Mikuni HSR review

[Pjackb]

copying from another forum I'm going to put this here and let the admin decide if it needs to be moved
This is a review of HSR carbs as Installed on my L28 but since I plan on a set for one of my roadsters eventually I thought this could be useful for some folks

If you are interested in this carbs I would highly recommend you speak with Jim Tyler (JT68) of Datsunrestorationproducts.com as he's figured the linkage for our cars and can make you a kit

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://imgur.com/fpoMjbU


Thanks for reading

[cbez]

Cool

[Gregs672000]

Really nice write up. They look like a great option that allow for more tuning and flow more air. If I did not have a Solex intake I would have likely gone there. Or some EFI version...
Inline 6's are nice smooth engines eh?

[theunz]

Good job on the carbs, but WOW that Z is gorgeous! Love that shade of green.

[Pjackb]

Good job on the carbs, but WOW that Z is gorgeous! Love that shade of green.

Thanks, this one and my 67.5 are my favourite both to drive and look at

[Pjackb]

Really nice write up. They look like a great option that allow for more tuning and flow more air. If I did not have a Solex intake I would have likely gone there. Or some EFI version...
Inline 6's are nice smooth engines eh?

Nice and smooth yes , mine is a little loppey at idle because of the cam but smooths out past 1500rpm
But that cam also pulls like a freight train, if you look at the dyno sheet you’ll notice that except for a small dip at 2000 rpm the engine is always making at least 90% of its torque between 1500-5500. I’m hoping the HSR will smooth things out even more while increasing output

ONe of the key feature/design of these carbs is that they’re made to be adjusted by throttle opening, so you can adjust mixture for idle, 1/4 open (cruise) and WOT (power). This give much more flexibility over the transition and removes some of the dips we see often with PHHs during acceleration

[ppeters914]

Damn, that sure looks like a clean setup. Rough costs?

[Pjackb]

Damn, that sure looks like a clean setup. Rough costs?

1200$ all in with air filter, extra jet kit and shipping
I spent another 50$ on odds and ends to get the linkage right
42s which are better for a U20 would be same price

[Pjackb]

Finally got a chance to drive my car for the first time this week following the HSR install and I have to say I continue to be impressed.
I took it for a 30min drive Monday from my friends house where it was to mine it ran good but was definitely way to rich at idle and part throttle (low to mid 11s) and too lean at WOT (14+)

One of the best things about these carbs is that they are specifically designed to facilitate tuning by degree of throttle opening .
Thus since I was rich idle and part throttle and lean when I floored it it meant my main jet was a little small and pilot jet too large and they needed to be replaced. this is very easily done, now Since it was my first time playing with them it took me about 30-40m to change the jetting but this could be done in way less time, I reduced the pilots from 25 to 22.5 and increase the mains from 210 to 220.

Now you’re looking at guy that could never tune his SUs himself to save his life but rebalancing and tuning these carbs after the jet change took me less then 10 minutes

With the new jetting idle to part throttle is still a little rich but at a much better 12.7-13.2 AFR and WOT is at a nice 12.5 average. the car pulls strong and sound awesome (see video) but it’s hard to tell if I have more power or if it’s the better acceleration of the new lower diff (4.11 from 3.34) that I’m feeling. (The dyno will tell objectively)

There’s still a little bit of fine tuning to do which I hope to do when I go to the dyno in the next day or two if all goes well

https://imgur.com/hLoAFVX


https://imgur.com/9e5xdNN

[Gregs672000]

That's really great! Car sounds good and smooth. The cars look like a great option that offers significant but relatively easy adjustments... win/win! Â wideband makes all of this so much easier eh?

[iloveredmeat]

Hey Jacques,

Good info and a good read.

Very, very awesome sound!

[todd lorber]

That looks great. I must say, if my Z ran 170 hp with SU's I'd probably just leave them alone. I have a stock, dished L28 in my car and I know the SU's are the limiting factor. I'd be surprised if I had 135 hp at the wheels, but the car is so streetable I haven't wanted to mess with changing them. These carbs might be a good answer. Looks like they use the stock intake manifolds?

[Pjackb]

That looks great. I must say, if my Z ran 170 hp with SU's I'd probably just leave them alone. I have a stock, dished L28 in my car

Even though it was making good power the SUs were really restraining the car and we couldn't really rev or make power past 5500 we tried a lot of different things but that was their limit on this engine. the new carbs pull very strongly to 6500 rpm with no issues

I have a stock, dished L28 in my car I know the SU's are the limiting factor.
These carbs might be a good answer. Looks like they use the stock intake manifolds?

They do fit the stock manifold , you can make very good power with SUs as evidenced by my original results , it's when you wan to make more that it becomes limiting IMO
I would consider these carbs as an alternative to having SUs rebuild or buying Mikunis at the price they are at now

[Gregs672000]

Hey Pjackb, did you ever get the car back on the dyno? I'm prepping and doing a bit more research, and was taking a look at your original pull. That torque drop between 2700 and 4500rpm could be due to a few things, but you mentioned having a 123 dizzy in it, and while your A/F ratios are a bit off there, I'm kinda wondering if you have more of an ignition timing issue than an A/F ratio problem. From what I've been reading/seeing, just about ANY ratio between 11 to 16 will produce nearly the exact output, but ignition timing changes of 1-2 degrees or more can make a much more significant change in power. Since you have the 123 and the ability to change that curve anywhere, you might try playing with the timing a bit in that area. I don't know what you came up with for a curve (stock?) but it may not be quite what YOUR engine wants there. Of course, you have to be careful in how much you advance things, but it could also be that it wants less advance... you just don't know. Fuel ratio is important for cooling the combustion chamber, so somewhere in the 12.5-13 ratio should be good at WOT, and that temp change helps keep it from knocking so be sure that remains good. I may be advantageous to create a few different curves there that you could upload while on the dyno, or to do what I am planning, which is a max safe (no risk of detonation) torque for every 500rpm at a steady state under high load from 2000-5000rpm. Are you able to use vac advance on the 123? If so, you could get your best WOT curve and use the vac advance to add timing under low load for better fuel economy and throttle response (or however the vac advance system works. My system interprets a drop in manifold vacuum as an increase in load).
Hope this helps!

[Pjackb]

Hey Pjackb, did you ever get the car back on the dyno?

I did but I had backed out timing to be safe on the first runs and jetting was not right and we were running way too rich
I spent a few hours at the dyno playing with different settings mostly because I did not properly understand the relationship between needle and needle jet on these carbs . Torque line was surprisingly consistent with what I had with the SU but was down significantly the first few runs we were down almost 13% and our best run was still down 5%. even with all this and timing at 4degrees less we still made 172whp

Since then I've added some stacks , nailed down the jetting and increased timing , acceleration has dramatically improved and the run from 60mph to 90mph is exhilarating

I had planned anther dyno run but it will have to wait later this summer as something broke this weekend in the bellhousing and i will be replacing clutch and flywheel.

base on the way the car feels now i expect to see power north of 185whp , as for the slight dip in midrange I've played around with different setting on the 123 and i can still feel its there and i'm thinking its a function of my cam profile