Mac 49E Carb

steveohara · Post by **steveohara** » Sat Sep 10, 2011 11:26 pm

Have any of you fellas run a Mc49 or similar 80cc motor with the stock 49E carb? It is the Walbro SDC carb with a fixed main jet and only one adjusting needle and mounts up on the simple sloper manifold with the two finger reed petal.

I would like to hear about your experiences with that carb and manifold, what size main jet you ran, if you ran it with a pipe or just a header or box muffler.

How reliable and consistent is it as far as idle and ease of starting? Will it deliver enough fuel consistently to keep a 49 from running too hot when driven hard on a race kart?
Do the reeds hold up well? How about the pumper... does it last or need to be replaced often?
Anything you can share will be appreciated.
Thanks.
Steve O'Hara

Butch Kavanagh · Post by **Butch Kavanagh** » Sun Sep 11, 2011 11:10 am

this could be interesting as us rookie fathers lied so bad back then. i loved it when one, flushed out his engine after the races with the same fuel used to race, just to play games. we've got some notes from our racing diary. son-vince, did have some luck back in the old wka rookie days. i'll find it mon nite.

Scott Kneisel · Post by **Scott Kneisel** » Sun Sep 11, 2011 4:54 pm

Hi Steve,
I have never run one but I know of two guys who tried to. Both of them had to partially choke the engine with a partial restrictor plate across the choke bore to get it to run rich enough. I have never had one apart but if they have a removable main jet, that was probably the problem they had. I would think that the very small venturi size and throttle bore would really limit the rpm which for me is the most important thing in getting a 49 to perform. Wacko advanced timing and a 1" bore carb with 8.5:1 gears.

Of course I did never try the SDC so I can't say for sure, maybe McCulloch had a better idea.

This will be an interesting thread. I am sure some will speak of the highly volatile additives that would evaporate out of the gas before the fuel was tested after the race. Do ya think!!

Scott

steve miller · Post by **steve miller** » Sun Sep 11, 2011 8:13 pm

the key to passing tech was .... i demanded 2 full hours of cool down time ..and in that 2 hours you drink as much beer as possible with the tech guys ..be suprised what evaporates outa your fuel and how hard your tires can get in 2 hours ...ohhh how i miss the 80s ..!!! hey wait are the 80s considered vintage ..?? anyways ..steve i have carb intake and a whole box of gaskets reeds cheater jets everything you could imagine ..that the proper racer needed when he was a rookie ..your more than welcome to try them ..

steveohara · Post by **steveohara** » Sun Sep 11, 2011 8:28 pm

This may ruin the fun but I am more interested in ease of use and consistency factors rather than max performance. The part about having to use a restrictor plate ties in with my finding that the fixed main jet in one carb I have here is a "28" and the other carb has a "31"
I'm helping a vintage karter with a setup for inexperienced kids and it needs to be able to run clean at low speeds, part throttle, different weights etc.
A good comparison might be to ask if anyone has used one of these carbs on a 49 on a mini bike?
Thanks for the replies.
Steve O'Hara

Rob Voska · Post by **Rob Voska** » Sun Sep 11, 2011 9:31 pm

Steve a friend of mine has a Dart with twin 49's & those funny little carbs. They run OK but always seem like something is wrong, to lean on low end don't want to take off etc.... I think you would be much better off with a small bore Tilly like came on the 580's or even the saw carbs. with the choke. I know you can set them up in your sleep, easy to get parts for or even use one of the modern small bore K80 tilly carbs. Bet there are a lot of those out there.

steveohara · Post by **steveohara** » Mon Sep 12, 2011 12:13 am

Rob,
Thanks!
That is just the kind of feedback I am interested in hearing about.
I have plenty of small Tilly carbs and parts so that is the obvious choice unless I were to hear some real good reasons to use the little Walbro.
Anyone else have any info?
Steve

Terry Armstrong · Post by **Terry Armstrong** » Wed Sep 14, 2011 1:02 pm

Hey Steve,

I have some very recent experience using a stock 49E with the Walbro SDC carburetor which may be helpful. Bought this engine last summer to put on a Bug Stinger for my 10 year old son. Engine had been sitting for years so I took it apart and inspected before trying to run it, has stock piston and porting. Had a former Walbro Engineer available at work who helped me put a rebuild kit ($15 from local saw dealer) in the SDC carburetor and we also replaced the inlet needle spring (more on this later!). Main jet in it is .031. Engine has a recoil starter and we're using pump gas.

When I first started it up, it would fire right up and run at high idle (4000 - 4500 rpm) and I couldn't adjust the low speed mixture to get it to slow down! It would run this way for maybe 25 seconds, then die out! Would always restart in 2-3 pulls, then repeat the sequence. Finally after some head scratching, I checked the pop-off pressure and found it was over 20 psi!! The inlet spring we had used had the same initial load as the spring we had removed which is puzzling. Goes to show you an experienced person can still miss the obvious, like checking pop-off! Anyway I replaced the spring and got the pop-off down to about 12 psi .

Now, the engine starts easy (2-3 pulls) and idles smoothly (2800 - 3300 rpm). I drove the kart first and was quite surprised at the performance, pulls strong and accelerates good even with my 200 lbs onboard! It does seem like it could be a little richer at the top end with my weight, so I've been putting my hand partially over the carb throat about 3/4 of the way down the straightaway which makes it 4 cycle before letting off. Since my son weighs about 100 pounds, I don't think it's a problem for him.

I was very skeptical about the carburetor at first and considered trying a tillotson, but for how we're using it it's OK.

Hope this helps your friend.

Terry

mcbob · Post by **mcbob** » Wed Sep 14, 2011 4:39 pm

Maybe the carb off a SP125 saw a SDC 20 would be a better bet as it has both low and a high speed mixture needle in the mid to late seventies Mac as a measure to stop the saw users from ringing the freckle out of the SP125c fitted a carb the SDC 124B to these saws as they had no adjustable high speed mixture only a fixed jet but the saw owners then screwed the low speed to lean as with careful fiddling it would alter the high speed a tad giving a few more revs but the saws didn't lean out as much

McBob.

steveohara · Post by **steveohara** » Thu Sep 15, 2011 4:16 am

Guys,
Thanks for the replies and sharing good info. My objective in this project is to remove as many variables for the user as possible. I'm attracted to the fixed jet carb for exactly that reason... once the correct jet is installed it is no longer an issue and I expect that the low speed needle setting should be pretty consistent too. As is common in almost anything that comes in contact with karters I have already seen some crazy things in the small sampling of the SDC carbs I have to work with. I took one apart and found the fulcrum arm was tweaked to be sticking almost straight up when I removed the cover plate. The user obviously didn't care about idle or clean low speed running.... just trying to shove as high a percentage of alky through as possible.
I have finished a rebuild of the first one and will be testing a little in the street pretty soon... the neighbors will love the sound of a Mc with an old Kendick pipe with a stinger and no silencing can

Thanks again.
Steve O'Hara

Mac 49E Carb

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