Relationship between idle jets and idle mix screws Options

[era vulgaris]

How much can one compensate for the other?
For example, with larger jets and idle screws further in, could you tune a set of carbs identically to another set of the same carbs that had smaller jets with the mix screw further out?

I want to experiment with going a little leaner at idle and seeing how my engine likes it. I'm trying to decide if I should order a smaller set of idle jets or if I can achieve the same result by just turning the screws in further. Basically, what is the difference? Don't they achieve the same ends? Obviously there's a limit on the rich side because there's only so much fuel a jet will allow to flow. But if you're trying to lean it out, what's the difference between going to a smaller jet or just turning in the screw?

[Justinp71]

On all the carbs I've worked on the idle screw just sets the mixture at idle, once you open up the throttle there are additional fuel holes that draw in more fuel on the idle circuit. The larger the idle jets the more fuel you draw overall. The idle circuit typically runs the motor up to 3k rpms.

However the farther out the idle fuel screw is you do get a little more fuel past idle.

I have my car tuned with a leaner idle, but larger idle jets. Then when you idling at a light your not burning extra fuel and its a little less smelly. Then when you get on it, its all smiles. I did notice its a little harder on cold start this way though.

[r_towle]

First answer, no, it does not work that way.
Then, no.

Look into getting an exhaust gas temp sensor.....even used temporarily while you tune....it will still do the job.

What you need to see is the temp of the exhaust where lean is hot and Rush is cool

Rich

[era vulgaris]

Thanks for the info guys!

I already have an onboard AFR gauge, so I know exactly how rich or lean I am.

My issue is that sometimes at idle the rpm's start to drop and the car wants to die. Happens maybe once every twenty or so times I come to a stop. Happens a little more frequently if it's the 2nd or 3rd time I've driven the car in the same day. But sometimes it doesn't happen at all. I thought it might be from running too rich at idle.
I have no issues at any other point in the rpm range. No stumbling, hesitation, or anything at all. It's just annoying to have to sit at a light every now and then with my heel lightly on the gas pedal to keep an idle.

I was just trying to find out how far up into the rpm range the idle jets had an effect. Sounds like I maybe just want to bring the mix screws in a bit then, if the only effect they have is on the idle and not any farther up.

[rhodyguy]

At what rpm does your car idle? Does your car still die if you roll up to the stop in neutral?

[era vulgaris]

It idles around 1100.
Yeah I normally roll up to a stop in neutral. I don't hold the clutch pedal down unless I'm shifting.
Like I said, it's only once every so often. But it's a slow dying when it happens.
It idles fine when I get to the stop, then down 100rpm, hangs out there for a few seconds, then down another 100 rpm, hangs there for a few seconds, etc until it starts struggling to stay alive. Then it's the lightest touch on the gas pedal and it comes back. Sometimes that's all it takes and it's fine again. But if it's like the 3rd time or more that I've driven the car that day, it usually doesn't come back to idle, and that's when I have to gently hold the gas pedal with my heel. But then maybe the next stoplight it's totally fine again. It's not a consistent or constant problem at all. Just happens enough to be annoying.

[somd914]

This doesn't answer your question but there is a lot of good carb tuning info in this thread

http://www.914world.com/bbs2/index.php?sho...hl=2056++timing

[PancakePorsche]

After 3-4 turns out, idle mixture screws are not effective. If you find yourself in that range you should increase idle jet size. I like to be between 1.5-3 turns range for good resolution. this will not effect transition. If your carbs do not have chokes and runs real good when cold, I can assure it will be too rich once warmed up. The 914-6 folks know all about that. Also, timing is everything with carbs. A lot of the time carbs take the blame for wrong initial timing and /or curve. Tuning carbs before timing is putting the cart in front of the horse.

[rfinegan]

It idles around 1100.
Yeah I normally roll up to a stop in neutral. I don't hold the clutch pedal down unless I'm shifting.
Like I said, it's only once every so often. But it's a slow dying when it happens.
It idles fine when I get to the stop, then down 100rpm, hangs out there for a few seconds, then down another 100 rpm, hangs there for a few seconds, etc until it starts struggling to stay alive. Then it's the lightest touch on the gas pedal and it comes back. Sometimes that's all it takes and it's fine again. But if it's like the 3rd time or more that I've driven the car that day, it usually doesn't come back to idle, and that's when I have to gently hold the gas pedal with my heel. But then maybe the next stoplight it's totally fine again. It's not a consistent or constant problem at all. Just happens enough to be annoying.

IT sounds to me like 1100 rpm is off the idle circuit and into the transitions. try to set the mixtures at a slower speed (less throttle plate opening)Last steps this set the idle speed

-Robert

[rhodyguy]

See if you can achieve a stable idle @ 950rpm.

[era vulgaris]

After 3-4 turns out, idle mixture screws are not effective. If you find yourself in that range you should increase idle jet size. I like to be between 1.5-3 turns range for good resolution. this will not effect transition. If your carbs do not have chokes and runs real good when cold, I can assure it will be too rich once warmed up. The 914-6 folks know all about that. Also, timing is everything with carbs. A lot of the time carbs take the blame for wrong initial timing and /or curve. Tuning carbs before timing is putting the cart in front of the horse.

Timing is set at 11 degrees at idle and 28 degrees at 3K rpm, which from what I could gather from searching is Jake Raby's recommendation for a Mallory Unilite. Idle screws are just over 2 turns out.
Carbs don't have chokes (DRLA 40's, in my signature), and run a bit rough when cold. Like I said, 99% of the time everything everything is fantastic. I just have this occasional dropout at idle. When I say I think it's running too rich, I'm talking like maybe 0.5 units on the AFR gauge too rich. Not a massive amount.

IT sounds to me like 1100 rpm is off the idle circuit and into the transitions. try to set the mixtures at a slower speed (less throttle plate opening)Last steps this set the idle speed

-Robert

See if you can achieve a stable idle @ 950rpm.

I'll try and get the idle down a bit. It just doesn't seem stable under 1K rpm, which is also what makes me think it could be due to it being too rich at idle. Every other part of the rpm range is absolutely killer. I'll try turning the mix screws in and stick with the jets for now, and see where that gets me.

[Justinp71]

Yes, AFR gauges are great.

Also make sure you don't have a fuel supply, vacuum leak or ignition issue (plugs or timing mentioned above). I've also had the tops of the spark plugs loosen up and create issues too.

[r_towle]

Talking about heat syncing and how it affects the mechanicals of a carb, as well as the fuel density....

Do you have phenolic spacers between the heads and the intake manifolds?
Do you have phenolic spacers between the intake manifolds and the heads?

When you get this issue to happen on the second or third time you have driven the car, please reach in and see if you can grab the side of the carb....without burning yrself.

See how hot the manifolds are and how hot the carbs are.

Also, please look at fuel lines, fuel pump and feel the heat if you can when this condition happens.

Rich

[era vulgaris]

I do have phenolic spacers between the heads and the manifolds.
Not between the manifolds and the carbs though. Do they even make them for that spot?

I'll check next time I've driven the car several times in a day and see if the manifolds, carbs, fuel lines, etc are getting hot.


No vacuum leaks, fuel supply, or ignition issues. Trust me, I just went through all of those systems. You missed my thread from a few weeks back (IMG:style_emoticons/default/biggrin.gif)

[r_towle]

I saw the thread, was not blind, just bored stiff staring at the screen all day.

They do make those spacers, but it really depends upon how hot things are getting.

It sounds like you have a list of issues.....and you may need to trace down more than one problem to make everything right.

I don't doubt that you have been looking at lots of things, but I would suggest you focus on heat, and only heat.

It runs good cold.
It runs worse after its hot.
So, what changes with heat.

To me, a few things come to mind.
Old distributors get really sticky and advance plates get stuck when hot, and to make it fun they get stuck in different positions, advanced, no advance etc....so look for advance plates that are stuck when hot...not a fun test but you will need to see how well it works cold, then try it hot...without getting burned.
Mine did this....it was 0.05 cents of grease, and my time to clean the distributor.
Found that after I replaced the entire fuel injection system.... (IMG:style_emoticons/default/smile.gif)

Valves.
Valves get longer when hot, so they will have no tappet clearance, or even worse they may be held open a hair when they are hot. This can make you have no compression, or worse an inversion...which will make it run like crap....
Btdt, check your valves cold, then see what they are hot.
Hot is not fun, but really a valid way to adjust valves....should be damn near zero, but not zero clearance... So when they are hot, the Valve does indeed close completely, but there should be just about no noise.
In my book, noises valves are happy valves, in aircooled cars.

Did a ton of work on the 911, still had a weird idle when hot....it would hunt.
It was the valve adjustment was too tight.

And fuel.
These engines get hotter when y shut them down. It's a heat sync.
On the 356 cars the issue was the fuel could boil, and by boiling it could go down into the engine, and create a hydro locked car...so much fuel in the cylinder it would not turn all the way over.. I have had to rescue one 356 at a show from this....we removed the plugs, got her running on three, then shut her down and got her going on four after we put the plug back in.
I was the youngest guy there, so I had to get dirty while being coached by my elders..
It was a hot day, show car, super tight new motor....

[era vulgaris]

Sorry, the missed thread comment was directed at Justinp71, as he mentioned ignition, vacuum leak, etc. I should have made that clearer, just didn't feel like cutting and pasting quotes.

I wouldn't necessarily say that it runs good cold and not when hot. I don't really start driving until it's warmed up. But even so, it usually runs great. It's only occasionally at a stop that this happens. Most of the time when I come to a stop the idle is strong and stable. It's just the few times when it isn't, that's annoying.

I'll have to check into that stuff. FWIW, my distributor is a 5 year old Mallory Unilite. I don't think it would be that dirty, but it's worth a check. Are they known to have problems with sticking advance plates? I know the stock dizzys can have that happen, as it happened to the one on my old car.
My pushrods are chromoly, so valve adjustment is zero anyway. I have the swivel foot adjusters. When I adjust them, I get them tight enough so the rocker arm doesn't move, but loose enough that the foot can still spin. Checking them hot reeeeally doesn't sound like fun! But sounds like it's worth it. I did a hot compression test a few weeks ago. All cylinders were within a couple psi of 140. If I had a stuck open valve, wouldn't I get a large reduction in compression in one cylinder?

Sounds like a lot of fun on that 356! I've definitely been in that position in the audio world! (IMG:style_emoticons/default/beerchug.gif)

I did go for a long drive this evening, and felt the carbs and manifolds when I got back. They manifolds were warm, but definitely not uncomfortable to hold my hand against. The carbs were even less warm. Fuel pump is in the front of the car, btw.

[r_towle]

Ok,
Sounds like a great car that runs perfectly fine.
What are you bitching about?
Just drive it....

(IMG:style_emoticons/default/smile.gif)

I realized today that my 911 was not nearly as smooth as my big Audi.
Oh, my 911 is over 30 years old and the Audi is less than ten....

My 914 was never without hiccups....just goes with the territory.

[era vulgaris]

Haha, yeah you're probably right. And if this were any of the ACVW's I've previously owned, I probably wouldn't even be bothered and, like you said, just chalk it up to it being a 40 year old car.
But this car, with the 4 channel digital CHT gauge and the digital AFR gauge, I've got so much more information coming at me constantly that I'd otherwise be oblivious to in any other car. Gives me a case of the "want-to-tweaks" and iron out every little wrinkle (IMG:style_emoticons/default/biggrin.gif)

[r_towle]

Thus why I have no gauges in mine....I took out the center console cause my feet were too big.

I can smell when an air cooled is not right...have owned quite a few.

TMI can be annoying and take away from the experience.