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Cross Pop-up Piston in MS390 topend

Terry Syd

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The carb would be a WJ, is that right?

If so, I looked at the metering on one of those once and there are three transition holes on the low speed circuit - so it should flow a lot more fuel down low (throttle response, torque). The actual size of the carb venturi wasn't that much bigger, but it did have a bigger section for the throttle butterfly. Did you have any issues blending the carb into the manifold, or did you just bolt it on?

Great mod.
 

94BULLITT

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The carb would be a WJ, is that right?

If so, I looked at the metering on one of those once and there are three transition holes on the low speed circuit - so it should flow a lot more fuel down low (throttle response, torque). The actual size of the carb venturi wasn't that much bigger, but it did have a bigger section for the throttle butterfly. Did you have any issues blending the carb into the manifold, or did you just bolt it on?

Great mod.

Thanks, I believe it is a WJ copy. I measured the 290 carb and compared it to the 660 carb but I could not find that stuff to put in the video. That is good to know about that carb. I guess my "seat of the pants" was not imaging things. You can really see the difference if you stop in the cut then take off again.

I also had a video showing how it fit the boot, the thing that hold the filter in alignment, and the filter. None of that stuff needed modded, the carb was smaller than all of that stuff. The boot is the tightest fit.
 

94BULLITT

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Thanks!

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Terry Syd

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I've tried the 460 carb with the offset discharge nozzle and it didn't produce the power that the stock carb did. Brewz was the one that corrected me on that stuff up. I had recommended it and he found it didn't produce as much power.

I was changing a few things at the time, air filter, timing and managed to fool myself. You would think after tuning for some 50 years that I would have learned to only change one thing at a time.

I think the larger venturi works on the bigger saws, but the fuel curves didn't work as well on a smaller saw. I wish I knew why as then I might be able to do something about it.
 

stretch5881

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P1010616.JPG Here is my pig with all it's lipstick.
Cross 039 cylinder with popup. I widened the exhaust port a tad and cleaned up the flash from the nikasil lining.
Timing advanced about 1/4 key.
Muffler modded.
New bar and chain.
Upgraded to mesh air filter.
Upgraded to drum and sprocket.
By the way, do NOT try an AM drum and sprocket. The drum seats about .050" too far away from the clutch, and even if you cut it down in a lathe, the splined shaft for the gear is shorter than OEM, so the gear can't walk to do it's job. Waste of money. Stihl only.
It'll be a week or two before it's in wood, but it seems snappy after tuning it.
 

stretch5881

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We'll see how it is when we start skidding. If it needs more grunt, then it's an opportunity to get a pin vise set and learn a bit of your carb magic Terry.
 

Terry Syd

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I don't know what the optimal set up on the 029-390 HD carb running on a 64cc jug actually is. I took it out to that size and it worked well, but it might be capable of working even better.

What we would really need is someone that has TWO HDs and then start modding one at a time. When the modding has gone TOO FAR, then we know the limits.

Once we know what the best jetting is, then it becomes easy for anyone on the forum to replicate the results with their carb.

I'm reluctant to mention it, but here's a real easy mod to start with - try rebuilding the carb before you start thinking of hacking away at it. I've had a number of guys want me to work on their carbs. Crikey, you wouldn't believe the worn out pump diaphragms and stiff metering diaphragms I've run into.
 

stretch5881

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I'm thinkin' the biggest difference we might have with jetting, is, where I live is around 2000' above sea level. I bet you might be just a little lower. Might be a hair warmer too.
 

Terry Syd

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I live at 2,000'. Since there is about a 3% loss in air/oxygen for every 1,000', I'm down some 6% compared with sea level. However, I haven't noticed a big enough change to worry about re-jetting if I cut down at a lower altitude.

However, some of the guys that are cutting at much higher altitudes might prefer to set up the carb for those altitudes. We just need a base-line jetting to work from and then tweak it as needed.
 

blsnelling

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Air filter testing

The nylon mesh filter will flow more air, but a lot of the difference in this vid is due to the tuning difference. The saw is running much leaner with the nylon mesh. You would need to tune the saw with each filter for an accurate test.
 

94BULLITT

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The nylon mesh filter will flow more air, but a lot of the difference in this vid is due to the tuning difference. The saw is running much leaner with the nylon mesh. You would need to tune the saw with each filter for an accurate test.
I did retune

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