Hi all,

I've recently reassembled my datsun 1200 gx motor in my spitfire mk1. I'm running SU's (2x 1 1/2"). Prior to stripping the car down for nut and bolt resto it was running fine.

Now... since getting everything back together there have been a few modifications external to the motor.

Firstly I've put on an electric fuel pump with a pressure regulator.

Secondly I've fit a programmable high energy ignition system.

Thirdly I've built and fit a large bore free flowing exhaust.

The problem:

I can get a decent idle smooth at 800rpm. Anything lower than this and it splutters horribly.

If I try and rev it it backfires horribly out the carb mouth - repeatedly so - and I can't rev it beyond about 1800RPM. With the throttle butterfly linkages undone I can try and rev each carb individually and they both do the same thing.

With the vacuum advance plugged and the motor idling at 800rpm I have it set to 10deg BTDC.

I've noticed that the advance moves to about 20deg BTDC at the max revs I can get i.e. sub 2000RPM.

Over the past 4 weeks of frustration I've stripped and reassembled the carbs - noting lots of crud in the float bowls and since cleaning everything with compressed air. This made no difference.

Observations

The pistons are not sticking.

The butterflies are closing properly - i.e. I can stall the motor if I wind each screw back out.

I am surprised that I can't get a smoother low idle. I used to be able to do as low as 500rpm prior to stripping the car down for the restoration.

I've checked float bowl heights and all is good (baring in mind that they weren't changed from when it used to run fine).

I have fuel at each jet.

I tried heavier piston damping springs and this made no difference.

I tried no piston damping springs at all .... no difference.

The pistons do rise when I try and rev it but they drop just after the backfire out the carb and then repeat the process of rise backfire rise backfire....

even after extended periods of what seems like a good idle the plugs are black and there's fuel up around the threads.

I've checked for vacuum leaks and haven't found anything.

Present conclusions / thoughts

I'm starting to lean towards the ignition side of things. Perhaps it's advancing too quickly. Could this cause induction backfires?
From my reading an induction backfire is usually caused by a temporary lean condition.

Perhaps the fuel pressure is too great and it's pushing by the floats and surging the carbs - this sounds unlikely as I can get a smooth idle at 800rpm - although I have noted it hunts up and down if I try an idle below the 800rpm mark.

As always with cars like this the obvious thing to blame was the carbs but honestly given that I've only changed ignition and fuel pump it has to be one of those things.

This is really giving me the craps.

Any advice would be much appreciated.