low power 69 427/390

[Bill C.]

1969 427/390 4spd. Car starts fine, idles and runs fine. That is...runs fine with moderate acceleration. The issue is when I step on it. It does accelerate but not like a 427 should. Acceleration feels flat. It doesn't bog or miss and no smoke but doesn't seem to want to go. Quadrajet is rebuilt and have pertronix ignition. plugs and wires are new. Any thoughts?

[Dick W.]

Re: low power 69 427/390

Timing? Spring too tight on the carb secondaries?

[Dennis S.]

Re: low power 69 427/390

First make sure the throttle cable is opening up all the way. If not, your accelerator linkage under the dash may be bent. Quite common.

[Clem Z.]

Re: low power 69 427/390

make sure the secondary lockout is released when the choke is open

[Bill C.]

Re: low power 69 427/390

Guys...I loosened the secondaries spring and I can get them to kick in. Acceleration improves but is still weak. The accelerator cable is new and clear. In the garage I can bring the rpm's up with no problem...it's under load where the problem occurs.
Bill

[Clem Z.]

Re: low power 69 427/390

Guys...I loosened the secondaries spring and I can get them to kick in. Acceleration improves but is still weak. The accelerator cable is new and clear. In the garage I can bring the rpm's up with no problem...it's under load where the problem occurs.
Bill

does the carb have the correct secondary metering rods because if it has SBC rods it will be very lean

[Clem Z.]

Re: low power 69 427/390

does the carb have the correct secondary metering rods because if it has SBC rods it will be very lean

pull the rods and post the 2 letter code on them

[Bill C.]

Re: low power 69 427/390

Clem
Carb is a 7029201 (originally for 69 396). Should be 7029215. Carbs are the same except for secondary rods. The 201 on the car has 'AN' rods, the 215 has 'AX'. I do suspect this might be an issue but wasn't sure there was enough of a difference between the 2 that performance would be as soft as it is.
Bill

[Clem Z.]

Re: low power 69 427/390

Clem
Carb is a 7029201 (originally for 69 396). Should be 7029215. Carbs are the same except for secondary rods. The 201 on the car has 'AN' rods, the 215 has 'AX'. I do suspect this might be an issue but wasn't sure there was enough of a difference between the 2 that performance would be as soft as it is.
Bill

AN is a .070 power tip dia and AX is a .040 power tip so you are lean at WOT. pri jetting is the same 45/71

[Bill C.]

Re: low power 69 427/390

Clem,
2 questions: would changing to AX rods deliver a 'significant' improvement in performance and any thoughts on where I can find a set?

Thanks...Bill

[Clem Z.]

Re: low power 69 427/390

Clem,
2 questions: would changing to AX rods deliver a 'significant' improvement in performance and any thoughts on where I can find a set?

Thanks...Bill

the secondary jets in a Q jet are fixed and are .135 dia. so calculating the area of that and subtracting the area of the tips you can see how lean you are. edelbrock sell a .041 tip rod,part # 1951 which is a close as you can get unless you can find some used secondary rods

[Bill C.]

Re: low power 69 427/390

Thanks Clem...ordered 1951 rods..

[Clem Z.]

Re: low power 69 427/390

Thanks Clem...ordered 1951 rods..

please let us know if it fixes the problem. a lot of problems are solved by the suggestions here on the board but we never hear back if it worked as this could help someone else down the line

[Bill C.]

Re: low power 69 427/390

Clem...received rods today and installed. No real improvement. Feels like it's just not getting all the gas it needs. Going to pull carb and check float level.

[Bill C.]

Re: low power 69 427/390

a suggestion ...

The secondary upper butterflies should operate very smoothly. If there is any binding, even the slightest, it will make them operate funny. They need to rotate without any effort (other than the little pressure from the spring).

I have seen carb rebuilders over tighten or tighten out of sequence, the screws holding the top carb section to the body.

Or you could have a warped top plate.

I had this issue on another car and it drove me NUTS. I replaced the top plate, adjusted the spring - and the car grew a set of brass ones!

Also - the carb gasket you use is really important. It may/may not influence the rate at which the vacuum falls off to the port the pull-off is connected to.