Carb Fine Tuning

[1strodeo]

I have an Edelbrock 1406 on my 302, just rebuilt it, got the timing set, got the idle screws dialed in pretty well. I had lost the fast idle linkage during rebuild, bought a replacement and since I put it back on, I've been having the following issue;

It drives great, accelerates great, but when I'm at very light throttle, like as little pressure as I can physically put on the pedal, it skips and kinda lurches, you know like when you're running out of gas. Can anyone tell me the most likely cause?

I also have an Edelbrock calibration kit, but right off the bat I'm confused on how to use it. It says normal (full) vacuum should be 14-22 mm/Hg, but mine is reading 29 mm/Hg. I read that high readings could be from clogged air filter, but i removed the air cleaner housing and no change. Just throwing that info out there. Also increasing RPM does not change the vacuum reading...if anything it goes UP a tad??

 

[SHX669]

Are you sure your vacuum Gage is accurate and reading correctly ?

 

[1strodeo]

well it starts out at 14.5, is that right? it's brand new Lisle gauge

 

[SHX669]

So the gauge only goes down to 14.5 - mine reads from 0-30 that's inches of vacuum-- {Hg } Hmm , been awhile since I've been in a Science class but seems i remember 29 hg or so is just about a " perfect vacuum " a little hard to achieve . Might want to check your gauge for calibration or what " units " it's reading.

 

[B RON CO]

Hey, your vacuum gauge should be steady @ 14 - 19 " (inches) of vacuum. I don't know what HG/mm is? Metric?.
When you wing the throttle the vacuum should drop like a rock. Vacuum is only steady when RPM is steady.
You can try the accelerator pump on a different hole on the lever and see if that helps.
I'm not home so I can't look at my Edelbrock book to see if there is a check ball for the accelerator pump.if you are missing a check ball, or maybe something else is keeping the accelerator pump well from filling that could cause your issue.
Good luck

 

[1strodeo]

yes i should have said inches Hg sorry

I think the gauge is defective, it does go 0-30 but when disconnected it is at 14 in Hg instead of Zero.

I will try a different hole in the accelerator bracket thanks.

 

[1strodeo]

and yes there is a check ball and weight under the main discharge jets, which were replaced during rebuild

 

[B RON CO]

Hey, you could retard the timing a tiny bit and see carb if it it helps. Good luck

 

[DirtDonk]

See if the outer dial ring of the gauge itself is adjustable. Maybe it can be "re-zeroed" this way like some altimeters and such. Long shot, but possible.

Like said, mess with the timing a bit too. Especially because it's just easier to do. It might still turn out to be jetting, but the timing could be too much when it's maxed out with both vacuum and centrifugal as well.
If it's jetting, it's sounding too lean.

What is the plug gap?
What type of ignition?
What is your initial timing?

Paul

 

[1strodeo]

outer ring of gauge was not moveable, tried that sent it back to Summit

changing position of accelerator linkage made no difference either.

It's an HEI distributor, plug gaps are at .050", plugs and wires are new, initial timing is 'about' 10 degrees BTDC... I say 'about' because I cannot read the degree marks, although I did mark 0 degrees at some point and it's about an inch above the pointer at idle. Compared to another balancer I have, that's about 10 degrees. When I rev the RPMs it goes to almost true North which I think is excessive.

 

[jckkys]

On very light accelerations you're getting fuel from the idle circuit and transfer slot. Since your idle is good, there must be something going on with the transfer slot. A piece of dirt maybe? Take the carb off to look to see.

 

[B RON CO]

Hey, the timing mark should go up under the water pump when you give it gas. That is the mechanical advance.
I would try opening the idle mixture screws like 1/2 turn each and see what happens.
Everything I suggest is easy to return to the way it was.
Good luck

 

[blubuckaroo]

Good that you've got the Edelbrock tuning kit for your carb. My biggest tuning improvements have come from changing the metering rod pull-off springs. They're very easy to change, and can make all the difference. Mine needed smaller jets too, but now it's clean enough that my clothes don't smell like gas anymore.
Edelbrock's owners manual and their phone support have been important to getting mine cleaned up.

 

[Skytrooper15]

So the gauge only goes down to 14.5 - mine reads from 0-30 that's inches of vacuum-- {Hg } Hmm , been awhile since I've been in a Science class but seems i remember 29 hg or so is just about a " perfect vacuum " a little hard to achieve . Might want to check your gauge for calibration or what " units " it's reading.

A vacuum gauge reads from 0 inches hg to 29.92 inches hg with 29.92 being absolute vacuum which you will find out in space but is not attainable with anything we have.HG is the chemical symbol for mercury.Usually everybody I've worked with just call whatever the vacuum reading inches and forget about HG.The last time I had a vacuum gauge on a 1405 Edelbrock and had it running right I was getting a reading of 16 inches vacuum.

 

[jckkys]

If the throttle is barely opened and held there, the idle circuit that includes the transfer slot is the only part of the carb that's relevant. The main and power circuits have no influence until the throttle is opened further. The accelerator pump isn't a factor either, unless the throttle is moving. Turning the idle mixture screws out (counter clockwise) to enrich-en things a little can cover a lean condition until the transfer slot contributes enough fuel to the mixture. There are carb makers that under cut the edge of the throttle plates to exposing more of the transfer slot to correct this problem. I wouldn't recommend you do that as an owner. You could call Edelbrock's tech line to see what they say. Study page 3 of the Edelbrock Owner's Manuel. It has a great illustration of the metering systems.
On the vacuum issue, remember the weather forecast of barometric pressure is also measured in inches of mercury so 29.92 in.Hg is all there is to create a vacuum.

 

[DirtDonk]

You might even close the plug gap the next time you have the plugs out.
Even though GM used .050 to even .060 inch, I think I remember some here saying that cutting that down to between .045 and .048 inch helped how theirs ran.
It's a combustion thing, so different cylinder heads/combustion chambers can act and react differently.

Paul

 

[1strodeo]

really appreciate all the tips and advice, I did suspect the idle circuit had something to do with it as I just BARELY open the throttle. I'll try the screw adjustment first, then pull the carb and check the transfer slot if no change

blu do you think Edelbrock tech support can help me identify the spring colors?? I'm always asking my wife or kids for help, especially with wiring %) The only springs I changed so far was to swap in the new versions of the old ones. From my understanding they are only for mid-throttle stumbling etc which I don't have a problem with. The main reason i bought the kit was for the jets because I was getting the gurgling and popping on deceleration, while in gear. Throw it in neutral when decelerating and it was fine. After rebuilding the carb however, about 95% of that problem went away.

Here's the gauge not hooked to anything....for some reason i was thinking "oh yeah the atmospheric pressure is 14 blah blah" yeah NO it's supposed to read Zero lol

 

[B RON CO]

Hi, yes the needle should be near 0. You read the outside scale on the left for vacuum and on the right is fuel pressure for old fashioned mechanical fuel pumps. Not high pressure fuel injection pumps.
You can blow and suck on the hose to see if the needle moves at all. If the needle is free when you suck on it there may be an adjustment screw on the back which will put the needle to zero. The gauge is defective unless the needle is freed up and goes to zero.
And, as mentioned, close the plug gap to .045. It might not help now, but it is better.
Good luck

 

[jckkys]

Your vacuum gauge is past tense. A new one should only cost you about 15 bucks. So get one. You said your Bronco drives and accelerates great. That rules out problems with the ignition. So don't worry about things like plug gap. It also rules out jets, metering rods, springs, etc. A large vacuum leak that's compensated for with the mixture screws while idling, may make the slightly off idle mixture too lean. There are plenty threads dealing with finding and fixing vacuum leaks.

 

[Skytrooper15]

I have an Edelbrock 1406 on my 302, just rebuilt it, got the timing set, got the idle screws dialed in pretty well. I had lost the fast idle linkage during rebuild, bought a replacement and since I put it back on, I've been having the following issue;

It drives great, accelerates great, but when I'm at very light throttle, like as little pressure as I can physically put on the pedal, it skips and kinda lurches, you know like when you're running out of gas. Can anyone tell me the most likely cause?

I also have an Edelbrock calibration kit, but right off the bat I'm confused on how to use it. It says normal (full) vacuum should be 14-22 mm/Hg, but mine is reading 29 mm/Hg. I read that high readings could be from clogged air filter, but i removed the air cleaner housing and no change. Just throwing that info out there. Also increasing RPM does not change the vacuum reading...if anything it goes UP a tad??

DirtDonk may be pushing the right button on the spark plug gap.Is your 72 model points ignition or did you change to stock electronic ford ignition.I had a problem a few weeks back where I had a pretty good stumble when you gave the engine a little gas.I finally pulled a spark plug and it was set at .044.I had changed the ignition back to points from duraspark 1 so I set the plugs on .034 like the book says to for a 1972 Bronco and all of my issues went away.So it would be worth a shot and can't hurt anything to check those plugs.To the guy above my comment what is a vacuum gauge that is past tense?