Dual Quad Intake Need help with Adjustments

[Takt]

The basics –

I have a 302, with a slightly aggressive cam, Pertronix Ignitor pointless conversion with flame throw coil (45,000 volt). I have a dual quad carb setup with high rise manifold and two Holley 4 barrel 390 cfm carbs (model 4160). The car has a C-4 Automatic and currently a 2.8 open differential (although I have a 3.8 posi on order and should be here in the next couple weeks).

I paid a guy to rebuild the carbs back to stock, and with his experience I’m confident everything is correct.

When I drove it for the first time, there was a huge hesitation when starting from a complete stop. Also, when I gave it some decent gas, there was a hesitation. After the hesitation (between 25 and maybe 45 mph) if I got on it, it had decent power. I was told that I may need to change the timing, so I experimented. Now, I’ve got it so there is no hesitation, however there is also no power.

A couple questions.

1. Should I stop messing with it until I get the new rear end in it? Then make adjustments.
2. Is there a rule of thumb as the process for adjustments in this situation? (i.e. Timing, then idle, then mixture, etc… )
3. Timing – any thoughts on where I should be?

Thanks in advance for the advise.

 

[66Runt]

I'd take it to the guy who rebuilt your carburetors

Timing is mostly dependent on cylinder pressure. 12 degrees @ idle should be close enough to make it run.

Mixture and idle speed are set together. Once above about 850rpms you come off the idle circuit. Both carbs need to be set in unison. It takes experience to get things balanced.

Did you give the carburetor builder your engine/car specs so he could set up the jetting and power valves? Did he ask for that information?

I hope the rest of your engine breaths well enough to take advantage of that intake, otherwise, you will not be impressed with the performance.
I'm thinking you may not have enough motor or stall to be happy with a tunnel ram. Looks cool though!

Good luck,

 

[2bav8]

Does it have progressive throttle linkage?
When I had dual quads it was such a dog if I didn't use the progressive linkage.

 

[dennis112]

Swapping gears from the 2:80 to a 3:80 is certainly a step in the right direction.

 

[Takt]

Did you give the carburetor builder your engine/car specs so he could set up the jetting and power valves? Did he ask for that information?

Good luck,

Yes, he had all the specs. He called Holley to get their opinions as well. Everyone thought this would be a good application for my car.

Does it have progressive throttle linkage?
When I had dual quads it was such a dog if I didn't use the progressive linkage.

I don't know what progressive throttle linkage is. I'm sure I don't have it though. I'm attaching a picture of the linkage (well before I cleaned everything up). The fuel lines are routed a different way too.

Thanks for the help.

 

[Spoofty]

Every Holley I've installed has had a hesitation. Usually stepping up to large squirters will fix it. I've never worked with a daul setup before. But in my Holley tuning book they recomend using a 50cc accelerator pump for automatic trannys but that's for a single carb. I would think as your using two accelerator pumps just going with larger squirters would do it.
Check to see what size squirter you have.
Or post the carb item numbers and I can look it up in my book.
It's probable around a 25 but I'm not sure with a carb of that size. My 600 came with 28 and I stepped up to 35 and it's perfect. I have a very aggressive cam and needed to go big

 

[Takt]

Spoofty, I thought the hesitation was due to too much fuel being dumped into the cylinders .. but per your post, I'm wrong?

I am going to take my car to the carb guy to have him help me tune. He lives in Illinois, I live in Wisconsin, so I thought I'd try it myself.... try to learn a little in the meantime.

Thanks

 

[Spoofty]

I always thought mine was from runing rich too. I ran around with a hesitation for probably 2 years. Then one day some one told me to get a set of jewlers hand drill bits and make the squirter larger. I went one size at a time and the hesitation got less and less. Until I went too far then I just went one size smaller on a spare squirter. After that I bought the Holley tuning book.
If your engine has a hesitation from being rich you will usually see some black smoke from the tail pipes.

 

[zookeeper]

Bogs are almost always fomr running too lean. In your case, I'd bet the problem is too little of an accel pump shot. Your carbs are a long way from the motor and it takes a pretty big shot of gas to get things rolling. I have a low-rise 2x4 manifold (the type that fits under the hood)with a pair of 390cfm carbs and the throttle response is immediate, even with straight linkage. BTW, by straight linkage, I mean both carbs are opened at the same time. One thing you need to be aware of is Holley offers some pretty neat stuff that helps the driveablity of 2x4 setups. For instance, Holley offers a kit that ties the secondary vacuum pots together with a vaccuum hose so they open and close together. Also, you may want to go up to the next stiffer spring on the secondary canister to keep it closed a bit longer. Finally, try to adjust your carbs with a vacuum gauge attached to the vacuum port at the front passenger side of the base. I disconnect the linkage, then attach a pair of vaccum gauges, then adjust the idle speed screws until the idle is at the right RPM, and the vacuum gauges are equal. Yes, it takes a bit of fooling around, but at least you know they're right and the throttle linkage isn't affecting your settings. Good luck!