Mikuni HSR 45 install

[PjackB]

Since I could not find any information anywhere almost on these, I thought I would share my experience here and hopefully this can help someone else looking for potentially better carburation.

 

 

Background:

 

First the background, my car is a 74 260Z with a “hybrid” engine specs are as follows

•      Stock N42 engine block with dished pistons

•      E31 head with larger 280 valves F Stamped OEM Camshaft from a P79 that was reground (no specs but measured around .460 lift)

•      Mild porting of cylinder head

•      123ignition distributor 35degrees total advance

•      240Z Roundtops SU with custom Z Therapy needles

•      6-1 headers into 2.5” SS mandrel bent exhaust

•      LW Flywheel , stock rear end .

•      170 whp / 170 Wtq

 

 

 

 

The car ran great with awesome midrange punch and decent throttle response but would run out of steam at 5500 no matter what we tried even though we were still in the meat of the powerband (see dyno sheet).

 

 

 

After multiple tests and diagnostics, it was decided carburation was most likely culprit and I decided to change my induction system.  I had Triple Mikuni PHH 40s on hand and that was the plan but I’ve been meaning to try Mikuni HSRs for years on a car so figured this was a good time to do so.

 

Mikuni HSR:

 

The Mikuni HSR is what is called a flat slide carburetor (as opposed to a CV for the SUs), these were originally developed by Mikuni as a performance option to replace SU and other CV carbs on Harley motorcycle their 2 main selling point was better performance with an average of 25% increase in power and ease of tuning.  

These carbs are more akin to PHH or in their jetting system (pilot, main, air)  and they also have an accelerator pump,  interestingly 2 HSR 45s will flow as much air as dual PHH44s with 37mm chokes

 

John Parker at V-Performance.com has been a pioneer at adapting them for car use and under his careful advice we chose the proper size of carbs and jetting for my engine specs.

 

 

 

Install:

John provides very detailed Installation instructions and even with my little mechanical experience I was able to get them installed on the manifold in less than an hour, it took me another hour or so to get the linkage to my liking and we were ready to roll.

 

We then put them on the car made sure the accelerator pumps and idle screws were sets were set equally on both carbs ( we did not connect the choke cables since they’re missing on my car )  , turn they key gave it some gas and that’s it the car started right away much easier  than easier than the SU , we let it run under gas for 30 second or so before it idled by itself . 

 

That was it for the first start , I went back to it a few days later same starting procedure but now we connected an LM 2 wideband , as suspected we were running a little rich (12.5)  so adjusted to 13.7 for now until we can get it on the street or a dyno to do further adjustment since it’s winter here.

 

 

 

 

 

 

First impression:

So far I’m extremely impressed with the ease of use of these carbs and the throttle response is just short of phenomenal, next steps is to get the car on the same dyno I did before making the change and see how they perform , the expectation is that I should see a gain similar to what I would have seen with the triple 40s which the consensus is 7-10%

I will update this thread as soon as I put I on the road with driving impression and the dyno numbers once that’s done also

 

https://i.imgur.com/fpoMjbU.mp4

 

 

 

Thanks for reading

Edited March 13, 2022 by PjackB

[Enzo250gto]

Hello from Australia, I have just picked up the same Carbs that you have installed. they look great. Have you finalised the jet selection yet. I will install mine towards the later part of the year on the new engine that is being built. Any advise you have would be appreciated.

Looking forward to any updates.

Regards

David. Australia.

[DuffyMahoney]

Gosh this should be a 10x easier upgrade over triples.  What did the kit run you?  

[Enzo250gto]

I picked my kit up here in Australia from someone that had purchased it for a Jensen Healey sports car. It came with new 2x HSR 45mm carbs, Adjoining throttle shaft, K&N air cleaners, choke cable adapter, Manifold adapters to bolt straight to the SU/Hitachi manifold and extra jets etc. The part are all new and unused. I paid $800.00AUD which is probably $1200.00AUD cheaper than if I had to try and buy from the USA supplier with the current exchange rate. I have had twin Hitachi/SU carbs, triple 40mm Dellortos, 390cfm Holley and Fitech 4 barrel EFI over the years on various engines but this looks neater, cleaner and less cluttered. I'm just looking for a reliable, spirited drive with reasonable economy. The feed back from customers at Vintage Performance seems positive. I know that Eiji at Datsun Spirit in Virginia has used 6x of the HSR carbs on his engines with great success.

David.

[Stephen Duff]

PjackB, did you ever get the HSR's dialed in?  Or run on a dyno?  Interested in your thoughts!  They look like a cool option!

[Ntel]

Hi PjackB 

 

I am looking at installing a pair of Mikuni HSR 45s on my 260Z, just wondering what Main jet and Pilot jet sizes you have finally settled on and are using.

Great looking car you have, very nice job.

Cheers from the Land down under.

 

 

[Ntel]

On 8/26/2022 at 10:49 AM, Enzo250gto said:

I picked my kit up here in Australia from someone that had purchased it for a Jensen Healey sports car. It came with new 2x HSR 45mm carbs, Adjoining throttle shaft, K&N air cleaners, choke cable adapter, Manifold adapters to bolt straight to the SU/Hitachi manifold and extra jets etc. The part are all new and unused. I paid $800.00AUD which is probably $1200.00AUD cheaper than if I had to try and buy from the USA supplier with the current exchange rate. I have had twin Hitachi/SU carbs, triple 40mm Dellortos, 390cfm Holley and Fitech 4 barrel EFI over the years on various engines but this looks neater, cleaner and less cluttered. I'm just looking for a reliable, spirited drive with reasonable economy. The feed back from customers at Vintage Performance seems positive. I know that Eiji at Datsun Spirit in Virginia has used 6x of the HSR carbs on his engines with great success.

David.

Hi David

 

Just wondering how you are going with these carbs,  I am looking at fitting them to my 240Z and was wondering what jet sizes you have settled on ?

 

Cheers Nathan

[boosted300]

Beautiful car!  But almost a year with no update? Tsk tsk...

 

Let us know how it's working out!

[PjackB]

Time has a way of slipping through one's fingers, and it seems 2 complete years have vanished since my last update. In the interim, my journey with the Mikuni HSR carburetors has been nothing short of revelatory. These marvels of engineering have consistently delivered joy with every press of the throttle , I can also I've gathered substantial hands-on experience to report back on their performance.

Not long after the install I went back to the dyno with them but need to preface my comments with that I had no complete comprehension of their inner workings  , I spent a few hours at the dyno playing with different settings mostly because I did not properly understand the relationship between needle, needle jet and main jet  on these carbs . regardless even though most of my runs were done with jetting that was mostly way too rich and realy  all over the place the Torque line was surprisingly consistent with what I had with the SU but initially down significantly , power was up and I could now rev better to 6500 . the first few runs we were down almost 13%  torque and our best run was still down 5%. Also because we had some very lean spots we kept timing at 34d compared to the 37 I ran with the SU even with all this we still made 172whp up 6hp from before . But performance on the street was Ok 

since then I spent a lot of time getting to know them better  and Through an iterative process of tuning—adjusting needles, pilot jets, and main jets and timing —I've dialed in the carbs to my engine's specific demands. One of the best parts about these carbs is with practice you can change needle position or swap needles completely in 5 minutes on the side of the road with few tools and try a completely different setting on the spot. 

This meticulous tuning has yielded a power distribution that favors mid-range torque, albeit with a slight trade-off in top-end power.   I’ve not gone back to the dyno but Real-world performance is significantly much stronger than when I initially went  and seat-of-the-pants feedback suggests a peak horsepower somewhere near 180whp which is 15 more than the Sus, I’m not sure about peak torque but  I would also say my average torque probably went from 151 with the Sus to somewhere around 165 ish . However, without a recent dyno confirmation, this is speculative.

One thing to note is that they seemed sensitive to engine bay temperature change so  a well-crafted heat shield proved essential for maintaining optimal air fuel ratio Also At some point Velocity stacks were added, which, while not increasing power, have provided a consistent air-fuel ratio across various driving conditions and seemed to help towards temp sensitivity 

Looking forward  the  focus shifts to a new project: a 3011cc stroker engine. The plan includes testing both 42mm and 45mm Mikuni HSRs to ascertain the optimal carburetor size for this new setup. Based on the experience thus far, the Mikuni HSR carburetors are highly recommended for their exceptional tunability and the overall enhancement in performance they offer,  although further testing is on hold as the new engine build takes priority.

Sorry again for the delay and hope this helps anyone looking at this 
Please feel free to reach out if the are any questions 
J.