Air/ fuel tuning

[Mattax]

No. I wrote this,

The pump shot is needed to cover the slowness of the idle circuit in responding to changes in throttle.

and I meant it as I wrote it, not as as you altered it.

 

[ap6street]

i scratched my last reply.
i gave the 76/86 with the 6.5 pv a try .

very similar to how it was with 78/86 and the 5.5

as for idle ratio , is this measured in gear or neutral?
how much affect would air temp have on these readings? driving from 50 -75 fahrenheit.

i returned to the 78/86 with the 5.5
my idle screws are a fraction less then 1 turn out.
in gear i get anywhere from 14 to 15. probably related to engine temp, my fans come on at 195 and turn off at 175 so the temp is always somewhere in there.
but if i put in neutral i will get about 13.8 on avg.

cruise at this setting is around 13.5 -14 that will stay at that anywhere from 37mph upto 55mph . then will richen up . 2000-2800rpm approx.

after changing back to this the plugs cleaned up nicer.
only thing that concerns me is that at idle the car doesnt sound very crisp although doesnt stall ?

whats best way to set idle ?
highest vacuum ? in gear ? rpm?
although in neutral i have my car at 1400rpm i have read that to set idle you should drop the rpm to around 900 , set max vacuum then put the rpm back to where you want it ???
also last time i did the vacuum it was probably very similar from 3/4 turn out to 1 full turn. so where would be best place to set in a situation like that ?

 

[Aaron65]

I always set my idle for best vacuum in gear, then I turn the screws in until the vacuum just starts to drop. I don't worry too much about the AFR at idle, but I do know that my stuff idles best and has the highest vacuum at 12.5-13:1 AFR in gear (or in neutral if it's a manual).

 

[Mattax]

Air temperatures will change the air density slightly, not enough to worry about, esp 20* F.
Engine temperatures make a big difference to the combustion and will effect the left over oxygen even if the AFR in doesn't change.

Exactly as Aaron posted, set the idle for best power in gear. If it noticbly drops rpm when going into drive, that's an indicator. Vacuum is a good indicator too. If there's a lot of exhaust dilution due to a radical cam, it will want toward the richer 12.5 and with more mild toward the 13.2 AFR.
(For stateside late 60s early 70s requirements, the AFR at idle was to be 14.2 and initial timing around TDC. If you don't have to go there don't.)

 

[famous bob]

How did you determine you need a 6.5 power valve? I have ~ 10 inches of idle vacuum and I'm using a 10.5 PV and may need to go to a 12.5. Seems to me unless you have very little cruise vacuum you still need an earlier opening PV.

ROSE! I can`t believe u said that ! Forever the word on power valves , have been "half" of ur vacuum . I didn`t read all the posts, because I have a timing controlled fuel .inj. set up, but do read some to see what others are doing , regarding mixtures. Maybe I missed something .?????

 

[Mattax]

ROSE! I can`t believe u said that ! Forever the word on power valves , have been "half" of ur vacuum . I didn`t read all the posts, because I have a timing controlled fuel .inj. set up, but do read some to see what others are doing , regarding mixtures. Maybe I missed something .?????

Yep. That's what was always said, although some said half of idle and others half of cruise. Half of cruise was probaly closer, its still not a good rule of thumb. YR is right, give it what it wants.
It needs to go rich when the engine says. Graph of AFR to Load

 

[famous bob]

Yep. That's what was always said, although some said half of idle and others half of cruise. Half of cruise was probaly closer, its still not a good rule of thumb. YR is right, give it what it wants.
It needs to go rich when the engine says. Graph of AFR to Load

I always set my sbc at 1/2 , at idle, worked perfect as far as I know .??

 

[Mattax]

?? Exactly. Unless its way off its hard for us to know. For most driving and drag racing, its close enough. Activities with lots of 50 - 90% full throttle is where it would be noticed. Pulling boats out of water, circle track, road racing...
Mike Urich recommended testing-tuning part throttle acceleration by "crowding" vacuum readings and timing the runs. More throttle should equal more acceleration. I've tried it with accelerometer and datalogger and its hard to do. A serious flat spot will show up though, just need the vacuum gage to verify whether it relates to power valve opening point or not.
Shrinker wrote that it can be seen in the exhaust with NOx reader - but how many dyno shops do you know that have one of those? So thats not an option for most of us.

 

[yellow rose]

The power valve never flows fuel at idle. Unless you have a bad diaphragm ( and this not a clutch or birth control device...lol). If you do have a bad diaphragm it will leak back into the PV hole in the main body.

The reason for using slightly less that cruise vacuum instead of half of idle vacuum becomes a big deal when you have low vacuum at idle.

As an example I have 9.5 inches of vacuum at 950 RPM idle. Conventional wisdom says use a 4.5 or maybe a 3.5 PV. At a cruise I have almost 16. With a 4.5 PV that is a huge gap to fill. That late opening PV will require a much richer T slot setting to keep from getting a tip in stumble. Some guys try and cover it with the accelerator pump, BUT if you barely touch the throttle, the accelerator pump does virtually nothing.

That's why I never set PV opening by idle vacuum. If you do it by cruise vacuum, you can clean up the T slot circuit and lean it out where it should be, and the quicker opening PV will cover the tip in stumble.

Mark Whitner has an excellent video on YouTube showing how the PV does NOT open at idle. I put a link to it in this post but I don't know how.


Damn technology.

 

[yellow rose]

?? Exactly. Unless its way off its hard for us to know. For most driving and drag racing, its close enough. Activities with lots of 50 - 90% full throttle is where it would be noticed. Pulling boats out of water, circle track, road racing...
Mike Urich recommended testing-tuning part throttle acceleration by "crowding" vacuum readings and timing the runs. More throttle should equal more acceleration. I've tried it with accelerometer and datalogger and its hard to do. A serious flat spot will show up though, just need the vacuum gage to verify whether it relates to power valve opening point or not.
Shrinker wrote that it can be seen in the exhaust with NOx reader - but how many dyno shops do you know that have one of those? So thats not an option for most of us.

You mean use a 5 gas on the dyno???

 

[Mattax]

You mean use a 5 gas on the dyno???

yes. Exactly. a 4 or 5 gas on the dyno.
The main reason I'm aware its doable was that Shrinker and some other guys discussed ways to do it on Speedtalk a few years back. So I know he's not the only guy whose done it.

Mark Whitner has an excellent video on YouTube showing how the PV does NOT open at idle. I put a link to it in this post but I don't know how.

Does this work?

 

[mopowers]

I've always had better luck using half of my cruise vacuum for selecting a PV. My 340 has 10.5" of vacuum at idle in gear, but cruises at 18". For me, an 8.5 PV works the best in my 4779. Any lower than that, and it goes lean at tip in. Sure I can bandaid the lean tip in by fattening up other circuits in the carb, but then it's fat everywhere else.

 

[yellow rose]

yes. Exactly. a 4 or 5 gas on the dyno.
The main reason I'm aware its doable was that Shrinker and some other guys discussed ways to do it on Speedtalk a few years back. So I know he's not the only guy whose done it.


Does this work?

It works and that's the link.

Thank you for posting it. Remember, he is using lambda on that A/F meter and not A/F ratio.

 

[ap6street]

could it be the jets taking over at 60mph or is that way too late ,
at 50mph 14 then at 60mph im getting mid 12
guess i could try some huge jets and see if that changes things.
otherwise im gonna hook up a vacuum gauge while im cruising to get an idea.

 

[ap6street]

also i'd like to add.
idle neutral we have 1500rpm 11" vacuum best
then in gear i get maybe 1100 with 7" vacuum
that left me with 13.7 neutral and then maybe 14.1 gear

i realise most people here said dont get too caught up with tuning the idle.
thought best place to start would be at least get this right.

biggest trouble i have is my electric fans .
engine heats up as temp changes so do vacuum, afr, and rpm.
then as fans come on at 195 and starts to cool over a 30 second period , all the reading are altered.
lower vaccuum lower rpm.
down to 175 then until it creaps up , i try to get my readings with fans off and on the way up from 180 through 190.

i ended up today with 1 full turn out and a touch . seemed to give best vaccuum and rpm. not a big difference but everyone seemed to prefer a touch richer idle .


can anyone tell me anything you can look for with spark plugs when driving on street and just using pump gas.
a lot of people say too hard to read anything

 

[Aaron65]

could it be the jets taking over at 60mph or is that way too late ,
at 50mph 14 then at 60mph im getting mid 12
guess i could try some huge jets and see if that changes things.
otherwise im gonna hook up a vacuum gauge while im cruising to get an idea.

If it's running 14:1 at 50 mph and 12.5:1 at 60, that is when your primary main jets take over (unless something weird is happening, like your power valve is opening too soon, which is unlikely).

What this says to me is that your IFRs and idle air bleeds are just about right, but your primary main jet is too big, or your main air bleeds are too small. Of course, if you lean out your main jet, you'll need to increase your PVCRs to maintain full power AFRs. Carb tuning can sure be a headache; widebands are nice, but there's a lot of peace of mind in saying "Man, this thing runs pretty well...I think I'll leave it!"

 

[mopowers]

You've got a 400 rpm drop in gear??? Damn!

Work on that first. Get your idle circuit dialed in (and timing) before you attack the main jets.

 

[d1970]

can anyone tell me anything you can look for with spark plugs when driving on street and just using pump gas.
a lot of people say too hard to read anything

In years gone by you could read how the engine was running buy looking at your sparkplugs, but with the way todays gas is formulated you can't use your sparkplugs as a guide

 

[yellow rose]

can anyone tell me anything you can look for with spark plugs when driving on street and just using pump gas.
a lot of people say too hard to read anything

In years gone by you could read how the engine was running buy looking at your sparkplugs, but with the way todays gas is formulated you can't use your sparkplugs as a guide

I read my plugs on pump gas. You can do it if you know what you are looking at.

 

[Mattax]

could it be the jets taking over at 60mph or is that way too late ,
at 50mph 14 then at 60mph im getting mid 12

As the other guys wrote, yes. As long as you're talking steady throttle cruising, somewhere around 60 mph the main circuits dominate the fuel contribution.
At part throttle acceleration, they come into play atsomewhat lower speeds.

idle neutral we have 1500rpm 11" vacuum best
then in gear i get maybe 1100 with 7" vacuum
that left me with 13.7 neutral and then maybe 14.1 gear

Agree with this too. If you can, work on this first.
Idle speed is high, but your idle mix screws are responsive, so its not out of the ballpark.
This is with a locked timing, so that's making it harder, maybe impossible with this particular combo.
It should run stronger in gear with a richer mixture matched with less timing.
I think if I was there, I'd try resetting the distributor at lower and lower timing. First try timing around 24*, then readjust the idle mix if need but don't adjust the throttle position (idle speed screw). Then do the same at 20 or 22* if it will run that way. See if you can bring the neutral idle speed to something around 800 - 1100 rpm. If you have to open the throttle (or close it), write down exactly how much (1/8 turn or 1/4 turn).

 

[ap6street]

When you say timing. What's the best way to know if it wants more or less. It's got locked 28 so for idle I would think that would be good.

Seems quite high for the jets to be taking over ?? 3100rpm can this be the secondaries at all starting to richen it up?

I could go a 76 . The WOT would still be pretty good like that. Just get a lot of mid throttle readings hitting 15s maybe even a 16 here and there.

 

[Mattax]

I read my plugs on pump gas. You can do it if you know what you are looking at.

I agree. I'm not very experienced at it and I can see certain things that are either obvious or have been getting better at observing.
For example, carbon fouling is obvious. Speckles on the porcelain, a little more subtle, but when I see them, I get a magnefier and see check for aluminum. I look at color too, both on the insulator and the strap.

 

[d1970]

yellow rose I know you are more educated on this subject then I am, I still like to learn more about this subject and I'm sure other guys can learn something to, could you please elaborate on how to read spark plugs with today's type of fuel

 

[Mattax]

When you say timing. What's the best way to know if it wants more or less. It's got locked 28 so for idle I would think that would be good.

Seems quite high for the jets to be taking over ?? 3100rpm can this be the secondaries at all starting to richen it up?

I could go a 76 . The WOT would still be pretty good like that. Just get a lot of mid throttle readings hitting 15s maybe even a 16 here and there.

Sorry I didn't finish above.
If you went and drove the car at 22* timing, it will feel sluggish above a 2000 rpm or so.
This would just be a test to see where it ought to be at idle. A richer mixture will burn quicker and needs less timing. I don't think yoiur going to be able to get it rich enough to have power in gear with 28*. You might, I just don't think so.
Knowing what that combo likes for idle timing, and assuming the dyno shop was pretty darn close on WOT timing, you can setup a distributor provide both.

It is not high for the main jets to take over. I'll tell you how this finally sunk into my skull. I had the car on a chassis dyno, and the last pull with leaner primary jets was just about the same as the richer. So I left it that way. On the way home, the engine started revving and then dying - but only above 60 mph. Slowed down to 55 mph (whicj was fairly safe to do there) and just drove it home that way. (OK I pulled over first and scratched my head LOL). Changed jets two steps up and it was fine.

Cars use very little throttle while cruising. Not much Hp need.

 

[Mattax]

dyno: 82/86

I could go a 76 . The WOT would still be pretty good like that. Just get a lot of mid throttle readings hitting 15s maybe even a 16 here and there.

I would not try WOT with 76 Jet after having dyno results that indicate it PMJs should be 82.
Even going winter to summer I wouldn't do it. It depends a bit on the carb how sensitive it is to air density changes. No choke tower will be more sensitive.

Test the leaner jets at cruise and gentle accelerations all you want. Go too lean under heavy load or throttle and you can cause damage.