Mikuni Carb

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Gary Wlodarsky
Posts: 158
Joined: Sat Oct 31, 2009 10:03 am
Vintage Karting items owned: Hornet SS, Dual Hornet
Expert II , Invader single and a dual, Rupp 301D,
User Agreement: Yes
Location: Wellington Ohio

Re: Mikuni Carb

Post by Gary Wlodarsky » Sun Mar 04, 2012 9:34 pm

Butch...

I don't think his true loyality will ever ""shift"', least it better not,,,family first.....but then I agree...that gearing sounds perfect for B'Ville...LOL....gonna bring many tires,,,prolly spent close to 2000 bux for tires
so far this year....had to replace many dunlops with vintage tires ...and had to replace some dunlops...psssssst...I'm bringing a set for the over 60 class kart just in case I/we decide to run sidewinder <6.1......

personally I think some don't like the Mikuni cause its not "Vintage""....sure works good on my over 60 kart....different strokes for different folks...

will keep my eyes open for a GP....

c ya at B'ville...

Gary

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steveohara
Posts: 411
Joined: Thu Jul 19, 2007 2:29 am
Vintage Karting items owned: 1969 Bug Sprint Mc 91B1
1965 Dart Gran Prix twin Mc100s
1963 Bug Scorpion ESll Mc45

Re: Mikuni Carb

Post by steveohara » Mon Mar 05, 2012 3:30 am

Just to clarify.... it isn't the brand that matters, just the size.
As two posters noted... with big carbs they have trouble at the low end of the rpm range and they have to struggle with tricky tuning or part throttle application to make them run off the corners.
Those problems are directly caused by a carb bore that is too big for the engine.
More specifically, the problem is the air speed in the throttle bore is too slow at low engine speeds to create the required pressure drop at the venturi to draw fuel in efficiently.
The big fallacy lies in the belief that the larger carb creates a lot more power up higher in the rpm band. That premise is only true if the carb it is replacing is too small. For a stock 91, anything over 23mm venturi is not going to add power up top but will make the carburation erratic at low engine speeds.
The BDC line of carbs used by McCulloch are are widely disliked for exactly the reason I mentioned above.
They are too big for the engines and to "solve" that problem they have the duckbill circuit built in as a crutch to help them overcome the poor low speed performance. They would have never fitted that carb to the engines if racing in the USA had followed the European model of direct drive sprint track racing where wide range performance it critical. Here in the US enduro racing was a big part of the karting market and the big carb is better when you are running 75% of each lap flat out at 12K rpm or more and you deal with the few slow corners with slipper clutches that keep the engine speed up at 9K or more and rarely come off the throttle.
Most vintage karters are using low stall clutches, running hard skinny tires and are off the throttle quite a bit so the smaller carb is a much better choice.
Steve O'Hara

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Bob Towarnicki
Posts: 122
Joined: Sat Jun 16, 2007 5:45 pm
Vintage Karting items owned: 65 Alley Kat Mc91
Narrow Hoop A Bone Mc91B
Various restoration projects
User Agreement: Yes

Re: Mikuni Carb

Post by Bob Towarnicki » Mon Mar 05, 2012 5:56 pm

Steve,

Thanks for the education. You just answered the question in my mind about why Mcculloch put the big BDCs on the karts if they are too big. Does this analysis hold true for alcohol as well as gas?

Also, curious if you have any experience with the Tillotson HL314G carbs on the Macs? I picked up a couple and they have a double stack from the factory, which I thought kind of strange for a gas carb.

Thanks again,
Bob

Gary Wlodarsky
Posts: 158
Joined: Sat Oct 31, 2009 10:03 am
Vintage Karting items owned: Hornet SS, Dual Hornet
Expert II , Invader single and a dual, Rupp 301D,
User Agreement: Yes
Location: Wellington Ohio

Re: Mikuni Carb

Post by Gary Wlodarsky » Mon Mar 05, 2012 10:32 pm

Steve....

as always you are extremely helpful when it come to carburation.....just an FYI I do run a 334a on my Mac 90( mild port job) and a mukini on my open 91...thats why I was wondering ????/but any how....I need another 334a or two...you mentioned 50-60 bux,,,the ones I see are closeer to 100......where do I find 'em fo 60 bux....thanks again...Gary W

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steveohara
Posts: 411
Joined: Thu Jul 19, 2007 2:29 am
Vintage Karting items owned: 1969 Bug Sprint Mc 91B1
1965 Dart Gran Prix twin Mc100s
1963 Bug Scorpion ESll Mc45

Re: Mikuni Carb

Post by steveohara » Tue Mar 06, 2012 4:33 am

Bob,

The issues with the carbs are the same regardless of fuel used. However, an over sized carb will tend to cause more noticeable problems when running gas compared to alky for one simple reason... alky is more forgiving when it comes to air/fuel ratio than gasoline. Alky runs great way on the rich side especially at the low end of the rpm range... not so with gas. Since the size of the carb is optimised based on volume of air flow, not cubic inches of displacement, other factors such as porting and exhaust system have to be considered. For example, a stock Mc 91 engine with a Hortsman muffler or some other simple box exhaust will not breath near as effectively as a fully ported 91 fitted with a well designed expansion chamber. Accordingly, the modified engine with the advanced exhaust system can be fitted with a larger carb than the stock ported, restricted exhaust setup because the air speed in the venturi at any given rpm will be much greater for for the modified engine.
Gary's two examples fit the model perfectly.
Gary... check with the kart shops near you that service the modern kart racers to let them know you are in the market for a used 334. There are always guys that blow up engines and have left over carbs or there are the real serious racers that buy ten of them to test to find the "good" ones and then they sell off the "bad" ones.... which are really no different than the "good" ones :)
Steve O'Hara

Gary Wlodarsky
Posts: 158
Joined: Sat Oct 31, 2009 10:03 am
Vintage Karting items owned: Hornet SS, Dual Hornet
Expert II , Invader single and a dual, Rupp 301D,
User Agreement: Yes
Location: Wellington Ohio

Re: Mikuni Carb

Post by Gary Wlodarsky » Tue Mar 06, 2012 8:37 am

Steve....

pretty smart guy...you read what I didn't evn write...LOL...my Mac 90(sportsman class) small carb ,alky,single six reed gem/or Hartman big 4 reed, Horstman muffler...my open Mac 91( over 60 class) ..good pipe , V12, alky and the single 34 Mikuni....

thanks for ther tipon the good bad good carbs,,,,never thought of looking for used...

thanks again...

gary

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