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Stihl 660

Allen Miller

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Just installed a cross mmws cylinder milled. 075 off base cut a pop up piston with. 010 Clarence. Saw will start but will not idle. Will stay running at part throttle and wide open. Saw sounds like a cam out 4 stroke right before it dies. To me timing sounds low . I thank it changed timing by dropping cylinder down. So we're doing it start thanks for any help
a37245a221f6df9060a4ecf23e210078.jpg


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brshephard

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Here is what needs clearanced below (Source: Mastermind). I took .060 off mine and had to grind two fins on the PTO side near the muffler also.
I also tried .010 piston to squish clearance and got contact. Most guys run .020" on these.
You might be able to install a .010 gasket to set your squish and get your case clearance at the same time.

If this was a 54mm jug......I'd be doing some clearance work on the case.

The black spots are where I grind on the 660......



 

smokey7

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.075 off the base has to yield huge compression. If port timing was not your goal..... Why did you mill that much off the base? Also at .075 off the base you may have a whole lot of free port in the exhaust. Im guessing your intake may have a whole lot of excessive duration. Ive caused this piss poor idle before with too much dmintake before. I think we need more info. Please take some pictures of exhaust port piston at tdc. Also take some pics of clearance around the cases.
 

MustangMike

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Im guessing your intake may have a whole lot of excessive duration

With my limited knowledge that is what I was thinking. These things often come with over 80 on the intake, it must be through the moon now.

I would also be in the camp looking for .020 clearance.

I think he needs a .055 gasket!
 

Allen Miller

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When I took cylinder off found oil on both sides of steel gasket never thought to check case clearness . I'll put it back together this weekend and post a picture of exhaust ports. Thanks for every body's input. Here's pictures of cylinder and piston.
5ecd3971b61cf024ae9cf2e82e1462e3.jpg
59b81506790ac1dd4301d103c2f13fe7.jpg


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Allen Miller

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.075 off the base has to yield huge compression. If port timing was not your goal..... Why did you mill that much off the base? Also at .075 off the base you may have a whole lot of free port in the exhaust. Im guessing your intake may have a whole lot of excessive duration. Ive caused this piss poor idle before with too much dmintake before. I think we need more info. Please take some pictures of exhaust port piston at tdc. Also take some pics of clearance around the cases.
I was trying to see how much compression I could make doing that. Saw back apart I'll put it back together this weekend and you some pictures.

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Moparmyway

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How much did you take off of the piston to give you that popup ?
 

smokey7

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What compressions did you get ?
 

brshephard

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[QUOTE="Allen Miller, 216 psi after a few minutes of run . You can't start it with out compression release valve.

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Sounds low for the work you did. Is your gauge bad?
I did .060 off the base and .080 off the squish .015 popup. Made 245 dry with one ring.

Maybe a leaky Schrader valve or something.
 

Wonkydonkey

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If I read it right, there’s a leak in the base, due to case clearances,. I think that’s why it’s only reading 216,,, prolly 265/or more once the base gasket leak is sealed,

If that is the case, you could be changing things. Otherwise your starter may not hold up for too long
 

Redfin

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If I read it right, there’s a leak in the base, due to case clearances,. I think that’s why it’s only reading 216,,, prolly 265/or more once the base gasket leak is sealed,

If that is the case, you could be changing things. Otherwise your starter may not hold up for too long
Comp builds above the ring. The intake is wide open at tdc regardless.
 

Allen Miller

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If I read it right, there’s a leak in the base, due to case clearances,. I think that’s why it’s only reading 216,,, prolly 265/or more once the base gasket leak is sealed,

If that is the case, you could be changing things. Otherwise your starter may not hold up for too long
There was a leak at base cylinder on bar side wear cylinder touching crankcase and the 216 was one pull couldn't pull a twice. So probably a lot higher psi than that.

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Mastermind

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There was a leak at base cylinder on bar side wear cylinder touching crankcase and the 216 was one pull couldn't pull a twice. So probably a lot higher psi than that.

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I'm thinking you are gonna have way too much compression
 

smokey7

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Do you have a degree wheel to measure the timing for us? I think your compression is a pretty big problem. I am also concerned with the timing if the cylinder after all the machining you did to it. It may be best to start over and take a much milder approach to modding this one. Its fun to play around but when they become so difficult to start and run poorly the fun is over. Before it gets pulled id love to see if the squish is even what the clearance us and what the timing is. Thank you for talking with us about your experiment.
 
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