Bonnewagon..quadrajet question

[pontiacgp]

Hi Mark, I ran a holley street avenger 670 for last summer to appease the race car owner. I sold that holly this weekend so I was going to re jet the primary on my quadrajet cause I wanted the rich it up a bit. I had one of my clients put my car on a sniff test when I had the Rochester on it and the readings was that it was running lean in the primary's. Plus when I hit the secondaries on the highway it felt like I dropped the car a gear and took off like a champ so that tells me the primary is too lean. When I took it apart I have 43k rods and 75 jets so there is no way that set up should be running lean. What do you think it could be. The holley was quicker on start from stop but the quadrajet kills the holley on the highway

 

[Bonnewagon]

Steve, that primary side is hardly lean, if anything it's too rich for a normal engine. A stock 455 might have only a #73 jet, and a 350 or 400 generally use around a #71. But crazy cams throw all that out the window. I've seen Chevy 350's demand #76 and #77 jets just to run, because of the cam. What kind of cam are you running? What secondary rods? The rod tip is the part that flows fuel, fat tip-lean, skinny tip-rich. A rich secondary side, with a properly adjusted air flap, will respond like you reported. But it may also cover up a deficient primary side. What kind of sniffer test? Just idle or IM-240 roller test? Leaned out idle screws can fool a sniffer but then NOX usually soars. Dual exhaust fools the machine too. Does it idle smooth? What about off-idle, smooth? Plugs running clean? Do you have APT? If adjusted to low, the primary rods stay in the jets too much and lean it out. Remember, jets affect fuel flow all across the primary range, but rods affect fuel flow at off-idle and mid-range. If you adjust the APT so it's up high, the rods will be just barely in the jets and ready to flow big time as soon as the throttle is blipped.

 

[pontiacgp]

Thanks Mark, I should have let you know that the carb was rebuilt by a quadrajet guru and he'd redo the carb but I would have to force him to take some money. He's known in this town and used to stock the speed shops with rebuilt quadrajets. He built the carb for the spec of my engine and the roller cam I am running is not wild and GM uses the same cam in their 604 crate and in the ZZ4 engine. As for the sniff test it was done at idle and at 2,500 rpm. I talked to the carb guy after that test and asked him about the lean condition. He said that's how he set it up so I wouldn't be going through too much gas in the city. He smiled when I told him the kick it had when hitting the secondaries but I can't remember what the rods are. With that carb cruising on the highway between 60 and 65 mph I get over 26 mpg. The plugs are on the light side but son't look bad.The carb idles nice and it's runs great but I'd like to get a little more livelier response from it taking off from a stop. I think I'm going to adjust the APT up a little and see what that does.

 

[G_Body_Enthusiast]

I like how he tuned the carb so you wouldn't use as much gas driving around town. I'd call it that a favor. He tuned it so it would be efficient while cruising but still fun at WOT. That's the best of both worlds for a daily driver. You could change to a thinner primary rod by one step down, MAYBE two at most. If you wanted to give it more oomph though I advise against it. No sense in making more trips to the gas station just to have a little more oomph taking off from a stop.

 

[pontiacgp]

I'm not looking to use more gas but I'd like to have a bit more response. The holley had better response from stop but the fuel mileage was still reasonable so I'm thinking the quardrajet can be tweaked just a little. I'll have a chat with the builder and see what he can tell me about the primary set up and let you know. I am really happy to be back with the quadrajet

 

[Bonnewagon]

Steve, I like to get what I call a "crisp" takeoff, and that usually means a fairly lean primary side. But those jets/rods you have are hardly lean, maybe for that cam, but not in general. I think the APT should be looked at as well as the accellerator pump shot. I have found around 4 turns from bottom about right for the APT but test-n-tune is really the best way. That is why sharpies remove the pressed in plug in the air horn that is right above the APT. They then use a screw-in plug so adjustments are simple. The factory adjusted it from there, then plugged it, but with it open you can adjust,drive,adjust, until it is best. By the way, it is a "double D" shape but a Q-jet secondary rod hanger with the flared end ground off fits PERFECT . It is almost mandatory to use the outer hole on the pump arm for any kind of big/fast motor. If your cam really needs it the pump discharge ports can be enlarged a little too. The Holley was probably better in that department because they knew it would be used on a modified motor, while the Q-jet was for a production car. In fact it would probably be best to let your guru make any changes since he was the one that set it up. Tell him you wouldn't mind losing some gas mileage to get some more oomph on takeoff.

 

[pontiacgp]

Mark, I'm picking up an AEM wideband gauge tomorrow so no more guessing games. I plan to talk to my carb guy before I make any changes. I originally brought him 2 other carbs that he would not use. He's particular in what carbs he uses cause he modifies them to suit. He has a couple of younger daughters and they each have a mini rail dragster that he builds and they race them. He also has a Grand National and a GNX in his garage that are both showroom condition, The GNX still has it's bench seat in it. ...

 

[Bonnewagon]

Sounds like he knows his sh*t so let him work the magic. I know I would not let anyone else touch something I set up. This gauge will it let you operate the car and get real time readings?

 

[pontiacgp]

the qauge installs in the dash giving you real time monitoring...

 

[Bonnewagon]

Pretty cool. Looked it up on Amazon, rave reviews. That would work great with that old Holley Pro-Jection. You could tune the injection right from the driver's seat! But with a carb even though you may know where the deficiency is, you still need to know what to do to correct it.