Ah, neat. I had forgotten about the secondary vac issue since I went mechanical secondaries before I cut my venturis. And yeah, the opening around 50% is correct. I'm not sure if the SA vs FB secondary timing is different, but yes there are some design differences. For example, I used SA throttle shafts in an FB t-body because the shafts are slightly less intrusive to airflow and they use smaller screws.

I'll be curious to see what you come up with for your "actuator". Many people simply weld the part you are going to wire-tie. I went an alternative route and chose to make a new linkage for my carb, but that's a bit of a hasle. Essentially it adds an extra arm to do the job that a wire-tie or weld would. I also took that opportunity to add one more arm to act as a throttle stop to reduce the torque on the shaft while closed (I cut half my secondary shaft off, which is a somewhat controversial choice due to a greater potential for flex).


Reworking the runners should help to offset the difference, although I can't say for certain whether the difference in flow is notable to start with. I would think that since they choke down to (I assume) the same diameter that flow would be relatively equal regardless. When I removed mine I also cut the channel into that side to closer mimic the SA intake manifold which is regarded as one of the better flowing. Doing this without a doubt made the power delivery feel smoother as well as slightly changed the sound of the car. Whether there were significant power gains I don't know, but it certainly didn't feel any slower; quicker if anything.

 

Yes, you are correct on the 50% throttle opening thing for how far the primaries must be open so the secondaries can start to open. Secondaries are pretty much digital. They are on or off; open or closed. Yes, the linkages and position of parts differ a bit from SA to FB. I recommend adjusting the activator or rather the "activated" piece, if you can do it, to allow the primaries to open to about 70% - 75% before the secondaries will be activated. It provides a a little bit stiffer feel in the pedal and can be useful for racing. But the best aspect here is that you can open the primaries to around 70% and drive on them that way instead of being limited to just a measly 50% primary.

It kinda sucks to drive around at only 50% max, and have to wait until friggin 4k until you cna open the secondaries, but sadly this is how all the previous mech sec carbs seemed to be.

But I can tell you this regarding a nice solution. If you follow my air bleed advice, you can open the secondaries earlier than 4k and it won't bog... if, and the key word here is IF you follow... my advice about air bleeds. It might not have much power at say 2k, but at least you could open it down there, and it should not bog on you. Ever. Unless the sec emulsion tubes are dry, and if they are, the micro delay is super short as fuel gets quickly pulled up and outputs through the boosters into the venturis and thus down to where it's needed.

It sounds like you are willing to maybe "edit" the secondary opening timing to something past the stock 50% of primary opening. Maybe you are willing to play with your secondary air bleeds too.

sec main from stock 140 down to 80 (requires machining work, which isn't super hard to do)
sec long slow: solder fill upper part only, leave lower part open
stock nickel plated 60s are ok but there is a seller on eBay that sells Mikuni 50s that fit here. Might be worth a shot.

As for your concern about the shudder valve in 81-85 manifolds, I would not worry about it because, using a finger or an eye, feel and look down past all the shutter valve stuff where the two runners start to look about the same size. That is when they become equalized. The runners can only flow as much as the narrowest point. Thus the really large area left over after the shutter valve and shaft get the old heave-ho, doesn't matter very much.

 

When changing the secondary main air bleeds you can use the same jets that are used for the fuel jets. They can had from JetsRUs listed as the Hitachi pilot jets - the largest size is 43 so you'll need to drill it out to 80. To install the new jets a 4.20mm hole should be drilled in the top of the old main air bleed (after shortening the top of it). Following that, an M5x0.8 tap is what you'll need to get the new jets threaded in.

For the nickle plated 60's the part you're looking for is the B42/55 Mikuni jets. There's a few places around that have the 50 (B42/55/50). Hey Jeff, at one point you mentioned using a solder-filled drilled 38. Is going that small really worth it or is anything smaller than 60 worth a try?

 

I don't recall saying that. Oh you mean a solder filled primary 150 through 190 that I drilled to .37... which is only necessary if boosting the carb and only if you want to restrict the unwanted air as much as possible.

I would not use solder filled air bleeds of any kind on an NA setup because tin whiskers can grow inside the hole over time with exposure to the atmosphere. It is super easy to clean them out with the same drill bit you used to drill them in the first place or a single wire from a wire brush or what have you. I suspect this is less of an issue on a boosted carb because it will have a carb hat directly on it instead of an open element air cleaner assembly. It also helps to use leaded solder instead of any of the newer reduced lead or environmentally friendly solders as those grow tin whiskers really easily. I use a special leaded solder meant for plumbing purposes and not for electronics when I fill air bleeds.

 

Oh, hmm. I could've sworn you once said you used a ~0.37 size no. 2 air bleed on the secondaries to further reduce the opening delay. I recall it was from something where you completely solder filled it which caused syphoning, at which point you drilled it out with the smallest bit you had. Looks like I'm confused though, oh well. My apologies if I misquoted you.

Interesting about the tin whiskers, I didn't think of that.

 

You pretty much got it right. Seeing 38 there didn't make sense until I read further. The smallest sizes I've worked with in the no. 2 sec slow position are .02 to .025 but you need the patience of a saint to go for this small of a size, and I think it might allow some unwanted siphoning to take place. The size is just too small to be practical. There were a set of brass blanks from some old dead Hitachis (one blank per carb). Then I later drilled them out to .40 for use in 32.XXmm secondary carbs, not that 50 couldn't also work, because I suspect it could just work fine. Maybe. Huge venturis for NA use only, are pretty experimental. It is all about speeding up the flow of fuel and keeping the AF curve within the range you want for a happy engine.

 

Thanks for the help guys. I have the tabs wired together and will see how it goes. Since there is no such thing as road racing at less than 4k (actually if you got as low as 4k you probably messed up), I am hoping that the simple primary venturi, main fuel jets and mechanical secondary modifications will be what I need. If not, I will definitely be back to look at all the other mods you guys have been doing. Hopefully I will have the car at the track sometime in the next month. Next race is in 3 weeks, but will probably be taking the Miata, since there is no "pre-testing" involved. Also lots more cars on track.

Anyway, thanks again.

Carl

 

Ok. Finally getting back to update the results of the modified Nikki in road racing service. As a summary of what happened before, I had my stock 12a rebuilt with a mild street port. Using a stock Nikki (minor jetting changes to stay at A/F 12.5-13), I was able to get 135 HP. With a modified Nikki where the primary venturis were machined to 124mm and secondary/tertiary booster support legs were removed, I was able to get 150HP (same dyno, within 20 minutes). Initially I had some issues with secondary throttle valve actuation and had to operate them by hand on the dyno. First, the gasket between the main and throttle bodies was installed backwards. Fixing that then Identified that the enlarged primary venturis did not provide enough vacuum to complete open the secondary TV's. I was originally going to just tie the secondaries and primaries together for a test, but it took a while to get to the track and I got antsy and just modified the mechanism. The attached photos show how I modified linkage (FB carb) on both the primary and secondary shafts to get all valves to be fully open at the same time. The secondaries start opening when the primaries are about 50% open. I basically just welded some additions to the existing linkage to get things actuating properly.

In actual operation, the carb was flawless. On track there was absolutely no hesitation or indication that there was any problem with the carb. Lap times were 2-3 seconds faster than the last time I had the car on that track. That despite the air temps being 100-105 degF, compared to the previous time when the air temps were probably closer to 50 degF. When driving it in the pits, there was a very slight hesitation when matting the throttle when starting at 1500rpm. A/F was steady at 12.5-13. The main fuel jets are: 140 primary and 170 secondary. All other jetting is stock.

I can't necessarily recommend this configuration for the street, but it is working great on track.

Because of the heat during testing another issue popped up. The oil was getting very hot (235-245 degF at the cooler outlet) and water was running at 210 degF. As such, I had limited runs and did not hang around to race the next day. Later testing showed that the thermostat in the oil cooler had failed. I pulled it out and compared it in hot water with another one I had. They both activated at about the same temperature, but the one that had been in the car could be easily pushed back in, which meant that the spring for the pressure relief kept the thermostat open. Either way I decided to upgrade the radiator (from the short-early radiator that has worked fine (-ish anyway) for 10 years) and also move the fmoc the front of the radiator. I may also remove the thermostat from the oil cooler (and plug the bypass of course) to avoid any failure problems in the future.

I may be able to get more power by opening up the secondary venturis too, but that will have to wait for another day (or year or two). Or maybe I can use the methanol that just came out of my home still ahead of the good stuff to boost my HP!

Carl

Modified primary and secondary linkages. The secondary mod captures the tang that is actuated by the primary

Primary linkage mod. On left is stock and installed is mod to get the primary and secondary synchronized properly

Stock secondary linkage.

Modified secondary actuator installed.

Vacuum secondary actuator removed.

 

Interesting secondary linkage method. I ought to post mine (not here as I don't want to clutter your thread).

Maybe I missed it, but can you tell us about where in the primary opening the secondaries begin to open? I see you said 50%. Well, stock is about 50% so I don't understand why you would go to this much trouble unless...

Did you work in a bit of progressive secondary opening or something? That's the thing these carbs lack is a progressive sec opening. These carbs are pretty much 0% to 100% back to 0%. It's pretty much digital. Either on or off. If you could work in some progressive opening action, that would be interesting.

What I've done is despite keeping them digital, I've allowed the primaries to open more closely to 70% or 75% before the secondaries will open. I like this way better than stock. It also requires a tad more foot effort to open which I've found is perfect during spirited driving because you can actually feel the hard spot in the pedal and know that this is where to punch a little harder to get them to open, rather than having to be kinda timid about it at the stock 50% opening if left simply mech with no further linkage work.

Not sure if what I've just described is making any sense to anyone else, but it feels like I explained well enough.

 

Here is a video I took of the TV's opening when I first put it together. I guess I would consider the actuation progressive.

I went the way I did with the actuation, because I was not comfortable with the reliability of wiring the tabs together. In a street car, it doesn't matter if the wire comes loose or breaks, since there is still plenty of power to get where you want to go (albeit a little slower). On the race track however, you are hosed, especially for endurance races (typically 7-8 hours per day). Sure, you can get back to the pit, but you will have just wasted a bunch of entry and consumables dollars (literally hundreds) going back to the pits and screwing up your race. I didn't want to weld the two tabs together on the secondary shaft, since that would have precluded making any future repairs/modifications or at least making it much more difficult. I originally only modified the linkage on the secondary shaft, but the "slop" on the primary side resulted in the secondaries not reliably opening fully. Increasing the width of the tab on the primary linkage eliminated that slop and made the TV's actuate consistently.

I am interested to see how you did your mechanical secondary actuation. It may already be posted somewhere, but I honestly had a tough time finding what other people had already done on line.

I don't want to imply that my modifications would be perfect (or possibly even acceptable) on a street driven car, but it seems to work quite well at getting lots of fuel and air into the engine under racing conditions with zero driveability issues.

Carl

 

I should take a picture of how I did it. Mine doesn't modify any parts it just has the tangs mated in such a way that the secondaries open around 2/3s of the primary and both are fully opened at WOT.

 

I would like to see how guys did the conversion. I have an early carb I want to finish up with mech secs and am interested in other ideas.

Carl