I got a 1985 Sportster XLX 1000 off a guy last week. It starts great, idles great and don't have any hesitation off idle once it is warm. The problem is only at highway speed / constant speed it will cough and miss out. It has a Screamin Eagle carb on it and has 160 and 58 jets. I took the carb apart and cleaned it and it made no difference. The numbers on the side of the carb says 27F8 HF30.

I have checked the timing and that it good. Also has an electronic ignition.

 

Whats the Plugs look like ?
The wires..to check them for voltage leaks get er In a totally dark garage NO Light.
Cover all the lights up and let yer eyes adjust to the dark.

Have a squirt bottle that will mist water,,
Start the bike,,See any Blue flashes of light round the wires??
Squirt It/ mist It,,look again.
Hold 3,000 mid throttle and do that again..
Lights,,wires leaking voltage,,
If nothing check the coils output per a service book.
Got a good CLEAN air filter In her ???

 

May be time to replace the rubber seals on the intake manifold. Where the intake seats to the heads and where the carb seats to the intake.


PS: On a bike that age you should be concerned about all things made of rubber. Did you change any the gaskets on the carb when you rebuilt it? IE: Rebuild kit.






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An air leak,,never thought of that,,good idea FreeBird XD