Two Barrel to Four Barrel Upgrade?

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dICKj

dICKj

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I decided to pull the dirty old two barrel off my 1974 318 and replace it with an Edlebrock Model 1405 600cfm 4bbl and a Weiand model 8007 dual plane manifold. I also added dual exhausts using the stock manifolds. The car ran really great with the two barrel. Now whenever I try to floor it, the motor just moans and the car actually slows. Any ideas?

 
I did the same thing only used an air gap Edelbrock and a 1406 Carb. Car ran ok with 2 BBl. P.O. replaced timing chain. I also thought vacuum leak. Took manifold off and saw all the push rods bent. Replaced all of the pushrods and runs great now. Some rods were bent pretty bad and others just slightly. Maybe chain broke and free wheeled and valves hit the pistons. When hot just turn the key and fires right up. Also a 74 318.
 
Does it stumble?
 
So take this with a grain of salt but here is my experience. Stock 318 rebuilt stock. I put a Edelbrock Performer and FiTech on it. I have never been able to get it is start quickly cold, it take 5 seconds of cranking every time. Also it had a terrible stumble. I had to figure out how to get into the low level calibration tables and I have the "accelerator pump" equivalent set to the absolute max that the FiTech unit can do and still it needs a bit more. It seems like the small port size in the 318 head does not like all that extra CFM. I have another 318 and 360 I put heads, Performers and 4 barrels on and they start instantly and don't stumble at all. I have also known 2 other people who put the 4 barrel on and then took it back off on the 318 because they thought it all around ran better with the 2 barrel.
 
Carb should be fine on that engine.

Check that the accelerator pump is working, and adjusted correctly.
 
I had an "all stock" 318 "EXCEPT FOR" in my Dart for a time, and it was snotty as hell

What was not stock:
Dual exhaust
Ed Intake
Ed AFB
A small cam which turned out to be a bit less than a 340 cam

Impossible to guess what might be the condition of that carb. New? used?

WHERE IS the timing set?

WHAT is the overall condition of the engine? Mileage on engine? Compression? Uses some oil?
 
dICKj said:
I decided to pull the dirty old two barrel off my 1974 318 and replace it with an Edlebrock Model 1405 600cfm 4bbl and a Weiand model 8007 dual plane manifold. I also added dual exhausts using the stock manifolds. The car ran really great with the two barrel. Now whenever I try to floor it, the motor just moans and the car actually slows. Any ideas?

Click to expand...
The pieces you added are great additions to your 318. I would guess something not right with the carb. Was the carb bought new or used?
 
I've done this deal also, with a Holley 600 cfm vacuum secondary on a 70 Dart 318 that was stock except for headers and dual exhaust. I used a Weiand single plane intake when I put the 4 barrel carb on as well. I found that the 4 barrel required about the biggest accelerator pump shot available to get the engine to not stumble, but tuning that variable and when the secondaries came in made made the car a LOT faster than it was with the tiny 2 barrel it came with. I eventually went to 360 heads, a decent sized cam and a Holley 750, which again made a big jump in performance at the cost of some drivability tradeoffs. Your carb isn't too big, but it does need to be optimized for your car.
 
What about your kickdown linkage? Anytime I see a 2 bbl to 4 bbl swap and no mention of the throttle bracket or kickdown linkage, they usually need help. Do you know you need it?
 
I put an 850Thermoquad on my 318 and it ran gangbusters.
But I used a small-port intake.
My guess is that the Secondary Air door just needs adjusting. And or the ignition-timing is late.

Here's what I would try:
check the air filter housing, and
check the firing order, and
Make sure that the choke cannot come on with the engine warmed up. Tie it open if in doubt. Then;
set power-timing to 34 *at whatever rpm it takes to max this number out, Ima guessing 3600rpm, and
Defeat the secondaries, and finally
Roadtest her.
If it now works great, then we just have to fix the Secondaries.
But if it still has problems; then
I would recommend a proper compression test, and
You may want to check the New dual-exhaust to make sure it really is free-flowing.
After you get it working as a 2bbl again, THEN you re-connect the secondaries and roadtest it s'more.
Finally, make the choke work right.
IMO, That's a great induction system for this combo.
 
It is not the carb size. You have tuning problem. If you are unfamiliar with tuning, get some help.

Basics first:
- Loosen met rod piston covers & swivel slightly. Push down on pistons & make sure they move up/down freely.
- Check that the velocity valve in the secondaries moves freely when you flick them
- Make sure the choke is fully off [ blade is vertical ] with engine hot
- Make sure the linkage goes to WOT!!!!!!
- Remove the airhorn to check float level, should be 7/16". Unlike brand H garbage [ & clones ] np gaskets needed if you are careful
 
I'll say this-

every single Edelbrock carb I've ever owned, including dirty, $50 swap meet ones-

ran acceptably upon initial installation with no excessive tuning other than setting mixture and idle speed.

I believe I'm up to seven now including 500, 600 and 625 on 318's and 600 and 750 on a 440 (the 600 did lean surge but otherwise ran OK)

The worst thing I've encountered was a clogged accelerator squirt nozzle.
 
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