MS660 stall/stumble when stopped in cut

[superduty5.4guy]

Hello all, I've lurked here for quite some time and have found just about everything I have ever needed just by searching. I would like to thank you all for the knowledge that you share on this site.

I have finally come to an issue that has me stumped.

This may be common but I have not experienced it with any other saws.

What happens is that when I'm in a cut and release the throttle quickly and the chain stops, the saw will almost stall/sometimes stall.

This condition can be replicated out of the cut by engaging the chain brake and hitting and releasing the throttle briefly.

A little background on what's been done so far. All parts appear to be oem and no aftermarket additions.

I did a slight muffler mod a while back by adding an exhaust hole just a tad smaller than the original, right next to the original. I also opened up the holes in the cage/baffle.

It originally had the WJ69 carb on it and I recently changed it to a brand new WJ76 with the .74 jet and no limiters. (The issue is the same with both carbs, I thought maybe the new carb would help)

I like to keep the idle around 2700 and WOT at 12,500. At this setting both H&L are about 1 turn out with the new carb. (About 1.5 turns out with the WJ69)

If I keep the idle at 2500-2700 the issue is much more pronounced and will actually stall if I suddenly let off the throttle in a cut. If I bring idle up to 3000-3100 the saw will still stumble if I stop in the cut but not stall. The issue I have with the higher idle is that the chain (correctly tensioned) will slightly creep on the bar.

I've tried fattening up the L and adjusting Rpm with LA, as well as leaning the L and adjusting with LA to see if this helps but it seems to be about the same no matter what setting I'm at.

I just replaced the spark plug (wsr6f), OEM fuel filter, OEM impulse hose and OEM HD2 filter.

Intake boot is in good shape, compression is excellent, starts and revs out nicely, clutch looks good, clutch bearing looks good etc. I have not done a pressure/vac but idle and WOT are not erratic and remain constant...Other than when I stop it in the cut. Rpm does not change when the chain brake is engaged at idle.

Everything else behaves correctly other than this.

So my question is, is this normal and should I just keep the idle at 3k or is there something I'm missing?

To recap, the issue is that when I'm in a cut WOT and let off the throttle suddenly, the saw will drop rpm too far and stumble/stall. This will also occur when the throttle is 'blipped' with the chain brake engaged. The idle will drop way down, saw will stumble, then if it doesn't stall, rpm will slowly rise back up to the correct rpm.

Sorry to be so long winded, I just wanted to try and cover all the bases. Thank you in advance for the help!

 

[plutus]

idle should be around at 2500rpm. try richening the mixture. adjust with speed screw to 2700idle then with mixture screw richen it and drop to 2500.

 

[Ozhoo]

It's way too fat on the low side. Set it to 1 turn out on both, reset the idle and test.

 

[superduty5.4guy]

Thank you for the replies. I tried, just now, bringing it in lean and adjusting the idle down to spec, tried it fat and adjusted idle up to spec, and also set it in the middle of the 'surge/load up' range and adjusted the idle.

What's done 'best' so far is bringing the idle down with LA, leaning L to peak rpm, back L out 1/4 turn and set idle again with LA, then set H to 12.5k.

Even at the 'best' setting, it still wants to die if I let off the throttle mid-cut if set at 2500. At 2800-2900 it will stumble down to about 1900 when I let off, and creep back up to 2800-2900, but if I set to the correct 2500 and do the same thing it will stall about 70% of the time when I jump off the throttle mid-cut. For reference I'm using a DTI TT20k tach that's always seemed spot on.

Also, just to cover a couple more bases, I'm using fresh 93 octane with Stihl ultra (silver bottle) mix. I've checked the fuel lines and all are nice and pliable with no obvious leaks. Tank vent holds a little pressure, as they all seem to these days. I also replaced the 'spring' inside the spark plug boot as the old one seemed a tad loose....I think that's about it

 

[plutus]

Buddy your problem is you are leaning out I recommended to drop the rpm by doing oposite richening it.

 

[smokey7]

If idle drops after slipping the throttle it has to be rich. Maybe the high speed set at the 12500 is really too rich for the saw. I tune a saw to what it runs right at. I never pay attention to a certain rpm. Your low and high combine to a total jetting number. If you went to a larger jetted carb it will be putting more fuel at the same turn setting. So I would tune it to alight 4 stroke on the high. Go back to setting low to a highest idle with no lean bog. Then adjust the la to a stable idle. Small fine tuning after like 1/16 a turn at a time after as needed to get desired idle throttle response and total function of saw. Right or wrong this is how I do it and always works provided nothing mechanical wrong

 

[Ozhoo]

Listen to Smokey... he's all over it. There's too much unburnt fuel left in the engine when you let off the throttle.

 

[superduty5.4guy]

I think smokey might be on to something. I leaned in the L to just shy (rich) of peak rpm, adjusted idle down and dialed in H to about 13,200. It definitely made the issue better. I'm almost wishing I would not have done the muffler mod now as I'm somewhat hesitant to run the rpm up to where they need to be, as I can see it would like to run higher as it's still 4 stroking pretty solidly at 13,200.

I greatly appreciate all the advice from everyone.

 

[Ozhoo]

I like to get a good solid idle before I even touch the high side. If you've got the low side set right, a saw will idle till it runs out of gas. If you're fat, it'll sit there and load up till it stalls. May take 30 seconds or 2 minutes... and as you've noticed, that's usually just short of peak rpm.

 

[smokey7]

Glad to help. Played that game before. 13200 seems safe to me.

 

[smokey7]

I'd be curious to see if it was all better if you went to 13600 13800. I know it sounds line its high rams but if its properly tuned there i would run it. Think about it saws never run no load in the cut. So max wot really don't matter. Tune it till its right and cut wood. I know it sounds harsh but proper is proper

 

[superduty5.4guy]

I will give that a whirl tomorrow afternoon. I will probably do a pressure/vac test before I run it like that for too long, need to buy a new mightyvac as mine has seen better days.

Tonight's project was replacing a fuel line on an 036. I went ahead and ordered the intake boot and impulse for the 036 while I was at it.

 

[smokey7]

Smart to vac test it. But just remember a saw or any engine for that matter tells you what it needs tune wise. If it runs proper and sounds good go fir it. I can't say I've ever used a taco to tune a saw. I have checked them later just to see what they are doing. Mainly my skis. Looking to see if I've found a max rpm. But that changes with pump load impeller, pump nozzle, jetting, altitude, and pipe. Saws are pretty simple. Keep in mind I won't rum wfo no load for more then 1-3 seconds. Maybe up to five if i have one that seems to lean out in cut.

 

[superduty5.4guy]

Well I think I got it. I was able to trim in the L slightly and get a nice solid 13.6 on the high. Decent 4-stroking out of the wood and cleans up quick once in the wood. I'm still apprehensive about running that high but I reckon I will get used to it haha, I mean after all it's 4stroking well.

I didn't get time to vac it yet but it seems pretty good and smooth.

Thanks again everyone for the help

 

[smokey7]

I've never ran a 660 but I've seen guys run 15000 on a stock bottom end no load. Just keep a good load on it and enjoy. You probably have a good timed jug that rubs good. I still think your safe as long as it still 4 strokes even lightly. Now that the issue is gone you will find yourself tweaking the tune here and there now.

 

[superduty5.4guy]

Wow 15k is smokin! Yeah I'm in a constant stage of tweaking unfortunately, for whatever reason the 660 is just a bit more sensitive than the others I have.

 

[smokey7]

Yea I can't say I agree with that high of rpms but I've seen some big saws do some high revs. I am a constant tuner of everything. It started in my dirt bike days and saw the difference. I am a fan of getting all a motor has to offer and am known to flirt with the edge. Also i tune for what I'm doing, as in lots of noodling or bucking large cuts back to back on a hot day I will back off the edge just a little. Same with dune riding is go up a jet or atleast open up the jets a bit. I've burnt down a few sleds and bikes finding out what works. It was worth everyone of them for the knowledge I've gained.