Mikuni HSR on L-series

[Jitenshakun]

Has anyone had a hand in running Mikuni HSR moto carbies on a L4 or L6? Dyno plots aside their compact shape seems to offer benefits for 620 and 720 owners that try to squeeze anything between the head and brake booster. 

 

If you don't know what I'm talking about read the Vintage Performance page. 

 

http://www.v-performance.com/products/air_fuel.html

[datzenmike]

I guess you would have to make your own intake with those???

 

Don't know about them but R-1 carbs from a Yamaha? Got to be cheaper.

 

You want the last of the 2000-2002 ish R-1 carbs before EFI. I got mine for $100. Spent $280 figuring out how NOT to make my own intake and $20 for materials.

 

 

 

 

Home made trumpets cut to length. It's just cheap muffler pipe.

 

 

The old 2bbl is slow from 4k up but this pulls to 6. Each runner can be optimized for length tuning. The sound is louder than the exhaust.

 

 

 

 

[Jitenshakun]

Apparently the HSR can be made to fit to a weber/mikuni side draft intake. 

[datzenmike]

How much for carburetors and intake then?

 

720? must be a 1980 720 then.

Are these slide carburetors?

[Jitenshakun]

My head comes with an intake and I'll be getting a quote after the holiday break. 

 

If they are small enough it might be a Z-motor setting. If not, it'll be a Z-motor with a L-series clocking (engine mounts and trans tunnel). 

[datzenmike]

 Confused.

 

 What head? Must be an L head as you mention clearing the brake booster.

 

What engine depends on the year the truck is. A 1980 had an L20B gas engine all other years were Z series gas engines.

[Jitenshakun]

It's the same engine we talked about. I'm well aware off all the challenges of my engine plan. 

 

Do you have experience running HSR on a L6 or L4 in place of triples or bolted to a Hitachi manifold? 

[datzenmike]

Ah LZ23.

 

No never heard of HSR carbs. But I like where this is going. Side draft is better as the air flows in a (more or less) straight line into the cylinder. Multi carbs can be 'ram tuned' by length.

[Jitenshakun]

They don't have main or aux venturi so less obstruction. Also, a much better seal on the butterflies. As a package they offer better attomization and response in a smaller package. 

 

Allegedly... 

[datzenmike]

Well if they are a slide then the venturi is there but the slide opens and closes to change the size. Also known as a constant velocity carburetor. The venturi adjusts larger as the air flow increases keeping the velocity the same across the venturi. They are very compact compared to a regular carburetor and have fewer parts and no accelerator pump or choke. My R-1 carbs are 38mm (1.5") or almost the same diameter of the U67 intake port.  (1.375") They are harder to start cold though.

[Jitenshakun]

I'm catching wind of the lack of an accelerator pump being an issue. 

[datzenmike]

If a slide or SU type they don't need it. Remember: constant velocity. Air passing through the venturi is always constant unlike a regular carburetor that if you floor suddenly there isn't enough RPMs to pull enough air past it's venturi to suck any gas out, and it bogs. On the slide carburetor the throttle opens fully but the slide in front of it is almost closed at idle. As the engine revs up the slide is pulled up opening the mouth of the carburetor wider and wider but tightly controlled to the engine's needs. There is no bogging a slide carburetor.

 

On my L20B I can drop to 1,500 in forth and floor it. I don't make a habit of it but it's the same going around a corner in town in 3rd. The engine immediately begins to pull slowly but smoothly with no hesitation or bucking. With the stock carburetor you would normally avoid this at all costs.

[Stoffregen Motorsports]

Accelerator pumps are nice on cold starts, but even without them, you can crack the throttle while cranking to help the engine suck fuel in. SUs are the same.

[datzenmike]

I'll try that. Mine will start after prolonged cranking. Not good for the starter life. I use a squirt bottle and once started good for the entire day. Gotta come up with something better.

[Jitenshakun]

I was wondering about a heater blanket on the intake manifold for winter starts. There's an attomization issue with really cold temps and heating the passage on the way in might help. 

[Stoffregen Motorsports]

I was born in Northern MN and the winters there are legendary. Cold starts on anything when it's 20 below are challenging. We used to do a ton of snowmobiling and would carry spare spark plugs and pull cords just in case, plus a can or two of starting fluid. I remember the smell and sound of the 2 strokes warming up. Brings tears to my eyes...from the fumes.

 

A heater blanket is probably overkill, but I'm sure it would do the trick.

[Tom1200]

I run Keihin FCR Flat Slide carbs on my 1200. Those have an accelerator pump; on cold days I just pump the gas pedal a bunch of times. I did a track day on the 2nd and it was 27 degrees at the track. I had to pump the pedal a few more times to get the car fired up but it wasn't a big deal.

 

I love slide carbs as they flow more; they also seem to be better made than the DCOEs.

[datzenmike]

I was thinking of an electric pump with small jets on each intake runner. Push button on the dash.

[Tom1200]

My Formula 500 (500cc two stroke powered single seater) doesn't have a choke. I put a T in the vent line to between the carbs; I use an oil can squirt bottle that I fill with fuel to simply pump fuel into the intake. I would imagine you could do this electronically with some type of servo that pumped fuel in the intake.

 

Slide carbs have an enricher circuit so pumping fuel into the manifold does the same thing...................should work great.