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Interesting how a turn on mixture screw can make increase temps.
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nextgen
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PostPosted: Sat Jul 24, 2021 6:59 pm    Post subject: Interesting how a turn on mixture screw can make increase temps. Reply with quote

My bug has had an bit more of the smell of exhaust fumes, so I did some test backing off on the mixture screws.

Actually backed off all 4 of my screws and the fumes did decrease but also noticed my oil temp increased.

Not bad, was taking a while to get to 170 f, car ran fine but did smell bad. Now it hangs out at between 180 and 190 f on 90 -95f air temp days doing 65 mph and some bumper to bumper traffic..

Smell is much less, not bad at all. I don't like over 210f

No difference in power.
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simon varley
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PostPosted: Sat Jul 24, 2021 8:39 pm    Post subject: Re: Interesting how a turn on mixture screw can make increase temps. Reply with quote

if you run leaner, you will run hotter. that's how engine combustion works all else being equal
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PostPosted: Sat Jul 24, 2021 11:09 pm    Post subject: Re: Interesting how a turn on mixture screw can make increase temps. Reply with quote

I've a 4 channel CHT gauge , it is amazing how much difference 1/2 a turn will make to CHT's (over 10C )and how balanced they are . here's a graph of temperature verses mixture .

The middle of the graph is Stoich 14.7 to 1 on petrol , peak EGT ,peak power is to the rich side normally around high 12's .with lower CHT's. That is why you don't want and air-cooled head running under load around the middle of the graph .


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PostPosted: Sat Jul 24, 2021 11:27 pm    Post subject: Re: Interesting how a turn on mixture screw can make increase temps. Reply with quote

simon varley wrote:
if you run leaner, you will run hotter. that's how engine combustion works all else being equal


Thats not quite right , you can run lean at cruise as long as you have a vacuum advance type dissy or programmable ignition . It's covered a lot in the Wideband thread .

I run around 16 - 17 to 1 and 42degrees advance at cruise , cool CHT's . good fuel economy . I just got 8.3/100km highway at 110km/3400rpm . 2600cc with 48mm dellortos in a Ghia . pretty stoked with that economy considering the cam is rather large .
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PostPosted: Sat Jul 24, 2021 11:58 pm    Post subject: Re: Interesting how a turn on mixture screw can make increase temps. Reply with quote

Wreck wrote:
simon varley wrote:
if you run leaner, you will run hotter. that's how engine combustion works all else being equal


Thats not quite right , you can run lean at cruise as long as you have a vacuum advance type dissy or programmable ignition . It's covered a lot in the Wideband thread .

yup, like I said "all else being equal" which is what the OP did by just adjusting the mixture. Timing changes are a whole other discussion as you point out.

that's why there are whole textbooks, theses and industrial research projects devoted to the subject Very Happy
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PostPosted: Sun Jul 25, 2021 8:15 am    Post subject: Re: Interesting how a turn on mixture screw can make increase temps. Reply with quote

Totally agree, yes I know lean will make the engine run hotter. As Wreck said it does not take much of a turn of the screws to start seeing a difference.

Happy it is in the safe range.
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PostPosted: Sun Jul 25, 2021 10:18 am    Post subject: Re: Interesting how a turn on mixture screw can make increase temps. Reply with quote

I was under the impression the mixture screws only mattered to idle.
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PostPosted: Sun Jul 25, 2021 11:18 am    Post subject: Re: Interesting how a turn on mixture screw can make increase temps. Reply with quote

Ellis' Bug wrote:
I was under the impression the mixture screws only mattered to idle.


That is correct
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PostPosted: Sun Jul 25, 2021 12:05 pm    Post subject: Re: Interesting how a turn on mixture screw can make increase temps. Reply with quote

simon varley wrote:
Wreck wrote:
simon varley wrote:
if you run leaner, you will run hotter. that's how engine combustion works all else being equal


Thats not quite right , you can run lean at cruise as long as you have a vacuum advance type dissy or programmable ignition . It's covered a lot in the Wideband thread .

yup, like I said "all else being equal" which is what the OP did by just adjusting the mixture. Timing changes are a whole other discussion as you point out.

that's why there are whole textbooks, theses and industrial research projects devoted to the subject Very Happy


You're both wrong. You're also both right, too. Laughing

Max heat is at stoichio, as mentioned, 14.7:1. That's what the science says.

"Lean out the mixture, and you overheat" is also true in the context of stock cars of the pre-emission era. So?

Leaning out a '50s engine meant going from 11:1 to something CLOSER to the 14.7 point of maximum heat..

If you're running at stoichio, 14.7:1, closer to what a modern car does, and you lean out the mixture, it will just get cooler as it gets leaner.

Peace out.
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PostPosted: Sun Jul 25, 2021 12:31 pm    Post subject: Re: Interesting how a turn on mixture screw can make increase temps. Reply with quote

A light car with a larger engine, and multiple carbs is still going to be running on the idle and transition phase of the carbs. Mine doesn't really start pulling through the main jets until 70 mph or more. So yes part of the fuel mix is still coming from the idle port and part from the transition ports.
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PostPosted: Sun Jul 25, 2021 2:36 pm    Post subject: Re: Interesting how a turn on mixture screw can make increase temps. Reply with quote

the mixture screws also affect the progression phase before the main jet starts working .
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PostPosted: Sun Jul 25, 2021 3:06 pm    Post subject: Re: Interesting how a turn on mixture screw can make increase temps. Reply with quote

Oprn and Wreck, exactly!!
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PostPosted: Sun Jul 25, 2021 4:49 pm    Post subject: Re: Interesting how a turn on mixture screw can make increase temps. Reply with quote

Wreck wrote:
the mixture screws also affect the progression phase before the main jet starts working .


Following this thread, so does anyone have a better procedure for dialing in the volume control screw than the usual highest rpm less 20-30 rpm? Can't quite get rid of a slight bog in the progression phase.
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PostPosted: Sun Jul 25, 2021 5:19 pm    Post subject: Re: Interesting how a turn on mixture screw can make increase temps. Reply with quote

beetlenut wrote:
Wreck wrote:
the mixture screws also affect the progression phase before the main jet starts working .


Following this thread, so does anyone have a better procedure for dialing in the volume control screw than the usual highest rpm less 20-30 rpm? Can't quite get rid of a slight bog in the progression phase.


There is a lot of info on the Wideband thread , what carburettors etc ?
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PostPosted: Sun Jul 25, 2021 5:23 pm    Post subject: Re: Interesting how a turn on mixture screw can make increase temps. Reply with quote

Ellis' Bug wrote:
I was under the impression the mixture screws only mattered to idle.


Mixture screws play an affect in all stages of the running engine. They don't stop adding to the total charge just because the throttle is opened further, the just become less signifigant. Try an experiment, close them totaly down then with your widebands take a drive, and note your readings at all throttle positions.
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PostPosted: Sun Jul 25, 2021 6:49 pm    Post subject: Re: Interesting how a turn on mixture screw can make increase temps. Reply with quote

Wreck wrote:
beetlenut wrote:
Wreck wrote:
the mixture screws also affect the progression phase before the main jet starts working .


Following this thread, so does anyone have a better procedure for dialing in the volume control screw than the usual highest rpm less 20-30 rpm? Can't quite get rid of a slight bog in the progression phase.


There is a lot of info on the Wideband thread , what carburettors etc ?


It's a 34 PICT 3, but I don't want to open up another discussion on tuning or hijack this thread, I was just wondering if someone had a more precise way to dial in the VC screw.
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PostPosted: Mon Jul 26, 2021 8:05 am    Post subject: Re: Interesting how a turn on mixture screw can make increase temps. Reply with quote

So much wrong here, I'm not sure where to start...

idle mixture screws affect idle mixture. Not much else.

once you open past the first progression hole, fuel is metered by the idle jet size.

Mixture screws are way beyond affecting anything once you start the transition to mains.

My method for setting the screws. Dial it IN until that cyl drops out. Back it out slowly until it comes back. Add an 1/8 turn to 1/4 turn. Done.
Nothing magic about that.

Most effective if you have your idle rpm nice and low.

Many times folks think there is a problem with screws not working because they have the idle speed screws so fast, the throttle plates are above the first progression hole.
They have the speed screws cranked up because the base idle spark advance is at 5deg BTDC. So they have to turn up the speed screws to get it to idle. So much so, they are into the first progression hole.
This consumes lots of fuel for no dang reason. It's still burning on the way out the exhaust and will light exh valves on fire.

I modify most every distributor so that it idles at least 12deg BTDC. This speeds up the idle all by itself. thus allowing the butterflys to be more closed and mixture screws more responsive. Things fall into place real fast when you have 15deg base timing at idle.

This all I'm going to say on this subject.
Folks have their own ideas and opinions.

Mine come from setting up more IDF, HPMX, DRLA, IDA, DCNF's than I care to count.
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PostPosted: Mon Jul 26, 2021 7:34 pm    Post subject: Re: Interesting how a turn on mixture screw can make increase temps. Reply with quote

Jake Raby when I got my Mallory Unilite told me it had duel timing setting and

set them a 12 and 28 top. With the correct gray springs from Aircooled.net,
my bug has been running great.

Except for me screwing around too much.
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PostPosted: Fri Jul 30, 2021 1:02 pm    Post subject: Re: Interesting how a turn on mixture screw can make increase temps. Reply with quote

Ohio Tom wrote:
So much wrong here, I'm not sure where to start...

idle mixture screws affect idle mixture. Not much else.

once you open past the first progression hole, fuel is metered by the idle jet size.

Mixture screws are way beyond affecting anything once you start the transition to mains.

My method for setting the screws. Dial it IN until that cyl drops out. Back it out slowly until it comes back. Add an 1/8 turn to 1/4 turn. Done.
Nothing magic about that.

Most effective if you have your idle rpm nice and low.

Many times folks think there is a problem with screws not working because they have the idle speed screws so fast, the throttle plates are above the first progression hole.
They have the speed screws cranked up because the base idle spark advance is at 5deg BTDC. So they have to turn up the speed screws to get it to idle. So much so, they are into the first progression hole.
This consumes lots of fuel for no dang reason. It's still burning on the way out the exhaust and will light exh valves on fire.

I modify most every distributor so that it idles at least 12deg BTDC. This speeds up the idle all by itself. thus allowing the butterflys to be more closed and mixture screws more responsive. Things fall into place real fast when you have 15deg base timing at idle.

This all I'm going to say on this subject.
Folks have their own ideas and opinions.

Mine come from setting up more IDF, HPMX, DRLA, IDA, DCNF's than I care to count.


x 1000. BINGO!
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PostPosted: Fri Jul 30, 2021 1:03 pm    Post subject: Re: Interesting how a turn on mixture screw can make increase temps. Reply with quote

Ohio Tom wrote:
So much wrong here, I'm not sure where to start...

idle mixture screws affect idle mixture. Not much else.

once you open past the first progression hole, fuel is metered by the idle jet size.

Mixture screws are way beyond affecting anything once you start the transition to mains.

My method for setting the screws. Dial it IN until that cyl drops out. Back it out slowly until it comes back. Add an 1/8 turn to 1/4 turn. Done.
Nothing magic about that.

Most effective if you have your idle rpm nice and low.

Many times folks think there is a problem with screws not working because they have the idle speed screws so fast, the throttle plates are above the first progression hole.
They have the speed screws cranked up because the base idle spark advance is at 5deg BTDC. So they have to turn up the speed screws to get it to idle. So much so, they are into the first progression hole.
This consumes lots of fuel for no dang reason. It's still burning on the way out the exhaust and will light exh valves on fire.

I modify most every distributor so that it idles at least 12deg BTDC. This speeds up the idle all by itself. thus allowing the butterflys to be more closed and mixture screws more responsive. Things fall into place real fast when you have 15deg base timing at idle.

This all I'm going to say on this subject.
Folks have their own ideas and opinions.

Mine come from setting up more IDF, HPMX, DRLA, IDA, DCNF's than I care to count.


x 1000. BINGO!
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