Adjustability of Newer Saw Carbs?

[SteveSr]

Hello,

I just got through rebuilding a Stihl MS251. It has the Zama C1Q-S295 carb on it. It is currently bone stock.

I carefully removed the limiter so that I could retune it if needed. In the process I measured that the factory setting for the "H" screw with the limiter full CCW was 1 1/4 turn from lightly seated. The SM called for a base setting of 1 1/2 turn but that was with a different Walbro carb. I put everything back together but didn't punch down the limiter so that I can still adjust the carb with a 2mm allen key.

So on Saturday I went to do some trail work and the saw performed pretty much as expected. At the end of the session we had a bar length plus trunk of beech on the ground. I wanted to see if I could hear the 4-stroke in the wood. Yea, this thing has a rev limited coil on it. After the first bar buried cut it was definitely running hot and the idle picked speed up after exiting the cut. I couldn't easily tell if I was hearing 4-stroking or the limiter.

I opened the "H" another quarter turn and tried again. Same results. I ended taking it another two 1/4 turn adjustments for a total screw opening of 2 turns. Again I still couldn't hear obvious 4-stroking. So it would appear that the "H" screw has very little effect on the overall mixture. Has anyone noticed if this is common on the newer carbs attached to strato saws?

I did bring my tach to the outing but couldn't tell if I had a weak pickup signal or if the saw was hitting the limiter. I think I'll wrap an antenna wire around the plug wire to make sure that I have a good tach connection and try again.

Your thoughts?

 

[huskihl]

It’s common on many new carburetors. They have a fixed jet inside that flows 80 to 90% of the required fuel, and then a small orifice of some sort feeding the adjustable H screw that may only make a subtle difference. But usually those that only make a small difference will continue to deliver more fuel up to about 4 turns out

 

[Backstage]

Interesting info huskihl, thanks for that. How far back has that been a thing?

I have trouble with tuning limited coil saws as well.

The strato limited coil generations are kind of going to be a half-breed. Not quite autotune, not quite properly tunable by ear. With the old carbs and unlimited coils, you essentially limited the top rpm by setting them to drown when they got too fast.

Limited coils are another animal. It’s limited from going too fast electronically, so the tach is basically useless for the H setting. Success I‘ve had with the H jet has always been by turning it down until it just starts to bog in the wood, then turn it back out rich a bit.

 

[huskihl]

Interesting info huskihl, thanks for that. How far back has that been a thing?

I have trouble with tuning limited coil saws as well.

The strato limited coil generations are kind of going to be a half-breed. Not quite autotune, not quite properly tunable by ear. With the old carbs and unlimited coils, you essentially limited the top rpm by setting them to drown when they got too fast.

Limited coils are another animal. It’s limited from going too fast electronically, so the tach is basically useless for the H setting. Success I‘ve had with the H jet has always been by turning it down until it just starts to bog in the wood, then turn it back out rich a bit.

The fixed jet plus supplemental fuel from the h and l jets has been around for awhile. Maybe 30 years. 288 comes to mind. But not to the extent that they are now with some carbs needing nearly a full turn to notice much of a difference

 

[Old2stroke]

I wanted to see if I could hear the 4-stroke in the wood.

Shouldn't expect to hear it 4-stroking under load in the cut, normally would get that sound when you lift the bar up to take the load off, but with a limited coil it's tricky, the best tuning is to start with it so rich that it sounds rough under load and won't speed up, then just start leaning it out until it runs crisp and clean. If you start tuning from being too rich it reduces the risk of ending up with a setting that is too lean.

 

[SteveSr]

Thanks for the info. Yes, the fixed jet "bypass" carbs have been around a LONG time. I have never seen one with this limited range until now.

It is interesting that Stihl is intending for 3/4 of a turn on the "H" screw to be able to compensate for a change between sea level and 10.000 ft. I would question whether or not this would make it... unless it is a compromised tune... too lean at sea level and still too rich at elevation. This may help explain why a LOT of these burn up.

I am glad that I didn't do the muffler mod on this saw that I was contemplating. It is beginning to look like you couldn't compensate it with this stock carb.

 

[North by Northwest]

Some new non m-tronic Stihl and pretty well all Echo's are extremely lean tuned from the factory . Your right on the ragged edge with the factory jetting ! lol.

 

[SteveSr]

Some new non m-tronic Stihl and pretty well all Echo's are extremely lean tuned from the factory . Your right on the ragged edge with the factory jetting ! lol.

This saw is intended for use by volunteer trail builders/maintainers. Therefore I would like to move it away from the "edge" as much as possible without compromising performance too much.

 

[sean donato]

Some new non m-tronic Stihl and pretty well all Echo's are extremely lean tuned from the factory . Your right on the ragged edge with the factory jetting ! lol.

Add in every homeowner grade manufacturer to thay as well....

 

[North by Northwest]

Add in every homeowner grade manufacturer to thay as well....

Yeah , its getting sickening the length that epa. restrictions reach out .

 

[Lightning Performance]

Tuning from rich to the limiter keeps the mix fat. Going the other way is a lean condition most times.

If you tune to the limiter it sounds very different then a fourstroke burble tuned below the limiter. Limiters near 13.5 are easy to toon imo. Ones at or below 12.2 are easier to pass and get on the lean side. As Kevin said you can tune out more on the H but I like to stay under 3 turns out for the finished product so it can go up in elevation from sea level here. If your running a restrictive air filter you have to tune leaner. No air filter knocks down the fuel signal. Tuning modified tools with stock carbs at or above 9,000 feet in fog can't be fun so you need to be from 1/2 open or more. If not your missing it going fat with even less air available to the stock air filter. It's not your friend on a tall mountain. Changing the main jet down in size or lever adjustment(s) will get you back on track most times.

With a limiter you essentially have a built in tac if you know how to approach it and adjust from there. Mostly overlooked by those who don't understand what is going on.

Milling is whole nuther thing with the added heat buildup as you mill long session like hot lapping the saw. The first tank will get you warmed up and usually heading into a lean condition. Climbing 1,000 rpms on tank number three of fuel vs a cold saw isn't uncommon.

Bonus info:
Blue carb diaphragm material sucks and not in a good way.

 

[Lightning Performance]

Interesting info huskihl, thanks for that. How far back has that been a thing?

I have trouble with tuning Limited...
Success I‘ve had with the H jet has always been by turning it down until it just starts to bog in the wood, then turn it back out rich a bit.

Your creating a very lean condition doing this and are likely to hurt parts in that state of tune.

 

[Backstage]

Your creating a very lean condition doing this and are likely to hurt parts in that state of tune.

Okay, I can see that. I haven't experienced a lean condition damaging parts, probably because I'm "turning it back to rich" more than you're thinking?

Perhaps a better way would be to turn it UP(out/rich) until it bogs in the wood, then lean it out a bit until it cuts hard?

 

[huskihl]

Okay, I can see that. I haven't experienced a lean condition damaging parts, probably because I'm "turning it back to rich" more than you're thinking?

Perhaps a better way would be to turn it UP(out/rich) until it bogs in the wood, then lean it out a bit until it cuts hard?

Yes