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Redbeard2 · #1 21st April 2022

I'm installing a new Mikuni HRS42 carb on my Sportster. It was originally an XL883 but I've upgraded it with an S&S 1250 Sidewinder kit, Screamin' Eagle heads, and Screamin' Eagle 536 cam set. What jets should I install in the new carb?

Redbeard2 · #2 23rd April 2022

I bought a Mikuni HSR42-18 from Parts Giant on Ebay. This is an Easy Kit which was apparently designed to fit on a Harley Big Twin not a Sportster. It was jetted with a 160 Main Jet and a #25 Pilot Jet. Part Giant didn't have the Mikuni HSR42-11 which is designed for Sportsters and Buells. But since I upgraded my Sportster I've thought of it as a Big Twin. I installed the carb and took it for a test run. No problems except I need to set the idle adjustment up as it dies when I come to a stop. I'll work on this tomorrow.

John Harper · #3 23rd April 2022

Looks good, wish I could help.

John

wrkdWS6 · #4 23rd April 2022

Tons of jetting variables, naturally..but here’s what I run and previously ran: 27.5 pilot, 167.5 main, 97 needle (top clip) on my 10:5-1 1275 with head work and cams. 22.5 pilot, 160-162.5 main, ‘97 needle at middle clip when bike was a cammed 1200. 1275 was tuned on the dyno, 1200 setup was tuned via Mikuni manual and look/sound/feel.

The 160 main seems light for a 1250 with head work and cams. The 25 pilot is likely close to correct. But there’s only one way to tell.

xena · #5 23rd April 2022

I added Mikuni to your title so it would attract the Mikuni folks. I’m of no help I run the original carburetor with my Hammerized sporty.

Redbeard2 · #6 23rd April 2022

One note here for anyone using a similar puke tube like I've installed on my Sporty. It's the chromed tubing going over the top of the carb. All Harleys have this problem to a greater or lesser extent. Most Harleys vent their splooge, a combination of air, oil and moisture mixture into the breather box. Then it's supposed to be sucked into the carb and then into the combustion chambers. Well, my Sporty never got with the program and splooge was always leaking out of the breather box onto the cam covers making a mess. So I installed the Drag Specialties splooge bipass kit [that's not the right name for it]. It's a good idea to put a filter element on the end of the hose because when your pistons go up they create a vacuum in the crankcase and so air is sucked back in. But I digress. When I installed the new carb the top of the rotary cable linkage was binding on the bottom of the puke tube and it took me a while to fix this. So beware of this problem if you're running this puke tube adapter.

Redbeard2 · #7 24th April 2022

I took the Sporty for a short ride down the road. This was to heat up the engine. This idle adjustment should be done with the engine heated up. The adjuster has a right hand thread on it so you turn the knob to the right to increase the idle and turn it to the left to decrease it. It was set too low at the factory so when I came to a stop if I didn't keep the engine revved up it would die. Also you need to wear gloves if the engine is hot because it will heat up the knob and burn your fingers if you don't. I got the engine to idle at about 1000 RPM which is good to go! Here's a pic of the Mikuni carb's idle adjustment knob.

Redbeard2 · #8 27th April 2022

On the Note on the Puke Tube Post I called it the Drag Specialties Splooge Bipass Kit which is the wrong nomenclature. The correct name is the Drag Specialties Crankcase Breather Kit. Here's the Dennis Kirk link. It's for a Sportster. I'm pretty sure DS makes one for the Big Twins as well. However the adapter hard points are about .750" wide and one needs to space out your breather box accordingly to install this correctly. I'm running an Arlen Ness Big Sucker breather box on my Sporty and had to install an aluminum spacer between the Mikuni carb adapter and the breather box. As I said before it's a good idea to put a small filter element on the bottom of your hose so it doesn't suck in dirt or sand. When your pistons go up they create a vacuum in the crankcase and air is sucked into the puke tube! One of my bros was riding his FXR on Daytona beach and hadn't realized his filter element had fallen off. Needless to say when he next changed his oil he realized he had sand in his crankcase! https://www.denniskirk.com/drag-spec...prd/H30007.sku This gizmo will end once and for all the problem of splooge leaking out of your breather box onto your cam cover!

aswracing · #9 27th April 2022

Quote:

Originally Posted by Redbeard2

because when your pistons go up they create a vacuum in the crankcase and so air is sucked back in.

Only if your breather check valves are missing or defective.

John Harper · #10 27th April 2022

Quote:

Originally Posted by Redbeard2

One of my bros was riding his FXR on Daytona beach and hadn't realized his filter element had fallen off. Needless to say when he next changed his oil he realized he had sand in his crankcase.

I doubt the sand got in through the breathers. I think there's more the story than that.

John