Front cylinder not firing up

[Forcerouge]

Hi everyone,

The front cylinder of my freshly rebuilt engine doesn't want to fire up

-I have compression (12.5 bars)
-I have spark
-I know for sure the carburettor is working because I flipped it onto the other cylinder and it worked just fine.

Weird thing is: the front cylinder did work a few days ago, actually at the very beginning the front one was the only one cylinder that worked.

I'm a bit puzzled, the only thing left would be timing, but there doesn't seem to be any way to adjust it on the bike, unless I missed something?
Any idea or suggestion would be welcome

[Forcerouge]

Swapped the front and back coils, nothing changed.

[Forcerouge]

Tuned the valve clearance, still the same.
I'm starting to run out of ideas

[DrPMC]

Its frustrating when that happens!

However, you have the advantage of knowing that the front cylinder, as set up, has the potential to work. Also, if you have a decent spark that would seem to cover that aspect. On that basis, I would suggest very carefully checking every element of the fuel mixture supply system. If your still using that old fuel, I would wait until you have some good stuff, at least that way you can be sure that isn't the problem. It may be worth rigging up a separate fuel tank to gravity feed the carb while you are testing. Its also possible that you may have flooded the front cylinder and it needs to be dried out.

There are much better people on here for these types of problems, but that's my contribution!

[Forcerouge]

Thanks mate,
Fuel was my first suspect, but since flipping the front carburettor to the rear cylinder worked fine I think fuel and mixture can be ruled out.
Also I got my hands on some new fuel so at least that's not a concern anymore

I checked the pulse generators and got 450 ohms for bot of them, which is still within specs. Also opened the clutch cover to visually inspect them and they looked okay.

Only thing I can still think of would be the CDI unit going bad. Unfortunately I don't have another unit to test it....

[Forcerouge]

I think I finally found out the issue....
Took me three days, I did countless of complicated things and in the end it was.... bad spark plugs!
Lame.

I was so desperate after a whole day of trying litterally everything, that in the end I grabbed a very old plug I had laying around in the shop, dropped it in the engine and, despite me expecting it to be a failure again....bingo, it fired right up!
That was a very tricky one, I never expected that.... These plugs were brand new, plus they were two on each cylinder, plus they were making what seemed to be decent spark when outside of the cylinder and also the other plugs which I bought together with these ones worked just fine on the other cylinder! What were the odds...

Anyway, the engine now seems to be firing properly on both cylinders, I'll buy 4 new plugs from a better brand and hopefully it should be fine.