Quoted from Joe Minton
(This is an archive, posted to preserve the knowledge and insight of Joe Minton. From my understanding he died long before I posted this article – added 7/2023)
Background:
Carburetors work by creating a partial vacuum in the throat (venturi or choke) of the carb body. The difference in air pressure between the throat and the outside air causes fuel to flow into the carburetor through any orifice it can find. Tuning a carburetor consists of controlling the size of those orifices so that the resulting air/fuel mixtures are correct.
The Mikuni HSR is an Amal-pattern carburetor, as is the stock Keihin. Amal-pattern carburetors have three basic air/fuel control element
s: idle, main (mid-throttle) and the main jet. The HSR also has an adjustable accelerator pump, which I consider an important fourth tuning element. All tuning elements are supplied with fuel by the float bowl at the bottom of the carburetor.
The idle system is a separate carburetor built into the main body. There are appropriate connecting passages, a replaceable jet, and an adjustment screw. The screw controls the air/fuel mixture at dead idle. The jet controls mixture as the throttle begins to open. By about 10 percent throttle, the idle system is delivering all the fuel it can. It continues to provide this fuel all the way to full throttle.
The main system operates as the throttle slide is raised above about 5 percent, and it begins to deliver fuel and assumes control of the mixture. Yes, there is an overlap between the idle and main systems. However, it is not difficult to get this overlap area right if one follows the manual’s advice.
The main system consists of the throttle slide, a tapered needle (jet needle), and a needle jet. The needle is mounted in the center of the slide. The needle jet is mounted in the carb body. As the throttle slide rises and falls, the needle moves in the needle jet. The needle is tapered. The relative sizes of the needle and jet determine how much fuel is forced into the carburetor throat at any particular throttle setting. More fuel flows as the slide rises, due to the needle’s taper.
The main jet is mounted to the bottom of the needle jet and limits maximum fuel flow. It assumes control at about 75 percent of the throttle opening and has no effect until then.
The accelerator pump’s purpose is to supply a squirt of fuel when the throttle is suddenly opened and there is too little vacuum for the main system to work properly. It has a replaceable nozzle and can be adjusted for starting and ending points.
Advanced tuning is a bit involved as it is for any carburetor. The individual steps are simple and clear. The great advantage of Amal slide-type carburetors such as the Amals, Bings, Dell’Ortos, Mikunis, and others is that their entire operating range can be adjusted in the field. We have a four-step tuning procedure for these carburetors that has served well for more than 80 years.
Tuning:
First Step
Adjust the idle system. Set the adjustment screw for best idle. Fit the idle jet that gives the smoothest response as the throttle is slightly and slowly opened.
Second Step
Adjust the main system. Accelerate with the throttle between 10-to-25 percent open. If the engine responds cleanly and briskly, the needle diameter is correct or nearly so. If the engine seems lean, a smaller-diameter needle is needed. Too rich needs a larger needle. In practice, needle changes are seldom required.
Third Step
Accelerate in the 25-to-75-percent throttle range. The needle taper controls the mixture strength in this range. There are five needle-height adjustments. If the mixture seems rich, lower the needle; if lean, raise it.
Fourth Step
Accelerate at full throttle. The main jet that makes the most power is the one to use. A dynamometer is not needed for this test. Accelerate between two points on the roadway. The highest speed at the second point defines the correct main jet.
Accelerator Pump:
As I said, the accelerator pump adds fuel when the throttle is first opened. This is to both richen the mixture for maximum throttle response and to compensate for low vacuum over the needle jet. The HSR pump has three adjustments: starting point, end point, and nozzle size. Nozzle size determines the fuel delivery rate and how long the squirt lasts.
The stock settings start the pump at about 1/8th throttle and end its stroke at around 1/2 throttle. The jet is a number 80 (0.80mm in diameter). I use and recommend different settings, which I’ll detail later.
This simple set of procedures works well and can remove most of the mystery from carburetor tuning.
Does Size Matter?
Yes and no. Those people who have worked with true Amal-pattern carburetors, which have round slides, know that the larger carburetors help make more high-rpm power by flowing more air. However, that greater power potential is accompanied by a loss of low-rpm throttle response.
The more recent flat-slide variations, such as the Mikuni HSR, greatly reduce this power/response trade-off. It is now possible to fit a “top end” carburetor and still get good throttle response in the lower rpm and throttle range.
A 42mm HSR42 adds about 3 horsepower to the output of a stock Twin Cam or EVO engine. The 45 adds 7 horsepower with no loss of part-throttle response, fuel mileage or tuning sensitivity.
The performance advantage of the 45mm carb means very little to most of us. Its power advantage only begins to show at well above 5,000 rpm. The HSR42 is easier to mount as it uses the stock manifold while the HSR45 needs the larger Harley Screamin’ Eagle or Mikuni manifold. It’s your choice. If you are chasing maximum power with cams, porting, etc., then your choice is obvious. Otherwise it makes no difference which one you choose.
The HSR48? Don’t bother. Our testing shows that the HSR48 only adds 2 or 3 top-end horsepower to an already powerful (120-plus horsepower) engines.
Tuning Recommendations:
Mikuni’s default jetting and adjustments cover most engine setups, as they should. Tens of thousands have been sold with very few complaints about how well they work. However, the stock settings are fail-safes and are on the fuel rich side of ideal. I have no complaint about the stock tuning, except for the accelerator-pump settings, which I am convinced are just plain wrong.
I have developed a set of jets and adjustments for the HSR42 and 45 that work better on most properly tuned engines. By properly tuned I mean that the engine is sound, there are no long straight pipes, drag-race cams, or stylish air cleaners that don’t flow much air. Many hundreds of Harley owners have adopted my recommendations and have gotten excellent results. They report that throttle response is improved, fuel mileage is better, and their engines sound and seem “happier.” The changes I recommend include accelerator nozzle size and adjustments, idle jet size, and needle size.
Here is what I change and do:
Parts:
•17.5 idle jet, leaner than the stock 25 or 20.
•98 needle, leaner than the stock 97.
•50 accelerator pump nozzle way down from the stock 80.
•The main jet is whatever it needs to be and the stock one is generally correct.
These parts are available individually or in kit form from Fox Distributing in St. Charles, Illinois, 630-513-9700.
The kit is called the Mikuni Mileage Kit and there are separate kits for the HSR42 and 45 carburetors because the needles are different. The kits come with simple installation and tuning instructions.
Adjustments (See The Manual For Details):
1. Adjust the accelerator starting point so that the pump starts working immediately. Adjust the end point for maximum travel.
2. Fit the smaller idle jet and adjust the mixture screw for best idle.
3. Fit the leaner needle with the clip in the middle notch.
Consequences:
1. Performance just off-idle is cleaner and more immediate. This is because the idle jet is more correct and the pump delivers fuel sooner but at a lower rate than stock.
2. The engine is more responsive, cleaner and “happier” sounding because of the leaner mixtures in the 10-to-25- percent throttle range, where we do most of our riding.
3. Fuel mileage increases, sometimes dramatically. Don’t be surprised if you record 48-to-49 mpg at 65 mph. Even baggers do better.
4. Midrange throttle response is greatly improved and the engine is much more pleasant to use. Expect bugs in your teeth.
However:
1. Engines with high cranking pressures (high compression) may detonate at low rpm when accelerating. The original 97 needle should fix this.
2. Nothing, not this kit, not this carburetor, nor any carburetor, is going to make an engine suffering with open straight pipes run well.
3. Big cams, those with intake-closing angles of close to 50 degrees ATDC, aren’t going to run well below 4,000 rpm. No Mikuni or any other carburetor can fix this. Either race the thing or install a more appropriate cam for street use.
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November 25th, 2019
Joe,
I must say, “Thank you very much”, for your generous
information on getting my old Evo (’97 Dyna FXDS-CONV)
girl running happily. The wife and I took her out for a spin
today, and I could hardly believe what we were experiencing!
The acceleration from idle (Once tuned right) was so smooth,
responsive, and powerful, that immediately a grin formed on
my face (I think my wife heard a happy expletive or two)!
Anyway, thank you for this info. It’s hard to believe it can run this good, and get better fuel economy, to boot.
Regards,
Krazy Kal
December 17th, 2019
THANK YOU Joe Minton. Earlier today, I installed a Mikuni HSR 42 on my 07 105″ 10.1:1 CR pistons, cammed Dyna. Runs like a Screaming Banshee!!! Following you Gas Mileage suggestion, all I can say is WOW. 17.5 pilot, 50 nozzle, nozzle squirts right off idle, 97 needle, middle clip was perfect. Feels like the speedo needle is attached to the throttle, give it a 1/4 twist and needle jumps up in unison with throttle. I’ll check back and give out my mpg numbers later . . . if I can keep off the throttle.
December 26th, 2019
Joe Minton, THANK YOU AGAIN!!!!! Your recommendations gave me 37.2 mpg and no loss of power. Decided to drop the needle down 1 notch in my quest for better mpg. Took the Dyna out on Christmas day for a long ride. Actually sounded better with no loss in acceleration. Kinda hard to maintain 65 mph because the slightest twist of the throttle had me at 75+. Gassed up around the corner from home. Got 42.3 mpg. That’s with the crappy “winter blend” of gas. Should be looking at around 42/45 mpg with good gas.
Joe Minton — YOU ROCK!!!!!
January 5th, 2020
When I think of all the gasoline I’ve wasted over the years because of the default Harley Davidson common knowledge to make it run so rich the tailpipes turn black inside… It makes me sad…
I wish I had found Joe’s wisdom years ago…
April 19th, 2020
I’m using the HSR42 on a 1275 cc classic mini. All is well except for popping, backfiring out the carb under acceleration. You can open the throttle wide with excellent response sitting still, but under load it backfires and spits until you get on the main, and then pulls like a train. Fooling with acel pump and needles. Larger pump nozzle?
April 21st, 2020
I would have no idea how to tune this carb for a Mini. I was using it on a motorcycle.
Does it do that regardless of whether the engine is hot/cold? The Harley needed some choke when it was cold…
If using the HSR42 on a Mini is a common thing, I would think there would be better tuning guides out there…
May 22nd, 2020
The mixture setting is done by sound…well…the course setting is like an old Briggs mower engine. All the way in then out 2 turns for the course setting. Engine needs
to be at operating temp…and as you go 1/2 turn in and out with the mixture screw you’ll hear it when its right. Might as well have a big ol neon sign that says RIGHT HERE BABY. Now if I can just figure out how to get the damned accelerator pump need bushing out. Grrrr.
July 20th, 2020
Thank you for your advice!! My 100% stock Evo has never ran better with your recommendations!
August 19th, 2020
Hi I have a hesitation ,no back fire , if I accelerate or blip the throttle , I put the needle from the top groove to the bottom and its better but still hesitates ?
August 26th, 2020
I’m sorry, I just posted to make sure it was archived for others. I’m not a carb expert, but, are you sure you accelerator pump is setup correctly? It is the accelerator pump that makes up the gap when you punch it.
September 21st, 2020
98 cu” Andrews 21 cam 2into1 propipe beans street ported heads 030 head gasket mukuni 42.carb set up stock needle and position,20 ideal jet,70 nozzle,80 main,was experienceing sneezing through carb.17.5 nozzle and set up was perfect
November 7th, 2021
? tc88, hooker tune flow exhaust slip ons, se a/c, se 203 cams, mikuni 42mm hrs ,looking for good baseline for this setup .. 97 middle clip, 27.5 jet, 165 jet, a/p 60 installed ty for any help
November 24th, 2021
Well that article just took all the mystery out of tuning my bike (99’flsts). Thanks for the laymens terms & explanation. I have tuned & rebuilt many carbs before (in my early years). It just seemed I drew a blank looking at that carb (more than likely early onset alzhiemers). But I’m still riding & will continue to do so even if i forget where I’m going!
September 4th, 2022
I just fitted a pair to a 2L Triumph car. The Lamba sensor I have fitted shows that as soon as I hold the throttle above the idle range, the mixture becomes far too rich at 10.5:1 The carb has the standard supplied needles and the needle jet is the standard 723-Y6 My float heigh is fine, so I can only consider that my standard Triumph air filters are causing too much of a restriction? I cannot locate K&Ns that fit inside the standard air box which I would like to keep as it draws in cold air rather than the heat from the exhaust manifold which is directly below. so my next step is to try leaner Y4 needle jets.
December 11th, 2022
I ended up going to the smallest needle jets and thickest needle with the result that every thing runs in the correct range according to the Fuel ratio gauge and it feels very good. The only issue I have is that occasionally, even though I have the largest accelerator pump nozzle #70, and it is adjusted to come on right away, if I am not careful with the throttle it can go far too lean, around 20 to 1 when pulling away if I open the throttle a bit too much.
Now I am used to Amal carbs on British motorbikes and for this problem I would fit alternative slides with a smaller cutaway that enriches the mixture for the first eight of throttle opening, seems I heed slides for the HSRs with a smaller cutaway but I see they are only available in the one size?
So I will try putting richer needle jets back in and lowering the needle?
January 29th, 2023
Hi I have converted my 96ci superglide to carb
I have gone for the HSR 42 I’m running a 575 slow jet and a 200 mail
The bike has v&H short shots with baffles and screaming eagle big sucker intake
Thunderheart standalone ignition
My problem is it idles good and runs great top end but low down I’m getting carb spit and flames and loud poping out the exhaust on deceleration
Needle is what came with the carb I haven’t touched it and ac pump is set to squirt more or less straight away any info would be great as there isn’t much out there in info for the 1584 converting regards andy
May 22nd, 2023
Your suggestions are way off. 17.5 pilot jet? Seriously? I tried your suggestions. Lucky it didn’t blow up my motor!
I undid everything thing you suggested! Went a 98 needle, raised it one notch and a 30 pilot jet. My bike is way happier. Everyone has a different setup. You just have to take time and figure it out. No formula works for everyone. I’m running 9.5 compression. Andrews N8 cams. Open air cleaner and opened pipes.
May 28th, 2023
converted my 2005 fxdci to carb mikuni air cleaner v&h big radius pipes hsr 45 runs but rough ordered mileage kit guess question is was hsr45 to big of carb for application thanx in advance
July 24th, 2023
I should probably make it a little more clear. I reposted a direct quoted article by Joe Minton. A man who knew more about the mikuni than most people who were not the original engineers. Unfortunately Joe died long ago. I thought I’d said that in the post, but after reading realized that I didn’t make it clear that he has long since passed, and that this was to archive his knowledge and not an article written by me.
I would however, like to point out that his article does mention aftermarket cams, and compression as considerations, that should drive people with such setups to consider that the advise may or may not apply to them. Something you seem to reference in your comment… somewhere after the “seriously” part…