I have a 2005 Road King that has a Mikuni Carb installed on it. I have been reading everything that I can find on the internet about the Mikuni HSR Carburetor. I have the installation guide, the tuning guide and many other documents that are out there on the carburetor, however, I have a few questions I was wanting to have answered.

1. How can I tell the difference between an HSR 42 or HSR 45 without removing the carb? From what I understand, the HSR 42 fits on the stock manifold and the 45 requires a Mikuni intake manifold to be installed. I have tried to see if there are any Mikuni markings on the intake manifold or looked for a model numbers on the carb and cannot see any.

2. I have not removed the main jet or jet needle yet, but plan on doing so to see what size is in the carb. I suspect the stock size jets and needle, but my question is this.
Are there any markings on these parts so I know what size is currently installed?

I have a 95 ci with 10.5 to 1 compression and a aftermarket cam, I'm thinking I would get more out of the motor if I properly tune the carb, I am suspecting it was just bolted on by the previous owners mechanic, but cannot verify that unfortunately. The bike runs good, but after performing the roll off method as described in the Mikuni Tunning manual, I believe the main jet is too small, which probably indicates a HSR 42 with stock settings.


Thanks!!

 

I guess all the carb knowledge is dead now.

 

Keep the 42mm for a 95", 45mm would imo be too much carb. But definately bump up your main to see what happens...

 

Not sure how to confirn the 42 or 45 but the jets, needles, etc. are marked. I replaced my Dual Edelbrocks with a 42 recently. I'm running a .030 over EVO, 10.0 comp., ported heads and .560 lift cam. It takes a little patience to get it tuned spot on but it's definately worth it. I bought a Mikuni kit on eBay with an assortment of jets, needles etc. which made it alot easier to play around with the different combinations.

 

Thanks scootrskum, I will check out ebay for the kit.

 

If the carb is an HSR 45 the mouth will measure 1.861". Just remove the air cleaner adaptor to check. The SE manifold for the 44mm Kehin will also work well with the HSR 45. While I have read here that a 45 is too big for a 95", I have seen a few hot 95 inchers with good heads and cams run great with 45 Mikunis.

 

I have tuned a bike with a carb that proved to be too small and replaced it with one that proved to be too big! It is easy to tell if the carb is wrong, either way, although only after it has been competently set up.

 

As Northside pointed out, the only way to tell the difference between the HSR 42 and 45 is to remove the air cleaner and measure the diameter of the mouth. When I was trying to determine this on a spare Mikuni carb someone had given me, I measured it in millimeters instead of trying to convert it to inches (42mm or 45mm). Actually, I thought the carb I had might have been an HSR 48. With regards to the intake manifold, I don't know for sure if the HSR 42 can be used with the stock intake and boot. However, the HSR 45 definitely requires the flanged intake manifold and flanged boot.

Depending on what you want from the engine the HSR 45 will work. If your primary focus is producing torque, the HSR 42 is a better choice. However, you will sacrifice a few HP on the top end.

I'm currently running an HSR 45 on my 95" build. I'm making 106 HP, 108 TQ. You can see the rest of my build specs in my sig. In the next few weeks I'll be changing my exhaust to a 2-1 and making another trip to the dyno. Based on what I see on the next dyno run, I may be installing an HSR 42 to try and get the torque numbers up. I'll keep you posted.

 

Take a look at the Harley Tech Talk forum. That's where I got a ton of info about tuning my Mik 42. You will probably have to register to use the search function. Using "Mikuni 42 jetting" will return a bunch of info for you. As I recall there's also been a lot of discussion on the 42 vs. 45 question over the years.

http://harleytechtalk.org/htt/index.php?board=3.0

 

Fletch, do you happen to have any before and after dyno results with that Crane HI4-TC ignition you're using? Just curious how much those igntion systems effect the overall performance of a modified and carbed engine. Thanks and apologies to those who may view this question as a minor hyjacking of the thread.