It took quite a bit of time to finally find this information. Feel free to download the pdfs'. Be sure to check Sears for the parts.
My ONAN is in a 1978 Westwind 24 foot motor home on a 4000 BFA gen set. The ONAN Cummings web site was useless to locate parts for this model and difficult to navigate. I located needle and seat prices from 60 dollars down to 15. The issue with tish unit was; the carb overflowed and hydrolocked the genset.
I removed the plugs and turned the thing over and created a a mess.
Be sure to change the oil and filter if this is your problem.
The needle and seat kit came yesterday. The seat is a small viton insert held in by a split ring. The needle is a common type. the kit includes a new spring. If you need to do a complete rebuild and soak then you need another kit.
Please feel free to download the following parts manuals. The ONAN-PARTS.PDF is 315 pages long so please wait for it to load.
http://www.steelerubber.com/