Fault diagnosis- help needed please!

[rossbraithwaite]

I'll try and keep this as short and factual as possible. The car is almost completely standard with points and distributor but SU fuel pump converted to electronic points 8 years ago. The first sign of a problem was in Oct.2022 when the engine cut out whilst going through roadworks at 40mph. It then restarted and cutout a number of times. Over the winter I changed the battery, coil, rotor arm, distributor cap & spark plugs. May 2023; The car started and ran for a while, went for a short run, 5-10miles and then cutout but restarted and drove home. I changed the fuel pump & ignition switch, checked fuel filters except those in the tank, the car seemed ok so I drove 25 miles each way to Donington Park for the historic festival, just a slight misfire when cold. I then drove about 25 miles the following day and again the engine cut out whilst off throttle over run. I emptied the fuel bowl & checked the pump was operating. The car restarted and I got to work where I replaced the terminals on the fuel pump feed wires. The car got me home but started misfiring occasionally after about 10 miles. Applying some choke or changing down seemed to clear it. I changed the points and set the gap at 15thou, checked and adjusted all spark plug gaps to 25thou (electrodes were beige), checked compression 150psi on all cylinders, replaced terminals on coil feed wires. The engine starts on the choke at a fast idle but quite soon starts misfiring and then slowly peters out and dies. I tried a separate battery feed direct onto the fuel pump and I tried a feed direct from car battery to the coil +ve terminal but the results were the same. I haven't touched the carbs but in 2019 they were overhauled by a reputable specialist with new choke rods, carb floats, piston damper springs and a service kit, balanced and tuned on a rolling road.
Any suggestions welcome
Thanks for reading,
Ross

[DWW]

Did you change the coil at any time or check if it was getting hot? Seems a likely candidate or possibly culprit from your description, that or the condenser.

[rossbraithwaite]

Yes, I've replaced the coil more than once. That was my first port of call and in total I have tried 4 albeit none that were known to be good but 2 were brand new. I have not replaced the condenser. The points I removed were pitted and blueish but I haven't inspected the brand new set that I fitted. I suppose I could swap the condenser and inspect the new points at the same time.

[mgcjag]

Hi Ross.....i have had a similar problem in the past. ....changed everything.....finally found a wire to the coil not crimped properly to the spade connector......i now solder all the low tension and fuel pump spade connections...worth doing just to eliminate them....even back to the revcounter/ign switch.....fuse block.....Steve

[chrisfell]

Could be a dodgy earth on the ignition. Check all the black wires to the frame, from the coil, from the dizzie. Do not rely on the dizzie earthing thought it's mount to the engine. Mine had this issue several times. Once was due to a PO using cheap lucar connectors, another was due to the earth terminal not being very tight on the frame. If ( as was the case with my car) the earth terminal also earthed the after market electric fan, this can get very hot, and that in itself can cause lack of continuity.

Also just check that somewhere a PO hasn't fitted an ignition cutout switch.

[christopher storey]

This is a fairly classic description of what happens with some aftermarket rotor arms which become conductive with heat. You mention having changed it : if it was not from the Distributor Doctor, then the first thing I would do is to swap it for a new DD one

[rossbraithwaite]

Thanks all
@mgcjag I have renewed the terminals to the coil and fuel pump but not the one on the back of the dizzy so I will do that one.
@christopher storey Yes my rotor arms, points and condenser are all from The Distributor Doctor and he refurbed the dizzy for me a few (5+) years ago

[mgcjag]

Ross....follow all the low tension side back and check....even the key switch can be hit/mis ...Steve

[rossbraithwaite]

Thanks Steve, I have replaced the ignition switch and the terminals to it are ok. TBH there may be more than one problem; last night I switched on the ignition, fuel pump primed and then stopped ticking, I released all three fuel banjoes and at the carbs and nothing happened, I hit the (brand new electronic) fuel pump and it burst into life. I changed back to my old pump and it was much more active. However, after changing the condenser, rotor arm and LT terminal to the dizzy there is now no spark at all. Also, I may have had the LT coil connections the wrong way 'round; does this matter? As I understand it is just a winding so shouldn't matter but they are marked +ve and -ve.

[mgcjag]

Hi Ross it should work either way.....but as per cct diagram is ideal......if your neg earth then neg goes to points....also shown as CB contact breaker on some coils....Steve