Okay so I know this may be a little complicated but try to understand and stay with me. I have just installed a 670 Holley street avenger that me and my dad rebuilt completely with a fine toothed comb! Anyways the Carb is performing almost perfect. A little minor tuning and it will be their no biggie. Anyways on to my question. So when I fire up the car it starts great! The fast idle is set to around 1600 rpm. So the fast idle drops down to 1000 rpm after a few minutes when I hit the throttle a bit (which is normal) but the car wont drop down to 750 rpm which is what the idle is set at! So I left the car running at 1000 rpm and toke off the breather. I noticed the butterfly wasn't 100% opened. It was only about 90-95% open. I manually opened the butterfly and held it open while I hit the throttle and the rpm dropped to 750! And I know what the first things someone might say is" you just have a bad thermostat choke cap". Well I already replaced it. When I replaced it I put the pin that's underneath the cap through the hole in the spring. So unless I installed the choke cap wrong that couldn't be it. Any help is appreciated!!

 

Any thoughts???

 

What I found on my 670 Avenger (and many other carbs) was that the choke thermostat was very touchy. I had to work at it several times to get it set to where it would do what I wanted. I had set to almost completely off/lean to ensure that was functioning as intended and then gradually bring it up from there. Problem is, as the engine gets warm, the whole setting procedure gets out of whack and you have to recalibrate again the next cold start. It took a while. Then as the weather changed, some adjustments were still needed. Further to that, although it didn't happen with the 670, I've found that the choke linkage will bind at times. Portions of the clips, linkages and fast idle cam were getting hung up on one another and giving me fits regarding idle speed.
One other thing you may want to check is your timing curve if you've altered it from stock. Some distributors I've been involved with had the springs so loose that at 1000 rpm, they had allowed the advance curve to start and that bumped the idle up several hundred rpm.

 

Well I have a brand new hei race distributor and it been total timed for 34 degrees. But I have recorded to do the manual choke conversion that way I know what I have at all times! But I apppreciate the help.

 

I may do the same with my BG 750. The choke linkage on these things is of the oddest design and causes interference issues with the air filter bases. At the moment , I've removed the entire choke mechanism and just deal with cold start issues.
Good luck.