DMSA Jetting

[aj81spider]

I have a 1974 1.8 liter with a DMSA. It has a rebuilt engine with a performance cam on the intake and a stock cam on the exhaust. The DMSA has been rebuilt in the last 200 miles. It runs OK, but not perfectly. I also have an AEM wideband O2 sensor in the exhaust with a wi-fi connection to my cell phone.

http://www.aemelectronics.com/?q=produc ... controller

I'm not sure how accurate the sensor is or how much to trust it.

I started with the stock jetting for a DMSA and found that the acceleration had a lag (especially at low RPM). The idle mix screw was out 5.5 turns when tuning it using the Artigue Engine Manual method. The O2 sensor said that the idle was extremely lean (bouncing around 15-17 AFR).

Based on that I changed out the idle jets from 0.50/0.60 to 0.60/0.70. It seems to drive a little better, but not exceptionally better.

The idle mix screw is now out 3.5 turns and I have the following readings on the O2 sensor:

1000 RPM: 12.3
2000 RPM: 11.9
2500 RPM: 11.6 - 13 at different times
3000 RPM: 11.2 - 13
3500 RPM: 12.5-13.5
4000 RPM: 13
5000 RPM: 12.8
5500 RPM: 14.5

This seems too rich across the range - but could simply be the sensor having an offset. When setting the idle mix I had a manifold vacuum gauge as well as the O2 sensor. The manifold vacuum was at a maximum when the engine was reving the highest (at the 3.5 turns). It might actually have gone a little higher at a little further out, but not significantly. However the 02 sensor gave me the 12.3 reading. As I turned the screw in it moved closer to 14.7, but the speed and manifold vacuum dropped.

I'm not sure where to go with this. The car does not have fantastic acceleration (but I may be spoiled as I was driving an FI 2000 while restoring this car - I wish I had never sold that!). The O2 sensor does approach 14.7 as I accelerate hard. It could be that the O2 sensor is simply calibrated so it reads richer than the actual mix, so I'm a little leery of relying on it too much. So some questions:

1.) Is 3.5 turns on the idle mix screw too many? Should I be going up in idle jet size again (but making it even richer if the O2 sensor is right)?

2.) Does the accelerator pump only squirt into the primary? Mine appears to do that.

3.) Is it normal that when I mat the accelerator starting at 2000 RPM that the motor takes a while to wind up and get speed going? It seems to accelerate reasonably well at higher RPM, but not when starting from low RPM.

Thanks for any thoughts and insight you can provide.

[carl]

I used an oxygen sensor from an injected spider hooked up to a digital volt meter to jet the dual IDFs on my spider. This test rig was useless for idle but very helpful in finding the proper main jet. I set idle mixture by the normal procedure of turning the screw in and out until max idle is found. Never really changed the stock idle jet on my IDFs or my DMSA. Make sure your ignition timing is spot on, if set at zero degrees as recommended by the factory for emissions, your car will always feel doggy. Set at 10-12 degrees. accel jet probably just shoots gas in the primary. Some carbs have just one accel pump nozzle which points in the primary barrel, some have Y shaped nozzles with one leg of the Y in each barrel but the secondary undrilled.

Also, make sure your cams are properly timed.

carl