Datsun Roadster Forum-311s.org

Has anybody tried the mikuni HSR42mm on their R16 1600 engines yet?

I've seen some pics of such carbs on a U20 I think. I just read a site that makes these sound like a really nice upgrade with improved power and throttle response. Cost runs around $1000+ and expect to do some work for linkage but clearly doable. They're not particularly nice looking, but the performance might be worth it.

i did not look to much into the type of carb that is but looks like a slide. i hear that they have amazing throttle response and kick but are a little more fiddly when trying to set them up. doing some digging i am trying CV type carbs on my r16. there is a bunch of info online with CV carbs on older engines. i decided to go that route as there are a bunch of info on them being converting to a car.

Unless you're doing a stroker or a lot of headwork to handle a lot more air 42s are too much carb for a 1600,
I've looked at these and researched them a lot and came to the conclusion that the HSR42s are actually perfect size for a hot U20

Thanks for the info, just wondering as I am having some problems with them, I also installed one of those 123distributors as well.

A 42HSR is going to be way too much carb on an R16 (stroker R is a different story), particularly when you consider the small size chokes you run in weber or mikuni setup on an R16. I am in the process of building a bigger U20 and plan to run 42HSR's on it, at least until I find the time to build an EFI setup for it.

The only real problem that I have going is if I slowly accelerate the throttle it runs great but if I go from idle to wot it bogs down and backfires thru carb. not sure if I have an accelerator pump problem or a leaking intake manifold problem

While I certainly understand the concerns about over carburetion, flat slides may be a bit different in that regard. From the site I explored, they had them on much smaller displacement engines that are not known to flow a lot of air to larger displacement engines with excellent results (of course, that's their site!). They didn't appear to kill bottom end and could flow plenty of air as needed. They were also reportedly a lot easier to tune than say a Weber DCOE as there are no emulsion tubes etc to consider.

Anyway, right off the batt I'd suggest you invest in a wideband kit for tuning ANY carb; the more tunable the more you need one. Second, it's a lot to ask of any aftermarket system to smoothly and effortlessly rev from idle to WOT without digging into and tuning it and perhaps the engine, maybe a lot. A wideband is a very valuable tool. Given this, based on what would typically be wrong in a carb, it does sound like you're not getting enough enrichment with sudden throttle changes, which is what the accelerator pump is there to cover of course. A lean condition will cause a backfire out of the lean carb(s)... watch your ears, they can pop loud! But you can also only ask so much of that system if your primary fuel supply is already too lean (and you may not know without a wideband unless you're really good at reading plugs). Intake leaks will likely show most at idle where vacuum is high in the manifold, so the old carb cleaner or WD40 spray looking for leaks may be in order. Any dual carb set up is very sensitive to equal airflow, and nothing less than at least a unisyn or better yet the Snail airflow meter is needed by most to get it right IMHO.

Hope this helps. They are an interesting alternative and I like the potential for performance increase without having to get a super expensive Solex manifold. I'm fortunate to have gotten mine 35 years ago, so I'm EFI now but if I'd been SU I would have looked closely at this. Again, they're kind of, um.. ugly, BUT I think they could be dressed up with some cool filtering or covers, etc.

ok, thanks for the info

I just noted the "...123 distributor" ... what do you have in it for a Curve? That can affect your running. Again, the more tunable-ability you have the more you may (will?) need to tune it! You can probably copy the stock curve and be good, but finding the sweet spot takes a dyno or a lot of street tuning to see if you can improve things, though that's what makes it fun (if you're so inclined!). However, you need data...and while a stock (assuming) R16 isn't a highly stressed engine, you don't want to be running any engine lean while messing with the timing, eh? Wideband! Or get your plugs reading nice and brown before you deviate a lot from the stock curve.

The curve was pre-set from the company I bought the 123dist from ( datsun restoration products) The curve that was programed is as follows:
500rpm = 10*
800rpm = 19*
3500rpm = 35*
6000rpm = 32*
8000rpm = 29*
I don't know if there is some data out there for the distributor curves, wish I could find some, if anybody knows of any it would be greatly appreciated

I'm no expert, but I've done some research. Ignition timing is very important for power and economy as well as emissions. The curve provided probably matches a standard stock curve which should serve you just fine. Research on a dyno has shown there are places where addition output can be had but to get it generally requires a dyno where you can objectively see the difference at different rpms. Also, you can add more timing and not get any knock but you may not be adding any power, could be staining the engine unnecessarily and reducing your safety margin. Typically the operator will run the engine to an rpm, hold it there under load and adjust timing until it doesn't show any improvement, then back it off a couple degrees for safety. That's done at many different rpms as each engine is different in what it wants. Ignition timing makes a much bigger difference in power than a "correct" air/fuel ratio. That's the advantage to a programmable ignition, but unfortunately a more tunable system requires more tuning to get the most advantage!

You can do some street tuning over time, especially when working on steady state stuff, like improving fuel economy at cruise and light load with vacuum advance if you're good at keeping track of it.

jparent wrote: ↑Mon Aug 30, 2021 2:37 pm The curve was pre-set from the company I bought the 123dist from ( datsun restoration products) The curve that was programed is as follows:
500rpm = 10*
800rpm = 19*
3500rpm = 35*
6000rpm = 32*
8000rpm = 29*
I don't know if there is some data out there for the distributor curves, wish I could find some, if anybody knows of any it would be greatly appreciated

This is a basic timing curve we provide. Since no roadster has a "new from factory" engine, optimal settings will be a little different for each.
Just so everyone understand the rationale behind the curve, here it is:

500rpm = 10* very easy on the starter. the more advanced, the harder the starter has to work.
800rpm = 19* depending on idle rpm, idle will be between 16 and 21 degrees
3500rpm = 35* linear digital advance from 800 to 3.5k. this is fairly conservative to avoid knock under load from 2k-3k. Depending on CR and the gas
you run, you can add a point at 2500rpm (or any other rpm) for additional advance or retard.

6000rpm = 32* power is falling off in most engines-time to shift. Linear decrease to 32* and then 29* avoids high rpm detonation which is nearly impossible to hear.
8000rpm = 29* street engines will be well out of the power band. Soft rev limiting is independently programmable too!

Sounds like the HSR's are showing classic signs of "too big carb". You may not be able to tune this out with a 1600. You probably will want to hook up the map/vacuum signal, but blog at full throttle and great performance otherwise are classic for oversized carbies. More fuel/timing may actrually make backfire and bog worse, but you can tinker with it.

Since you are aware of it, you may simply be able to modify your pedal technique. Nailing the throttle with small displacement engines usually doesn't provide the best power curve since the intake velocity goes to nil.

J, Great talking with you the other day, let us know if we can further assist!

At Vperformance.com they ran a dual set of 42s on an MGB with before and after dyno runs etc, so while I too thought you might be over carbed, it seems that flat slides may be more forgiving. Also sounds like you could do some head porting and a cam and pick up some power. They also sell the 123 dizzy.
On the MGB they replaced two 38mm SUs.

Thank you all for the help. I'll do some tinkering and let yas know what I come up with.