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MG MGB GT V8 Factory Originals Technical - Drivability Questions

I've had my car on the road for a year and have just passed emissions for the second time, and I'd like to start addressing some drivability problems. Any suggestions would be appreciated. Here's the engine configuration and symptoms

The engine is SD-1 with the 'federal' emission system from the '80 Rover. It is pretty much how I took it out of the car with about 70,000 miles on it. While the engine and FI are both from SD-1, the engine and FI were sourced from different cars.

At the emissions test station, the idle HC read 32PPM and at 35 MPH on the dynomometer it read 17 PPM. The CO reading was '0.00%' at both test conditions. (Well within the requirements)

On the road, the car runs up to about 3500 RPM under rapid acceleration (pedal to the floor) then it hits a 'flat spot' where it won't accelerate the car. If I accelerate gradually it will pull strong right up to the redline.

On the highway, it will be cruising along at 65MPH and the engine will 'hickup' several times, then go on to run fine for a little while, when it will repeat the hickup.

It sounds to me as though the engine is running lean eneough to pass emissions, but out on the road it hits spots in the operating range where it is two lean.
Phil

What are your emissions limits? Your actuals seem very low, don't you have some leeway to increase the mixture but still pass? My factory V8 needs every ounce of 4.5%CO (my limit) if I am to avoid a similar (but not as bad) flat spot. I also get intermittent back-firing through the carbs after a spell of weak running at 70/80 mph.

PaulH.
Paul Hunt

Phil
Try resetting on 1.5% CO and let us know how you get on

All the best

Roger
Roger Walker

How does one adjust the CO on the Federal fuel injection system?

When I had the car tuned for emmissions to get it registered a year ago, the mechanic adjusted a spring in the airflow meter. Is there a procedure to richen the mixture and produce more CO? Does the feedback from the O2 sensor have eneough authority to put it back where it is now, even if I adjust it? Does the bypass screw on the air meter adjust only the idle mixture or does it work in combination with the spring/potentiometer in the airflow sensor and shift the mixture thruought the operating range?
Phil

By the way the standards are 250 PPM HC at idle and at load.

CO is 2.2% or less at idle

CO is 1.65% or less at load
Phil

Does the car have a catalyst? If so the emission test is not measuring engine out.


Based on other fuel injection engine's I'd test:

a) throttle position sensor (can cause hiccups while cruising) check for smooth voltage signal.
b) air vane (you seem to be getting no fuel or air at high loads)
c) hose between air vane and throttle (no leaks not collapsing)
d) insure that the ignition system is properly functioning (not sure of the best method here

Good luck
Chris Moog

The car has a cat.
Ignition is new conventional Buick distributor (points style)
Replaced inlet tube with mandrel bent tube and new rubber and clamps.
Checked TPS when I installed system, but will check for continuity through the range of motion of the throttle again.

Thanks for the suggestions.

Phil
Phil

Hi Phil
The air bypass screw will make a change in mixture to a point. The best that I can remember is that an additional 100 ohms in the water temp circuit will provide an added boost to the mixture to prevent the common starvation problems of this type of system. This is done by soldering a 100-ohm resistor in one of the leads to the water temp sensor. You may want to start by checking to make sure the water tenp sensor and the fuel pressure are in spec before any modifications are made.

Kelly
Kelly

The airflow meter adjustment will give you what you need - try the richer mix in the first instance - you may find that there is another problem afterwards but one adjustment at a time (although be prepared to turn down the idle speed adjustment see the parallel thread)

Roger
Roger Walker

This thread was discussed between 15/08/2001 and 16/08/2001

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