Anyone else have minor hesitation/jerking at part throttle??

NWPT70

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Hello guys,

I noticed my 1970 Chrysler Newport with 383 engine and 2 barrel Carter BBD has always had a very minor hesitation/jerking upon a certain part throttle while usually cruising . I'd like to finally figure out if this is just a thing this particular engine carb combo does (I've heard this from another member) or if this is something I can resolve. Stock engine, original air cleaner in place, rebuilt carb, dual exhaust.

What happens exactly is at part throttle while cruising, the RPMs go up and down just enough to feel like you are going against a strong wind gust head on, on and off. Not terribly noticeable but enough to be annoying. It goes away if you step on the throttle a bit more. The car idles nicely. 18 inches of vacuum. Has plenty of power at full throttle. I've tried richening up the mixture screws just a bit to see of that would help but no luck. Any advice?
 

HWYCRZR

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If you are going from idle or a rolling stop to accelerate and have the hesitation, your carb plunger could have shrunken, not pumping enough fuel to accelerate quickly. But you said it is at mid range when steadily cruising.
This Could be a timing curve thing. Mechanical advance, could be advancing too early for your cruise RPM and smooths out when the rpm to catchs up to the advance.
Mechanical advance is fairly easy to check if you have a dial back timing light and tachometer (many newer timing lights have a built in tach) and can do a little math.
Distributor curve is in your service manual in the electrical section. And lays out the distributor specs.
Remember if checking in the engine with a tach the engine turns twice for every distributor revolution (400 distributor rpm is 800 engine rpm.) and distributor advance is also 1/2 of what shows on the crankshaft. If the distributor spec is 4 degrees @ 750 RPM, You would set your engine rpm at 1500 RPM and you should read 8 degrees plus initial advance (7-10 degrees depending on the engine, year and carburetor)
 

CBODY67

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Sounds like a bit of "lean surge", as it used to be termed? Not a big deal, but once you find it, then "key" on it, it seems to be worse that it actually is, from my experiences with a '70 Skylark 350 2bbl with CA emissions.

I suspect you are using E10 fuels? Which require a bit richer fuel mixture to have the same power content as E0 fuels, which might be jsut enough to make things a bit lean in a particular rpm range and load. What road speeds is the issue occuring?

As for things to try, advance or retard the base ignition timing 2 degrees from stock and see if it changes. Might need to contact Dana at Woodruff Carb for a pair of main jets one notch richer than stock?

On the Skylark, my mother and I flew to LA to drive an aunt back to TX. On that car, it had a lean surge right at 60mph. 62mph, it was not there. 58mph, it was not there. Not wanting any CHP lights in the rear view mirror, with a 60mph speed limit, the 62mph was not an option, but 58mph was not either. So I finally just drove 62mph and that was that. After we got on higher-speed highways, no problems. After I inherited the car a few years later, did a good tune-up with new wires (the ones one the car were brittle, but it did not miss and always started on the first try, even with THEM), that lean surge was gone.

I also found a Carter add-on device that appeared to be a variable vacuum bleed of sorts. Took it off as it was not OEM, but I have a stack of CA emission test receipts where it always passed.

Just some thougths,
CBODY67
 

Gerald Morris

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S

As for things to try, advance or retard the base ignition timing 2 degrees from stock and see if it changes. Might need to contact Dana at Woodruff Carb for a pair of main jets one notch richer than stock?
...

. After I inherited the car a few years later, did a good tune-up with new wires (the ones one the car were brittle, but it did not miss and always started on the first try, even with THEM), that lean surge was gone.

Just some thougths,
CBODY67

Wise words here. I currently have Mathilda retarded about 2.5 degrees, running at 10 BTDC to stop pre-ignition pings and the resultant loss of power when I stomp it to WOT. This works WELL, though it runs about 10 degrees Fahrenheit hotter as a consequence when rolling on the highway. I've also caught a couple insulation breaks on the plug wires in the last year, and have replaced those wires to good result, when performance had mysteriously declined. While I like solid copper core plug wires, I won't buy the house brand insulated in soft vinyl from Speedway again. I got the Packard 440 stuff (EPDM) now and plan to replace the rest of the transparent showroom stuff this autumn, after I get some new wire loom repops. (or I might just dip the old loom stuff in some plastic to re-insulate them as an alternative... ) Anyway, little things like this can prevent or cure a host of performance ills.

Good luck!
 

NWPT70

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Sounds like a bit of "lean surge", as it used to be termed? Not a big deal, but once you find it, then "key" on it, it seems to be worse that it actually is, from my experiences with a '70 Skylark 350 2bbl with CA emissions.

I suspect you are using E10 fuels? Which require a bit richer fuel mixture to have the same power content as E0 fuels, which might be jsut enough to make things a bit lean in a particular rpm range and load. What road speeds is the issue occuring?

As for things to try, advance or retard the base ignition timing 2 degrees from stock and see if it changes. Might need to contact Dana at Woodruff Carb for a pair of main jets one notch richer than stock?

On the Skylark, my mother and I flew to LA to drive an aunt back to TX. On that car, it had a lean surge right at 60mph. 62mph, it was not there. 58mph, it was not there. Not wanting any CHP lights in the rear view mirror, with a 60mph speed limit, the 62mph was not an option, but 58mph was not either. So I finally just drove 62mph and that was that. After we got on higher-speed highways, no problems. After I inherited the car a few years later, did a good tune-up with new wires (the ones one the car were brittle, but it did not miss and always started on the first try, even with THEM), that lean surge was gone.

I also found a Carter add-on device that appeared to be a variable vacuum bleed of sorts. Took it off as it was not OEM, but I have a stack of CA emission test receipts where it always passed.

Just some thougths,
CBODY67
Thanks for the advice. The lean surge you are describing is exactly what I'm experiencing. It only is noticeable at certain speeds and at a part throttle position. Hardly noticable until you key on to it and it does seem worse. Go a bit faster or slower and it will dissapeared.

I am using E10 gas unfortunately. Do you recommend any good Ethanol stabilizers? I will also take a look at my timing and plug wires!
 

Ripinator

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Thanks for the advice. The lean surge you are describing is exactly what I'm experiencing. It only is noticeable at certain speeds and at a part throttle position. Hardly noticable until you key on to it and it does seem worse. Go a bit faster or slower and it will dissapeared.

I am using E10 gas unfortunately. Do you recommend any good Ethanol stabilizers? I will also take a look at my timing and plug wires!

At the risk of being repetitive: Ninety percent of all carburetor problems are electrical. . .
 

halifaxhops

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mech.gif


Seriously check the distributor curve can be sticking at that rpm.
 

CBODY67

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Agreed, one of the things mentioned in a 1966 MOTOR Manual was just that, that fuel issues CAN also be electrical issues, in relation to engine performance. In a lot of cases, even acting very similarly. Additionally, some fuel issues can also be "small vacuum leak" issues, too. Whether it might be vacuum lines with small cracks no big enough to really cause issues yet, various vac actuation diaphrams, etc.

Capping the vac ports on the carb, except for the one for the vac advance, at first, might help trace things, IF this might also be an issue.

I have been using the Lucas ethanol additive in my lawn mowers for many years. It's green, is about $10USD for the larger bottle, and is fine with "over-treat" concentrations. Keeps the spark plug new-clean and the insulator with a tint of green on it. Others might have similar products they like, too.

Enjoy those "diagnostic drives",
CBODY67
 

NWPT70

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I do have an orange box mopar electronic ignition system installed on the car. With thick 8mm wires and E3 plugs. These were all installed a few years back so I will recheck the timing curve and look at the wires for burn spots and possibly change the plugs.
 

CBODY67

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For good measure, re-check the torque on the intake manifold bolts and the carb base, too.

CBODY67
 

halifaxhops

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Hey Just FYI some of the orange boxes had funky issues at certain rpm's. Any chance you have a box to swap out and try?
 

halifaxhops

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I do have an orange box mopar electronic ignition system installed on the car. With thick 8mm wires and E3 plugs. These were all installed a few years back so I will recheck the timing curve and look at the wires for burn spots and possibly change the plugs.
The wires and plugs if there was a issue I would think would be all ranges. Easy check on the curve sticking take the cap off and slowly turn the rotor and see if there is a say stiff spot in the motion for the advance. Shh don't tell any on that!:rofl:
 

CBODY67

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If the vac advance is "gone", fuel economy can bre in the 10mpg range all of the time, rather than cruise eco in the 15+mpg range, from my experiences on our '66 Newport 383 2bbl Stromberg. But it still ran just fine.

CBODY67
 

69PHOENIX

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G'Day,
Ain't no Expert but back in the 70s I had a383 2 bbl that had a really annoying habit of "Nudgeing". When you were cruising on Trailing Throttle the car would "Nudge Nudge" as though it was sort of missing but not really, it was REALLY Annoying. Your Foot wasn't really "On" the throttle but neither was it off. Long Story short one of my Club members told me the air filter Bolt on a Carter 2bbl Carby doesn't go all the way into the body of the Carburettor and if you have a habit of over tightening the Wingnut it ends up pulling the top plate away from the Body of the Carby allowing fuel to splash over into throat. I removed the top and used a sheet of Glass & sandpaper and sure enough you could see where the Top had pulled away from the Gasket. A bit of work gently Tapping & Sanding it level, a New Gasket & that cured my Problem. Might be worth checking. Hope this Helps.
Kind Regards, Tony.M (Oh & Tighten Gently in the Future)
 

halifaxhops

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LOL you just answered a question on a NOS gasket I got that is labeled thick or thicker, was wondering WTF that was for.
 

Big_John

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Hello guys,

I noticed my 1970 Chrysler Newport with 383 engine and 2 barrel Carter BBD has always had a very minor hesitation/jerking upon a certain part throttle while usually cruising . I'd like to finally figure out if this is just a thing this particular engine carb combo does (I've heard this from another member) or if this is something I can resolve. Stock engine, original air cleaner in place, rebuilt carb, dual exhaust.

What happens exactly is at part throttle while cruising, the RPMs go up and down just enough to feel like you are going against a strong wind gust head on, on and off. Not terribly noticeable but enough to be annoying. It goes away if you step on the throttle a bit more. The car idles nicely. 18 inches of vacuum. Has plenty of power at full throttle. I've tried richening up the mixture screws just a bit to see of that would help but no luck. Any advice?

While I agree about carb problems being electrical, this does sound like lean surge.

One thing that I see is you've installed a dual exhaust. Improvements in exhaust flow can make for a leaner mixture. Couple that with the gas that we have now and it wouldn't surprise me if you are running on the lean side.

Richening the mixture screws will only affect the idle.

I would start looking for vacuum leaks. All the hoses good? Carb bolts tight? Brake boosters can be a source of vacuum leaks too.
 

70bigblockdodge

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If you put a new electronic dist in it most likely has too much vacuum advance at cruise rpm. Use a 3/32nd Allen wrench right though the nipple and tighten it up test, tighten it up test, tighten, test until you get rid of the surge. Could be lean as mentioned, but with exhaust manifolds I'm guessing not.
 

mopar440

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I wouldn't say that, I had a 5.4 3v that randomly bogged out. Other garages said it was fuel related, so replaced almost all fuel related components. No change

Came to me, it felt like a missfire in a way. Customer said the plugs were new. These plugs have a tendency to break in the head

I pulled one, completely shot. New plugs fixed it. The data on the scan tool showed a lean condition, I almost went down the wrong road too, no missfire codes
 

Gerald Morris

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If the vac advance is "gone", fuel economy can bre in the 10mpg range all of the time, rather than cruise eco in the 15+mpg range, from my experiences on our '66 Newport 383 2bbl Stromberg. But it still ran just fine.

CBODY67

Such has been my experience. The first 2 distributors I ran after buying Mathilda both had only "decorative" vac advances. I get 14 mpg on the highway these daze with the sino-repop. In town its still 10.
 
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