MS290 with 390 topend. Can't tune.

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Summitlt

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First part, been a lurker for a while.

I've got an MS460 magnum and a MS290, 290 I bought for $50 with a bad top end. While lurking here saw a 390 top end fits. So it should be a 65cc saw. I know my 460 is a different animal but the "390" has no low end and just seems pathetic. Compared to an older 036 its not even in the same area code. I've removed the limiters in the carb and tuned. Still didn't seem like it really has anything for power. Granted I'm comparing it to my 460 with a dual Port, drilled carb and an 036 that's stock. It's a Chinese top end, but are they really that bad? In the cut it takes nothing to stop the chain and doesn't seem to have any bottom end.

Should I get a 390 carb?
 
I did my dads 290 a few years back. Left the stock 290 carb and installed nwp 390 top end. Was a turd. I told him to run it to see if it wakes up the more it runs. He put 6-8 tanks through it with nothing but disappointment. Tore it down a few weeks back and installed oem 390 rings along with a new oem 390 carb. Night and day difference. Runs much stronger and more like expected.


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There is a bigger main jet in the 039/390 carb. This is a cheap part and may be your issue getting it to tune. Make sure there's no vacuum leaks, check intake boot, and AV bumpers. People like to yank around more than they should and RIP $#!^ up in the process.

Always check the easy stuff first. New top end, the rings may just need to settle in. Tune it well and then just run it.
 
Key word "Chinese Top End". you can stop there, the consistency between piston and cylinders batches are not very good so takes some customizing. Anyway i didn't see a compression reading, so that would be a good start, i agree with the post that it may be too much squish but ultimately i always prefer used oem cylinders that either i clean up or my dealer works up with their ball hone. This is assumings is 4-stroking at around 13K rpm.
 
Haven't run compression on it yet, my gauge is at work. Went with the Chinese stuff because the previous owner ran it until it seized. Either no oil or leaned it way out. I will say the cylinder wasn't beautiful by any stretch of the imagination. I'll check compression and check back.
 
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