Mikuni Carb

[Don Tomich]

Can anyone tell me how well a 34mm Mikuni gas carb will work ok on a stock 91B. Would the Hartman manifold that uses an iame reed block be of any benefit?
thanks
DonT

[ted johnson]

Don, the Mikuni will work great on a 91. It's easier to tune, holds its tune better, and is far easier to overhaul than a Walbro BDC. Remember that if you're running it on the Mac manifold, you must do the mods described in the REAR tech article. Rather than cut and beburr tubing to make your stud spacer bushings as described in the article, go to McMaster Carr and get some 1/4" I.D. X 5/16" O.D. X 1/4" long bronze bushings. They're cheap, and ready to use. The IAME reed is a good unit, but the Mac manifold as modified will work fine. Easy modification. Ted

[steveohara]

Don,

That big Mikuni carb is way too big for a stock Mc 91B1 engine. Even the factory BDC 14 is too big.
If you want to make your 91B1 run really strong and be easy to tune and keep tuned I recommend the Tilly HL334A/B carb that is standard on all of the Leopard Tag motors. You can pick them up cheap... usually $50-$60. The carb is a 25mm bore, 23.5mm venturi. The Leopard will make close to 30HP and rev to 17K on gas with that carb. The Leopard is a 125cc motor of a very advanced design and I would estimate it would move close to twice as much air as a stock 91 series motor.
With the smaller carb you'll get much better low and mid range performance and lose little or nothing on top end depending on how you gear your kart.
The air speed is just too slow in those big carbs to efficiently meter the fuel so you will never get good performance at engine speeds below 10K on that 91 motor.
Steve O'Hara

[Don Tomich]

Thanks Ted & Steve for your great responses.
Regards
Don

[Butch Kavanagh]

steve-interesting. my gr son last year, ran a stock 91b1, gas with the 334ab, at vintage events (junior class). no world beater times, but the engine responded well. my son is trying to talk me into the mikuni carb on a mac manifold , 92 stock for b-ville (60 and over class). got a nice-clean 334 on a reed sloper. to some, b-ville is considerred a short to medium rpm track. now if i could only find some good gas and a couple cans of blendzall gold label

[steveohara]

Butch,

No surprise you like the 334 carb. The most powerful kart engines I have ever used were my Rotax rotary valve 100cc Formula A engines. They made over 30 horsepower and revved to 20K and used a 25mm carb and the venturi was even smaller. One of the great plagues of the early and now vintage karting era was the over carburation of the engines with sometimes 4 carbs!
At Adams last month I ran a mildly modified 101 in the sidewinder 8.2 class and I was the only kart on the grid with a single OEM carb. There were 820s with 2 of the big Mikunis, others with one big Mikuni or Tilly, 101s with dual carbs or the huge single Mikuni.... all the trick stuff.
I started last in the first heat and it took about 100 feet to pass everyone at the start... no one even close.
Stock BDC 16 running alky and it would sit and idle like scooter after running the race.
Over carburation is one of the leading causes of damaged motors because they are so hard to tune when the carb is too big. At low engine speeds the air flow through the big carb is just too slow to draw fuel properly and atomize it effectively so you end up with jet settings that won't work through the whole range of engine speeds. Net result is lots of stuck pistons at one end of the spectrum and lots of fouled plugs at the other end.
Steve O'Hara

[Rob Voska]

The HR Tillitson is a bit smaller than a Mikuni. With that being said Steve's pointing out air flow vs RPM is a good debate. We ran 135cc direct drive engines with Ibea slide carbs at 20,000 + RPM. When you drove off the corner you needed to ease the throttle open, not jump on it, to get the bst lap times.

[Scott Kneisel]

I am running a gas HR191 with a big sloper on my 101A and it tuned nicely and is very responsive. I ran it hard last season with no issues but I always run it rich both on the low and high speed. I always set the HS so it 4 cycles right at the end of the straight. I do notice that at Avon if I have the LS set too lean it hesitates coming out of the tight hairpin that slows you down to a crawl. If I richen up theLS that goes away. The HR 181 and 191 carbs have a separate little ring in the bore around the HS dump tube that must also act like a separate venturi, that may help the transition.

I built an enduro last year with a rotary valve Parilla 100cc and took Steve O's advice and used the HL334AB carb and it ran sweet, tons of R's and top end and good low end even with the enduro gears. I have been meaning to try one of those on a Mac but hesitate because they are not a vintage carb but I may try one anyway.

My friend Mike S. uses nothing but single Tilly HL carbs on his 91's and 101AA and swears by them. He uses the Hartman variety though which can throw in more air and fuel both.

I still like the HR carbs though and have had a lot of success running them on both my modified 91 and my 101.

My 2 cents

[Gary Wlodarsky]

Hey Butch...I agree,,,stay with the small carb........LOL....you know why!!!!!

GW

[Butch Kavanagh]

lol-good gary, when spk vince today, he had me running 20degrees ignition timing and 12-52 gears for b-ville. guess my son's loyality has ah, 'shifted' lol. asked him to bring used front sl4s, may go other mac class. talking about a dart gp frame or latest bug rear engine kart for tbo this summer. still not sure which carb, would be gas