Still hesitates off throttle...

[Ryan68Dart]

Hello, I have searched and tried just about everything I have found. 225 slant six with the holly. Accelerator pump seems to be working and rebuilt the distributor. The problem seems to be the vacuum advance. Timing goes from 5 btdc to about 22 degrees with the vac advance disconnected and just running the mech advance. Connecting the vac advance, the timing shoots to like 15 deg Before tdc. According to the haynes manual and the OG dodge manual everything is correct. Honestly stumped. Runs pretty well with the vac advance plugged. Falls on it's face with vac advance hooked up.

 

[volaredon]

Is the vac advance plugged into the right place? Usually you want ported vacuum not manifold vacuum

 

[teringer]

Timming marks sliped on.balancer?

 

[AJ/FormS]

It seems like you saying that the Vcan is retarding the timing?
Did you set the base timing with the Vcan disconnected?
In Neutral;
when slowly opening the throttle, the vacuum advance should not start until something like 1600 or even 1800 rpm, and, should not be all in until near peak vacuum develops, say around 2200rpm or perhaps more.. They are usually adjustable.
While driving;
But as soon as vacuum begins to drops off, no matter the rpm, the timing should begin to drop out too..
Cruising along in Drive, and slowly applying throttle, the Vcan should drop to zero long before the trans kicks down.

 

[slantsixdan]

You should not see a jump like that at idle when you hook up the vacuum advance. Either you're hooking it up to the wrong place as Volaredon suggested, or there's something the matter with your carburetor such that the throttle plate is above rather than below the spark advance port at idle. Carburetor operation and repair manuals and links to training movies and carb repair/modification threads are posted here for free download, and find tune-up parts and technique suggestions in this post.

 

[Ryan68Dart]

Thanks for the replies guys.
The balancer is new and I also confirmed the timing mark is correct.

I had the carb off and checked that the vac advance is plugged into the ported connection. I confirmed that it has zero vacuum at idle with the throttle plate closed then builds to about 18 in/hg as the throttle opens, then backs off to 15 inches at about 2500ish rpms. It stays at 15 inches to full throttle.

the vacuum advance should not start until something like 1600 or even 1800 rpm

I think this may be the problem. The vacuum on that port goes shoots to 12 inches almost immediately - before I hit 1400 rpm. The vac advance starts to move at about 8 inches. I will check the manuals about adjusting the vac advance to get it to kick in later. Thank you.

 

[Ryan68Dart]

You should not see a jump like that at idle when you hook up the vacuum advance. Either you're hooking it up to the wrong place as Volaredon suggested, or there's something the matter with your carburetor such that the throttle plate is above rather than below the spark advance port at idle. Carburetor operation and repair manuals and links to training movies and carb repair/modification threads are posted here for free download, and find tune-up parts and technique suggestions in this post.

Yes, I realize how I wrote my original question was a little inaccurate. The vac advance does not kick in at idle. I meant that just off idle right when you hit the throttle, the timing shoots to 15 btdc. Sorry for the confusion.

 

[slantsixdan]

I had the carb off and checked that the vac advance is plugged into the ported connection. I confirmed that it has zero vacuum at idle with the throttle plate closed then builds to about 18 in/hg as the throttle opens, then backs off to 15 inches at about 2500ish rpms. It stays at 15 inches to full throttle.
I think this may be the problem.

No, I think not—the way you describe is the way it should work.

The vacuum on that port goes shoots to 12 inches almost immediately

Yup, that's correct. At idle, the spark advance port is just barely above the upper edge of the throttle plate. It takes very little throttle opening to uncover it and expose it to vacuum. That thing about how vacuum advance shouldn't start til 1600-1800 rpm is not correct.

I would return your attention to the carburetor. The formal thing to do would be to double-check that your accelerator pump link is in the correct hole in the throttle lever, and double-check the pump stroke adjustment. For a quick diagnostic check, move the accelerator pump link to the outermost hole in the throttle lever. This will give a longer pump stroke (assuming there's nothing the matter with the accelerator pump system). If that significantly reduces the stumble/hesitation, you know you're on the right track.

 

[Ryan68Dart]

move the accelerator pump link to the outermost hole in the throttle lever

It is currently on the innermost hole. That's where it was when I bought the car so I left it. I will give that a shot this afternoon. Just to be clear, the outermost would be toward the passenger side correct? The pump diaphragm is new.

What about the vac advance starting to move at 8 inches? Is that typical? FYI my advance (Standard motor products aftermarket) does not seem to be adjustable.

 

[slantsixdan]

Try the middle hole first and see if that clears up your hesitation. If it's reduced but still there, try the outer hole. "Inner" and "outer" are with reference to the throttle shaft, so yes, the outermost hole is nearest the passenger side of the car. Note that most of today's gasoline contains ethanol as an oxygenate, which leans out any fixed fuel-air mixture. That means an accelerator pump setting that might have been adequate with pure gasoline might not be enough with the oxygenated fuel and you may have to dial in a larger pump shot than called for in the original specs. Make sure to check and correct your bowl vent adjustment after getting the accelerator pump to work the way you want, or else you'll cost yourself unnecessary gas money (refer to those linked carb manuals).

Most of the pre-'72-type vacuum advances are not adjustable.Yes, 8" of vacuum is within the active range. This very old (2005) post lists the characteristics of some Slant-6 vacuum advance units.

 

[Ryan68Dart]

Tried the middle hole and didn't make any difference. Went to the outermost hole and it seems to have helped a little bit, sort of. Doesn't hesitate quite as bad, but it lasts much longer. I will take off the carb and go through it again. Still runs like a champ with the vacuum advance disconnected and port plugged.

Other bits of info.. I borrowed my neighbor's old maxi-tune to more accurately measure the timing curve. Initial timing is 5 btdc at 750 rpms. Mechanical advance maxes at 28 btdc @ 1800 rpm. Total with vacuum advance is 38 btdc @ 2000 rpm. Not sure if it is relevant, but just fyi.

I will see what I can find in the carb and report back.

 

[slantsixdan]

Interesting that the hesitation lasts longer with the bigger pump shot.

Doesn't look like there's anything wrong with your advance curves.

 

[Ryan68Dart]

Interesting that the hesitation lasts longer with the bigger pump shot.

Doesn't look like there's anything wrong with your advance curves.

Yep it is not as extreme, but it lasts for about 3 seconds or so. Previously, it would fall flat and almost die, but only for about a second and a half before it would catch itself. Is it possible that the pump spring has lost it's tension and just isn't giving the pressure it should? Maybe that accelerator pump spring is bad.

 

[AJ/FormS]

It stays at 15 inches to full throttle

this right here tells the story.
If your vacuum advance-can is seeing 15inches at full throttle. my guess is that you have somehow found a venturi-vacuum source, cuz this is MORE vacuum than is in the intake manifold itself .............. at full throttle.IMO, this can't possibly be right.

 

[slantsixdan]

Well, yeah, the accelerator pump spring's tension ought to be given some thought every five decades or so, whether it needs it or not, LOL. I routinely stretch 'em a bit before reinstallation.

You might have crossed over from not enough of a pump shot to too much of a pump shot, or you might have more than one cause of the hesitation and your pump shot adjustments are addressing one cause but not the other. There might be something still to be found in the distributor…

…errrrr, yeah, hold on a sec. Going by your screen name, you've got a '68 Dart. Those came with CAP, the Clean Air Package, which was the auto industry's first effective exhaust emission control system. Is your car still equipped with the vacuum advance control valve perched atop the cylinder head, just to the driver's side of the valve cover, about 3/4 of the way back? Fat hose to the № 6 intake runner; thin hoses to the carburetor and distributor? If so, that needs to be working properly and adjusted correctly. If not, you'll have great difficulty getting the engine to behave nicely for you. As described in the linked article, that setup is intended to have very retarded basic timing (around 5° ATDC). The distributor is configured to kick in a bunch of mechanical (centrifugal) advance just above idle, and there's a long-throw vacuum advance. If you're running a CAP-curved distributor without the valve and/or with the base timing advanced a bunch of degrees from spec, then yeah, the car will run reasonably well with the vacuum advance disabled (but not as well as it could, and with increased fuel consumption) and will exhibit weird driveability faults with the vacuum advance enabled.

Good options would be either to put the system the way it was built, or recurve the distributor to non-CAP specs, or kill multiple birds with one stone and do the HEI upgrade (which involves installing a new distributor, and the electronic distributors did not have the unusual advance curves found in the '63-'69 CAP distributors).

 

[Killer6]

this right here tells the story.
If your vacuum advance-can is seeing 15inches at full throttle. my guess is that you have somehow found a venturi-vacuum source, cuz this is MORE vacuum than is in the intake manifold itself .............. at full throttle.IMO, this can't possibly be right.

He's talking in the driveway in park/neutral, raising it, then winging it. That would be close to on the nuggets.

 

[AJ/FormS]

He's talking in the driveway in park/neutral, raising it, then winging it. That would be close to on the nuggets.

well, I get what you saying
but when you "wing it", manifold vacuum still falls to "very low" ; not 15 inches at the Vacuum port at full throttle.
Ok let me rephrase that: I've never seen that behavior.

 

[Ryan68Dart]

That's a lot to digest. A lot of really cool history there also. Thanks.

First off, yes. It is a 68 4dr auto dart.

When I rebuilt the carb, I did give the spring a bit of a stretch. I usually do when I mess with things like that but to be honest I have no idea what a 'good' tension should feel like for that particular spring. The former owner gave me stacks of records from when it was sold new till now and I know there are at least 2 carb rebuilds done by a mechanic in there somewhere. I know that doesn't hold much water, but it's something.

I can't tell you if the car came with the CAP package, but I can tell you that none of that extra stuff is there now. There is what looks to be a 3/8" NPT port on the number 6 intake runner that is plugged with a hex plug. I would assume this would be for the fat hose. Vacuum advance hose goes directly to the port on the passenger side of the carb in between the 3/8" PCV port and the accelerator pump linkage. The distributor has been converted from points to electronic with a modern kit. Says NV5M on it.

Just for fun I just put the initial timing at 5 ATDC, 0, 3, 5 and 7 BTDC and it is happiest at 3 BTDC. 5 AFTER was almost undrivable. This was set with vac disconnected then connected for driving.

 

[Bewy]

15* of vac @ WOT suggests the carb is not getting anywhere near WOT.

 

[Ryan68Dart]

15* of vac @ WOT suggests the carb is not getting anywhere near WOT.

How do I check that? The throttle plate and choke are almost vertical. Is there something else I should be looking for?

I'm going to have to make a video and link it. I must be doing something very wrong.

 

[Dave999]

15* of vacuum advance at full throttle might indicate a totally blocked up air filter or a filter where the lid is too close to the carb intake horn, made worse with a dirty filter of marginal depth . are you using a filter with a base that is offet down for hood clearance?

try it with the filter off and see if gets better

in a dirty filter situation
If the float bowel is vented into the filter, the carb venturi and the fuel in the bowl see the same pressure/vacuum, the car will still run, it has some impact on mixture, but vacuum is high. The pressure differnce between top and bottom of carb is reduced, the inlet into the carb is now not at atmospheric pressure. so you run a bit leaner.

if the float bowl is vented to the atmosphere like on a carter BBD, and is not seeing the restrictive effect of the dirty filter, it is seeing atmospheric pressure, and the carb venturi isn't, becasue its under vacuum due to the work necessary to draw air through the filter. Then you will run rich and it will get worse the faster you go.
the atmospeher crams fuel out of the bowl into the air stream due to the fact that the venturi in the carb is under a level of vacuum that is caused by the restriction at the filter

and a hestitation can be a lean spot a rich spot or a problem with igntion timing and they can all feel exaclty the same

sorry i don't know holley carbs

5* initial and 28 total mechanical advance the 28* is probably ok but i can't see the 5* BTDC being enough
try it at 8* and see if it gets better with 8* BTDC initial and 31 total 31 should still be safe enough.

one other thing, does the carb have an auto or manual choke and does it work? is the flap open when the car is warmed up.
if standard there will be a bimetallic spring in a box on the side of the exhaust manifold connected to a rod that is connected to a choke flap lever near the top of the carb. if the spring breaks, or the box rusts up the choke gets stuck in the ON/cold running position

if the choke is on all the time you will see vacuum at high throttle openings

Dave

 

[Killer6]

well, I get what you saying
but when you "wing it", manifold vacuum still falls to "very low" ; not 15 inches at the Vacuum port at full throttle.
Ok let me rephrase that: I've never seen that behavior.

I should clarify "winging" after revving it up in stages being less abrupt than "snapping" the throttle WO from a low rpm, it sounds like He progressed to WOT more gradually, I hope it wasn't at WOT too long!!!!

 

[Killer6]

How do I check that? The throttle plate and choke are almost vertical. Is there something else I should be looking for?

I'm going to have to make a video and link it. I must be doing something very wrong.

Nope, 2 vertical blades with the pedal on the floor is good.
Some pics of what We are working on would be nice.
What was the float height setting You used?
A low fuel level will delay the mains kicking in.
The "economizer" valve is a notoriously finniky unit, it may be partially obstructed or not moving freely in operation.
I may have missed it but, what is the fuel pressure if checked?
Were there any issues with throttle pressure linkage/adjustments?

 

[Dartswinger70]

What about no /broken advance springs in the distributor ...?

 

[Ryan68Dart]

I have been doing all of this testing with the air cleaner off. The mechanical advance springs are in good shape and everything is lubed up and moving freely.

I think I put a bandaid on it for now though. I went through the carburetor again and didn't find anything, but I did change two things. The first is that I put a bigger main jet in. It was a random jet that I had in my parts bin and it is not stamped so I don't know what size, but it is slightly larger than the 552 jet that was in there.

The second thing was set the float slightly higher than it should be. The dodge manual I have says 27/32" or 0.84" wet height which is what I had set it to. I double checked it before adjusting and it was still there. I then adjusted it a hair and it is now about 0.80" while idling.

The problem now is that it is clearly running rich because all I did was just richen the entire rpm range and didn't actually address the low rpm stumble... hence the 'bandaid'. Probably wasting a bunch of gas but no more hesitation and it seems to be happy. I also set the timing back to 5 btdc.

As for the vacuum at WOT, I have no idea why, but it is definitely there.

I will get a jet kit so I can more accurately tune that.

I am not convinced that I actually found the culprit, but I am going to call it good enough for now until I have more time to tinker. Could have just been the float level and the main jet change was unnecessary. I will report back when I figure it out. Thank you all very much for your help!