Idle surge issue on qjet...smooths out if I tip in the secondary air flaps

a couple things, first the secondary air valve shouldn't open so quickly with no load on the engine, something seems wrong. also did you check for primary nozzle drip with a flashlight at idle, that will give you fits. a last thing to check is taking the carb off and seeing where the primary throttle plate is sitting at idle, the transfer slot should just be slightly exposed if timing and mixture screws are properly set. good luck

 

also don't rule out, vacuum leak, cracked spark plug or other tune up issues. good luck

 

Opening the secondary flaps shouldn't make any difference if the secondary throttle blades are fully closed....so I sense that the secondaries are not fully closed at idle. I had the same issue on my 7027248 Q-Jet on my trip to Colorado Springs.

Check the linkage from the primary to the secondary throttle blades...I had a groove worn in the rod which actuates the secondary shaft, and it was hanging up & keeping the secondaries open a bit...making my idle get stuck at 1200-1500 RPM. You may have the same issue. I changed the rod, and the problem went away.

Try pushing the secondaries closed while idling and see if your RPMs go down/smooth out.

 

Don't give up. Never surrender. With these vehicles, answers can be found, experience built and knowledge gained. With today's rolling laptops, all we get is frustration with cheap designs and electronics with counterfeit parts.

 

I'm not sure I follow the "vertical cuts" statement. What rods have vertical cuts in them?

There always seems to be a lot of confusion on the secondary hangers. They have letters indicating the placement of the secondary rod holes. That placement changes where the rod tips sit in the jets. There is only one correct hanger for each carb. The same carb number could end up with any hanger. The reason is that one of the last procedures on the assembly line was to gauge the secondary metering jets to a location on the air horn. That reading corresponded to a certain hanger, so the secondary rods would sit exactly where the engineers wanted them too in the Jets. Manufacturing variances, gasket thickness, etc, all made each carb unique. There is no way to measure that distance that I'm aware of as a service procedure, and that's probably why everyone usually plays it safe by using a secondary hanger in the middle of the alphabet.

Glad you issue is resolved.

 

The early quadrajets used the secondary rods with the cuts. The tolerances were tight and the fear was that the rod would seal the origice and cause a delay for fuel delivery in the secondary circuit. So the slots would allow fuel to pass and fill the well. After a couple years, the secondary orifice was made a few thousands larger and slots were no longer added to the rods.

Glad it is running better for you now. Hurry up and get that cam in that beautiful car!

 

I'm a little confused, are you saying that changing the "secondary" rod hanger made the car idle better. this is strange because they are totally separate circuits. am I missing something?

 

Mark just did a carb for me and I got it yesterday. Looks very nice and professionally done. Going to test it in BG.

I have two more correct 70 Stage carbs Im going to send him soon.

 

Adan. The carb that was on when you got it was built by Ethan and it ran like a top. I'm pretty sure it was basically stock however.

 

I just love threads like this that describe a problem, everyone chimes in with possible causes and solutions , finally a solution is found and shared with everyone......I store it all away in my brain because sooner or later as with all Classic cars I'll experience the same problem.

Thumbs Up to everyone!

 

I'm waiting for a response also............ummmmmmmm...............

 

Technically, the secondary rods should have no effect on idle. Unless the secondary blades are not closing all the way. We checked the linkage to verify it wasn't preventing full closure. I will probably send out another baseplate to swap then realign the originals. I always completely disassemble each carb and these blades have to be centered just right to prevent too much air passage. Between me being human, and shipping trying to destroy anything in a box, things can get a tiny bit off.

 

Base plate problem, that makes sense. Sometimes my quadrajets have just hung up unexpected on their own, especially the upper air flap.