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Help with getting MC-9 to run - Printable Version

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Help with getting MC-9 to run - Michael Drechnowicz - 07-07-2021

I am trying to get my MC-9 with a flatback carb to run to take to a kart event this month. It starts right up with part throttle and choking the carb. It runs well for about 5 seconds and then starts to die like it is out of gas. If I choke it (cover the carb) it picks right back up. I do not see any bubbles in the fuel line so I don't think there is an air leak but I replaced the fuel line anyway. Changes to the carb adjustment screws do not help.

Engine was last run about a year ago and ran fine then.

Any suggestions for what to look for?



RE: Help with getting MC-9 to run - Terry Armstrong - 07-07-2021


I have limited experience with Mc flatback carburetors, it was on a chainsaw that I converted to a Mc49 repop.  The diaphrams were shot and it behaved like you describe on your Mc-9! 

I would check (2) things: 1.  Make sure your pulse hole is unobstructed from the manifold all the way down to the reed block.  Check by removing the carburetor and using a small wire (straightened paperclip, etc.) to see if there are obstructions.  Then make sure the carburetor to manifold gasket pulse hole is onobstructed.

2. Carburetor diaphrams;  Make sure they are in good condition; Not stiff or torn and that the diaphram plates are flat (a piece of flat glass works well as a poor man's surface plate!).

I have a Mc-9 on a Bug kart, but switched over to a Tillotson carburetor.

Let us know what you find!


RE: Help with getting MC-9 to run - David Luciani - 07-08-2021

all righty then.
when the flatbacks are working they are pretty reliable.
 what you described sounds like your gaskets got a little dry.
that leads to alot of air being in the carb.

 no big deal on a saw but on a fixed kart motor a little more hassle.
the air in the carb is hard to get out without running it a bit.
but keeping it running is bitchy because its half full of air.

but it can be done.
the easiest though sloppy way to rewet the carb by letting fuel out of it.
 loosen the four screws and let a little fuel pour out. 
retighten and you should be good to go.

or disasseble the carb soak the gaskets in fuel oil mix then reinstall.
my method is done trackside in about 1 minute. other method is more fussy takes longer and gets same result, you chose.
hope this helps
dave l

RE: Help with getting MC-9 to run - ted johnson - 07-08-2021

At one time, Paul Booth was a flatback whiz. See if anyone here knows how to get hold of him. I remember when we had boxes full of flatback parts! Most of the guys switched to Crescents, Christensens or the like. Basically Tillotsons with mods. A tilly HL250 or 227 would be a good swap for gas, an HL293 for alcohol. On a stock motor, even a HL232 from an 820 West Bend would run like a flatback, and parts are readily available. The 227 and 250 are gas carbs that came stock on rotary valve 100cc Italian engines like Komet, BM and the like. The also make dynamite alky carbs in pairs with the proper mods. Ted

RE: Help with getting MC-9 to run - Michael Drechnowicz - 07-10-2021

Thanks everyone who responded both here and through email.

I tried a couple things:

1. Swapped out the carb for an HL232 off my West Bend. Engine ran great. I wanted to prove to myself that it is a carb not an engine problem.

2. Soaked the gaskets in fuel/oil mix overnight and cleaned the carb thoroughly with carb cleaner. I did notice that the diaphragm in the carb (I think that is what is called, the one that hits the fulcrum that opens the fuel inlet valve) is a little stiff (would soaking it longer help?). Engine ran a little better (more time between choking) but still would not run well without choking.

Based on this I'm sure it is a carb issue. I don't have an extra diaphragm to try and haven't been able to find any online. Does anyone have a source they can point me to for that?

Thanks again,


RE: Help with getting MC-9 to run - David Luciani - 07-10-2021

first question i should've asked. did you flat plate sand the flats on your carb. 
if you didn't do that first.
after you are sure its sealed, TRY venting the carb by loosening the screws and letting gas seep out.
i had a flatback i tried everything on till i tried that old chainsaw trick. it went from the most annoying carbs to one of my best.
BUT i ALWAYS have to vent it at beginning of year sometimes several times. 
theres some weirdness that gets them airlocked on occasion and that tip fixes the air lock.

also soak the diaphram in tranny oil. 
sometimes they come back to useful.
or i think if you double the gasket you can use a newer mcculloch carb diaphram. 

d Cool

RE: Help with getting MC-9 to run - Tom Smith - 07-11-2021

RE: Help with getting MC-9 to run - ted johnson - 07-11-2021

Tom, this should be known to every old Mac guy! Good catch. Ted

RE: Help with getting MC-9 to run - Michael Drechnowicz - 07-12-2021

Thanks everyone for the help and thanks for the link to the gasket Tom.

I think I tried a combination of all suggestions including soaking the pumper in trans fluid. In any case, one or more of the suggestions worked! The engine now runs without choking. I ran it up and down my street a few times (sure the neighbors were not impressed) and it did not die out, acceleration seems a little slow so I think it needs some adjustments but I am not really good at determining what to do so I will wait till I go to the track and maybe someone can help me out there. I am not one to try to get everything out of the motor, just being on the track is good enough fun for me.

Thanks again,


RE: Help with getting MC-9 to run - ted johnson - 07-12-2021

Michael, the switch to a plain old Tilly HL is still the best idea. Kits are plentiful and the HL is a simple beast that runs as well as the flatback, with none of the parts issues. A nice HL232 or the equal would bolt right on. You might have to fool with the throttle hookup, but that would be all. I put the dual alky flatbacks on the 101 just to make it look like an Mc20, but otherwise, I'd have used my alky HL250's. Ted