McCulloch Chain Saws

[Jeffrey F.]

I was very kindly given a Tillotson carb kit along with a printout of a McCulloch Flatback shop manual by Jeff today, thank you sir!

so I'm looking through the pages and what's this?? A rubber one-way duck valve in the impulse passage? If this is located somewhere inside my Mac Flatback carb, it is almost certainly hardened up and my guess is not allowing the crankcase pulses to make the carb function properly....

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Sorry about that thought maybe book would help
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[MacAttack]

The book is very interesting! Lot's of good info about the theory on those carburetors.

I'm guessing the flatback and pressure pulse carbs work on very similar principles, they appear very similar.

 

[Okie]

Macattack:

more hands on experience with Mac's

referring to post 49,916 about carb pulse priming (getting gas to the engine)

One thing I like about the HL Tilly carbs is if you suspect the fuel pump is not working the air box cover STUD that is on top of the carb is also a fuel pump test port.
Mac had a test barb that screwed into the top of the carb after the existing stud and COPPER SEALING WASHER is removed and a low pressure gauge (10 lbs or so) could be attached to the stud and as the engine is cranked several times (about 10 times) the gauge should show some pressure of around 1/4 to 1/2 lb EVENTUALLY as the engine is cranked. Do not remember the exact specs for pressure but main thing is if any pressure is noted the pump is sending gas to the carb.
I've made a test barb using a threaded stud and drill a small hole in the stud for attaching a pressure gauge.. Also a hint that the saw is not getting gas is if you removed the stud and the port hole is dry the carb is not pumping any gas. (the stud threads into the carb top are standard SAE fine threads nd the test port can be made from a bolt)

Also the majority of my 1-40 through 1-50 Mac saws start easier when the carb throttle is wide open, both cold and hot. To do such I place the saw on the ground and hold the throttle open with my shoe toe OR I take a #10 gauge SOLID copper wire and make about two turns around the handle and throttle trigger to hold the throttle open and when the saw starts just push the wire down and leave it one he handle.
I seen on this site sometime back where a guy installed a throttle advance on his old Macs but do not remember the suggestion.
The old Mac's when they start do not high rpm scream and are slow coming up to speed therefore more time to come back to idle when the saw starts.
On a cold Mac I usually at first just try manually manually priming them through the muffler and not have to removed the air box cover.

 

[MacAttack]

Okie,

Thank you for that excellent write-up! Very useful info on the HL Tilly carbs.

I'm hoping to have time tonight to start putting my HL back together, I'm very cautiously optimistic this will get my 450 running.

I also got some parts from Mark yesterday for my build that I will share later.