360 with Carter AFB, tuning Qs

[1974DartSwinger]

Hey guys, I'm having a blast with my new ride! Had my '66 Barracuda for almost 3 weeks now and put an easy 700 miles on her! haha, love it!

A little background: from what I know its a stock 360 (1974) block/heads with original '66 Carter #4120 273 hi-po carb and intake. Stock distributor (points), duel 2 1/2 exhaust with turbo mufflers and factory manifolds.

I pulled the carb the other day to give it a clean up and rebuild, checked the floats and sprayed all the passages out. The choke pull off was broke so it got a new one and the linkages are all good from what I can tell. I tried setting the mixture screws with a vacuume gauge hooked to the carb vac line for the choke pull off once the engine was warmed up. At 800rpm I couldn't get the vacuume to change really at all. I cranked the idle up to 1,500 and it was at about 15 psi vac I think. Still couldnt get it to change with the mixture screws. I could hear the RPMs change a tiny bit so set it at the highest revs and called it good. I noticed while driving at a steady speed the engine would surge slightly. You could feel, very slightly, the power come up / down. I re-set the mixture screws back to 2 1/2 turns out and it got a bit better but there is still a slight engine surge while keeping the gas steady.

I'm not really sure what to make of it , what it means carburator wise or if it could be something else like timing which I havent checked yet. It never did this before I redid the carb. I'm thinking about getting a good quality tach for tuning under the hood, maybe that will be better than a vacuume gauge.

On a smilar note, the thought crossed my mind that most likely the jets in this carb are factory 273 jets. It runs great, has gobs of smooth power from idle to about 1/2 throttle but I did notice above 1/2 throttle there is a flat spot where the power seems to drop off or level out. Maybe I need to fine-tune the metering rods or step the main jets up a bit. The primarys and secondarys are opening all the way and the float height and drop is fine.

In the near future I'll be updating to an electronic distributor. Any of them better than others? Just go with Mopar performance or Mancini's ? Trying to fine-tune everything and make this puppy run her best.

 

[65Val]

The surge is probably a lean-surge, if the jets and rods are still stock 273. That 273 intake also has pretty small runners for a 360, so that can contribute to a surge too. I would go to a 360 port-size intake(to match the heads) and a slightly larger carb...600cfm anyway. Stock 273 AFB will be 450-500 cfm.

Check initial dizzy timing and make sure the vacuum advance can on the dizzy is hooked up and functioning. Inadequate or no vac advance will cause a surge.

If this just started after the carb was pulled, check for vacuum leaks at the carb base and that all vacuum ports are hooked up correctly or plugged off.

When adjusting your idle mixture screws, get the engine to idle as low as possible...300-400rpm in neutral, then adjust mixture. At too high an idle, the mixture screws become ineffective.

 

[Muad'Dib15]

I'm having almost the exact same problem. Same engine, same carb, same surge. I talked to some one from my dad's speed shop and he said that it was air/fuel mixture. However after adjusting those two screws at the bottom, it runs much better how ever it still surges a little and still doesn't like being stomped on at low speeds. Could someone give me a little walk through on how to tune the carb? I"m a complete noob at this.

Maybe unrelated, but after I did this I can only start the car by cranking it then going WOT until it fires up.

 

[65Val]

Those 2 screws only adjust the idle mixture,,,anything above idle will not be affected. You have to change rods or jets to affect cruise mixture.

Re: the hard starting...
1: Check your accelerator pump for a good shot. No/little shot will cause the bog and hard starting conditions

...or...

2: Do you get a cloud of black smoke after it finally starts? If so,check that the choke isn't coming on too far before you start it. If it is, it will choke the air off to the engine.

 

[rumblefish360]

Muad; To tune a carb to the engine is not a quick write up in anyway shape or form.

 

[moper]

Disconnect the vacuum advance and plug the line to the carb. See if the surge goes away. If it does, use a 3/32" allen wrench (IIRC thats the size) inserted into the nipple on the distributor's vacuum can to adjust the preload on the vacuum advance. If it is surging, insert the wrench until it seats in the screw inside there and turn it counterclockwise 2 turns. Then replace the vacuum hose to the carb and drive it again. If it still surges, go another 2 turns. Eventually you run out of adjustment and the screw inside just turns and you feel the snap as the threads pass each other. But you should be able to tune out the surge. You should get a strip kit for that carb and richen it slightly.