Carburetor question on old car...

[sasilverbullet]

When adjusting your idle mixture screws, when screwing in to where the "engine speed drops"...
1. Is it a pronounced "drop"...
2. About how many screws out from all the way in is yours set to?

On my Edelbrock 1406 on my 70 Mk III's 460 they don't seem to "drop" the rpm until i have them screwed all the way to stop.
3. Anyone know what my problem might be?

 

[FL_Rob]

You need to bring the throttle up to around 2k rpm and listen carefully.The 460 is big and idle rpm is low ..thats why you can't hear the change.It's really a matter of feel than anything else.hang on ..there's a knock at the door.....nevermind it's just the ricers' giving me heat.

 

[The_Jeff]

Sounds like gas (air?) is getting in another way. Choke sticking? Non-stock jets?

Can you get a tailpipe analyzer? They're pretty cheap on ebay since noone does this anymore.

 

[OVERKILL]

Use a vacuum gauge. Adjust for the best vacuum reading.

 

[mpvue]

Originally Posted By: OVERK1LL
Use a vacuum gauge. Adjust for the best vacuum reading.

yes. it's been a long time, but that's how I always adjusted them. on a holley I had, I would turn the screws in until it stumbled, then back them out to get the highest vacuum.

 

[UNC017]

Originally Posted By: sasilverbullet
When adjusting your idle mixture screws, when screwing in to where the "engine speed drops"...
1. Is it a pronounced "drop"...
2. About how many screws out from all the way in is yours set to?

On my Edelbrock 1406 on my 70 Mk III's 460 they don't seem to "drop" the rpm until i have them screwed all the way to stop.
3. Anyone know what my problem might be?


Its a gradual drop, and more gradual if the intake manifold is either a single plenum design or you have a carb spacer that lets the plenums "share" air. If its a fully divided plenum, you can hear the engine start to "4-cylinder" as you take one of the idle screws offline.

When tuning carbs, I find it very helpful to have a clip-on dwell/tach (one of those tools not much in demand these days!) under the hood with me so I can see a drop of 50 RPM or even less, which can be hard to hear.

Most Edelbrock (Carter) carbs I've worked with seem to wind up with the idle screws somewhere between 2 and 4 turns back off the stops, but that's just a rough guideline. As someone else said, the 460 is a pretty big mutha, and in order to get it to idle it requires the idle speed (not mixture!) screw turned a lot further in than a 302 would, so the effect of the idle mixture screws can be masked by flow through the mains. When I get really careful with the mixture on my 440s, I tend to turn the idle speed down lower than normal so that I'm really and truly just on the idle circuits. Then I set the idle mixture, and THEN I set the idle speed back up to normal. Don't have to do that extra step on most smallblocks.

 

[Chris142]

Originally Posted By: sasilverbullet

2. About how many screws out from all the way in is yours set to?


3. Anyone know what my problem might be?


#2. Every carb and every engine will need the screws at a different place.

#3. Lose that Edelbrock piece of junk.

Yes I hate those carbs. They took a decient AFB and ruined it.

 

[ARCOgraphite]

Sounds like the throttle opening is too high. I would adapt a junkyard 900cfm rochester quadrabog off a BB caddy onto that 460. You running stock "too mild" cam in that? where did the motorcraft carb go?

 

[flanso]

Temporarily pull the crankcase vent hose off the front of the carb, plug the nipple and see if the adjustment on the idle screws becomes more sensitive.

 

[mechtech2]

First get your idle set - it may be too high.
Ignition timing must be OK and the distributor advance working properly.
About 1- 1/2 turns out is normal.
But whatever works!
The highest idle is NOT the correct way. Set for the highest Idle, then go for a 25 RPM drop by going leaner. You may not have a lot of room - your car is exceptional .

 

[onion]

Originally Posted By: ARCOgraphite
Sounds like the throttle opening is too high. I would adapt a junkyard 900cfm rochester quadrabog off a BB caddy onto that 460. You running stock "too mild" cam in that? where did the motorcraft carb go?


I did exactly that one time. Had a ford truck with a 460 several years ago. The carb was gonna cost too much to fix- and I had a perfectly good quadrajet off an Olds 403 sitting in the basement. Bought an adapter plate and made an adapter for the throttle lever. Even hooked up the heat-riser tubes for the exhaust heated choke (used compression fittings). It ran like that carburator was made for the truck.

 

[UNC017]

Originally Posted By: Chris142


#3. Lose that Edelbrock piece of junk.

Yes I hate those carbs. They took a decient AFB and ruined it.


Wow... tell us what you REALLY think ;-)

I've got a pile of vintage Carter AVS's, a couple AFB's, a couple of Thermoquads, and I've run a late 1980s vintage Carter "9000 Series" AFB, and an Edelbrock Thunder Series (AVS clone). I really can't see that Edelbrock "did" much at all to the AFB and AVS, other than save them from the dustbin of history. I particularly like the fact that they resurrected the AVS in their Thunder Series, since I always preferred it to the AFB anyway. Carter themselves put that non-effective counter-weighted secondary air valve below the venturis in the AFB when they brought it back in the 80s as the 9000 series, that wasn't an Edelbrock thing.

But then I must be a masochist because I really think the Thermoquad is the niftiest carburetor ever made. Quirky as all heck, but when its dialed in and nothing's broken, it'll give 3-4 more mpg than most any other carb on the same engine, and still have all the power when those jet engine sized secondaries open. A Quadrajet is also a very good and underrated carb that's very similar to the TQ (actually the QJ came first and the TQ copied its features), but the TQ tops it in "coolness factor" with the insulating phenolic body to keep the fuel cool.

 

[punisher]

Ditto on the thottle opening being too large at idle.

There is an "idle transfer slot" on most carbs that is there to enrich off idle response before the accel pump kicks in. If the throttle opening is too large, the engine pulls fuel through this slot and essentially makes your idle needles next to useless. More of a problem on large displacement engines as they require more air, hence throttle opening, at idle. Our solution was to drill a small hole in each primary throttle plate to let more air bypass.

Be sure the basics are covered. Engine timing makes a BIG difference in idle RPM.

 

[Chris142]

Originally Posted By: 440Magnum
Originally Posted By: Chris142


#3. Lose that Edelbrock piece of junk.

Yes I hate those carbs. They took a decient AFB and ruined it.


Wow... tell us what you REALLY think ;-)

I've got a pile of vintage Carter AVS's, a couple AFB's, a couple of Thermoquads, and I've run a late 1980s vintage Carter "9000 Series" AFB, and an Edelbrock Thunder Series (AVS clone). I really can't see that Edelbrock "did" much at all to the AFB and AVS, other than save them from the dustbin of history. I particularly like the fact that they resurrected the AVS in their Thunder Series, since I always preferred it to the AFB anyway. Carter themselves put that non-effective counter-weighted secondary air valve below the venturis in the AFB when they brought it back in the 80s as the 9000 series, that wasn't an Edelbrock thing.

But then I must be a masochist because I really think the Thermoquad is the niftiest carburetor ever made. Quirky as all heck, but when its dialed in and nothing's broken, it'll give 3-4 more mpg than most any other carb on the same engine, and still have all the power when those jet engine sized secondaries open. A Quadrajet is also a very good and underrated carb that's very similar to the TQ (actually the QJ came first and the TQ copied its features), but the TQ tops it in "coolness factor" with the insulating phenolic body to keep the fuel cool.


I've had 5 or 6 of those Edelbrock AFB copies in the past and had no luck with them. Tried them on everything from a 302 Chevy to a 392 IHC.

The last 2 were nothing but trouble. Spent 3 hrs on a Chassie dyno trying to get the jetting correct. No luck after that long. Changed Jets, metering rods, springs etc.

Couldn't get it to work. Took it back and got a 2nd. This one spent 5 hrs on the dyno. Same thing.

Found a used baseplate for my broken old Holley, put the Holley on with stock jetting and immediatly got 60hp and 90ft lbs of TQ over the Edelbrock.

 

[demarpaint]

Originally Posted By: mechtech2
First get your idle set - it may be too high.
Ignition timing must be OK and the distributor advance working properly.
About 1- 1/2 turns out is normal.
But whatever works!
The highest idle is NOT the correct way. Set for the highest Idle, then go for a 25 RPM drop by going leaner. You may not have a lot of room - your car is exceptional .


+1 Also make sure the engine is fully warmed up and at operating temperature. I'd make sure it was driven at least 30 minutes before messing with the carb. Make sure the timing is right and there are no vacuum leaks as well.

 

[The_Eric]

Originally Posted By: punisher
Ditto on the thottle opening being too large at idle.

There is an "idle transfer slot" on most carbs that is there to enrich off idle response before the accel pump kicks in. If the throttle opening is too large, the engine pulls fuel through this slot and essentially makes your idle needles next to useless. More of a problem on large displacement engines as they require more air, hence throttle opening, at idle. Our solution was to drill a small hole in each primary throttle plate to let more air bypass.

Be sure the basics are covered. Engine timing makes a BIG difference in idle RPM.


Excellent point. I believe the general rule of thumb is to expose (lengthwise) no more than the width of the slot.

As for timing, I agree with that and will add that it affects idle quality as well.

 

[SS1970chrysler]

I've used small pliers to make a small outward vent on the throttle plates. It's a lot easier to adjust or undo if you need to later.

I set mixtures with car fully warmed up. Remove air cleaner and plug vacuum lines, same with distributor advance hose. I set a quarter under the idle stop and turn screws outward(counterclockwise) until highest idle is reached, then I back off(in) 1/4 turn.

 

[mechtech2]

Originally Posted By: SS1970chrysler
I've used small pliers to make a small outward vent on the throttle plates. It's a lot easier to adjust or undo if you need to later.

I set mixtures with car fully warmed up. Remove air cleaner and plug vacuum lines, same with distributor advance hose. I set a quarter under the idle stop and turn screws outward(counterclockwise) until highest idle is reached, then I back off(in) 1/4 turn.


Lots of bad advice here.
Bend the throttle plates? Are you kidding?
Plug the necessary vacuum advance? Wrong.
Put a quarter in the throttle so you are artificially high and exposing the transfer slots? Silly.

 

[SS1970chrysler]

Better than drilling holes in throttle plates as in the post above mine. Smog era big block carbs respond a bit better with a few unconventional approaches.

 

[RH+G]

If im reading this corectly u have a 1970 460 Big Block FOrd motor and u put 1406 which is a 600 cfm tuned for fuel economy on it.

Right their tells me u will never get that motor to run at its best with that carb.

I would recomend a 750 or 800 eddy. I personally like eddy over holley. QJets are the best to me but u good quality replacement parts are hard to come by.