MGB & GT Forum

I have a 1980 B with twin HS4 SU's and a PECO header/ansa exhaust. I have a petronix ignitor. It has the stock head and cam i believe. What timing should i set the car to? (dynamic) Also, i do not have the vacuum advance hooked up (from the carb to the dizzy) - I always felt that it advanced it too much - like way off the scale - under revs. The centrifugal advance on the dizzy advances it another 10-15 degrees but with the vacuum advance the timing goes off the scale - is this normal/desired? Also, the tube that i used for the vacuum advance is very thin (pulled it off a wreck) - would it lessen the amount of advance with a slightly larger diameter tube?

I have the engine set at 10 degrees ( as per the manual) currently but do get some pinging on warm up (no choke) or on trailing throttle. Should i move it to 12-14 degrees?

thanks,

tc

What is trailing throttle?? What octane are you using?? If it is pinging, either your octane is to low or you are too far advanced. The other possibility is that somebody has put the wrong distributer in it and the curve is wrong.

Tom, if the scale you're referring to is the one on the engine checked with a timing light, the only time it should be ON the scale is at about 600 RPM with the advance disconnected. If you have your original distributor, your advance should be connected to your intake manifold, not to the vacuum port on the rear carb. This will make a HUGE difference in your advance curve! If you are using the port on the carb with the the later dizzy, then I would expect some pinging, as you are advancing at exactly the wrong time. If you have a port on the manifold with a plug in it, find a fitting that will screw into it and run your vacuum to the dizzy from there.

Ken

I'm currently running without any vacuum advance b/c the advance seemed wrong, I'm just using the mechanical advance of the dizzy. The specs say a 1980 engine should read 10 degrees at 1500 rpm - which my engine does. The Dizzy is the correct one for a 1980 car. I will check the intake manifold...i believe that there are a few plugs in it.

Tom,

OK, the factory spec should be pretty close, as a 1980 spec distributor uses quite a bit of centrifugal advance when measured up through the 4000 rpm range. The vacuum advance originally ran through a "Transmission Controlled Spark Advance" (TCSA) unit, which only allowed the use of intake manifold-based advance in 4th gear (good for mileage). After setting it up to the specs you now have, you can go ahead and directly connect the vacuum advance to the intake manifold port - yes it IS quite a bit of advance (depending upon your distributor, you could be adding anywhere from 10 to 20 degrees more advance at idle, using intake manifold vacuum). However, this advance will fall away as you increase the throttle opening (which is when pinging tends to occur anyhow). Moreover, you'll tend to get better fuel economy at cruise conditions.

In my case, I use the carb port for my vacuum advance, however, my distributor was curved for that type of advance...yours is not (assuming it's the original). There are a number of other ways to attack this, such as obtaining a better distributor, however, since you probably have an LC engine, setting your timing up for factor spec and using the intake manifold for vac advance will get you in the ballpark. When you reconnect your vac advance, you'll probably need to lower the idle setting as well. If you really want to optimize things, then you can always try the "hill" test, adjusting for more advance until she just starts to ping, then backing off a couple of degrees, until she stops.

FWIW,

Barry