Ok stumped up for the crank posisiton sensor... still no fix.

How likely is it to be the ignition module? That's a £265 part here in the UK so I REALLY don't want to risk it if its not the problem.

so to recap... air intake is fine, I changed gaskets today. Petcock, fuel filter and hoses are fine. Carb seems ok, diaphragm and needle are good, jets are clean. Spark plugs spark, there's compression. Battery is fully charged and fairly new.

Symptoms are the bike will idle reasonably, but sputter and cough under any load and die very quickly when riding. Once dead it will restart and gradually worsen to the point it won't fire at all.

leave the bike for ten mins and I can repeat the above procedure.

any help here would be hugely appreciated as I'm totally stumped

 

How about the carb to header seal or header to jugs? Is everything tight?

 

Yep, all good, checked the intake manifold and gaskets look fine.

So I think it's fixed!!! I'm not entirely sure what it was but I stripped the carb again and cleaned it up, cleaned the jets, replaced the needle with an 88 Sportster needle.

got it back together and fuelled up and it fired.

HOWEVER, it's currently only running with the petcock set to reserve... but there's a chance I didn't put enough gas in to pass the reserve level (I was reusing the fuel I drained before). Haven't had the chance to go to the gas station yet but either the petcock is indeed screwed, or it's all fixed fine and I just need more gas.

glad I didn't stump for the ignition module now....

 

How does the fuel tank look inside?
A loose piece of tank liner or trash can cause all kinds of trouble that is difficult to pin point.
Trash moves to fuel inlet and stops fuel flow...bike turns off and piece floats away...bike starts and piece moves towards inlet again...bike turns off and piece of trash moves away...and repeat process with frustration building.

 

Good call. I'll take a look and see if anything looks dirty. Nothing came out with the fuel filter and the filter itself was clean as a whistle.

 

Glad to hear you are on the right path good job

 

Drop the bowl and clean the carb. It sounds to me like your main jet is clogged.

 

before you pull the carb make sure it's not your gas cap. the best way to check is loosen it . ride it around. If the cap vent failed it'll starve the motor for gas. Sounds like 1 of 3 possibilities #1 gas cap to easy and no cost to check. #2 bad gas Drain it fill with good gas and fight to keep it running until the good gas catches up #3 trash in the gas pull the tank, remove petcock, carb clean everything out. install a fuel filter so you don't have to remove the carb for trash again