Thread: Back Fire Now Won't Start

Originally Posted by d2dave

Still not fixed. On a cold start today it would not go. I primed and manually lifted the diaphragm and got it going. Once running it was ok and then when switched off it would re start.

I figure that there is not enough vacuum at cranking to lift the diaphragm.

When running it has around 16 inhg at idle which is a tad low. Cranking produces about 5 inhg. How much should a motor have when cranking?

1. Fit a proper primer with a button inside.

2. Richen the idle slightly. The model 200 mixer idle screw is an air bypass so screwing it in richens the mixture. This also increases the vacuum to the diaphragm.

3. Use a small amount of throttle for starting. About 1" down on the pedal once cranking has commenced.

4. Make sure the timing is set at 10 degrees BTDC.

5. Set the plug gaps at 0.7mm

6. Make sure you've identified all the vacuum hoses and ensure they are properly fitted.

7. Fit a bigger & better battery and/or starter motor. Early Lucas starters are too slow compared to a good Bosch one.

Originally Posted by d2dave

This would certainly do the trick. How do I go about it? Switch and wiring in to the car I am ok with, it is just the procedure at the gas end.

as previously posted by me

a straight gas engine usually has a manual primer consisting of a petrol solenoid screwed to the test pressure port on the converter. It's found between the water connections on a model L converter or similar. You plumb it to the inlet side of the mixer.

as to your other q

Is 5 inhg enough vacuum at cranking? I did a bit of googling and one site I came across said that 3 inhg is ok.

Wouldn't have a clue, however anything that raises the cranking speed will obviously raise the vacuum reading.