AFR reads rich at idle

Hello,

after dropping in the recurved DSII from Bill, I started a AFR measurement series with some variations on the carb setup.
At the end I am fine with cruise, light acceleration and WOT AFR values. Everything between 12.5 (WOT) and 15 (Cruise) on the Innovate gauge. Engine performs very well.

What makes me wonder is that I read significant rich AFR values at idle.
About 11.5 is the best I can achieve without going too far in with the idle mixture screws (actually 1 1/4 turns out).
Going leaner with ims leads me up to 13 AFR but has negative effects on cold run and off-cruise acceleration.

So, is this only a false reading at idle from the 260 /260 cam with 110 lobe center?
Or do I oversee something?
Idle in general is slightly rough with shaking vacuum gauge needle 13.5 in.hg. +/- 1 (shaking only at idle).
Pls. see engine spec below.

Cheers
Joerg

65 Mustang Fastback, 68 Inline 200, OZ 250 Head, Holley 2V 7448 350 cfm, Comp 260 cam, Hooker Headers, 2” dual exhaust, DSII, MSD 6A. 4 speed toploader, 3.2 rear end.

Joerg, try manifold vacuum at idle. Set idle speed to 1000 rpms. Adjust idle mixture screws on the lean side, but still retain a smooth idle.
Worst senerio is mods to the idle orifice in the metering block. Air bleed also.
What is your initial advance set at?? Should be at least 18-20 degrees. Bill

Hi Bill, I am on manifold vac. Initial advance is at 20.
I will try ims adjustment at 1000 rpm idle and look what the afr gauge tells me.

Cheers
Joerg

Joerg, you can also try a 7.5 power valve & or a larger discharge nozzle for the accelerator pump shot?? Either one will fill in the lean hole on initial acceleration.
What color is your accelerator pump cam?

You mean I should screw in the idle mixture screws until I get a reasonable idle afr (lets say 14) and tackle the thereby appearing slight acceleration hesitation with a higher rated PV and/or acc pump noozle and/or cam?

Noozle and cam are 7448 stock: 31 noozle and orange cam in hole 1. My current PV is a 10.5 which made the former hesitation in some situations with the stock 8.5 PV disapear.

Cheers
Joerg

Georg, just screw in the idle mixture screws to the point you still maintain a smooth idle. If the AFR is 13 that is fine.
I will do some homework on pump cams, but you could go .002" larger on the discharge nozzles. Make sure you have pump shot as soon as you open the throttle.
Your main jets should be 60-59, your carb comes with 61's, which seems a little to rich.

Okay, got it. I have a 35 noozle here that I can try. Acc pump lever is free of play. Main jets are already on 59.

Cheers
Joerg

Joerg, you could go up to 60 jet closest to engine & go 61 jet outer position, go back down to a 8.5" power valve before you install the discharge nozzle. Only problem your stock metering block has fixed PVCR so your WOT might be too rich.
I install screw in jets in the metering block PVCR to lean the WOT.
Try different combinations & see which works the best.
Go to Holleys web site & check out the various pump cams. When considering pump cams what is better to use the chart or the amount of fuel per stroke ?
Thanks ......... :2thumbs:
Cams from lightest to the heaviest :
cam color #1 #2

white 17cc - 19.5cc

blue 18cc - 20cc

red 18.5cc - 20cc

orange 19cc - 24.5cc

black 19cc - 18cc

green 24cc - 30cc

pink 30cc - 37.5cc
Summit sells a kit which contains all different cams. However your orange one is the one i would use. Bill

I am watching this thread. I have always run rich with the same specs as the OP. I just learned to live with the black soot on my driveway

65coupei6":1kspgmw0 said:

I am watching this thread. I have always run rich with the same specs as the OP. I just learned to live with the black soot on my driveway

Click to expand...

Joerg, you've got a perfect cam, igntion and cylinder head. The solution is air fuel calibration, but the problem is port size of the 250 2V head.

The 2V head is very much like the old 2.0 Taunus 4 cylinder head...its port size is huge for its cylinder capacity.

If the distributor is a DSII, unless you use spark delay via the vacuum ports, you can't easily "tip in" the advance like you can with, say, the TFI CFi 5.0's (the AOD automatic M code 1984-1985 GT 5.0 HO Mustang and Capri RS engine); they just pulled in or out 6 degrees of timing to control the idle, and then used a steel plate to make the 2-bbl throttle body flow the right amount of air to each cylinder. Megasquirt and Megajolt guys like Jack Collins have done this on there little Ztec engined MGB's, and that always works as another trick in the tool kit.

On an I6, Bill has got totally the right idea; a nice fat spark, and then adjust the air fuel as best as possible. The IAB's then fine tune it further.


Its due to the port size being 100% of the original 1.65" valve size. The port diameter of 1.65" is the same as the valve head, and people have a nasty habbit of polishing the intake port, the single worst thing you can do to a 2V head.

Comparitively speaking, the 250 2V ports are bigger than a 4v 302 head on a Boss Mustang.


The all mighty Falcon XY/XA/ XB 250 2V's seams to be a lot harder to take a slog around the traffic, they lost a lot of air speed and always end up pulling a lot more more revs to make torque, because of those huge, 2.13 sq in intake ports, the same area as the 1.65" diameter valves. A port 100% of the valve. The torque grew only past 2200 rpm, 600 rpm higher than the 1v 250, and that was with very good headers and a really mild 256 degree cam.

By way of reference, the renown "ragged low end torque" Boss 302 and Cleveland 4V had about 2.7 sq in port area to a 3.7 sq inches (for a 2.19" diameter intake, or a 4.0 sq in 2.25" diameter intake).

A port 67 to 73% of the valve area. The heads always had a torque hump at 2800 rpm, then they'd go crazy with power gain. "505" 300 degree and K code 310 degree cams made this thing take off at 2800 rpm and make torque at 3500 rpm.


If you've got huge ports, you go easy on the cam duration and lift, and then look at a really good Transistorised igntion and matching fuel delivery system. I think your very close.

Hello, thanks for all the input!

Yes, I think I am close to the optimum, as said, the engine perfoms very well with the current setup.
Only the rich idle reading makes me wondering.

@Bill: I will give the approach with leaner idle, slightly bigger jets and lower rated PV a try. In case lean hesitation appears, I will try to cure it with acc. pump cam and noozle change. I have all parts here for that.

@Marco: Seems that we have a nearly identical engine combo. Any other engine tuning tips for this particular setup?
E-Choke tuning for a good cold run gave me a hard time in the past. Cold run means at 40-50 degr. F. in spring/autumn.

Cheers
Joerg


65 Mustang Fastback, 68 Inline 200, OZ 250 Head, Holley 2V 7448 350 cfm, Comp 260 cam, Hooker Headers, 2” dual exhaust, DSII, MSD 6A. 4 speed toploader, 3.2 rear end.