Valkyrie

It suddenly occurred to me why my idle has been so freaking low lately, when I was thinking about the fact that the carburetors that the company I work part-time for (a subsidiary of Nikki, who made the FC's throttle body) makes have two separate, but equally important idle adjustments:

The TAS (throttle adjustment screw) and the IAS (idle adjustment screw).

It occurred to me that the reason my idle is so low when the car is warmed up, yet idles *great* if I simply give it a TINY bit of gas is because neither of my throttle stop screws (primary OR secondary) are touching the throttle.

The reason the TAS is important is because every batch of throttle plates on small-engine carbs has to be lathed to fit the right tolerance of each batch of carb bodies, do to various differences in sizes. This is slightly different with larger engines, with computer controls, but it's still the same concept. And more importantly, the throttle is never supposed to be completely closed.

Essentially the issue is that the IAS alone on these small-engine carbs does not provide nearly any air to maintain the proper idle. They simply control the amount of fuel flow at idle. Basically, the TAS gives you the idle speed, but the IAS can change the speed by changing the fuel ratio. However, it needs to be adjusted to give the best performance and emissions.

It's a bit different on the FC's three-bore, because it's EFI. The throttle-stop gives you sort of a ball-park (baseline) idle speed whereas the air bypass (idle adjustment screw) either on top or on the BAC for the turbo gives you fine adjustment. On the S5, the idle fuel flow is automatic whereas it's manually adjusted (as far as I know) on the S4 with the resistor thingy. And then the BAC compensates for engine load at idle (whereas on a small engine you compensate for load by simply opening the throttle...because there is no load at idle).

Thus, a properly-adjusted TAS SHOULD touch the throttle just enough to open it a tiny bit (this is according to my boss, an engineer who was with Nikki when they were still making the three-bore throttle body, as used in the FC). Meaning, if you hold throttle up to a light, you should see light coming through, but not all that much. It should not be pitch-black.

On the other hand, mine is completely closed, so the engine is basically trying to idle by breathing through a straw... hence the reason why my idle drops even more when I press the brakes, turn on the lights, etc. I opened the secondary stop just a little bit, and it made a slightly improvement in idle, but I'm going to have to take the TB off to adjust the primary stop.


So, it is NOT to prevent the throttle from sticking as many people seem to think. It is to open it just a tiny bit, and provide the proper amount of air for idle (as well as to compensate for the variations in the carburetor or throttle body itself), whereas the idle screw is purely for fine adjustment.

Of course, don't forget to clean it as well, as carbs and throttle bodies are very precise and very sensitive devices (for a car part that is), so if it's not clean, adjusted, and in good condition, you're bound to have idle and driveability issues (albeit slightly less so with an EFI car).




For the record, the reason that the throttle stop screw adjustment isn't in the FSM is probably because it was never intended to be adjusted by the user. Like I said, the general guideline is to set it so it opens the throttle just a tiny bit, unless you're going for a "bridgeport idle."

Valkyrie

Addendum:

Ok, even with the screw completely not touching the throttle, there will still be *some* light coming through.

So basically, only turn the screw until the EXACT point where you can detect it opening the throttle. And don't open it more than you need to (the screw should be protruding a bit above the lock nut), thinking you need to have a perfect circle of light (you don't, it can be a little bit uneven).

I opened it up a tiny bit too much (to the point where the screw is perfectly flush with the lock nut), thinking I needed to and now I'm idling nice and smooth ... at 1200 PRMs (with the idle adjust all the way in)...

(smooth RPM wise, it still hiccups a bit bit because it's idling too high)

So I need to unbolt it and readjust it... oh well, not that big of a deal.

Meh, idling solid at 1200 is still better than wimpy at 500 (that'll kill your BAC...).

stylEmon

iTrader: (2)

very helpful research
thanks

iceblue

I thought this was common knowledge of what the set screw does.

Valkyrie

I would have though so two, but all of the threads I could find on it had people thinking it was there to keep the throttle from sticking. (IIRC even HAILERS didn't know what it was for )

RotaryEvolution

iTrader: (30)

yep, pretty common knowldge but the FSM is not very conclusive about how to set it. i personally haven't seen any worn enough to affect the idle that much so i am willing to bet someone dicked with your idle set screw or the throttle body is stripped out from constant on/off throttle applications(seen that as well.). when modifying cars that single screw is usually all we can rely on for proper idle.

don't mistake the primary throttle stop screw for the secondary throttle stop screw though, the secondaries should be closed completely when no throttle is applied and the stop screw is only there to keep the plates from wearing/jamming up. that may be where this misconception came from.

Valkyrie

I'm pretty sure I was the one who dicked around with it

Valkyrie

Doh, this is what I get for not listening to my own advice... I readjusted it and it's a bit better, but still too high, at 1100...

Note to self: Seriously, only turn it enough so you can detect it opening. And if you think you've turned it too much, you have.

Now I've gotta wait for it to cool off before I can work on it again...

divinefist81

this is valuable information to Rtek chip owners not holding idle issues such as myself at the moment. i will try ReteD method by losening the jam nut & adjust it with the flathead scredriver within the 8mm jam nut.

divinefist81

professionalpyroman

Hey divinefist,

one thing I noticed in your picture here is that it looks like you aren't using the coolant line that runs through your throttle body. That line is meant to warm up your throttle body so that the intake air temp sensor functions correctly. The stock ECU, or modified one uses coolant temp as the reference to know when it can rely on the temp obtained from this sensor. Once the engine is warmed up per the coolant temp sensor, the ECU believes that the sensor is warm and relys on the intake air temp sensor at the throttle body. That sensor changes resistance when it is cooled down by the intercooler air from the reference of the coolant temp. This is the entire reason that the first set of secondary throttle plates don't open until the engine is warmed up. It keeps throttle input low prevents boost from going through the throttle body before it can be correctly measured by the temp sensor. There are a few systems that need to work together to function correctly. This is why you do all the engine tuning at operating temp.

GtiKyle

iTrader: (5)

@divinefist81 Did this solve your idle problem? I'm just now diagnosing what's going on, followed the idle procedure in the RTEK and still cannot get solid idle. I'm leaning either towards vacuum leak or this throttle adjustment screw.

divinefist81

thats a good observation there i never thought about that because the car was given to me like this after the new Jspec engine swap. do you have installation instructions because i have no idea where the hose connects to? thank you

divinefist81

im gonna work on the car friday i cant give you update; somebody said to replace the vacuum hoses once again to be on the safe side since checking for actual air leaks is like finding a needle in a haystack

divinefist81

***update***

Got the car to start with a Liittle help holding the gas pedal for 15min. made some adjustments with the BAC & idle screw but the moment I let off the gas it STILL won't hold idle on its own so I'm gonna change the vacuum hoses & compression test

Valkyrie

The car being referred to in this post from 17 years ago was resting (rusting) peacefully on a farm for the last 13 years until it was recently hauled away by the owner of the farm, without my consent.
I was given $100... Sure, the paint was ruined, and the engine needed to be rebuilt, and the interior was sun bleached to death, and the air conditioning (which I installed) didn't work, and I'm pretty sure the brakes were out..... but....it was MINE.

Anyway, if anyone wants a cheap S5 NA FC roller, call around all of the junk yards in the Mobile/Baldwin County Alabama area. If you rescue it from the crusher, I will give you to the title for free. lol.

WANKfactor

iTrader: (1)

It took years of working at a carburettor factory to come to this revelation? Holy shhhhhit!

Valkyrie

don't judge me

WANKfactor

iTrader: (1)

Lol, sorry 😔

Valkyrie

I think the reason I started this thread was because there were two adjustment screws, but the FSM only had specs for one… the other one was never supposed to be adjusted anywhere but the Nikki/Mazda factory.