carbs (both walbro and tillotson) are much better than the rwj4 on a ported saw if your wanting to do a straight bolt on without mods to the carb.

 

[XP_Slinger]

Now that would be a great comparison, especially if it could be run up on a dyno. When Poleman did a comparison of a stock HD-12 to a modded Zama it was night and day. First he did a 30-35 on the 7900 then he did a 30-40 on the Zama - he was all for the 30-40 on the Zama for a 79cc engine.

I never knew if he cleaned up the venturi on the Zama for the tests. I don't think he did, if that is true, then there is that more than can be added to the equation.

Yes dyno testing would be great but I won’t be going that route, at least not any time soon. Hopefully there will be an appreciable difference in time/rpm for all this tinkerin

 

[XP_Slinger]

Did some comparing, both carbs stock. Slight time difference but that could either be variation in my cutting or the fact that the wood was getting a little bigger with every cut. Both carbs ran very good, win /win imo, tough to tell a difference with the short bar. Next step is to mod the Zama to see if the margin widens.

 

[sixonetonoffun]

That's a suprise. Zama looked to pull great. Unless ya hit some kinda twisted grain with the Walbro.

 

[XP_Slinger]

That's a suprise. Zama looked to pull great. Unless ya hit some kinda twisted grain with the Walbro.

Wood was clean and straight...white ash. I was surprised as well, especially after feeling like my saw lost some power when I first switched to the Zama. She felt good with both carbs, I knew it was close before I checked times. But, times don’t mean a whole lot to me, the difference was minimal

 

[sixonetonoffun]

Just found out the hd30 I ordered isn't coming. Probably skip it.

 

[Jedd]

Here is a link to where poleman posted his Zamia mods on AS. Wrong site I know but still good info there.

https://www.arboristsite.com/community/threads/zama-carb-mod-cliff-note-version.275905/

 

[Terry Syd]

Great thread that Rich communicated all the mods - especially the difficulty to getting that idle/transition jet out so you can drill the first transition hole in the carb to .55mm.

That part of the mod has been the biggest problem of all the mods. Rich, me, Randy and a heap of other guys tried to quickly get that jet out - and we ALL cut the top of the jet off.

You need to get a good strong grip on the sides of the jet and TWIST it out. If it doesn't twist, STOP, and go back and figure out what you are doing wrong and what you need to do to get it to twist. NEVER, EVER, try to pull that freaking jet out, just twist it like it was screwed in.

Rich did a heap of work on the Zamas and came up with some brilliant recipes for different size engines and configurations.

I wish he had posted all his recipes, but he did post enough that you can work up to what can work for you.

 

[XP_Slinger]

Here is a link to where poleman posted his Zamia mods on AS. Wrong site I know but still good info there.

https://www.arboristsite.com/community/threads/zama-carb-mod-cliff-note-version.275905/

Yup I’ve had that thread ear marked in my browser for quite a while. Thanks for posting. Info is info man, I personally don’t care where it comes from.

 

[XP_Slinger]

I’m gonna order another C3M to mess with. For only $40 I’d rather have a spare if I mess it up, at least I would be able to put the stocker back on and not have a downed saw. Sounds like there’s a steep learning curve for removing that jet, chances are high that I’ll suffer the same breakage despite the very clear guidance from Terry. Fingers crossed. I’ll also thin shafts and smooth the casting line in the Venturi. It’s not bad in my carb, just a slight ridge.

 

[qurotro]

There's the HD30 from the 375K power cut. Bolts right in with zero changes.

part number 503 281 811

The diaphragm vent is different.?

 

[XP_Slinger]

The diaphragm vent is different.?

Yes it is. It’s identical to an HDA199 (357xp). It has that black spacer on the diaphragm side. No vent hole in the cover.

 

[XP_Slinger]

Just ordered another Zama, $38. Given how it performs I don’t see the sense in spending $70+ on an HD12B ever again. I’ll attempt modding once my back up arrives...to be continued.