I recently bought a Thunder Series 750cfm carb after I got tired of re-building the original Thermoquad that came on my Duster w/340ci (original owner).....

I can't get it to work. It runs best when I adjust the vacuum secondary to a quarter turn, but the performance is still way off.

The current engine puts out 450hp at 5750rpm and 460torque at 3750rpm. At 6000rpm the dyno meters 69gph through the carb.

Anyone have any experience or suggestions on jetting and vacuum secondary adjustment.

how big is the cam...how much vacuum you have at idle?

you need to check the metering rods...in that went it is idling the rods are down...if they are bouncing up and down you need a light step up spring to keep the metering rod down in the jet.

also make sure your timing is set correctly too...

The base settings on that carb OOTB should be pretty close. I have the thunder series 625 on my 360 with a 509 cam and it runs like a champ. Don't over adjust the carb, my initial thought would be that you do not have enough tension on the secondary throttle flap and it is opening too early. All I use is a vacuum gauge on the manifold vaccum port with the idle raised to about 1,300 rpm. adjust the idle circuits till I pull the most vacuum and leave it alone. You should not need to mess with the step up springs or metering rods till you start reading your spark plugs.

there have been NUMEROUS posts on this board about problems with the 750. some people have been able to make them work, but it seems like the vast majority of people have problems with them.

before going any further, i would search for those threads and see if your problems are similar, and if they are, you may want to ditch the carb and get another.

i'm a big carter/edelbrock fan, but have heard so much bad stuff about those 750's i don't think i would ever go near one.

They Dont Make A Thunder Series 750
650 AVS And 800 AVS Thunder

They Are Mechanical Secondaries With Adjustable Air Doors Like Our Original Carter AVS Carbs



Now They Make A Performer Series 750 COUGH COUGH


So Lets Start With A Carb # First ???

Isnt there any former edelbrock employees on here that can shed some light on the 750 carb thing? I read the Carter Comp series 750 was fine but yet the Edelbrock version falls short.

The 750 1407 carb has been junk for years. Plenty of time to figure out that somebody pulled some crap in the production. They are just junk.