Carb or ignition ?

-

mbaird

mbaird
Joined
Dec 10, 2005
Messages
9,341
Reaction score
6,555
Location
Boise, Id
I have been chasing a persistant rich condition and eratic afr reading .

Tried Holleys and AVS (both 600 & 800)
Various jetting...

Experimented with intial timing and curves.

Right now I have a new 800 AVS on it.

I can get the idle set correctly so it runs around 12.8-13.5 with 15" of vacuum but at 1500-2200 its still running 11- 12.5. Cruise rpms around 2900 are running 13.5ish.
Right now it will idle around 12.8 one moment then hit 16.5 the next ...
If I lean it out a little more the afr jumps to 20ish.

This is a magnum 360
IMM EQ heads
Eddie RPM intake
Dougs headers
9.5:1 with .029 quench
Mopar ignition ( tried several boxes)

Its eating fuel and hurting my eyes .

Could this be a weak ignition spark ?
 
I can get the idle set correctly so it runs around 12.8-13.5 with 15" of vacuum but at 1500-2200 its still running 11- 12.5. Cruise rpms around 2900 are running 13.5ish.
from 1500 to 2200 its still on the "idle cicrcuit" but the pressure differential is higher. probly 16 to 18"Hg.
Cruising at 2900 is highway speeds - probably 60 mph ish in top gear. Pushing enough air the boosters (main circuit) is supplying most of the fueling.

How to lean out or shut down the "idle circuit" as the throttles open varies with the type of carb. But that's what has to be done. Might be able to get away with increasing just the idle air bleeds.
 
Ok ... so the current carb is a 800 avs .
Do I need to drill the primary plate ?

Seem drastic for a 221/229 .513 on a 112 hyd roller cam .
 
I have been chasing a persistant rich condition and eratic afr reading .

Tried Holleys and AVS (both 600 & 800)
Various jetting...

Experimented with intial timing and curves.

Right now I have a new 800 AVS on it.

I can get the idle set correctly so it runs around 12.8-13.5 with 15" of vacuum but at 1500-2200 its still running 11- 12.5. Cruise rpms around 2900 are running 13.5ish.
Right now it will idle around 12.8 one moment then hit 16.5 the next ...
If I lean it out a little more the afr jumps to 20ish.

This is a magnum 360
IMM EQ heads
Eddie RPM intake
Dougs headers
9.5:1 with .029 quench
Mopar ignition ( tried several boxes)

Its eating fuel and hurting my eyes .

Could this be a weak ignition spark ?

What fuel supply system is being used?
Pump/regulator?
What type, and pressure.
 
I have been chasing a persistant rich condition and eratic afr reading .

Tried Holleys and AVS (both 600 & 800)
Various jetting...

Experimented with intial timing and curves.

Right now I have a new 800 AVS on it.

I can get the idle set correctly so it runs around 12.8-13.5 with 15" of vacuum but at 1500-2200 its still running 11- 12.5. Cruise rpms around 2900 are running 13.5ish.
Right now it will idle around 12.8 one moment then hit 16.5 the next ...
If I lean it out a little more the afr jumps to 20ish.

This is a magnum 360
IMM EQ heads
Eddie RPM intake
Dougs headers
9.5:1 with .029 quench
Mopar ignition ( tried several boxes)

Its eating fuel and hurting my eyes .

Could this be a weak ignition spark ?
So I had similar issue with mine, 72 340 thumper cam/LT/stall/ Holley knock off 750cfm double pumper/Edlebrock heads and air gap intake/MSD blaster 2 coil. adjusted timing to 15deg initial 30deg all in. new plugs gapped at 42, adjusted carb to roughly 14" vacuum plus 1/4 turn..no more burning eyes but carb is still **** installing 1906 AVS2 carb this weekend.
 
So I had similar issue with mine, 72 340 thumper cam/LT/stall/ Holley knock off 750cfm double pumper/Edlebrock heads and air gap intake/MSD blaster 2 coil. adjusted timing to 15deg initial 30deg all in. new plugs gapped at 42, adjusted carb to roughly 14" vacuum plus 1/4 turn..no more burning eyes but carb is still **** installing 1906 AVS2 carb this weekend.
I have encountered similar problems and results from not enough initial timing. I would suggest bumping your timing up a little and see if that starts to clean it up I would continue to do that till you reach maximum manifold vacuum then back it off maybe a smidge and see where you're at.
 
As fussy as it seems you might have a vacuum leak since changing carbs isn't fixing the issue.
 
I am at 18* now .
I have encountered similar problems and results from not enough initial timing. I would suggest bumping your timing up a little and see if that starts to clean it up I would continue to do that till you reach maximum manifold vacuum then back it off maybe a smidge and see where you're at.
 
time to start tuning the step up rods and springs.
Welcome to the world of fine tuning a Carter.
 
I am at 18* now .
How do you know that? I just don't trust timing marks that's why I would use the vacuum gauge method and then compare it to your timing mark, timing marks can lie vacuum is telling you what the engine is actually doing just my opinion.
 
Checked for leaks and replaced intake gasket with an embosed edelbrock one

Anything out side of that? Power brakes? PCV valve? Possible intake to head angle leak, I know that's grasping at straws and you'd see the oil traces. Its the instability that seems odd, other than possibly a issue with the distributor. Have you tested how strong the spark is? I had a coil go to poop and give a weak blue ark. Possible ECU ground?
 
Checked tdc to timing mark during assembly
Changed step up springs / jets and metering rods a dozen times . I have both Eddie and Carter strip kits.

I am starting to think weak spark maybe .... but the engine runs strong . It will roast the tires even with the 2.76s I have in it now.
 
Alternator keeping up? Engine grounds? What are you using for a fuel pump? Tank venting?
 
I have been chasing a persistant rich condition and eratic afr reading .

Its eating fuel and hurting my eyes .

Could this be a weak ignition spark ?

Yes. When the ignition doesn't have enough power it can look like its running rich when in reality its not. I have seen my engine go from running really good with clean plugs with 14+ volts at idle yet drop the voltage to the 13.5 and it fouled plugs and ran like it was rich.

I know others that have seen the same. Ignition can have a huge effect on combustion quality

This is what I use: https://obd2australia.com.au/produc...MIobfFxdHI8AIVf4NLBR1Z_QGIEAQYAyABEgJQefD_BwE
 
Ok ... so the current carb is a 800 avs .
Do I need to drill the primary plate ?
I wrote maybe increasing the idle air bleed will help.
Adjusting the initial throttle position to be more closed is different. Could it help? Maybe. My suggestion was based only on the AFR information and assumed the idle readings are somewhat correct.
Seem drastic for a 221/229 .513 on a 112 hyd roller cam .
That's fairly radical. Compared to stock it would not surprise me in the least that there is a fair amount of exhaust dilution and somehat cold combustion conditions at idle. That can shift a bit depending on the details and exhaust but bottom line is its not going to act like a stock engine.,

IMO 18* BTDC at idle is a reasonable ballpark for that combo. Details such as when the IV closes matter but its with 2* of where it ought to run strongest IMO.
I can get the idle set correctly so it runs around 12.8-13.5 with 15" of vacuum
What's the idle rpm this 15"Hg vacuum is obtained? Is this an automatic? If so what's the rpm and vac in gear?

Right now it will idle around 12.8 one moment then hit 16.5 the next ...
If I lean it out a little more the afr jumps to 20ish.
Sounds like the idle mix screws are highly sensative. probably remains true even if the WBO2's AFR intrpretation is not correct.

So based on this info here's an approach you could try:
Goal here is to reduce the eye watering exhaust at idle
Since the idle mix screws are sensative and the vacuum is strong the throttles may need to be opened a bit more at idle rpm. This will increase the fuel contribution from the transition slots. Then the idle mix screws should be less sensitive. Open them up until the engine is strongest in gear (least drop in rpm when placed in D).

If the idle speed is too high, take a degree out of the timing and then adjust the throttles and idle mix.
Taking a little timing out is another way to reduce the idle mix sensitivity since it will reduce the manifold vacuum.

As far as Carter Idle Air Bleeds go, see this:
Operational theory of the Carter idle restricions & air bleeds
Note:
Change any links in that thread that go to imperialclub.org to .com
If the there is a Tapatalk popup asking you to join them, just click the X to close it.
 
Its eating fuel and hurting my eyes .
Classic too much Transfer-slot exposure, usually because of lack of bypass air.
Try this;
Reset your T-slot exposure to a lil taller than wide. Reset your mixture screws to 2.5t on a Carter type carb. Make sure your fuel-level is correct and stable. Make sure your V-can is plumbed to the Spark-port. Make sure your PCV is NEW and correctly plumbed to the front of the carb and below the plates. Make sure the Secondaries are closed up tight but not sticking. Make sure the Brake booster is not sucking air and that the suction hose goes to the PLENUM, not one single runner.
Finally start it up, and DO NOT TOUCH THE SPEED SCREW. Set the idle-speed with Idle-timing. I suggest 12 to 14 degrees.
That is your baseline for the 221 cam.
Now;
if the rpm is too slow at 12/14+ timing;
She will want some bypass air. To find out how much; I tee into the PCV line with a smaller line. Sometimes I install a wooden dowel, plug into the end of this line, and drill the plug out until I get the rpm where I want it AND, very importantly; the exhaust no longer burns my eyes,lol. hen I calculate the area of that hole, divide it in two, and then drill the primary plates to nearest smaller size, one in each . I install the holes on the PCV side, about half way to the edge and between the two ports of Transfers and Idle discharge.
Warning; if your fuel level is not right, don't make trouble by drilling the holes too big. But if you do, don't panic. I have dropped solder into too-big chamferred holes, and peened it into there then dressed the high-centers down. and Finally just moved over a bit.
If the idle-rpm is too high at 12/14 Idle-timing, with the T-slot exposure set right; Something is Wrong! You probably have a vacuum leak. Be sure to check the crankcase for vacuum after flipping out the PCV and sealing the covers. If you have cut the decks it is easily possible for the engine to pull air from the crankcase atb the intake to head interface.
That's the best I got.
Killer low-speed cam that 221 will be.
IMO forget trying to target the idle AFR. Find out at what rpm in neutral, that your V-can comes on line, and work from there.

BTW-1
I ran a Hughes 223/230/110 cam for 4 years; What a great hi-torque cam.But I had to modify the V-can to bring in 22* to satisfy her craving for cruise-timing, with alloy heads. My Idle-timing is just 12/14 with a manual transmission.
BTW-2
If you want a great coil, I highly recommend the Accell big yellow square top, and it's matching ballast resistor. Mine was new in 1999. It does a great job to past 7000, yet doesn't burn up the plugs. My plugs were refreshed in 2004,lol. I mounted mine horizontally on the apron with the terminals down.
BTW-3
the jumping around AFR is probably misfires, or the O2 sensor seeing an up-stream air-leak into the exhaust system.

OOPs Sorry Mattax, I didn't read to the end.
OP, Mattax has you covered
 
Last edited:
4 corner Idle carbs are super sensitive. Like 1/8 turns on all for will have an impact. With the AVS I don't know how sensitive these are to changes.

When it jumps lean, is there a change in drive ability?
 
@Mattax
I don't think I can get the plates closed any further . I have been conscious of that .
Rpm at idle is about 750. RPM drop of approx 30 in gear .

I will check out the thread .

@ aj/forms
I happen to have one of those coils and will give ita try.

I will try both of your guys procedures and report results... my take a few weeks as I am busy for a while with other projects.
 
I don't think I can get the plates closed any further . I have been conscious of that .
Rpm at idle is about 750. RPM drop of approx 30 in gear .
That pretty much tells the story.
Ima guessing
Your timing is just too much, and your throttle is just too far closed.
Sync it up and it will be fine.
 
I don't think I can get the plates closed any further . I have been conscious of that .
Rpm at idle is about 750. RPM drop of approx 30 in gear .

If it came across like I was suggesting that, I'm sorry.
Bold added to the crux of what may be the issue.
Since the idle mix screws are sensative and the vacuum is strong the throttles may need to be opened a bit more at idle rpm. This will increase the fuel contribution from the transition slots. Then the idle mix screws should be less sensitive. Open them up until the engine is strongest in gear (least drop in rpm when placed in D).

RPM drop of 30 is good. A/J may have a better sense of this cam than I do.
We're both suggesting more throttle and that may mean less initial timing. I was thinking 16 but maybe its going to be strongest at 14* 750 rpm.
Open the throttle, see what happens with respect to rpm.
Then take a few degrees out of the timing or whatever it takes to get back to 750 rpm. Fiddle with the mix screws - should need more open.
Test it shifting into Drive.
Also whether its burning your eyes.
Then see what the AFR meter says just for the record.
 
Last edited:
-
Back
Top