MCCULLOCH The official McCulloch thread

[heimannm]

For the record, the big bore Walbro SDC carburetors (SDC 18 & up) all have the same throttle bore and venturi diameter; 0.812" throttle bore and 0.750" venturi diameter. Most of the Tillotson HL carburetors and the Walbro/MAC carburetors used on saws have the 0.812" throttle bore but a .687" venturi diameter. The smaller Tillotson HS carburetors (i.e. 43) matched the smaller SDC carburetors at 0.750" throttle bore and 0.625" venturi diameter. The larger HS carburetors (70, 71) matched the HL and MAC carburetors at 0.812" throttle bore and 0.687" venturi diameter.

The HL272 used on the welder is 1.000" throttle bore and 0.875" venturi diameter.

Some of the MAC BDC pressure pulse carburetors (BDC2 & BDC4) were 1.375" throttle bore, The BDC14 was more conventional with 1.375" throttle bore and 1.125" venturi diameter.

It was interesting that there were a number of the smaller 10 Series saws that came with the bigger SDC's, and some of the bigger 10 Series and large frame saws came with the smaller SDC and HS carburetors...go figure.

I have and SP125 that came with the smaller HS, installed a bigger SDC and the difference was remarkable.

Mark

 

[the 28inch mac man]

For the record, the big bore Walbro SDC carburetors (SDC 18 & up) all have the same throttle bore and venturi diameter; 0.812" throttle bore and 0.750" venturi diameter. Most of the Tillotson HL carburetors and the Walbro/MAC carburetors used on saws have the 0.812" throttle bore but a .687" venturi diameter. The smaller Tillotson HS carburetors (i.e. 43) matched the smaller SDC carburetors at 0.750" throttle bore and 0.625" venturi diameter. The larger HS carburetors (70, 71) matched the HL and MAC carburetors at 0.812" throttle bore and 0.687" venturi diameter.

The HL272 used on the welder is 1.000" throttle bore and 0.875" venturi diameter.

Some of the MAC BDC pressure pulse carburetors (BDC2 & BDC4) were 1.375" throttle bore, The BDC14 was more conventional with 1.375" throttle bore and 1.125" venturi diameter.

It was interesting that there were a number of the smaller 10 Series saws that came with the bigger SDC's, and some of the bigger 10 Series and large frame saws came with the smaller SDC and HS carburetors...go figure.

I have and SP125 that came with the smaller HS, installed a bigger SDC and the difference was remarkable.

Mark

I was suprised how easy it was to make a flat back/ hl to sdc adapter. Its impressive how much better the sdc carbs run then the flat back.

 

[Duke Thieroff]

My saw had a flat back before the sdc. I also tried a tillotson hl, but the big bore sdc has no compairison.....

And what SDC did you use?

 

[the 28inch mac man]

And what SDC did you use?

I used a sdc 20. I was going to use a sdc 18, but that required a idle speed screw mod. The sdc 20 worked great. The only thing I had to change was the diaphragm plate to one that didn't have a impulse fitting in it.

 

[edju1958]

You are close in thinking and the motor is the same displacement, but the Super 44A has a larger bore and shorter stroke, the Super 250 has a slightly smaller bore and longer stroke.

The longer stroke should equal more torque,even though the bore is smaller,correct?

 

[Duke Thieroff]

The longer stroke should equal more torque,even though the bore is smaller,correct?

I couldn't say with any real authority

 

[hacskaroly]

My saw had a flat back before the sdc. I also tried a tillotson hl, but the big bore sdc has no compairison.....

What does the SDC do better than the flat back? More fuel, more efficient fuel throughput?

 

[the 28inch mac man]

What does the SDC do better than the flat back? More fuel, more efficient fuel throughput?

Better everything. The flat backs are very tempermential. The sdc carbs run much nicer and get better rpm.

 

[the 28inch mac man]

The longer stroke should equal more torque,even though the bore is smaller,correct?

The problem with the 250 is whatever port design mcculloch used on them they have low top rpm. The s44 gets more rpm and cuts faster. I would love to port a 250 and see what I could do with it.

 

[hacskaroly]

Better everything. The flat backs are very tempermential. The sdc carbs run much nicer and get better rpm.

I will have to get a SDC 20 and swap them out when I get my saw running to see the difference. I have to get my two larger saws running first. Working on new gaskets for them. Just got a gasket cut to go between the cylinder and flywheel cover on my 200. It took a couple of hours to figure out Fusion 360 and drawing out the gasket and then getting it cut on the Cricut. Turned out a whole lot nicer looking than me trying to do it free-handed. Next I will be working on the cylinder to fuel tank gasket. This one fell apart, so it will be a bit more of a challenge.

 

[the 28inch mac man]

I will have to get a SDC 20 and swap them out when I get my saw running to see the difference. I have to get my two larger saws running first. Working on new gaskets for them. Just got a gasket cut to go between the cylinder and flywheel cover on my 200. It took a couple of hours to figure out Fusion 360 and drawing out the gasket and then getting it cut on the Cricut. Turned out a whole lot nicer looking than me trying to do it free-handed. Next I will be working on the cylinder to fuel tank gasket. This one fell apart, so it will be a bit more of a challenge.

Yeah it isn't bad to make a sdc adapter. You probbly seen the pics I posted of the one I made.

 

[cowroy]

 

[cowroy]

Can someone explain why this hole is open into the cylinder of this 795? The top of the piston is broken also unfortunately

 

[hacskaroly]

Can someone explain why this hole is open into the cylinder of this 795? The top of the piston is broken also unfortunately

My guess is that it is one of the drill holes used when the cylinder was machined and then plugged. It is possible the person before you attempted to drill out one of the plugs in the hopes that they could do something more than they could from the bottom or through the sparkplug hole. I had a 10-10 that lost a ring and then seized in the cylinder. It was a BEAR to get out, but it finally come out.

 

[hacskaroly]

I supposed I could have showed the other side of the cylinder where the holes are that had to be machine out.

 

[Stump Shot]

 

[qurotro]

That McCulloch girl at the end

 

[the 28inch mac man]

Can someone explain why this hole is open into the cylinder of this 795? The top of the piston is broken also unfortunately

Yep that is were the port plug should be. That is a common problem with the big macs.

 

[hacskaroly]

That is a common problem with the big macs.

Common problem that they fall out or are drilled out? Is there a way to reinstall them?

 

[the 28inch mac man]

Common problem that they fall out or are drilled out? Is there a way to reinstall them?

Its a steel plug that has a jb weld like coating over it. The jb weld cracks and causes air leaks. My guess is that it cracked on cowroy's 795 causing the plug to fall out......