Mac flatbacks on chainsaws

[Terry Bentley]

On a lot of the saw motors that use flatbacks carbs, some of the newer models have a bombsight over the high speed pickoff. Anybody putting these to use? I have a box full of them. I noticed the venturi is smaller than on kart models. Thats easy enough to fix if it nets any performance gains. These carbs are fitted with primer pumps instead of choke plates, which can just be removed and plugged. So there are no holes for choke shaft drilled, which always looks better on kart engines. What are people doing for new diaphrams? Is anyone sitting on a hoard of kits? Would there be enough demand to reproduce them if not? BTW, am I asking too many questions at one time?

[mcbob]

Fellas are selling those kits on ebay the primers are near impossible to find not that it would worry you though

[Scott Kneisel]

Most of the saw flatbacks also have considerably smaller throttle bores than the kart models. I think one or more of the bigger saws like the SP125 have full size flatbacks but not many. The saw flatbacks have a spring, orifice and ball check valve under the 1" welch plug instead of the capilary seal (screen) on the kart models. They do take the same diaphragms.

I rebuilt a saw flatback with the site, eliminated the primer and plugged the feed hole and installed a kart throttle shaft arm and spring which bolted right up. I tried it on a dual carb mani (VanTech) on an Mc20 and used it on the 3rd port side. Those carbs seem like a nice size for the 3rd port intake . I ran it with a kart flatback on the reed side. I never could get it tuned, the carb never really performed like I thought it should have. I replaced it with a kart carb and it ran perfect and tuned easily like it should. So the carb may have had a problem that I didn't know about but I just gave up on them at that point. Someday I may try another on an Mc5 or Mc6 as a single and see how it does because I have a bunch of them too.

Anybody else have any experience with these carbs on kart engines???

[mcbob]

The SP125 carb's are tillerton 124's SDC the smaller square carbs the ones your mentioning are off the 797 saws.

[Terry Bentley]

The throttle bores are .813" instead of .940". My objective(if its worth any gains) is to bore at least the venturi to 3/4" and match kart models. Drill passages to meter fuel. It would remove the bomb sight, but not important if carb responds like the kart model was known for. The carbs seem to work at idle and full throttle. Anything else forget it. That is something I am dealing with on all of these old saws I still have in use. Both Tillotson and Mac carbs. I guess pushing 10%+ corn was not an issue back in the early 60's on chainsaws. With both needles about 2 turns out I can almost cut wood....well at least for a little while when its not very cold outside. I expect to open the idle and transition, but need to add an air bleed to the low speed circut and enlarge passages on the saws withTillotsons first. The bodies of flatbacks are getting tumbled to a nice shiny finish and the stacks polish to a mirror shine, so they are slowly being replaced with Tillotsons.

I obviously dont need the primers for kart conversions. But the parts are so simple it would be very easy to copy pieces on any saws I still have using them. I would prefer changing over to a choke anyway. McBob, are the SP 125 carbs like a WB3A like used on KT 100 or WB20 Walbro? Also, which 125 models have the chrome bore?

[mcbob]

The tilly SDC 124's on the 125's are either with 2 adjustable needles or as on the 125c with fixed high speed
The 125C was the chrome bore model. As regards the walbro dont know for sure.

[Terry Bentley]

Here you go on the carbs.