Modded saw - lacking power

[Tree Sling'r]

Okay, I have a MS460 w/bb which is modded. There is lower and upper transfer work, finger ports, intake and exhaust has been opened, the cylinder gasket has been removed (Squish .016). I advanced the ignition (cleaned the grease off the crank shaft when done removing the Woodruff Key) - Fired it up and eventually tuned the carb to a safe but, perfomable RPM.

It would not even come close in performing like my stock 460 barrel and piston with the same port job.

I did notice a slight fuel leak where the tank was cracked this past summer (falling incident) - I inspected the fuel line - impulse and everything else I could think of.

Any advice of where to begin troubleshooting? Would the slight fuel leak (which am I going to re-expoxy) lack me of that much power.

Don't get me wrong - it runs - but not like it should - powerwise. I am thinking carb stuff, but who knows.

Thanks - Tree Sling'r.

 

[Crofter]

First move would be to degree it to see if all the timing events are within the realm of possibility. Any chance you retarded instead of advancing timing?

 

[Freakingstang]

How much was the timing advanced?

Are you sure the carb is set right? I remember casey having problems on a BB460 that the main jet in the carb was too small. He used a jet from an older non epa 046 carb. his BB460 was stock, to the best of my knowledge.

Good luck, lets us know what you find out

 

[Tree Sling'r]

First move would be to degree it to see if all the timing events are within the realm of possibility. Any chance you retarded instead of advancing timing?

When I advanced the timing - turned the flywheel appox. a 1/3 of the way accross the key slot - counter clockwise. The same I do on all of my saws.

Please explain degree.

 

[rahtreelimbs]

Am I to understand that all was well before the ignition change???

All the modded saws I have have stock ignition timing to the best of my knowledge.

 

[Four Paws]

Well, I am not an expert by any means. I have only modded one saw, and although I have made a few cuts with it, it isn't broken in yet, and I want a 8-pin sprocket and a good chain to really test it against my stock saw (same model).

As far as your BB460 goes, I have this to offer. Perhaps the BB kit has different port timing to begin with. Thus, even when modded, it might not be possible to attain the same numbers as you can with the stock cylinder. From what I have heard, .016" squish is pretty tight for a woods saw. I can't think of anything that would make the saw not perform other than timing numbers that aren't quite right - either that or the saw needs some break-in time. You have mentioned that you covered the obvious things (impulse hose, fuel line, etc.). Does it perform worse than a stock 460 with no mods? I couldn't imagine your leaking tank causing a loss of power - as long as you have a full tank, your fuel pick-up is under fuel and wouldn't be sucking air. What RPM are you running? I have heard of saws running better when the carb is adjusted slightly richer than peak RPM. Maybe some of the big boys will stand up and offer their advice. I am just a novice, learning as I go.

 

[Crofter]

When I advanced the timing - turned the flywheel appox. a 1/3 of the way accross the key slot - counter clockwise. The same I do on all of my saws.

Please explain degree.

That would advance the timing OK. By degree it, I mean put a degree wheel on it and check what the intake, transfer and exhaust open duration is.

 

[Tree Sling'r]

I wonder if the port timing with the BB is diff. than a stock kit. I modd my saws the same. My 460 with the stock jug and piston rips to holy hell. My 660s with the same port job do the same.

Yes the advancement of the timing helped the saw. I ran it before and after. And yes I know how to tune a carb - so it is right as well. As far as RPMs I did not put my tach on it but, it is plenty blubbery (4 stroky) when wound up. I would say no more than the mid to high 13s. I normally run my stuff right around 14 - they are RPM saws - with the low end torque done with addition porting.
Thanks for the efforts so far.

 

[ol'homey]

What's your compression psi? If some of the mods intended to increase flow resulted in a drop in compression that would explain the lack of HP.

 

[Tree Sling'r]

What's your compression psi? If some of the mods intended to increase flow resulted in a drop in compression that would explain the lack of HP.

It will rip the handle out of my hand. That is a good enough test for me as far as compression .

 

[ol'homey]

That confirms you've got the timing advanced but it doesn't tell me what your static compression psi is. We can rule out retarded timing, lean mix, and poor air flow as the cause of poor HP. There's not much left other than compression. Anybody have a clue what psi a stock 460 BB top end normally runs? If your compression is low you've found the problem.
If the compression is nice and high say 190+ then I would guess that the high compression along with lots of spark advance is causing the saw to detonate under load resulting in the loss of power. If that's the case you might try some 105 octane av gas and see what happens.

 

[Crofter]

Removing the same amount of material in roughly the same areas could give you greatly different duration timing with an aftermarket piston and cylinder than what you might get with the same dimensions on Stihl parts. If you could establish exhaust and intake duration and measure blowdown you would know whether that was your problem. As long as you have enough compression to start well, that should not affect high rpm operation.

 

[Freakingstang]

That confirms you've got the timing advanced but it doesn't tell me what your static compression psi is. We can rule out retarded timing, lean mix, and poor air flow as the cause of poor HP. There's not much left other than compression. Anybody have a clue what psi a stock 460 BB top end normally runs? If your compression is low you've found the problem.
If the compression is nice and high say 190+ then I would guess that the high compression along with lots of spark advance is causing the saw to detonate under load resulting in the loss of power. If that's the case you might try some 105 octane av gas and see what happens.

I'm going off of memory, but the BB460 I put together was about 155-160psi.

 

[ol'homey]

I'm going off of memory, but the BB460 I put together was about 155-160psi.

That's sounds about right. Removing the cylinder gasket should bump it to what 175-180 ?

If he's looking at say 135 psi it will still start OK but it won't have the power he's expecting. If it's up there pushing 200 psi and detonating it's not going to make any power either.
If you don't have a compression gauge try putting the timing back to stock and see if that helps. If you make more power with the stock timing then you were detonating.

 

[ol'homey]

It will rip the handle out of my hand. That is a good enough test for me as far as compression .

Tree, I've had old worn out Briggs lawn mower engines rip the handle out of my hand due to advanced timing caused by a partially sheared flywheel key (customer hit something with the blade) which is more or less what you've got on your saw.

 

[fishhuntcutwood]

Have you asked Dean? Didn't he build it? He'd know well about port timing and the internals.

He put in new crank seals and all that right?

 

[Jacob J.]

Thing to remember about those big bore kits for 046/MS 460- The combustion chamber design is huge. Even though they displace more volume than the stock cylinder this volume is somewhat offset by the much larger combustion chamber. The result is less "snap" no matter how tight you set the squish. And the transfer port design is anemic, although the 046 case doesn't really have the volume to support the additional displacement. So I'm guessing the manufacturer kept the transfer ports skinny to keep the velocity up.

 

[Tree Sling'r]

Have you asked Dean? Didn't he build it? He'd know well about port timing and the internals.

He put in new crank seals and all that right?

This was the first saw I got from Dean. He bought it up in WA, built it and sent it down. There was no need for crankseals, it was new.
Since then I have had this saw torn down prob. 10 times. His methods have since evolved - and after spending four days with hime last week. I completly re-did the saw.
Dean, is not around much on the weekends. I am going to PM him too.

Well, I'll tool with it - may just put the stock set up back on (Modded of course) Gettin' plenty of rippage with my other set up.
For now I gotta go get an X-Mas tree.

 

[Tree Sling'r]

Thing to remember about those big bore kits for 046/MS 460- The combustion chamber design is huge. Even though they displace more volume than the stock cylinder this volume is somewhat offset by the much larger combustion chamber. The result is less "snap" no matter how tight you set the squish. And the transfer port design is anemic, although the 046 case doesn't really have the volume to support the additional displacement. So I'm guessing the manufacturer kept the transfer ports skinny to keep the velocity up.

I will bet money its a timing deal.
Thanks guys!