372XP X-torq 75cc Conversion & Porting

[mdavlee]

If the whole ring isn't exposed it will be fine. Rings drop into the port on a lot of saws

 

[RIDE-RED 350r]

That's good to know Mike. He has been wanting to W-ify his XT for awhile now and got this 375k top end from GKS's pile for a really good price.

 

[XP_Slinger]

If the whole ring isn't exposed it will be fine. Rings drop into the port on a lot of saws

Thanks for the info Mike. It's no where near the whole ring being exposed. I'd say at the peak of the arc there's about 2/3 the thickness of the ring visible. Still lots of support as the arc curves downward.

 

[mdavlee]

Thanks for the info Mike. It's no where near the whole ring being exposed. I'd say at the peak of the arc it's about 2/3 the thickness of the ring visible. Still lots of support as the arc curves downward.

Then you'll be fine

 

[XP_Slinger]

That jug is one nice looking casting. It should run great after the work you did. Are you going to thin the divider any?

Left the divider as is. Trying out some theories discussed in Randy's porting series of threads. I'll never know if I don't try.

 

[bikemike]

I'm too broke to take that kinda risk. But if it's not near the centering pin and ring tips I'd run it

 

[XP_Slinger]

Put it back together using the intake and block for the 75cc cylinder. Got it to fit but, the throttle cable was getting cranked on so hard that the throttle wouldn't return to the idle stop as seen in the second pic.

 

[mdavlee]

That can happen. Trim a little of the casing on the cable

 

[XP_Slinger]

Put the X-Torq carb block on it and the throttle problem went away. I'll be keeping an eye on this block, there looked to be plenty of rubber from the intake to seal the carb but we'll see how it goes. Also, finding the correct position for the intake clamp was another slight problem to work through. Put it on the left and the clamp hits the air injection duct and prevents the cylinder from seating. Put it on the right and the impulse line is guaranteed to be punctured via chaffing after some run time. The only place it's out of the way is on the bottom, clears everything.
Pic with XT carb block installed.

 

[XP_Slinger]

That can happen. Trim a little of the casing on the cable

That would've been easier. ha! I used the XT block to fix it.

 

[mdavlee]

That would've been easier. ha! I used the XT block to fix it.

Little things like that I forget to mention.

 

[XP_Slinger]

Had to trim some rubber off the intake where the impulse line passes through as Mike said earlier in the thread. This pic is before I trimmed it down so the top cover would fit over it.

 

[XP_Slinger]

Little things like that I forget to mention.

It's not a problem brother. Except for the fact that I was dead on my feet near the end of the project, I had a lot of fun figuring these little things out. Trial and error is fun when it works out in the end.

 

[mdavlee]

Sometimes it's fun. Sometimes it takes forever when you're going for a fully functioning work saw

 

[XP_Slinger]

And the result. Pardon my expression in the video, I'm tired as hell. Starts, idles and revs good. Didn't tune it yet, checked with a tach after the video out of curiosity and it piss revs to 13.4k and is readily 4 stroking. We'll see how it cuts tomorrow if the weather turns for the better.

 

[XP_Slinger]

Sometimes it's fun. Sometimes it takes forever when you're going for a fully functioning work saw

That's a fact. This is the first saw I've modded start to finish in the same day. I didn't intend on starting it tonight but that intake boo boo had me curious so I kept on trucking. I was at the bench about 7 hours. I think I'm getting chicken legs after all that...lol!

 

[XP_Slinger]

Eventually @RIDE-RED 350r and I will compare our saws. There are a few differences in our builds and it will be interesting to see how they stack up to one another.

Intake: Joe squared his intake to a greater extent and left the timing at 75* BTDC. I widened mine slightly and set timing at 79* BTDC. Applying the theory mentioned in "Part 2: Intake" thread in Randy's series. "If it doesn't have area you better give it some time." And also the statement that bigger carbs need more time. My cylinder is also a little closer to square with intake and exhaust durations at 158 & 160 compared to his at 150 & 160.

Exhaust: We're about dead even here. Both open at 100* ATDC and are widened to 70% of bore. Although in my tired rush to get it running I forgot to match the gasket and heat shield...oops.

Transfers: We're close to each other here also. His open at 122, mine at 123 with 22 and 23 degrees of blowdown respectively. Shapes and angles are the same because we were both stuck with a dremel.

Lower transfers: Big differences here. Joe did extensive work on the lowers. He completely removed the big humps in the primary's, knife edged the divider and cut the cylinder down flush with the case at the entry. As seen in this thread, I only funneled the humps in the primary's, that's it. Another curiosity about volume vs velocity.

Carb: Joe has the stock XPW carb and I am running the larger XT carb.

Although there are similarities in our builds such as the exhaust and transfer timing, I think these saws are going to have different personalities due to the listed differences. Just hope mine isn't a dog, but I think it will run strong. How strong is the question yet to be answered.

 

[blsnelling]

Drop the bottom ring. I don't run it. It adds a significant amount of friction.

For the intake you can either trim a standard 372 intake and rotate it, or, run a 385/390 intake.

I run 98°-100° on the exhaust, 18° of blowdown, and 78°-80° on the intake.

I entirely case match the cylinder base/transfers to the case.

I thin the divider, but do not knife edge it. You want it rounded for the best flow.

When the carb wouldn't return to idle, I suspect the cable housing was not seated in the trigger.

 

[XP_Slinger]

Drop the bottom ring. I don't run it. It adds a significant amount of friction.

For the intake you can either trim a standard 372 intake and rotate it, or, run a 385/390 intake.

I run 98°-100° on the exhaust, 18° of blowdown, and 78°-80° on the intake.

I entirely case match the cylinder base/transfers to the case.

I thin the divider, but do not knife edge it. You want it rounded for the best flow.

When the carb wouldn't return to idle, I suspect the cable housing was not seated in the trigger.

Never thought to check the other end of the cable at the trigger. Between Mike and yourself Recommending it, I'm thinking I'll be tearing it down again to work on the lowers. I'll still get a video of it running in its current state for analysis. Thanks for the info Brad, much appreciated.

 

[XP_Slinger]

Had time to make just a few cuts with it this afternoon. First impression, strong saw. Didn't have any big logs left so I cut a few at once. Saw is wearing a 24" bar with Stihl RS chain, tuned to 13.9k no load. After the video I tuned it to 14.2 but it was barely 4 strokin so I turned it back down to 13.5 for work. Overall very happy with how it came out. Has that XPW feel that I was looking for. Dare I say it feels very close to Joe's saw, but I know he can tune his a touch higher. Maybe we'll get a good video of both our saws this weekend.