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MG MGB Technical - Misfire when applying gas

Facts- 1974.5 MGB RB non OD Roadster. Weber DGV conversion (PO installed), Petronix electronic ignition, 1 year old Lucas Sport coil, recently rebuilt head, valves adjusted to .015 cold, new gaskets all around. Less than year old, plugs, cap, wires, fuel filter. Fairly new fuel pump. New K&N air filter and housing. Attempted to time the car, it was a little off, set it for 15 degrees btdc (is that correct?). Antway, It appears to be running good, a little lumpy at idle. When out driving the other night I noticed when coasting down a slight grade, in 4th gear with the engine braking, about 2250 rpm, when I give it a little gas it would backfire. I was able to recreate the problem each time I coasted down a slight grade and gave a little gas it would backfire. If I stomped on the gas it wouldn't backfire. When on a straight and in 4th gear I could stomp on the gas, let up, and stomp down again, over and over, and no backfire. Coming down a hill with RPMs over 2500 and it would not backfire when given a little gas. Could it be I didn't set the timing correctly, or a vacuum leak somewhere, maybe ignition or card misadjusment?
Thanks in advance
John M
John

Did you disconnect the vacuum advance when you set the timing? It sounds like the ignition is retarded. With the vac hooked up you should be around 20-22 degrees, with it disconnected, around 10-12. Remember, this is with the idle set at "500rpms." Or as low as you can go. If you're higher than 700 or so, the mechanical advance will be kicking in already, so the actually static timing will be less than what the light shows.
Jeff Schlemmer

Jeff you are right on the money, I did forget to disconnect and block the vacuum advance, dope that I am, I guess I will doing it again tomorrow. Thanks.
John

This thread was discussed on 29/06/2005

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