372XP X-torq 75cc Conversion & Porting

[XP_Slinger]

Got another small project for my 372. As the title states I'm going to swap out the XT cylinder for a 75cc cylinder that I grabbed off of a junk 375K. I know the P&C is a direct swap; but what about the carb? I have the 75cc intake and it looks like it will work with my carb. Any body ever run the larger XT carb on an XPW cylinder? I've seen it talked about and I'm curious if anyone has actually tried it.
My XT cylinder is ported and runs real strong. That being said, my brothers, @RIDE-RED 350r XPW is also ported and I like the feel of his a little better. Seems to have more torque although my saw in current form still runs neck and neck with his. This swap isn't about building a faster saw, just having some fun. Yes I will be porting the cylinder.

 

[XP_Slinger]

Something I found interesting. My friend @afleetcommand has always expressed concern about how heavy the XT piston is compared to OE 372's. This concern comes to the front when you're talking about spinning the XT up to and beyond 14k like the old saws and his concern is bearing load with the heavier piston. Out of curiosity I weighed the XT piston and the 75cc piston, and the big bore piston is lighter! Not by much, but the fact that the larger bore piston IS lighter gives a little perspective on the subject. That XT piston is a heavy pill for its bore size.

 

[old 040]

Will the non XT carb/intake bolt right up to the XT version?

 

[XP_Slinger]

Will the non XP carb/intake bolt right up to the XP version?

No, the XT intake cannot be used on the XPW cylinder. The clamping area for the intake is much larger on the X-torq to accommodate the strato ports. But upstream where the carb bolts to the intake appears to be the same. Kind of fishing for some carb experts to chime in with bore size info and such. If I remember correctly @Mastermind has tried this?

 

[old 040]

I believe it will. But I am curious to see how the bigger carb runs on the 75cc cylinder. Kind of fishing for some carb experts to chime in with bore size info and such. If I remember correctly @Mastermind has tried this?

I actually meant the XT not XP Which I corrected in my post.....

 

[XP_Slinger]

I actually meant the XT not XP Which I corrected in my post.....

Ha ha! I think we were both editing our posts at the same time.

 

[XP_Slinger]

I actually meant the XT not XP Which I corrected in my post.....

I fixed my original answer, sorry I made this a mess lol

 

[Derf]

The XT has a RWJ4 carb, the xpw uses the same 372xp carb (walbro hd12b). After port work some people have tried to look for additional gains with the XT carb. If you have both it will be very cool to see you run it both ways and hear your impression on how it affects the powerband. A lot will depend on how you port the jug. Randy @Mastermind may have tried it, as well as others. I can understand the mentality that the larger carb of the XT might pair better with the 75cc ported jug than the original carb which was on the 365 and 372xp. The XT carb has a 2-3mm advantage in the throttle bore iIrc.

The big issue is the xpw intake boot and hd12 bolt holes are horizontally even to the intake, and the rwj4 (like most of the larger throats carbs) offsets the bolt holes one high and one low. Using the rwj4 carb requires you to use the XT intake boot and filter holder (you can cut out the divider if you so wish), the blue choke lever and to ditch the rubber carb suspension piece. @TerrySid and @Poleman might be able to suggest other carb modifications to the HD12b to make it run better with the larger jug if you want to keep the hd12b carb.

 

[old 040]

I fixed my original answer, sorry I made this a mess lol

What I meant was, will the non XT carb/intake fit on a XT saw when using the non XT top end?

 

[XP_Slinger]

What I meant was, will the non XT carb/intake fit on a XT saw when using the non XT top end?

I believe it will.

 

[Derf]

What I meant was, will the non XT carb/intake fit on a XT saw when using the non XT top end?

Yes. If you change the XT cylinder, intake boot, and carb over to non XT versions they will fit fine. The XT top cover is actually larger than the non-XT, because everything is a little bigger. You can see this in the taller top handle and brake lever. So non XT pieces fit fine, it's when you do the opposite and put the XT bits in the non-XT case, things get tight, hence why you have to ditch the carb suspension piece.

 

[XP_Slinger]

The XT has a RWJ4 carb, the xpw uses the same 372xp carb (walbro hd12b). After port work some people have tried to look for additional gains with the XT carb. If you have both it will be very cool to see you run it both ways and hear your impression on how it affects the powerband. A lot will depend on how you port the jug. Randy @Mastermind may have tried it, as well as others. I can understand the mentality that the larger carb of the XT might pair better with the 75cc ported jug than the original carb which was on the 365 and 372xp. The XT carb has a 2-3mm advantage in the throttle bore iIrc.

The big issue is the xpw intake boot and hd12 bolt holes are horizontally even to the intake, and the rwj4 (like most of the larger throats carbs) offsets the bolt holes one high and one low. Using the rwj4 carb requires you to use the XT intake boot and filter holder (you can cut out the divider if you so wish), the blue choke lever and to ditch the rubber carb suspension piece. @TerrySid and @Poleman might be able to suggest other carb modifications to the HD12b to make it run better with the larger jug if you want to keep the hd12b carb.

Excellent info thank you. I've only eyeballed the fitment at this point and how the bolts are "clocked" was one of the concerns. Hard to tell exactly what's going on with everything still assembled on the XT.

 

[mdavlee]

All you really have to do is trim the rubber where the impulse comes through and rotate the boot. I built a few with that carb.

 

[XP_Slinger]

All you really have to do is trim the rubber where the impulse comes through and rotate the boot. I built a few with that carb.

How'd it run Mike?

 

[mdavlee]

How'd it run Mike?

Pretty good. Good torque. I have tried some different numbers and like them better now.

 

[XP_Slinger]

Pretty good. Good torque. I have tried some different numbers and like them better now.

My initial thoughts on porting is widening but keeping exhaust a little lower than I would normally put it at, looking for good torque, around 104 or 105, about 22ish degrees of blow down and minimal lower transfer work. I'm thinking intake around 80 for the bigger carb. But, all this of course depends on how the cylinder times before grinding.

 

[RIDE-RED 350r]

If I recall, the exhaust on my XPW is at 101 and 70%. Some of my old text messages to myself for stuff like that didn't transfer to my new phone...including the one recording the port timing on my XPW

 

[XP_Slinger]

If I recall, the exhaust on my XPW is at 101 and 70%. Some of my old text messages to myself for stuff like that didn't transfer to my new phone...including the one recording the port timing on my XPW

Yep we can't rely on memory anymore that's for sure. Lol! No biggie anyway, kinda wanna try my own thing on this big bro

 

[mdavlee]

My initial thoughts on porting is widening but keeping exhaust a little lower than I would normally put it at, looking for good torque, around 104 or 105, about 22ish degrees of blow down and minimal lower transfer work. I'm thinking intake around 80 for the bigger carb. But, all this of course depends on how the cylinder times before grinding.

I like 96 on them. 118-120 and 78 widened.

 

[RIDE-RED 350r]

Yep we can't rely on memory anymore that's for sure. Lol! No biggie anyway, kinda wanna try my own thing on this big bro

That's for sure! One thing we need to do or at least I need to start doing is keeping a little better record of what I did. Thought texting to myself was pretty reliable