Retune my entire carb after installing new heads?

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DrCharles

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I finally installed my aluminum heads (low 290's cfm) to replace my hand-ported iron heads (doubt they made 250). Now my 451 runs a lot leaner everywhere, around 1 full point AFR. Cruise is more like 15.5:1 but no surging, so that's ok. The idle is now much too lean, showing around 16:1 after full warmup, and continues to spiral in (gradually lean out and slow down until it dies), especially when fully hot. Used to be more like 14:1 and my [email protected], .652" cam was happy with that.

So do I need to redo the entire carb tune now? At idle or cruise, aren't the throttle blades and the bypass air the primary restrictions to airflow, not the head (so the increased head flow at WOT should not affect it)? Vacuum seems about the same.
 
Well, that was certainly illuminating ;) :realcrazy:

I realize it will need larger secondary main jets at least since I'll be making more power up there. Primary is on the edge of being too lean, although good for cruising, so may go up 2 sizes there also.

But, as I asked above, why would the idle and transfer slot requirements change?
 
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To get the AFR, idle speed and decel behavior back to the way it was, I ended up adding just under 1/4 turn out on my four-corner idle mixture screws, and maybe 1/8 turn on the secondary idle stop screw.
 
Well yeah, more like retune the entire engine. When you change something, especially something that has that big an effect, you'll want to recheck all tuning attributes.
 
Your new heads probably have larger intake port volume, which reduces velocity AND carb signal. This explains your symptoms.

Getting leaner when hot does not sound right.....expect it be leaner when cold.

What:
- idle vacuum
- idle timing
- idle rpm
- t/slot position
- carb
 
8"
27 deg
11-1200 rpm
square t-slot
950 QF body, BLP baseplate, Holley HP metering blocks/bowls
Also, ME Wagner tunable PCV valve which I have not readjusted.
 
Your new heads probably have larger intake port volume, which reduces velocity AND carb signal. This explains your symptoms.

Getting leaner when hot does not sound right.....expect it be leaner when cold.

What:
- idle vacuum
- idle timing
- idle rpm
- t/slot position
- carb
Thanks, that makes sense. I'm sure the intake ports are bigger, although still the standard Mopar big-block size.

Naturally it's lean when cold and needs throttle tickling to keep it alive until it gets warm. But after a 30 minute drive on the highway, the AFR gauge showed leaner again and it wouldn't idle without attention to the throttle.

Only thing I know changes with fully hot engine would be the alum. heads (lash grows an estimated .006", I haven't had time to check it when hot yet). I have a 1/2" phenolic spacer and a 1/2" nitrous plate between the manifold and the carb as well as a six-pak hood scoop - don't think it's fuel boiling.
 
1100-1200 idle? That's awfully fast.
 
I've spent quite a bit of time trying to find that "fairly easy" tune without success. All combinations of ignition timing, idle mixture, bypass air, especially without a stinking rich, plug-loading exhaust.

So next time you're in my neck of the woods, come on by and show me how easy it is... this is the Show Me state, after all :p
 
That is a BIG cam & the large rod/stroke ratio of the 440 doesn't like big cams.....
So your high idle speed is not surprising & I think it is about right. I doubt there is anything you could do to get it to reliably idle slower....
I would expect the lash increase due to heads warming up would cause the idle rpm to increase because you are in effect shortening cam duration & increasing vacuum [ carb signal ].
I would also remove & plug the PCV to eliminate it as a problem.

What heat range plugs are you running? Have you got another carb you could try?
 
I've spent quite a bit of time trying to find that "fairly easy" tune without success. All combinations of ignition timing, idle mixture, bypass air, especially without a stinking rich, plug-loading exhaust.

So next time you're in my neck of the woods, come on by and show me how easy it is... this is the Show Me state, after all :p
I've tuned a 13:1 528 stroker with a .780 lift solid roller to idle at 950. It can be done.
 
A 528 is sucking harder on the carb than a 451 for the same RPM. Lack of signal at low rpm especially with 92 degrees of overlap is what's making it difficult.

Same 1100 rpm stable idle with my old Holley 800 DP before upgrading to the 950. The PCV is needed for the bypass air.
With a four-speed, a higher idle speed is not really a problem per se since I don't run in parades ;) In the past I have searched every post I could find by those who have actually run a Mini-Express cam, and that rpm seems to be the norm.

Back to the original subject - I guess I am going to have to go through the whole carb again. MAB, IAB, TSR, IFR... sigh. At lleast jets are easy to change through the plugs in the fuel bowls.
 
Posted it a couple years ago... doesn't sound like much on a laptop, but the dual 3" with x-pipe does have a pretty good rumble.
 
The biggest cam I have tuned is the 292/292/108 in my 367. So what I'm about to say, may not apply to your combo. But I'll say it anyway, cuz you just never know.
I'll tell you what I think. not necessarily what I know.
As to the Idle-speed;
I think
27* of Idle-timing is conspiring to close your primary throttles too far, thus killing the transfer-fuel delivery, and I think this is causing the Mixture screws to have to opened too far, to get enough Idle fuel. The result, with a 4-corner system, is extra air getting inhaled thru the slightly open secondaries , and so the Idle-speed has to go up.
The above is what I think. I mean the air could be getting in there from other sources too....

Now here is what I know.
I know that your engine/ any engine, needs a certain amount of fuel to make enough power, to idle on. And I know it is normally supplied by a balance of transfer fuel plus mixture screw fuel. And that means, you can adjust either way for best results ..... at idle.

Theory
So then, you can increase fueling from the T-slots while simultaneously subtracting fuel from the mixture screws, This then maintains the fuel, while simultaneously subtracting air, from the secondary cracking screws. By doing this, you can also subtract Idle-timing, and then the idle speed will go down.
Now, the Ica on my 292 was between 70 to 74 degrees, which puts the piston well on the way up the bores on the compression stroke..... which means, at idle, some of that just inducted mixture is gonna be pushed back up into the intake which then causes the idle vacuum to run pretty low. Sooner or later it will get to be too low for the engine to continue running. For my combo, this occured at 500rpm in gear and pulling.

IDK
how low your idle-speed can go, or
how low you might want to go,
but idle-timing for those engines that I have tuned is not that big deal, at least, not nearly as important as getting the Idle air right.
For instance;
I had no problem setting the idle-timing on my 292-cammed 367 , all the way down to 5 degrees, and this allowed her, with the A833 and 3.55s to idle down to 550rpm in first gear and pulling herself along.
This is just one example of many that I have tuned.

Another thing IDK,
is how hard you have already tried to get this beast running. So it may be that I'm not telling you anything that you don't already know. In which case, I'm sorry to have wasted your time.

Edit
I forgot about these;
I once tuned a big Mopar Crate-hemi 4bbl and
a really stinky, really big-cammed solid-lifter 440 too .
I forget a lotta stuff these days.
So I was thinking
that if you have read this far, I could at least tell you a joke;
When I was a kid, my Momma told me that too much masturbating would make me go blind. Well by age 15, I was diagnosed with astigmatism, and Momma bought me my first pair of glasses.
Wait, I'm not finished yet.
So seven years ago my eyes began to get better and five years ago, I retired the glasses. There's been no change in my habits.
Wait, still not done.
Now almost 70, my wife of 46 years, tells me too much self-pleasuring will cause me to lose my hearing.
What?
Getting old is killing me, lol.
 
A 528 is sucking harder on the carb than a 451 for the same RPM. Lack of signal at low rpm especially with 92 degrees of overlap is what's making it difficult.

Same 1100 rpm stable idle with my old Holley 800 DP before upgrading to the 950. The PCV is needed for the bypass air.
With a four-speed, a higher idle speed is not really a problem per se since I don't run in parades ;) In the past I have searched every post I could find by those who have actually run a Mini-Express cam, and that rpm seems to be the norm.

Back to the original subject - I guess I am going to have to go through the whole carb again. MAB, IAB, TSR, IFR... sigh. At lleast jets are easy to change through the plugs in the fuel bowls.
Then leave it where it is.
 
Leave it where it is?? Gee, why didn't I think of that :D

AJ, thanks for your input. To get this cam to idle (using the old seat duration/overlap with the regrind, it's now 312/312/107 in at 105) it takes a lot of air since the stable rpm is, as I've said, at least 1100. It will idle slower than that (by a little), but not for long. Signal to the carb gets weaker, idle leans out, rpm and vacuum drops, signal gets weaker... it might idle a little lower if I made it deliberately over-rich, but as I mentioned that is smelly and not good for plugs. Remember this is street/strip...

The throttles would be open too far for the primary t-slots to be square. So the primary idle speed screw is set for proper t-slot exposure. Then there are .100 holes in all four blades, an adjustable PCV, and the idle speed is set with the secondaries.

Your theory sounds interesting, but does not account for the fact that only so much fuel can be supplied by the t-slots. When using up too much of that fuel at idle, there will be a lean flat spot before the mains come on.

Once again, I am not worried about whether this engine idles at 950 or 1100... all I wanted to know is, do all the fuel parameters have to change. Apparently they do :)
 
The suggestion to disconnect the PCV was just temporary, to see if it was malfunctioning.

With engines with big-gish cams, I gut the factory PCV & weld a plate in, with a 7/64" hole. The hole represents the area at idle; it eliminates any problems with the PCV & looks 'legal'.
 
I am not sure a fixed orifice is the best thing, because it may be too big at idle but still too small at wide-open...

I found my carb log with the most recent settings: (pri/sec)

IFR .028/.028
IAB .078/.079
TSR .074/.074
MAB (HSB) .026/.026
MJ #71/#86
PVCR .066
PV 7.5"

Tomorrow is supposed to be a nice day, hoping to get the Dart out and get a few AFR readings at low-speed, high-speed cruise, and a very brief WOT on all four barrels :eek:

Since the head swap, it's a little too lean at the start of the transfer slot (that small flat spot JUST off idle is now worse and leaner), but highway cruise at 60-65 mph is good and lean (15-15.5:1) without surging. I may need to open up my TSRs a little bit.

Depending on what WOT looks like, maybe go up two sizes on the secondary jets?

Hey @Mattax are you around? I've also posted on racingfuelsystems Holley forum :)
 
In your initial post you said that the afr leaned out one full ratio across the board, So if it was my carb the first thing I would do is up the main jets 2 sizes all the way around ( if I remember right 4 jet sizes is one afr ratio). While you have the metering blocks off for jetting, up your idle feed restrictions(idle jets) by .002, see where your afr is. Then keep going up in in these small increments till you get the afr back where it was before the head change and yes you will most likely be tuning every circuit to get the carb perfect. You opened up a large restriction in the air flow in that 451 when you went to the higher flowing heads, so now it needs more fuel for all that extra air. At least that my take on the problem.
 
Thanks. Got to start somewhere. Actually I don't have to take the bowls/blocks off to change jets, I've got those little removable plugs. Only thing that comes out is the gas in the bowl :)

I got the idle AFR back by just opening each corner screw about 3/16 turn more... so my plan is to change jets first, see how it drives and AFR readings, then mess with the small orifices. :thumbsup:
 
Are you still persevering with that combo that's too low on the compression for the camshaft duration you're running.....
 
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