Jeff20B

iTrader: (13)

Yes I've searched and from what I understand, the duty of the air bleed is to bleed off the vacuum signal entering the carb. This affects mid to high RPM more so than low RPM and has no effect on idle, because the idle circuit has its own set of air bleeds.

The larger the air bleed's orifice, the lower in RPM it will start to affect things because it begins to work sooner than an air bleed with a smaller hole.

So a bigger hole = lower RPM operation, and a smaller hole = a higher RPM-capable carb and may potentially offer more high RPM performance/power over the carb with the larger air bleeds. Right?

Then what about low RPM operation and power? Can it get all screwed up? Or is the stock Mazda carb pretty much idiot-proof for low RPM? Heck, reading Sterling's writings and you'll start to think that.

I have an intended victim uh vehicle in mind that would benefit from a good understanding of low RPM tuning, but I don't want to count out mid to higher RPM either. More info is coming...

Jeff20B

iTrader: (13)

If I got the above correct, I can go on to tell you that I have six Hitachi carbs; five ranging from '74 to '78 and one J-spec with a strange set of air bleeds which came off of a tiny ported 13B out of some strange automatic car in Japan. The e-shaft had no blind plug and the rear plate had an oil return fitting (looks similar to a bellhousing alignment dowel in the wrong place). The oil return drains right into the sump. The tranny must have used engine oil in some way. I installed a blind plug in the shaft and will add an NPT plug to the rear plate.

Just a brief list of the carbs and the vehicles from whence they came.
'74 REPU
'76 Cosmo
'77 unknown
'77 unknown
'78 RX-4
Jspec - unknown

The two '77s could have come from RX-4s or Cosmos.

If you're an old school rotorhead or have had any experience with Hitachi carbs, you may know that the '74 carbs had the biggest jets and the intake ports of those engines were larger than the '76-'78 13Bs and '76-'85 12As. They are what I lovingly refer to as '74 spec ports and they work very well and provide a noticeable power improvement over stock on a '76 or later 12As or 4 port 13Bs without hurting low RPM. Finding a carb suitable is not as much of a challange as finding one for a streetport as the port opening is the same and the closing is only like 10° later. The improvement mostly comes from the reduction in casting flash and general better "breathing" capability of these ports which retain more velocity than typical streetports. The exhaust ports can remain stock or slightly enlarged (recommended on a US spec 12A as their exhaust ports are too small from the factory).

REVHED

The air bleeds or air correction jets are basically a controlled leak in the main run circuit of the carb. The air mixes with the fuel in the emulsion tubes and is drawn out by the venturis. Changing the size of the air bleed mainly effects the mixture at mid to high rpm. Smaller air bleed = rich, larger air bleed = lean. It's not a case of one size being right for either low or high rpm operation but of tuning the mixture to suit your engine.

Jeff20B

iTrader: (13)

Ah yes, thankyou REVHED.

The J-spec carb has 90 primary fuel jets, 140 secondary fuel jets, 90 primary air bleeds and 160 secondary air bleeds.

The J-spec engine had super small intermediate (primary) ports no bigger than FC primary ports; short on the outside, tiny on the inside. For those that have seen ports that small, you find yourself wishing they could be enlarged safely at least to stock 12A spec.

You know ports that small would be good for low RPM operation, and is probably why that carb has such good tip in, like what I found when I tested it on my automatic Cosmo engine the other day (Cosmo has 12A-sized ports). It just runs out of breath way sooner than I want.

Jeff20B

iTrader: (13)

I blame the air bleeds on that...

One option is to build an engine with the tiny ported J-spec intermediate plate and leave the front and rear plates unported and use this carb and its channeled manifold (I should mention the Cosmo manifold does not have channels, which may have messed with my testing results) on a specific-use engine. The engine would probably produce more low end than your typical rotary and have great tip-in, but would most likely have pathetic high end. I intend this engine to go into a baja project. However there are just too many unknowns and I'm affraid the loss of high end power of this carb the way it sits right now would make for a very boring baja.

The other option is to swap jets and air bleeds and make a carb that would do better at higher RPM, but hopefuly not kill the low end.

Perhaps I could take a lesson from the '74 carb: Awesome performance without any noticeable loss in low end power. High end like a street port, midrange like a well tuned setup, and low end like what would benefit an REPU. That would be the perfect setup for a baja since I don't want to attempt a fuel injected setup just yet.

Jeff20B

iTrader: (13)

For comparison purposes, I tested the Cosmo's carb on my streetported 4 port 13B REPU yesterday and I've gotta tell you it's very nice, even on the larger ports. It tends to run out of breath sooner in the RPM range than on the Cosmo, which was to be expected due to the smaller fuel jets, but I didn't expect just how much more fun it would be to drive than the '77 carb I'd been using.

I don't recall the secondary sizes of the '77 carb, but I think the fuel jets were 140 and the primary fuel jets were upped to '74 spec - 105 and the primary air bleeds are stock for '77 which is 80; just like '74 spec. So in other words, with the emulsion tubes and venturis being equal (I believe), the primary side of my '77 carb is the same as a '74 carb.

By the way, the '74 carb is a monster. It scared the crap out of me when I briefly tried it in my MG Midget 13B project last year. Whoa Nelly. Believe me when I say it's seriously fast. Even on a buddy's REPU, even with an automatic it is surprisingly powerfull! Both vehicles had '74 spec ports and carbs. It's definitely the way to go.

Jeff20B

iTrader: (13)

My REPU certainly isn't as quick as his auto with the '74 spec ports and carb. I could either do a different engine with '74 ports and carb, or just drop a supercharger on the current engine. I highly doubt an NA Holley setup, even properly tuned, would offer this kind of performance.

Another twist is I think the '77 and later carbs were meant to run on manifolds with channels cut connecting primary to secondary runners below the carb spacer like '79 and later 1st gen manifolds (well, '79 to '80 and some '78 and older manifolds; you FB guys have that dumb flapper valve thing in one side). As long as I've had the '77 carb, it's been run only on seperate runner manifolds. So I just recently cut some channels into a spare '74 manifold and I'll try it out pretty soon. My other '77 carb has been working great on a channeled '78 manifold and has plenty of power in an RX-4 with streetports identical in size to the REPU and a full Racing Beat single pipe system.

Jeff20B

iTrader: (13)

Back to the J-spec carb.

In the Cosmo, I couldn't tell whether the secondaries were even opening so I removed the spring from the secondary vacuum actuator and then got a bog and a really throaty sound at low RPM. They were definitely opening, but not doing anything for power as far as I could tell, appearantly regardless of the RPM. It's gotta be those air bleeds...

For comparison purposes, the Cosmo's stock carb has 100 primary fuel jets, 130 secondary fuel jets, 60 primary air bleeds and 90 secondary air bleeds.

The Cosmo carb's low end power isn't worth writing home about, and has less tip-in probably because it's got a bad accel pump (will get fixed when I get around to it), but the secondaries are a lot stronger than the J-spec carb. You can really feel them when they open (yes, even on an automatic!). The Cosmo's exhaust is totally stock complete with a thermal reactor.

Jeff20B

iTrader: (13)

Writing all this out and gaining an understanding of what air bleeds actually do has helped. I think I will go ahead and attempt to copy what's in a typical '74 carb and install it in the J-spec carb for use on a '74 spec ported 13B for a baja project. It will have adequate low RPM; at least close enough to a bug engine with a stock carb so as to be driveable at low speeds, and the mid to high RPM should be a real kick in the pants like the other two have been.

Of note: the engine I built for 7percentC's '83 GSL received '74 spec ports and was able to idle all the way down to 500-something RPM within 20 minutes of break in time (and that's only because it took me that long to grab the RPM/tune-up guage in between all the other first-startup check list things, checking for leaks etc). It was a 12A with a channeled old school manifold and a slightly modded Nikki with stock air bleeds and secondary jets; the primaries were upped from 92 to 94 and it has an AP mod.

I want that kind of 'performance' out of this 13B, and I think I can get it with the correct understanding of... whatever it is I'm doing here.

Since I don't have the baja yet, I think I'll break in this engine in my MG.

vipernicus42

iTrader: (4)

As I recall, air bleeds aren't about making power, but rather about fine tuning the afr at the top of the primaries, then again at the top of the secondaries.

So you JET for the type of driving style and performance characteristics you want, then you use the air bleeds to help insure a smooth transition from primaries to secondaries, and a proper fuel mix at high rpm/load in the secondaries. (the transition from primaries to secondaries is also helped by accel pump and power valve if your carb has one. I know the FB nikkis don't have a power valve, but I think some of the hitachi carbs do).

Now my understanding of how to tune is still relatively rudimentary, but one thing is certain. You JET first, and do the air bleeds later.

Mixing and matching air bleeds from carb to carb is a tricky thing, and not something I'd end up doing. You don't know exactly what size any of them are, or if you *do*, chances are you only have a few to choose from and you can't really tune it well since you're missing a bunch of sizes that you could try/might need.

Someone should get Sterling in here, he'd know

I found these useful when trying to understand air bleeds.

http://www.yawpower.com/carbtech.html
http://www.yawpower.com/niktest.html

Jon

Hyper4mance2k

iTrader: (40)

Get a weber and well make some real power!

peejay

The purpose of the air bleed is to allow air into the emulsion well, so emulsified (foamed) fuel comes out of the booster. This is one of the reasons why carbs have better atomization than fuel injection.

The size of the air bleed does affect the fuel curve. It's not so much "when" the boosters come on-line so much as the shape of the fuel curve, smaller bleeds = more fuel on the top end, larger = less at the top end.

Jeff20B

iTrader: (13)

So the air bleeds are for tuning at the top of each 'system' in the carb. I had the basic idea.

I don't think Hitachis have power valves, but I'm not sure what to look for.

The type of driving will consist of daily driving; low RPM, part throttle cruise and some full throttle runs up to redline. So far, the '74 carb's performance has been top notch and is what I'm attempting to copy in the rest of my carbs.

I think I've gotten it right on the '77 carb in the RX-4. While it would have been better if the engine had '74 spec ports, it still performs rather well on streetports. It's my guess the increased vacuum signal of the channeled manifold is partly responsible. That would also explain why my REPU doesn't perform as well with the same carb, same port job and a non channeled manifold, but why the Cosmo carb, which was factory tuned for a non-channeled manifold, worked better (before I maxed out the fuel jets, heh).

The RX-4 will be getting a rebuild and then a supercharger in a few months so the carb will be going onto a spare 13B which will also get '74 spec ports.

I'm sure some of you are wondering why I'm even trying to use these old carbs. I mean a Sterling carb works on a stock ported 13B, right? A 74 spec port is only a little bigger. Why not try a Sterling carb then? Well, unlike a stock Nikki, the Hitachi actually has changeable air bleeds like a Sterling carb, and already posesses larger venturis... I think. Maybe they're only 20mm like a stock Nikki. I'm not sure.

I'm trying to get six projects to run at their best and each one has the potential to be great. Three have street ports and the other three have or will eventually have '74 ports. I'm almost fully convinced it's the key to a great running rotary when you're underfunded but have lots of old carbs sitting around. I'll know more in a few weeks when I've gotten some more work done.