Another SP-125/101b build

[rwoods]

Getting down to it on parts. Still a couple items to get and some that will be here in "4-6 weeks"...

So, what do you guys think, too much?

View attachment 222161

Probably not too much for the 101B but how about the clutch. I believe Jacob J. had a post about his dad's kart saws eating clutches. Ron

 

[Brian13]

Probably not too much for the 101B but how about the clutch. I believe Jacob J. had a post about his dad's kart saws eating clutches. Ron

It probably will wear the clutch out faster, but I dont think cpr is going to use it everyday:hmm3grin2orange:.

 

[cpr]

I better stock up then. I have a NOS 4 shoe here to thrash. Guess I'm looking for input on gearing. Too fast/get more? I was going to start with this because I am concerned about over-revving an 8. My sdc20-equipped 125 with little weasy reeds rips an 8 with authority...

 

[RiverRat2]

Probably not too much for the 101B but how about the clutch. I believe Jacob J. had a post about his dad's kart saws eating clutches. Ron

Yeah I thought those were 797's and CP125's w/101B hybrids Im sure he will be along shortly to straighten us out....

 

[Jacob J.]

Probably not too much for the 101B but how about the clutch. I believe Jacob J. had a post about his dad's kart saws eating clutches. Ron

Yeah I thought those were 797's and CP125's w/101B hybrids Im sure he will be along shortly to straighten us out....

Yep and yep. He bought a whole case of clutches one time, 24 in a box I believe. He was working some big, big wood down by the coast. The job was several
months long and when it was over, he had four good clutches left. I can't tell you how many drums he went through, probably 50. He used to buy the drums
12 at a time.

 

[cpr]

Any input on that duckbill? Carb is whistle clean and the kit is here with a new one. It goes in there, but which way? Throttle shaft is going to get cut to narrow it up.

I still need a new rear a/v mount, get it set up, make a throttle rod and return spring, a bar (ordered), and a different clutch cover. I posted in the swap meet thread. If anyone has a plain 125 clutch cover to trade for brake style one, pm me. It'll come together soon enough. I hope to drag it out at the central Ohio GTG and whip some ass!!!

 

[cpr]

I really don't get all the flatback fear around here .

View attachment 224132

Turns out this kit is for the double pumper... now I need the regular flatback kit for the fuel pump parts. You'll also notice that the throttle shaft is cut. I took about 5/16 out of it. Be careful cutting it, it bends easily (yeah, yeah, YEAH, I straightened it). It's back together. More in-depth pictures will come on the carb when the other kit arrives. There's a lot going on in there.

 

[cpr]

I'm far from an expert, but let's take a tour of the BDC carburetor.

The impulse is lower left just like all the other flatbacks and HLs. The gasket is cut at all four corners and the reed V is drilled, too. So the fuel pump can drive in any position.

This is a no-welch-plug carburetor. Here's how it looks on the wet side of the diaphram with the cover, gaskets, and valve in place. Fuel enters through the metering nozzle at the top as always. It's fed through the 2 ports identified. The Main has a check valve just like the bottom fuel pump. I think it is there so diaphram position changes won't pull fuel back through the main and lean out the engine.

Now it gets a little confusing. I'll try my best to explain. First the idle circuit. Fuel enters at the left of the bowl-shaped channel through the top plate. The idle vacuum port at left, I believe, relies on low pressure inside the delivery channel due to vacuum from the jets to pull fuel over. It's ducted there through the gasket/valve/gasket sandwich. Or maybe some kind of bleed? Not really sure. Anyway, fuel moves to the right and then turns up along the low speed nozzle into the low speed metering chamber. At idle, fuel is delivered through the idle jet. Part throttle opens Accleterator jets #1 & #2 in succession.

Now it gets more complicated. Accelerator jet #3 takes fuel out of the main jet nozzle. It's delivery channel is parrallel to the carb's bore and goes from the Acc. #3 jet chamber up at an angle, through the main jet, and to a vacuum port in the carb bore in front of the venturi. What I believe happens is that at part throttle, the pressure differential siphons fuel off the main jet and into the acceleration #3 jet. At WFO, it is inactive and pulling all from the Main. The High needle threads right into the Main jet stand pipe and the needle constricts as necessary. The only other thing is the duckbill feeding the dry side of the diaphram instead of an atmospheric vent. Haven't figured that one out just yet. The fuel pump section is just like a saw flatback. A double pumper is, I imagine, much more complicated...

Clear as mud?

 

[heimannm]

I have the Pressure Pulse carburetor manual in hard copy, if you can wait a month or so I can scan it in when I get back. Otherwise, I think they come up on e-bay on a regular basis and do a pretty good job of explaining the operation fo those carburetors.

Mark

 

[cpr]

Yes! Brown says I have a tracking number. Motivation to complete this thing is restored.

 

[RandyMac]

Clutch/drum wear is a given with hard use. Of the two 797s the modified one went through a drum in two weeks, it pulled a 60" bar like the stock one did a 48", both had 8 pin .404. I quickly learned to ease off at the hint of a stall in a bound cut, it would turn a drum blue in an instant, it would also try to blue an oil wet chain on dry knots. Another McCulloch that was tough on clutches was the 660, a drum a month and a clutch every two months, that was most likely the dry DF that was it's steady diet.

 

[cpr]

I'll keep an eye on the clutch, but the run time you pros put on in one day, this thing won't see in a couple of years. That, and the bar I ordered is nominally 30", but factoring the spikes and wanting to keep the roller nose out of the cut, it will never see more than 24" timber, but what a ride it'll be!

 

[RandyMac]

Why fool with .404, you really need 9/16" full chisel.:hmm3grin2orange:

 

[cpr]

It isn't chisel, but I have some 9. 3/8 for comparison.

View attachment 229129

 

[stinkbait]

How come no one ever uses the walbro WB carb on big saws like this? They have a large bore in a small package. The ones I have have about a 1-1/8" throttle bore. They also have the double pumper pump diaphragms.

 

[Eccentric]

.404-8 with 48" and 60" bars Randy? Yikes. No wonder you went through clutches. I guess I'll be fine with an 8pin .404 rim on my 790 with a little 31" RN afterall...

 

[Eccentric]

How come no one ever uses the walbro WB carb on big saws like this? They have a large bore in a small package. The ones I have have about a 1-1/8" throttle bore. They also have the double pumper pump diaphragms.

Good question Mitch. The Homelite 750 uses a WB9.

 

[cpr]

Hope it's better than the WJ in my 655... ready to Verlander that thing into the field :hmm3grin2orange:.

 

[RandyMac]

Its all in the chain filing Aaron.

9/16"

I also have the bar that goes with that chain, a nearly new 34" OAL hardnose.

 

[Eccentric]

Its all in the chain filing Aaron.

9/16"


I also have the bar that goes with that chain, a nearly new 34" OAL hardnose.

Beefy. Is that going on the project that I'm contributing to? The loop of 5/8" chipper that went to Mark with the 77 was HUGE.