I've run a 600cfm and 770cfm on my 351c, and both were similar in performance, after fine tuning that is.

I'm wondering if your 600cfm has a small blockage somewhere, or if it needs an overhaul.

Maybe thoroughly clean the 750cfm before you install it, and then checkout(overhaul?) the 600cfm once it's removed.

I was going to ask if your air filter is clean, but LOL, I won't as I'm sure it is.

Good luck for this weekend when you do the carb change !

 

My air filter is brand new as is the 600cfm carb. Just bought em a month ago. The 750 I have gone through a few times so it's nice and clean for the install. Funny thing about this motor in general is that no tater which carb is on it, to get her to idle steady I have to have the passenger side idle screw turned way out arundel 3-4 turns and the drivers side turned out only about 2.

 

On top of the carb(Edel) and in front there is a nozzle/air passage to each air screws.Every soon often I have to take the screws all the way out, spray some kind of carb cleaner in the screws holes and at the top nozzle/air passage.That clears a lot up.

 

I'm starting to feel YOUR headache !! LOL

In the absence of the compression readings, two things come to mind, but I could be way off.

If you have an EGR system installed, is it working 100% ?
(I'm grasping at straws with this one.)

Hydraulic lifters, when adjusted too tight, can cause a less than smooth idle.

You might have to check the valve train.

Assuming the new carb is 100% trouble free, 2 turns versus 4 turns is just wrong. (But you knew that! LOL)
I'm wondering if that compensates for lifters being too tight. (Second opinions needed.)

I assume your intake manifold is a 4 barrel to match the 4 barrel carbs.

 

Thinkin' about it some more, I get the impression that changing the carbs this weekend won't solve the problems.

Read Fasthauler's comments in post 45 on page 3.

I'd make that my next move.

A worn cam lobe or lifter issue will show up very quickly.

Assuming it's a stock cam, all you need is the exh and int lift figures as a benchmark.

Now, where's those headache tablets when ya need 'em ?

 

Currently there is no EGR system on this truck. PO must have removed it, as I don't see any sign that there ever was one!

This trucks idle has always been odd. Some days it's smooth. Other days it's shaking the steering wheel all around and shuddering while idling (mainly in drive). Idle is usually rhythmic, meaning it idles smooth for a bit then hiccups, then continues to idle smooth till it hiccups again. Certainly sounds localized to a single cylinder. When I had the intake off do do the intake gaskets, I saw the had have a bunch of heavy carbon build up on the valves. I recently had the valve covers off the do their gaskets as well, however I saw so no sluge or grime on the top side of the heads. Frankly, the valve covers looked new inside! My dad (40yr ASE mechanic) and I are under the impression the slight shimmy felt occasionally at idle is due to to coke up heads.

The part about having to have my foot stuffed way into it on the highway though is my reason for wanting to swap to the other carb as a simple fact check. I have to head out the same way I went wednesday, and figured I could toss it on to check what my mpgs and throttle response would be with both carbs. With the 1406, I burned about 5 gallons in 40 miles out on the highway. Equates out to 9mpg. Foot was halfway to the floor on that drive, and I'm assuming I ended up having the secondaries cracked open the whole time.

At this time I don't have a dial indicator to check the valve train with. When I had the valve covers off I cranked the motor by hand just to verify everybody was moving (and everything was) and check for anything out of the ordinary. I'm gonna be quote a bit sad if it turns out to be a dying cam, as I just had both the intake AND the entire front off the engine within the last month and a half. If I get the time this weekend (c'mon we don't want the girlfriend getting jealous of this thing attention wise), I will try to do a compression test just to see how she looks. Double and triple checking the running voltage at the coil is another quick easy test to perform as well.

Wish I could be getting paid to fix my own damn truck instead of all these heavy duty box trucks for Snap-On! Hell it's like working a second full time job to try and get my truck back to where it should be haha.

 

​​​​​​Excellent point about the air bleeds. I will be sure to clean them on both my carbs. I did pull the mixture screws the other day though and shot em full of brake clean. This had no effect on how many turns out they are.

When I perform the carb swap this weekend I will get a definite number of turns for each screw. That being said, I would still hazard a guess the passenger side will be out more than the driver a bit.

If I set my idle mixture in PARK, I get 17" of steady vacuum at 800rpms. both screws turned out about 2.5 turns. Runs rather smooth, only an ocassional bump. To cure the bump, dad suggested setting the mixture while in DRIVE. So I had my helper hop in and toss her into drI've while I tuned her. While in drive, she pulls about 15" steady of vacuum at about 600rpms. Lots of shimmy all over the place, until I turned the passenger side mixture screw OUT another 2ish turns (forget exact number), and turn turn he drivers side screw out another half turn. That got my shimmy down considerably. It's still there sometimes, but not nearly as bad. Glad it ain't just me thinking thats kinda strange.

Only other weird thing I have to note is that when I have pulled all my plugs in the past, I have noticed the 2 in the middle on both sie of the engine are always a couple steps leaner than the outer two on both sides. Not too lean that the plugs are white and burnt, but definitely a noticeable color change between say the #1 plug and the #2 plug. 1 can be any color from gray to tan, 2 is usually gray/white. Doesn't matter which carb I try or how this motor is set up, my plugs always have looked this way.

 

Intake manifold vacuum leak?

 

You having the problem with the odd adjustments of the air bleed screws just woke me up. I had a problem with mine a while back. When I had to slam on the brakes hard the truck started running like crap. I had to get out and turn the air bleeds to even get it to idle. It still ran bad and stunk to high heaven but I was able to get it home. It turned out the rubber tips on the float needles were deteriorating from the ethanol and if I stopped hard one of the needles would stick and the float bowl wouldn’t get enough gas. It happed a couple of times and I went nuts because I would pull the carb apart looking for dirt or anything plugging it up and couldn’t find anything and as soon as I put the darn thing back together it would run fine as soon as I readjusted the air bleeds back to normal. I replaced the needles and it cured the problem. I know its far fetched since you have two different problems but could the float levels be off in both carbs?

Also the brake booster vacuum only comes from number 4 and 7 cylinders not the entire engine. I had a vacuum booster that was leaking air so bad that I warped the intake valves on these two cylinders. The machine shop guys were the ones that let me in on this. You might plug the line going to the brake booster and see if the air bleed screw adjustment changes. If they do then you may be heading for warped valves like I did.

 

I have double and triple checked the float levels in both carbs to both edelbrock specs. When I bought the truck the booster was on its way out the door (slight hiswing then turned much worse after a few days of driving) so the booster is brand new too. I've sprayed brake clean on everything including the intake and have found no leaks at all. Even tried unplugging and capping everything vacuum operated and that didn't fix it either. Vacuum gauge is rock solid while the truck is running.

 

Well my friend I am about out of ideas here. I have been going through 460’s for 40 years and I am puzzled. I have seen, as I mentioned earlier, engines coming through the shop with flat cams that were really dogs until the cam was replaced but never seen that in a 460. And a flat cam doesn’t cause the bogging you are describing. The only other thing I can suggest is going to a bone yard and grabbing a complete distributor and replacing the existing one and see if the trouble goes away. You mention the vacuum is steady. That is usually the sign of a healthy engine. Is it excessively high or low? If its excessively high it could mean a flat cam.

 

It's right in at 17".

I have the opportunity to purchase a running 1968 429 that's complete carb to pan for $500. Considering purchasing it and transferring my new water pump, timing cover, and timing chain into it and stuffing it in the truck, but I'm afraid of having 11:1 compression afraid trying to run 87 octane gas because I can't afford constant fill ups with premium. Not sure if it would be worth the hassle of swapping from my current 460 down to a 429, even though the 429 has substantially more power. What would you guys do? It's winter in wisconsin, I dont have a huge budget, and the motor being offered hasn't been run in 5 years but turns over fine and the guy is willing to check the main bearings and spec it over (without actually firing it up) before purchase. Or I could keep whacking everyone's heads together over this smogger 460. Thoughts and opinions?

 

Another dizzy from the bone yard would be a cheaper option. If that doesn't work then that is another option. the 429 runs pretty strong just doesn't have as much torque do to the shorter stroke. But not enough to really notice unless towing strong.

 

At this point the sag is mostly gone. Motor revs instantly only reason she isn't moving or burning tires is due to the 3.25 rear end I'm assuming. The 429 seems like a good option, but a lot of work and money for something that will only run premium gas.

 

Agreed!!!!