I've got a Holley 4150 850CFM with vac secondaries. Leaking from accelerator pump. Want to get a new diaphragm, but they seem to have 2 sizes, 135-9 which is 50cc, and 135-10 which is 30cc. Any clue which is stock for that carb? Also, should I be buying Holley brand or some 3rd party? TIA

Having the Holley list number would probably help

50cc on the left 30cc on the right. Noticeable difference. According to this article...all 4150/4160 carbs came with 30cc pumps on the primary side

https://www.holley.com/blog/post/accelerator_pump_tuning_for_holley_carburetors/

As stated...enter your carb list# here(on the airhorn) and you should find all the original specs for the carb

https://www.holley.com/support/carburetor/

Look at the well that the accelerator pump fits in. If it's deep and the taller diaphram fits in perfectly, then it's the taller diaphram. If it's shallow the shorter fits in.
Also, make sure you use the appropriate spring for the diaphrams. Tall spring spring for tall diaphram, short spring for the short diaphram. If you accidentally threw away the diaphram just look at the well and spring and that will tell you which one.

The throat is stamped:

80531
1588

Thanks I'll pop it in there.

Also, make sure the cover is flat - they get distorted from over-tightening.
Grab some 320 or 400-grit, place on a dead flat surface, and rub the cover across it.
It'll be easy to see if it's flat, and/or when you get it that way.

I'm having a devil of a time figuring out what # that web page input wants.

I've tried 80531, R80531, 4150, 0-80531, and I get "no results." I will take it apart and eyeball it using the tips posted here - thanks!

Thanks, will, do!

I think your question has been answered. If its not 30cc, it would be because someone changed it.

I agree, I've had to file many accelerator pump covers when a new diaphragm wouldn't seal. Common Holley problem.

50cc cover on the left and 30cc on the right.