Yes, it's a copy but I was told that Holley produces it for summit and is branded as a summit carb.  But you could be right. Supposably Holley uses to make one and discontinued it a while back.

HOLLEY produced the CARB some years ago. It didn't sell well and HOLLEY discontinued the CARB, destroyed molds and tooling and sold the rights to someone else that began making them in CHINA. There is a previous thread here somewhere that describes all of this.

The interesting part is if original HOLLEY SERVICE PARTS would fit if available.

To me (IMO), it was a great design for a driver/street car. It combined features of the F4100 and H4150.

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You went down from 68s to 62s?  That might be your culprit...the 500 gets less air so the 62s even it out...your 600 gets a bunch more air so going that small with jets I would think you're way lean.  
Pull a spark plug and post a photo of the end that screws into the heads.  

Years ago I was taught that for initial tuning to move 3 jet sizes at a time. Easier to read the plugs. So going 6 lean would probably show up as a flat spot. I had a new 4160 Holley once that was mis shipped with 53’s instead of 63’s, and that left an undriveable flat spot off idle.

So I’d agree, get the original jets back in, and check the secondary spring just to be safe. Also make sure you’ve got enough vacuum for whatever the stock power valve was, and that it’s right.

miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ

As aways great response to my issues. With the weather getting better here I will take up the tuning with some great input you all suggested and hopefully get it figured out.  Still my carb. a phobia will not get in the way.  

Don’t be afraid of a carb, just be careful and follow the instructions. In case you don’t have them, here’s a couple links from Summit.

https://static.summitracing.com/global/images/instructions/sum-carb%20instr.pdf 

There’s a note here that rarely will you need to go over 4 jet sizes up. I interpret this to mean they ship them on the lean side (not uncommon for street units). So you might be more than a little lean down as you are.

https://static.summitracing.com/global/images/instructions/sum-m08500vs_bp.pdf 

This one has a note on adjustment of the secondary throttle plates. I’ve read several posts on Holley’s where these were shipped adjusted wrong. It’s simple, and worth a look.

I was hoping to find a manual like ships with the Edelbrock Carter clones that gave detailed instructions. It may be out there if you search around a little more, or maybe someone can refer you.

Good luck


miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ

See if this helps - https://static.summitracing.com/global/images/instructions/sum-carb%20instr.pdf

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