'76 Newport 400-2 Off-Idle Stall

Austin Barnett

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I've got this wonderful issue that has been present ever since I purchased the car 2 years ago, and I'm getting quite sick of it. Holley 7366 Smogerator. Car will fall flat on it's face when you give it anywhere from 10%-80% throttle coming from a stop, and regularly stalls when I go to leave any stop sign/intersection. And it will also act funky when giving part throttle trying to accelerate at all speeds. Give her the beans, and she'll scream away all the way to 50mph thanks to the wonderful 2.76 rear end. But part throttle? Wants nothing to do with it unless you're already cruising. We rebuilt the carb after purchase, and have been through it an additional 4 times now, trying to figure out what the problem is. Accel pump is shooting damn good adjusted to factory specs. Float level has been set. New fuel pump. Ignition module was replaced so I could drive the car home when I bought it. Cold, hot, dry, wet, it doesn't matter, it's never happy trying to accelerate on part throttle. The power valve piston/cylinder looks good, not too loose, not binding up. All the ports look good, we cleaned out every passage thoroughly. Can't figure it out for the life of me. Got great vacuum at idle, 26in. Timing is set to factory spec. If I'm a moron please let me know, with every day I aspire to become just that much less stupid! I'm at the point I'll swap a 4bbl carb/intake if I can't get this figured out, but I know if I get to the point of putting those in, a new cam/lifters would be so easy! I try to avoid the project creep when possible haha.
 

CBODY67

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Just for general information, when you pulled the air horn off the throttle body of the carb, how much of a compressed area/indentation was there in that gasket at the rear of the float bowl? No imprint would indicate a warped air horn, which would also result in the power piston keeping the power valve deployed all of the time, which also means "10mpg, no matter what". BUT, that situation would not cause the issues you mention.

It sounds to me like the transition ports in the throttle body (the vertical slots above the idle mixture screw holes) are not working, for some reason. There might be some hard deposits in those channels which normal carb cleaner mixtures will NOT remove.

The other issue with those slots is that when at hot, base idle, there is a particular relationship between them and the throttle plates. If the throttle plates are too-closed at base idle, or too much of the slot is uncovered at hot, base idle, sags and flat spots will happen. Seems like there should be no more than .040" of slot below the throttle plates at hot base idle?

Back in the 1950s, many FSMs advocated for a full disassembly of the carb, even removing each throttle plate from its shaft. Wondering if that might have happened before you got the car, for some reason? End result would be an incorrect re-assembled relationship between the transition slots and the throttle plates.

There are probably some fuel circuit illustrations in some of the FSMs at www.mymopar.com for the Holley 2245 2bbl. Doesn't need to be specifically for your model year, but probably since the first 2210-family carb in 1970. 383/400 or 360 would probably not matter, due to their similarities. Key thing is to see where the fuel for the transition ports feeds from. Then you might get a stiff wire and probe those passages, checking for obstructions by hard deposits. Using spray carb cleaner to check for flow can be deceiving, from my experiences, so "flow" can be good, but not tell the whole story.

In normal driving, the Holley 2bbl will provide good performance with the advantage of a 4bbl happening only above about 3000rpm or so, given that the Holley and most approx 600cfm squarebore 4bbl have the same size (1.56") throttle bores as the Holley 2bbl you have.

Enjoy!
CBODY67
 

70bigblockdodge

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I honestly would add 2 degrees of timing and see if it improves. Then add another 2 degrees and test. Add all the way up to 12-14 initial.
Waiting for your report back.
I honestly would not put another dime into that carburetor.
 

CBODY67

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I honestly would add 2 degrees of timing and see if it improves. Then add another 2 degrees and test. Add all the way up to 12-14 initial. Waiting for your report back. I honestly would not put another dime into that carburetor.

In one respect, I concur on not spending any more money on the existing carb. To troubleshoot/diagnose any unseen problems (especially if somebody else might do it), can take time and money.

I know there can be a temptation to just change everything out to a 4bbl (and other things mentioned), BUT that also might cause more issues than suspected, especially in adapting the throttle/kickdown linkages to the 4bbl format. Nothing that hasn't been needed to do in such things, but another complication into the mix, by observation. If the car was already a 4bbl, no big deal, just a few adapter brackets to bolt on and "done", plus any fuel line changes.

But in this case, it might be best to procure a quality rebuilt carb of the existing type/model. A simple replacement that bolts-on with new gaskets AND a warranty. If that carb fixes things, so much the better! But it might also be that there might still be a few issues, as sometimes, ignition issues can have the same or very similar issues as fuel issues. With all of those lean-mixture, part-throttle issues, some of those might be agravated by a weak spark from a weak coil (or related items in that circuit), as a lean mixture is harder to fire than a rich mixture.

Unfortunately, the price of a reman carb at RockAuto seems to have increased a bit from what I last recall, tending to make a 4bbl more of a viable altermative, just for the carb, not including the intake manifold and related items. In chasing "problems", it's always best to change ONE thing at a time (in this case, the carb) to see what works. Just as trying different base timing settings would be ONE thing.

When I was chasing a hard-deposit fuel circuit restriction issue on my '80 Newport BBD 1.25, I had very few carb options. I wanted to fix it myself, rather than start replacing things. After a good bit of research, finding out just where the "Low Speed Jet" was in the idle fuel feed tube, I got it fixed for the cost of a set of twist drills from a local hobby store. But I also realize that my solution might not "fit" for others, for which a reman carb might have been better.

Keep us posted on your progress,
Thanks,
CBODY67
 

CBODY67

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One of my customers called one day, wanting me to come by after work to check out a probvlem on one of hia customer's cars, a '77 or so Newport 400 with about 36K miles on it. Complain was "no power" off idle. I got in it, fired it up, sounded normal, but with doggy throttle response.

When trying to drive it, NO power off idle, no matter what. Even torqueing it against the brake made no difference, UNTIL the rpm got to about 2500 or so, then it took off like a rocket. Strange!

As it turned out, the owner's grandson had just put new plug wires on it. When my customer double-checked the firing order, he discovered that the grandson had swapped #5 and #7 wires. But to look at it casually, nothing seemed amiss. When he got the wires/firing order issue fixed, it laid rubber, as it should have.

As with a SBC, #5 and #7 fire one after the other. Perhaps that's one reason #7 wire is usually routed over the back of the valve cover rather than otherwise?

One other thing to check for!
CBODY67
 

Austin Barnett

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I'll go through the ignition order and bring up the timing after work today. Hopefully daylight savings time hasn't screwed me like it always does by leaving me no daylight after getting off work! Need to get the garage cleaned up a bit so I can fit the yacht in there for maintenance! Thanks for the suggestions guys.
 

commando1

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Blah, blah, blah, blah, blah....
Swap in an Edelbrock.
I won't spend an hour telling you...
1. 1,000 possible problems.
2. 500 solutions to each of the 1,000 possible problems.
You are looking for a solution, not an Engineering analysis of internal combustion engines.
 

rapidtrans

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Dad had a couple of cars in that era that had that stumble right out of the gate as brand new cars. Most never did get fixed by all the engineers and mechanics in Highland Park. In those days car companies slapped stuff on a car to meet that year’s emissions or cafe rules with very little development time. I do know it made for very tense left turns in traffic! I recall the slant sixes suffered from this the most.
 

Austin Barnett

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Checked the firing order, all is good there. I did see that the ignition module I replaced 2 years ago is only a 4 pin instead of a 5 pin, is that ok? Coil tested good.
Base timing was 12 degrees, w/ vacuum at idle 31 degrees.
I added 8 more to get 20 degrees base, 40 degrees w/ vacuum at idle. A quick drive around the block and it was feeling a fair bit better, definitely snappier! The stumble is still there, but not as severe and doesn't last nearly as long. It's always been finicky, sometimes it'll be damn near undriveable around town from the stalling, sometimes it's completely normal. I'll drive it some more this week and see if it stays good or not. Thanks for the suggestions guys.
 

CBODY67

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Is the vac advance hooked to manifold vacuum, rather than ported vacuum??? If so, that would mean that the throttle plates are closed too far from what they would be, to keep the idle rpm where it should be. End result, transition ports are too covered-up at base idle. How does it respond when it's on fast idle, relative to the hesitation?

CBODY67
 

CBODY67

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To a ported vacuum tap, with little to no vacuum at hot base idle?
 

Austin Barnett

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At the location the distributor gets its vacuum, it's got about 15in of vacuum at idle w/ advance disconnected. When reconnected, timing brings vac up to about 26in.
 

CBODY67

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That amount of vacuum at idle, basically manifold vacuum, is not the ported vacuum port with little to no vacuum at idle. Swap it to the correct ported vacuum port, readjust the idle speed screw and mixture screws as necessary to get the idle speed to specs. Yes, you'll probably need to open the throttle up for it to idle, from where it is now. Report back.
 

Austin Barnett

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When you're right, you're right! Found a line that is ALSO on the throttle plate, that had no vacuum at idle, with a cap on it. Hooked gauge on it and tapped the throttle, and there's vacuum! The true ported vacuum port. Swapped dist over to that and did some quick idle speed and mixture screw adjusting. I've got about 16in of manifold vacuum at idle, seems a little low? Base timing is still set to 20deg, I'll leave it there and see how it drives and if it pings any. Idle screws will need finer adjusting tomorrow, it got too dark for me to do much quality work anymore without pissing off the neighbors.
 

CBODY67

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Thanks for that update. Manifold vacuum at idle can vary from engine to engine, gauge to gauge, and with altitude. Lower compression ratio can also affect it. I know that 18" used to be something of a standard, but I've not been able to get any of our vehicles to that level, even back in the later 1960s when they were new. Main thing is that the vacuum is pretty steady and the needle is not jumping around.

Thanks, again.
CBODY67
 
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