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What jets are you running???.....mikuni 42....1200.....

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Old 10-27-2009, 08:28 AM
Chopster1200 Chopster1200 is offline
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Talking What jets are you running???.....mikuni 42....1200.....

High flow intake? High flow pipes? Mikuni 42 carb? 1200? WHAT JETS ARE YOU RUNNING????

Hey guys, I finally got my ignition system freshened up and timed properly. Now its time to tune the carb correctly. I have a 95 1200 with a Mikuni 42 carb. SE heavy air filter and in a couple days, there will be some Wet Dreamz pipes on her that are 2" diameter with 4" long baffles. I would like to know what jet sizes you are running so I can use that as a baseline. Thanks for your experience!!

Last edited by Chopster1200; 10-27-2009 at 08:32 AM.
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Old 10-27-2009, 10:39 AM
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OldFenderGuy OldFenderGuy is offline
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First off, I've used a lot of carb's over the years, and a properly set up Mikuni will give you the best possible performance, period, no question. They are also very easy to work on, and you don't need to 'pull the carb' or turn your bike upside down to make adjustments.

To answer your question, the HRS-42 is not a EPA Approved carb (like the CV) and come jetted pretty rich right out of the box... You should also realize that the big pumpers like the 42 flow very well, and I've had a number of good mechanics tell me that you would need to go with something like a CV-46 to get the same type of flow rates as a HSR-42.

In stock condition the HSR-42 comes with a 25 Pilot, and the available range of replacements is 17.5 to 32.5. As you probably know, the Pilot comes into use from idle to 1/4 throttle.

The stock main is a 160, and can be replaced with anything from 150 to 210. The Main is used from 3/4 to full throttle.

Stock needle jet is a 97, and replacements range from a 95 to a 98. The main jet is used from 1/8 to 3/4 throttle.

The accelerator pump can be adjusted, and in stock condition comes with a 60 nozzle for Sportsters and a 70 nozzle for the Big Twins. Replacement nozzle sizes run from a 50 to a 70.

If I were you I would get the model listed for the 1200 Sportsters (with the 60 accelerator nozzle), and not change anything right away. Unlike the EPA CV carbs (and EPA DOESN"T stand for Enhanced Performance Accessory) you don't need to assume that it will be jetted too lean..

You can also purchase a complete Jet Kit (PNKHS-025) which includes;

Main Jets, 2 each, 150 thru 170
Pilot Jets, 2 each 20 thru 32.5
Needles, 96, 97, 98
Pump Nozzle, 60, 70
Needle E-Ring Clips and Washers
O-ring, Pump Nozzle

The HSR also comes with a long idle adjuster, in which you can just reach down and adjust the idle speed while sitting on the bike, but that can be replaced with a regular short screw type adjuster if you want.

Anyway, good choice is going with a Mikuni. Easy to work on, instant throttle response because of the adjustable accelerator pump, and they will give performance that can't be matched by even a modified CV.

Only 'negative' thing is that although the Mikuni can't be set up for best possible fuel economy most people set them up to run slightly richer and give best possible performance. Because of that they probably won't get the same gas mileage as a CV carb.
My 2003 WideGlide went from an average of 48 MPG with a well jetted and dyno tuned CV carb to an average of about 44 MPG with a dyno tuned HSR-42. Not a big difference for the performance increase, but just something that should be mentioned.

(Almost forgot to mention, but the Mikuni 42 comes with an excellent tuning manual that provides all the information needed to 'fine tune' the carb for your specific applilcation)..

Last edited by OldFenderGuy; 10-27-2009 at 10:43 AM.
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Old 10-27-2009, 12:53 PM
Chopster1200 Chopster1200 is offline
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Wow, well that is about everything I needed to know. Thanks!! The Mikuni 42 was on the bike when I got it. Whomever tuned this bike didnt quite know what they were doing based on the condition of the ignition system and lack of VOES. I assume the jets are wrong in the carb as well. I'll find out shortly. I only rode it for 10 min or so after getting the elec system back together and it seemed ok except for a random low rpm hiccup when starting to roll on the throttle at really low rpms. Also noticed the WOT at low rpms is a bit boggy and that WOT in general seems a little boggy.....you can feel and hear it bog just a touch past 3/4 throttle....there is almost more acceleration at 3/4 throttle than there is at WOT. Either too much or not enough fuel there. I'll have to find one of those tuning manuals. Sounds like what I need!
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Old 10-27-2009, 02:12 PM
drum412 drum412 is offline
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The tuning manual is available on mikuni's website. Took me about a month of tunning to get mine just right at all throttle positions. It's a tedious carb to tune because of all jet/needle combinations, but once you get things right, it's awesome.
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