Mac 101

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ted johnson
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Re: Mac 101

Post by ted johnson » Thu Feb 09, 2017 10:00 am

Back in the late seventies, a rookie friend of mine was running a stock 101M/C in street racing. Just for the helluvit we took off the BDC in the Ft. Myers street race pits, changed out the reed cage for one I'd modified for the Mikuni carb, and installed one of my gas 34mm Mikunis. He ended up running in the top five at the huge course in B Limited. We put the stock setup with the BDC back on for the second day, and he ran third in 125cc reed with the same engine. Not bad for a rookie against Florida's best in 125cc Mac. The Mac ran great with the Mikuni. It was faster and easier to tune than the finicky BDC ever was. TJ

Brian Naufzinger
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Re: Mac 101

Post by Brian Naufzinger » Fri Feb 10, 2017 4:37 pm

Ok I'm going to get the hr-181. Will this bolt up to the stock manifold. Or will I have to get an adapter for it.

ted johnson
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Re: Mac 101

Post by ted johnson » Fri Feb 10, 2017 7:46 pm

Somewhere there is a photo article showing what you need to do. The big Mac reed cage has four pulse holes in the corners of the flange. These must be plugged. If memory serves, the existing holes are exactly the diameter of a No. 8-32 tap drill hole. Get a No. 8-32 tap, tap these holes and install four 8-32X 1/8" long socket head cap screws in the holes. Apply a little epoxy to the threads to thoroughly seal the threads, and wipe away the excess epoxy. Get the proper gasket for the HR carb mounting flange. Buy two 5/16" outside diameter X 1/4" inside diameter X 1/4" long bushings to place on the studs. The HR has 5/16" holes where the BDC has 1/4" holes. Place the bushings on the studs, place the gasket over the bushings, making sure the pulse hole is oriented correctly as to the way you're going to mount the HR, and mark the pulse hole. Drill the reed flange .141 diameter for the pulse. DO not get drill shavings into the engine! Now install gasket and carb with two nuts and hook up a throttle cable or rod. That's it! The existing Mac manifold works fine, as does the reed when modified as above. The 8-32X1/8" LG screws and the bushings all can be purchased at http://www.mcmaster.com. Ted

REAR
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Re: Mac 101

Post by REAR » Sat Feb 11, 2017 7:51 am

Brian,

The original 'TECH' article along with many others that Ted mentioned was a feature we offered before we experienced a web crash. Perhaps its time for a new article on this conversion.

Ted's instructions on what you need to do are exact and very simple to follow.

The 5/16" by 1/4" bushings as Ted described are pretty much self explaining since the original mounting holes in the carb flange are larger then the manifold studs so you are basically creating a sleeve to slip over the studs to increase their diameter. Cut the sleeve's slightly shorter in length then the thickness of the carb mounting flange so that the carb mounting bolts will draw the carb to the manifold and not bottom out on the top of the sleeves once they are slipped over the original manifold studs.

Best way to drill the pulse hole for the carb is to take the manifold off of the engine.

If you have any more questions feel free to ask.

R.E.A.R.

ted johnson
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Joined: Fri Jun 15, 2007 7:59 am

Re: Mac 101

Post by ted johnson » Sat Feb 11, 2017 10:35 am

The bushings I use as spacers are McMaster Carr part No. 6391K126 They're oil impregnated, but that doesn't hurt anything. They are already the right length and cost 43 cents each. The screws are McMaster 91251A179 for a package of 25. $5.48 for enough to last a LOOONG time. The small diameter of the socket head ensures that they won't interfere with the carb flange. The flange of the reed cage is very easy to tap. Like REAR said, take the reed out of the manifold to drill the pulse hole and tap the old pulse holes for the plugging screws. TJ

Brian Naufzinger
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Re: Mac 101

Post by Brian Naufzinger » Sat Feb 11, 2017 11:12 am

Thanks a lot guys..

Rob Voska
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Re: Mac 101

Post by Rob Voska » Sat Feb 11, 2017 1:19 pm

I want to add that after pulse hole is drilled in reed cage stock gaskets need modified & I usually use a small round file to chamfer the 1/2 of a hole in the manifold so I get a good pulse.

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