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Trying to figure out proper carb tuning
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Author:  Jopapa [ Wed May 02, 2007 1:48 pm ]
Post subject:  Trying to figure out proper carb tuning
Right now my Duster has a 1945 on it, and since it's running well and I'm hoping to acquire a Super Six package before long, I'm planning on leaving it there. My question is that for properly tuning the idle mixture, I want to use a tach, enrichen the mixture (turning screw CCW, currect?) until the engine starts to stumble, then lean it out to get the highest RPM, adjust the idle speed screw to the proper setting (700, correct?), go through the mixture adjustment process again, then readjust the idle speed screw again if necessary. Is this the correct procedure? I've also heard of adjusting the mixture while using a vacuum gauge and setting it for the highest amount of vacuum, but I'm of the impression that the tach and idle adjust method would yield better results.

Thoughts, tips, advice? I still consider myself a newbie to carburetion.
Author:  SlantSixDan [ Wed May 02, 2007 1:58 pm ]
Post subject: 
Tach-and-idle is a good way to do it. Aside from idle screw location this post contains procedural notes.
Author:  Aggressive Ted [ Wed May 02, 2007 2:19 pm ]
Post subject: 
If you have much of a cam, I like to put it in gear and block the wheels.
Make sure the engine is real hot!
With the slight load you can get the mixture dead on at 750 rpm or what ever works for your cam.

You can also test to see if your vacuum advance is screwed in to far and coming on to soon. That will throw off the idle alot. Just unplug the distributor line and see if the timing changes. If it is adjusted correctly there should be no change. If the timing jumps up a few degrees when you plug it back in, then the advance is coming on to soon. Back it off a turn until there is no change. You need a 3/32 allen wrench to stick in the distributor canister. I just went through this when I replaced my vacuum canister (the old new one was stuck) and it made the idle go whack no matter how much you adjust the mixture screw! Mine was 2 and half turns off. Now it idles perfectly.
Author:  SlantSixDan [ Wed May 02, 2007 2:22 pm ]
Post subject: 
Ted's suggestion is a good one, to make sure there's no significant vacuum advance at idle.

With a stock or near-stock cam, the idle mixture and speed get set in Park or Neutral (not in gear), with high beam headlamps on.
Author:  Jopapa [ Wed May 02, 2007 2:25 pm ]
Post subject: 
Quote:
If you have much of a cam, I like to put it in gear and block the wheels.
Make sure the engine is real hot!
With the slight load you can get the mixture dead on at 750 rpm or what ever works for your cam.

You can also test to see if your vacuum advance is screwed in to far and coming on to soon. That will throw off the idle alot. Just unplug the distributor line and see if the timing changes. If it is adjusted correctly there should be no change. If the timing jumps up a few degrees when you plug it back in, then the advance is coming on to soon. Back it off a turn until there is no change. You need a 3/32 allen wrench to stick in the distributor canister. I just went through this when I replaced my vacuum canister (the old new one was stuck) and it made the idle go whack no matter how much you adjust the mixture screw! Mine was 2 and half turns off. Now it idles perfectly.
Stock cam, but I've never heard of doing that to simulate a load. Seems like a pretty obvious way to go now that it's explained to me. Will the load presented at 750 RPM in gear be the same as (or at least pretty close to) the load at WOT when getting on the highway? Also a good idea with the vacuum advance. I'll be sure to check that tonight if I get around to messing with the timing.

Edit: Just read Dan's second post. That actually makes sense, but why exactly would I need the high beams on?
Author:  Bren67Cuda904 [ Wed May 02, 2007 3:04 pm ]
Post subject: 
Load on alternator
Author:  Jopapa [ Wed May 02, 2007 3:06 pm ]
Post subject: 
Quote:
Load on alternator
Will that actually put much of a load on the engine?
Author:  Bren67Cuda904 [ Wed May 02, 2007 3:10 pm ]
Post subject: 
I am just guessing but I don't think its so you can see what your doing better. :D
Author:  SlantSixDan [ Wed May 02, 2007 4:25 pm ]
Post subject: 
Quote:
Quote:
Load on alternator
Will that actually put much of a load on the engine?
About 0.2 horsepower -- enough to affect idle speed.

As for your question about WOT and load and such: Irrelevant. You're setting the idle mixture and speed here, which has almost nothing to do with the mixture under load or at speed, and this particular adjustment you're doing is done in Park or Neutral.
Author:  Jopapa [ Wed May 02, 2007 8:45 pm ]
Post subject: 
Well my vacuum advance was good. At idle with and without the vacuum line attached the timing was the same. I found out why it would backfire on deceleration and still wouldn't idle very smoothly: IT WAS SET AT TDC! So I set it to 10 degrees initial, tuned the carb (correctly this time) and set the curb idle to 700 RPM. I thought it was running well before this, but now it's fan-freakin-tastic. Even driving up a long steep hill, I can floor it and still feel it accelerate. I LOVE IT!
Author:  SlantSixDan [ Wed May 02, 2007 9:00 pm ]
Post subject: 
Good work getting it all dialled in.

TDC might've been spec for your car's year. Keep an ear open for pinging; if you hear it, back off on the initial timing a little.

You can do a quick 'n' easy check if the vacuum advance is doing its job by pulling the hose off the carb with the engine idling and applying vacuum (as in, suck on the hose). If you can suck air, the hose and/or advance need replacing. If you can't, and the idle speeds up when you apply vacuum, it's all good.
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