Old fashioned carb and ignition tuning

One very common mistake is setting the air screws too lean by using the 'highest vacuum' or highest idle rpm method, but on a manual trans car there is no way to simulate the load an auto trans has in gear, so this method on an auto trans car usually can net a richer idle mix that has more torque and responsiveness than on a manual trans car. (iow, setting to highest vacuum at idle in neutral results nets large drop in rpms or stall in gear, and stutter, lurch or stall upon pressing accel, and is therefor a poor state of tune independent of type of tranny))

Set too lean at idle, the mix goes leaner still as soon as the throttle is touched, so starting off leaned out created a dull off-idle condition till the accel pump catches up.

Although you set the accel pump clearance, make certain the accel pump actuates and squirts right off idle

Try closing the air screws closed 1/4 turn from where you set and see if the symtom improves.
Alternatively
If the air screws are not equally responsive and can stall out the engine when too lean, the throttle is open too much and getting off the transition slot at idle, and needs more work to investigate, typicaly the idle circuit in the metering block can be buggered up.

Usually a pop when you jab it is a dull lean condition off idle and/or not enough initial timing. Usually another degree or two can make it an entirely different animal.

I gather you likely could tolerate a few degrees more initial advance. If it detonates at high rpm, you can always adjust the span of the advance to reduce the final and keep the same advance at idle.

Thanks.
I tried 1/4 more idle screw, no change in drivability, but plugs are quite black. Has changed back.
Then I changed from 6 deg BTDC to 10 deg BTDC (which is equal to 40 deg BTDC at 4000 rpm). The drivability improved a lot, and no backfire . But still a small bog. I've now increased to 12 deg (someone painted a red stripe on the harmonic balancer at 14 deg), but I haven't had time to test. I've also found a bigger acc pump nozzle, 35 versus the 25 I have now, and I've turned the orange cam 180 degrees, this gives quite a lot more acc pump arm travel. I wont have a chance to test the car before Sunday, hoping for the best. But still worried about detonation, the car is so noisy and the engine behind me, I'm not sure I can hear the detonation if it happens.

There is almost nothing you can do to the air screws to turn the plugs black, that is a symptom of something else amiss.

I agree 40 final is much too much.

You are setting static timing with the vacuum advance pot plugged, right?

It also may be wise to confirm TDC and the timing pointer before distressing over the raw number too much.

40 degrees total advance is not too much advance with a Cleveland motor, depending upon the camshaft and octane of fuel you are using. Your motor has either 9.7:1 compression ratio (66 cc closed chamber heads, flat top pistons), 8.8:1 compression ratio (75 cc open chamber heads, flat top pistons) or 8.0:1 compression ratio (78 cc open chamber heads, dished pistons). In the US a '73 Pantera would have the 8.0:1 Cleveland motor version, however 6 degrees static distributor setting was the recommending setting for the 9.7:1 version of Cleveland motor. By the way, the motor with 9.7:1 compression ratio was advertised by Ford as having 10.7:1 compression, this was most likely a blatant lie.

Ignore Ford's recommended static advance setting which is designed to pass US emissions testing. The motor will want up to 20 degrees advance at idle.

To set it, connect the outer vacuum advance connection to "ported" vacuum. Do not connect the inner connection to anything (this is a vacuum retard connection). The vacuum advance is to remain connected during this proceedure. Loosen the distributor hold down bolt. Start the car & let the motor reach operating temperature, twist the distributor in the advance direction slowly, as you do, the motor's idle speed will slowly increase, the vacuum in the intake manifold will slowly increase too. You will eventually reach a point where twisting the distributor no longer increases the idle speed. The setting that just reaches this point is the ideal advance setting for idle. It is the setting that achieves maximum intake manifold vacuum, smoothest idle, most power, least amount of waste heat (i.e. lowest exhaust gas temperatures). You will have better throttle response and better low rpm operation. Readjust the motors idle speed, as it will be idling too fast now. I like to idle Clevelands at 1000 rpm. Since the car has a manual transmission this will not hurt a thing. Remember to tighten down the distributor hold down bolt.

Fussing with the carb before you have the ignition dialed in can lead to false conclusions.

cowboy from hell

Today I tried a 8.5 Powervalve and slightly higher fuel level. General drivability better than ever, but still a short bog and occasional backfire. And plugs are all black now. Then I tried tying down the secondary throttle blades, they were slightly open so the idle was higher but couldn't move. Then the bog and backfire was gone. So now I've ordered a spring kit, hope a sligtly stiffer spring will fix it. If it does, I'll go back to the 6.5 Powervalve.

Also check and bend the linkage that forces the secondaries closed, it shouldn't have any slop when the primary is on the idle stop. Check it every time the idle stop is adjusted, for binding or slopiness.

What PV was in there?
What is your vacuum at highway cruise?
typically, select a pv that is 1" numerically lower than cruise vacuum.

Finally it's as I want it, instant throttle response at all rpms and in all gears. I ended up changing the following:
-Accpump nozzle from 25 to 35 and the non-solid bolt (from a Dominator I had from a boat)
-Black instead of orange pump cam
-8.5 Powervalve
-Made sure the linkage closing the secondaries when primary throttle is closed were correctly adjusted
-Bought secondary quick-change and spring kit, but I ended up using a spring very similar to the one I started with

I don't like Holleys because they never work properly from the start, while the Chevrolets I had with Quadrajets were always perfect, and this Cleveland ran better with the Motorcraft until it started flooding. So why do I use a Holley? Well I had one on the shelf, and I have tons of jets, powervalves etc.

Thanks for the ideas

quote:

Originally posted by No Quarter:
..., but still a short bog and occasional backfire. And plugs are all black now.

So, what causes a backfire? (I assume you mean a loud bang from the exhaust.)

1. HUGE blast of raw fuel into the intake manifold
2. Failure to ignite, due to insufficient oxygen (this is the bog)
3. Unburnt air/fuel purged into the exhaust system
4. Mixture leans out again, so ignition occurs again
5. Air/fuel in the exhaust system finally ignited... BANG, and car lurches forward

And on inspection, the plugs are black.

My guess is that your squirter is too big, not too small. The black cam will give you a good volume and early delivery, and with a smaller squirter the delivery will be spread out a bit.

(I used to play in a band in the 1970s, and we had a Chevrolet bus for our gear. One of our pranks in the bus was to turn off the ignition, pump the gas a couple of times while coasting in gear, and then turn on the ignition again. BANG!!! Scared the crap out of pedestrians, ESPECIALLY in an underpass. Ah, youth...)

Backfire from the carb when accelerating off idle is a too lean condition.

quote:

Originally posted by mrr:


So, what causes a backfire? (I assume you mean a loud bang from the exhaust.)

I used to play in a band in the 1970s, and we had a Chevrolet bus for our gear. One of our pranks in the bus was to turn off the ignition, pump the gas a couple of times while coasting in gear, and then turn on the ignition again. BANG!!! Scared the crap out of pedestrians, ESPECIALLY in an underpass. Ah, youth...)

Hey, I remember you guys. That wasn't nice Marcus.