A little backstory: I've got an SP12A in an 85GSL, 2 1/4" straightpipe from the header all the way back to a cheapo muffler, with a 2GDFIS and a Sterling carb bought second hand on the forum. The carb tuning was pretty good, but seemed rich and somewhat ill-mannered at cruise, and it was recommended that I switch to a .043 primary airbleed. Upon opening the carb up, I saw that the previous owner had installed .043s in the secondaries, and was running wide open emulsion tubes in the primaries, which seemed pretty weird to me.
I figured dude must have had it backwards, so I put the 43s in the primary and left the secondary bleeds open, and tuned from there. The primary circuit now seems pretty happy with 47s (but a 45 might be still better), but I believe the secondary circuit is now too lean, running on open emulsion tubes. Factory spec had the secondary bleeds at twice the size of the primaries, which would put me in the neighborhood of .093.

My questions: How big are the emulsion tubes by themselves? And/or where's a good place to start on the secondaries (.089,.091, etc?)? I'm aiming for slightly lean on the primary circuit and slightly rich on the secondary circuit.

 

Anybody?

 

By ill mannered do you mean it was bucking at low speeds? Explain a lil bit more on whats going on.

 

You can throw in a set of stock air bleeds if you need to get it going right now. Then get yourself a set of Holley jets of appropriate size and screw them into the Sterling-modded air bleeds and reinstall. I don't know the sizes but I do know the metric stock sizes are anywhere from 60, 70, 80 and 90 on primary air bleeds and the secondaries are almost always 140. I'm not sure what that equates to in 'standard' sizes, which is what Sterling and Holley use.

 

The first thing I would do is contact Sterling about it. If by "wide open" you mean that the emulsion tubes were there but there wasn't a jet screwed into it, that definitely isn't right. He'd be able to give you an idea where to start, though all the tuning is gong to have to be done starting from there because every motor, driver and location is different.

The Sterling uses Holley 10/32" Air Bleeds as both its air bleeds and its jets. Jegs has a kit that has more jets then you will ever need - I split a kit with someone and got 2 of every size. That will give you much more tuneability.

Jon

 

By ill-mannered I mean stumbling, bumbling, and refusing to display nice cruise behaviour. With the setup that the carb had, it was really only happy at WOT. I now have the primary circuit running very well, and have beautiful spark plugs after cruise, but the secondaries still seem lean. To me, this also all but rules out the possibility of incorrect fuel jetting.

Jeff, I read that too, but am equally in the dark as to what those translate to in standard sizes, and also suspect that they may need to be larger on the modded carb.


That is exactly what I mean, Vipernicus. I've seen this before on other cars; my understanding is that in situations where the required jet size was equal in diameter to the emulsion tube, one can run without it. On the Mazda it does seem to be pretty close, but the open tubes are just a hair too big. Nobody knows what that diameter is? The thing is, it's close enough to dead-on that I suspect I can get it there with a range of 3 or 4 sets to tune with, and would hate to purchase a full jet kit for this. I just need an idea of where that range should fall.

Thanks much for the responses, fellas.

 

I just got mine, Sterling sent it with .63 secondaries, and it runs very well up high. I have not started tuning it yet so I don't know if it's ideal, but I'm happy with the way it's running.

 

Thank you! I would've started quite a bit higher- perhaps the secondaries are not enlarged to the extent of the primaries...
Best of luck with tuning.

 

The secondaries not being gapping has absolutely nothing to do with it.
The venturis in the Sterling Nikki are OPTIMIZED in their SHAPE to maximize the pressure differential created at the apex. This is done by maximizing the efficiency of high air flow through the venturi. The bigger the venturi for a given engine (constant), the lower the velocity, the less the pressure differential, the less "suction" provided to the fuel circuit.
The gapping secondaries of a Holley need larger fuel jets to deliver the same amount fuel for the same mixture at WOT than a Sterling Nikki does.

I never recommend more than .063 for secondary fuel jets. For the suction the Nikki provides to the secondary main circuit, that's a big jet.

Emulsion tubes; Sorry, but the person you bought the carb from must have just needed those Holley jets. Nobody in their right mind would run with open E-tube jets.
The emulsion tube is named as such because it's job is to create an emulsion of air and fuel as the fuel makes it's way up the tube and out through the booster venturis.
Earlier, this was just a plain tube to pipe the fuel up. High pressure differential would cause such a suction that an air bleed jet was installed so that at high suction the fuel wouldn't siphon directly into the engine, flooding it.
Then the carb gods realized they could harness that energy by cross drilling holes in the bottom of the tube so that, as the fuel level went down in the reservior (bowl), it would uncover these holes gradually, and mix in air bubbles with the fuel . Premixing the fuel with air is difficult, but increases atomization greatly, and thus efficiency (and economy).

Having an open emulsion tube bleed means that there's no signal to your secondary fuel circuit. The pressure differential created in the secondary venturis is just sucking air out the feed tube in the booster venturi.

Put an ,042 jet in the secondary E-tube, and don't go past .063 for secondary fuel.
Use .036 for primary air, and .048-.055 for primary fuel.
Start there. Make sure your accelerator pump is not over shooting causing a stumble.

 

Great info as always, Sterling.

 

sterling should have been the first person you talked to he did the work he is also usually willing to help any one with carb problems don't be afraid to ask him or anyone else questions he is also the most knowledgeable on this subject so listen to him good luck

 

I was wondering where Sterling was.

 

Thanks much for the comprehensive info- it is greatly appreciated.