Hello, I have a S5 13B with an aggressive street port and a 48mm weber DCOE. my car runs like **** right now and i have figured out that its because of my emulsion tube (F7) I am kind of new to this so I could use some suggestions on what tubes jets and air correctors to get, I have had a few people suggest F8, F11, F12, and F16 tubes with the 16s being the most popular. let me kow what your thoughts on this are, i'm tired of my FB sitting in my garage and just want to finally take it for a drive, so any suggestions on what emulsion tubes, fuel jets, and air corrector would be highly appreciated, thanks

 

i find the only way to tune a carb is with a process. you need to start with idle, and work your way up, as things interact, so if you start in the middle, you can just get lost. plus when you work your way up, in a logical fashion, you get to learn what each piece of the carb actually does.

so step one is the idle jet, it controls the amount of fuel from idle to approximately 25% throttle, or 3000rpm, whichever occurs first. if you drew a graph, with throttle opening on one axis, and rpm on the other, and drew the line from 25% throttle to 3000rpm, the idle jet handles fuel there.

the test for this is to take the main jet stack out of the carb, and go drive it. you do need a closed course, or an unbusy street or something, as you only get 25% power. ideally you try a few different jet sizes, it seems like most engines end up around 55-65 for a fuel jet. if you have a wideband you should be able to get it to run somewhere between 14.7:1 and 13:1, the final mix is up to you, but richer is just wasting fuel, and it probably won't be happy any leaner.

obviously the idle mixture gets adjusted to suit, however it is based from the idle jet, so the idle jet is important

the next step is the main jet, like the idle jet it controls the amount of fuel you get, however it is not throttle based, it simply works from air flow. if you have a wideband you're looking to see about 12.5:1 to 13.2:1 AFR. if you don't have a wideband, then richen it up until it stops running, then lean it out until it feels the best, then then go a jet richer. you want best power, and then slightly richer, just to have some margin.

once you have that, the next step is to get the transition between the two circuits correct, and this is the "hard part". so you've driven without the main jet stack to get the idle jet where you want it, and then the next test is to see when the mains come on. this one you need to be in a high gear, and a low rpm (you need to be at an rpm under where the idle jet is working), and you floor it. the car will run on the accelerator pump, and then if its way too lean or way too rich it'll stop running. if its a little lean or a little rich it'll stumble.

there are a few ways to go about this, but basically you now need to change the air correctors. a smaller air corrector in the main jet, will have the main jet come on sooner, and a larger one will have it start working later. this also applies to the idle circuit, so you can tweak both. i do find that the rotary likes a slightly smaller idle air than the weber default, with the IDA we change the jet holder, or fill it and redrill it, from 120 to 100, but the DCOE has an air you can change.

i think the Emulsion tube has an effect here, but there is a chart of cylinder size to EM tube, and a 12A should get an F7 and the 13B gets an F8. the F11 is for a vw beetle, and has no place in a rotary. i do notice most people want to run the F11 for some reason, and they usually have a lean stumble around 3000rpm because the idle circuit is tapering off, and the mains aren't going yet. i'm running an F7 (its a 12A), and i can make the transition so rich the car stops running.

 

well it seems that for street port engines racing beat recommends the F11 tubes, all I know is i have had no luck with the F7 tubes i have as soon as i get on the gas my engine starts to stutter and it just gets worse as i give it more gas, i cant afford to buy every single tube and jet combo so im trying to narrow it down to one or two tubes and a couple jets and air correctors

 

how do you know that stumble has anything to do with the E tube? mine was worse than that, and it was just the main jet.

part of going through the process is that you learn what the parts of the carb actually do on your engine, which is helpful later, plus you learn something, and that is neat

 

well i have tried going with smaller jet i stepped it down two sizes with no noticeable difference, and when i finaly gave up on trying to figure it out and asked around online most people said that my Etubes were the cause, one guy even said his car did the same thing when he had the F7s and it went away when he swapped to the F16s, im new to working on cars and rotarys in general, I have nobody that knows these engines and i have a limited budget at the moment so im trying my best to figure this out before spending money so that i can buy the thing that I know i need, and overwhelmingly it seems like people are saying that the F7 etube is the cause