Hi everyone!
1971, 4.2, twin 175CD2's, with 37,000 miles, was running great, then it "smoothly" quite running while driving down the road with no indication of any issues. No roughness, just died. I was able to get it started after a dozen tries, but had to keep the rpm's up, and brake-torque it to get home. The next day, it started and idled fine. Couple of days later, idled fine, smoothly shut off, and will not start.

Here is what has been replaced
new battery, plugs, plug wires, pertronix electronic ignition conversion, swapped out carbs years ago, new fuel pumps, one new tank, K&N air filter, fuel filters, etc...

One thing that happened prior to this was the smell of gas, one of the float bowl plugs was halfway out and leaking, which I replaced with new, and went ahead and replaced the other just because.

There is gas in both tanks, there is gas to the carbs, starter spins fine and fast, there is 12v at the coil, switching either pump makes no difference.

If you spray brake cleaner in the carbs it momentarily catches. Otherwise, nothing happens, it just spins.

I put the originally carbs back on. The only difference is the ones on it had the receptacles for the large emission "Y" pipe that goes between them, but I never had that and had both plugged, and it ran fine like that for 8 years.

It was running, and the beauty of this is it is either "gas" or "spark" lol!

I am at a loss, and need your assistance.
Thank you!

 

I don't see any check on the ignition coil. You may have a bad connection to that, or it's internally failing. I had something similar on an Ariel Square Four motorcycle in the late 60s. The coil was earthing itself. I wrapped the coil in insulating tape and then managed to get home OK. New coil fitted and then we were back in normal business !

 

Thanks Fraser!
I will check that out tonight, and I have a spare coil as well.

 

Put in a new coil, 12 volts at the coil, and nothing......

 

Sorry this didn't work. I think now you've just got to go back to basics, and check for spark at the spark plugs, correct timing, fuel getting to the plugs, cylinder compression values, etc.

 

You say that the engine responds to brake cleaner sprayed into the carbies. That indicates that the ignition system is working properly.
The problem appears to be in the fuel system BUT you appear to have overhauled that as well.
I am wondering about the possibility of water contamination in the fuel system. That will definitely stop the engine from starting
All of the XJ series are prone to water getting into the fuel system if the drain holes adjacent to the fuel filler caps are blocked.
I would have a serious look for water starting by pulling a couple of spark plugs to see if they are dry or wet,
Also drain the carbies into a glass bottle. Water will sit below the gas in a well-defined separation line

 

Again, thank you all for your assistance.
Okay, checked fuel, which has the fuel fresh stuff in it, and is 93 octance, and the plugs looked great and dry.
Went back to basics, pulled a plug wire, attached a spark plug to it, and grounded the electrode on the head of the engine.... no spark at the electrode.
So.... even though the rotor, cap, and pertronix ignition components are only a couple of years old with about 1,000 (yes that is correct lol), miles on them, I guess one of them could be bad.

 

If you remove a plug immediately after trying to start the engine, the plugs should look a bit damp if the fuelling is working and the engine is not firing.