Saw hits an rpm "wall"

[Pioneer]

I have a saw that starts and cuts great, but seems to hit an RPM "wall". The carb starts to spit out the fuel fog at high rpm like it is plugged up downstream. It's a reed valve Pioneer, it has no baffle in the muffler, just a large spark arrestor screen that's nice and clean. My other thoughts are point bounce and reed flutter. The saw has a lot of power, but I would like to get rid of that built in "rev limit".

 

[happysaws]

I have a saw that starts and cuts great, but seems to hit an RPM "wall". The carb starts to spit out the fuel fog at high rpm like it is plugged up downstream. It's a reed valve Pioneer, it has no baffle in the muffler, just a large spark arrestor screen that's nice and clean. My other thoughts are point bounce and reed flutter. The saw has a lot of power, but I would like to get rid of that built in "rev limit".

I would check the RPM with a tach.

Have you polished and re-gapped the points?

Advancing the timing can help with this problem; file the flywheel key a little, turn the flywheel as far as it will go CCW WITH THE KEY IN, tighten the nut while holding the flywheel CCW, and you're good to go.

 

[alexcagle]

To me it sounds like the carb governor flooder valve kicking in.

 

[kevin j]

I don't now if this applies to saws, but older two stroke motorcycles could have reed valve float or flutter and stay open. Some were designed to float and stay open, less restriction, and they essentially became piston ported engines.
However, some were designed to rely on the reed valves to work properly, and when the reeds floated partly open, there could be a reversal of air pressure pulse or wave, such that some air actually went backwards through the carb a bit, then went forwards again. It was called 'triple fueling' because each pass it picked up some fuel. The riching of the mix brought the rpm down, just like in saw tuning. Sometimes there could be a standing vapor cloud at carb entrance if no air cleaner, or a lot of wet areas in air cleaner or filter.

So, does this saw depend on the reeds to work right? The reeds might be lowering crankcase compression, less transfer, and less rpm. Or, the richening of the mix might be limiting the rpm.

Like I said,I don't know if/how it might relate to your engine, but it's something to mull over.

 

[Pioneer]

Points are good, correct gap and clean. Mixture is good, plug color verifies it. I see no way to adjust the timing on the points or coil, other than slotting the mounting holes on the latter, and I really don't want to start fooling with offset flywheel keys. Without a load on the saw, the rpm will hit the wall and then drop quite a bit if you continue to hold open the throttle. In the cut however, it seems to rev pretty good until you let off the load a bit, and then it hits the "limiter".

 

[alexcagle]

What Pioneer are we talking about anyway?
This valve opens at higher RPM causing it to let enough fuel into the carburetor venturi to unbalance the fuel/air ratio, causing a drop in RPM.

 

[s sidewall]

If it doesn't have that type of carb, check your reed valves to make sure they are seating and haven't gotten weak.

Steve

 

[Sepia]

Learned a few things on this thread - great job guys.

 

[s sidewall]

Got a old Poulan 2300 top handle that would soak your hand down with the cover off. Pulled the handle assm and found reed valve not seating, pulled reed, bent slightly and problem was fixed. Saw ran a lot better after that.

Steve

 

[Pioneer]

It appears to have the governor. The brass plug on this carb? Pioneer P-40 by the way. Found this on the net about the Tillotson HS governor.....

"A low tech rev limiter. It works off the vibration of the saw and will pour in extra fuel when the rpm/engine vibration go past a point. Also known as a "buzzball". When the saw is under enough load it will stay closed, but under light or no load it will open to keep rev's down."

This describes the conditions perfectly, so just leave the carb be?

All that aside, Pioneer really went out of their way to make it easy to work on the saw. Notice the square notch cast into the cover frame just left of the mounting screw at the bottom of the first picture? It's there to clear a screwdriver to make it easy to access the left mounting screw on the carb! Great engineering, the whole saw is made with easy maintenance in mind.

 

[s sidewall]

Well, seems like it's working as design.

Steve

 

[pioneerguy600]

I would need to know the RPM that the governor is kicking in at to say its working as designed. As these carbs and governors age they are known to start malfunctioning and the governor will open too early killing the real usefulness of these saws.

 

[kevin j]

I have a Husq 2100 project in process, Tillotson with the brass plug governor. I've learned many/maybe most o wners in the time cut a tiny piece of alum or beer can or small welch plug and put in the tip in the hole, then screwed the check plug back in, to plug the drilled orifice into the venturi. Defeated the governor and allowed 'normal' tuning.
Pretty unique and simple/cheap system if it works. Just depends on the resonant frequency of the check ball and the spring to allow the ball to open up the fuel path at a certain engine frequency.

 

[s sidewall]

Couldn't you just put in a stronger spring so it would stay shut?

Steve

 

[kevin j]

On the Tilly at least the spring and ball are crimped or pressed into the body somehow. It just looks like a set screw or plug with holes in end and side.

 

[Modifiedmark]

It sounds to me like the governor in the OP's saw is working as its intended. If working correctly they are no detriment in the actual working of the saw. Piss revving high rpms in the garage don't make them cut any faster under load out in the woods.

Let me give a word of advice to anyone thinking of removing the governor from there saw. First there were two different designs in HS choke shafts, the one pictured on the OP's saw requires the choke shaft to be removed from the carb body in order to have room to get the governor out. Do not loose the ball and spring that will come out of the carb body with the shaft removed. Some have a flat on the choke shaft lever and have enough room to remove the governor without removing the choke shaft.

Second, the governor is installed with a sealant and is very tight in the threaded hole, and since the governor is brass, the slot in it is very soft so if you do not use the correct screwdriver to fit that slot correctly you will strip the slot out. You will play hell getting it out if you do that.

So unless your governor is malfunctioning, there is little advantage to removing and blocking it off. Most HS carbs are set at one turn out from closed and if you need to adjust much farther from that, you know there is something wrong with the carb or something else wrong with the saw.

Just set the H screw at one turn out, then put it in the wood for the final fine tuning. Set it for max power then pull up off the cut just to check that it's still four stroking.

 

[Big_Wood]

when those governors go bad you'll know it. you've never seen a saw 4 stroke in the cut until you have an old HS governor dumping fuel. i don't run saws with the carb governor anymore really but when i did i plugged them all. i never had one blow up but back then i probably ran them richer then i had too. worth it to me cause the saw behaves much better for every task. could only really stand them for bucking with the governor functioning. easily reversable as well as it is just a aluminum disc cut out of a soda can to block the governor from fueling. i would only do this to a beater you intended to run the piss out of for firewood or something. if your a collector i'd keep it original.