Edelbrock 650 Thunder Series Carb... Issues! HELP!

I bought a new edelbrock thunderseries 650 carb. Everything works great until you stomp on it. if you stomp on it from an idle it hesitates, stumbles for about 1/2 second then it comes to life. The higher the rpm you are when you stomp the less severe the stumble.


Here is what I have done, I read the entire manual on the carb and followed every tunning setup guide to the T. I adjusted the mixture screws exactly how described. Didn't change anything. I thought it may have been timing at first so I adjusted and tested made no difference. Called a friend told me to try to cover the secondarys with my hand and crack open the throttle. This did reduce the the hesitation. So I changed the secondary butterfly to open as late as possible. This helped but didn't cure the issue so I don't think that this is the right way to go.

So the way I understand it at this point is if you cover the secondarys to reduce air flow and it improves the issue the motor is not getting enough fuel to go with the amount of air?

So I ordered 2 sets of jets for the secondarys to test this. The secondary jets from the factory are .98" I ordered 1.01 and 1.04 to test.

Please let me know if anyone has any thoughts on this. Am I going the right direction? Or am I way off? Any input would be great!

the secondaries are controlled by an airvalve that can be adjusted...
open too soon...bog and stumble... especially from idle or low speed ...not enought air flow ...

tighten up the spring..

also check your acelerator pump...see that is is squirting gas as soon as the throttle is moved...also check to see how big the pump shot is...move the rod to get the most pump shot...

and then what size squirter do you have?

I have tightened up the spring to the point I thought it was just way beyond the typical adjustment. The result was less of a stumble / hesitation but it was still there. Squirter size would be stock I am not certain what it is though. I did also try moving the accelerator pump to the highest setting. It did not change anything.

Something else I should mention is that we had a new edelbrock performer 600 sitting around and we threw that on to see and it worked fine.

Thanks for the response.

Not an expert with the Thunder series, but generally a WOT stumble is a lean condition.

This can be caused by

1 - a sloppy air valve as 70aar stated , or

2 - Just too plain lean in the jetting (too much air, not enough fuel when it opens)

Put the air valve back close to factory spec if it didn't change it much - and concentrate on jetting.

I'd be copying the AFB jetting to start with - especially the primary side....that way you know you're in the ball park.

FWIW - If that 360 is running a mild cam - at a hunch Id be running .110 primaries and .104 secondaries as a start point...with rods around 73x.42 or whatever is close in "thunder speak".

Also take a look at EB's web site under "tech video's". Very informative and usually included with the performer series on DVD, I assume your thunder series would be on there too. Good Luck!