71 dart hesitation at steady cruise

[Belvedere66]

Hey guys, having a hesitation (feels like fuel starvation) while cruising at a steady speed....most notably around 40mph+. 360 4bbl Holley, 727 auto, 8-3/4 3.23 suregrip. Electronic ignition, small cam and performance intake. Stock distributor and coil. Initial timing set to 10 degrees Btdc. Vac advance hooked to port in carb. Holds a steady 18" of vacuum while idling(with the advance hose off) It idles perfect/accelerates fine after moving timing from 5 to 10....only thing that got worse is a hesitation symptom. I did notice that the idle speed and timing mark do not move when I plug/unplug vac advance. Could this be causing my problem? Is my vac advance not working? Or could it be my fuel mixture? Kinda stuck on this one and don't want to lose my head checking 4 million different things.

 

[TrailBeast]

If your vac advance is connected to ported vacuum (meaning a port with no vacuum to it when the carb is at idle) you won't have any vac advance at idle, but only as the carb opens.

Sounds like a mid throttle lean surge just off the top.

 

[Belvedere66]

If your vac advance is connected to ported vacuum (meaning a port with no vacuum to it when the carb is at idle) you won't have any vac advance at idle, but only as the carb opens.

Sounds like a mid throttle lean surge just off the top.

OK, makes sense so far. How would I go about checking for this and/or how do I fix this "lean surge"?

EDIT: I found some information pertaining to the power valves located in the carb that if too low a value or busted may cause a mid throttle surging. I remember reading that you take your highest possible vacuum reading after adjusting your idle mixture and then divide in half to get your PV value.....if so then it would be a 9?? Seems high to me but I am far from an expert!

 

[TrailBeast]

OK, makes sense so far. How would I go about checking for this and/or how do I fix this "lean surge"?

EDIT: I found some information pertaining to the power valves located in the carb that if too low a value or busted may cause a mid throttle surging. I remember reading that you take your highest possible vacuum reading after adjusting your idle mixture and then divide in half to get your PV value.....if so then it would be a 9?? Seems high to me but I am far from an expert!

I'm not a Holley expert, but I seem to remember the same thing.
Got to get that mid throttle fuel richer.
(if we are on the right track anyway)
Am A/F meter is a really nice thing to have for stuff like that, as it would tell you exactly what is going on with the fuel mix at all ranges of throttle.

A manual choke would tell you also, if you slightly apply it during that surge it would straighten out if that was the problem.

 

[AJ/FormS]

At 50 mph or less in top gear,with 3.23s your carb will be working on the low-speed circuit only. You could take the main jets out, and it wouldn't make a difference.
So what to do?
Well firstly you need to be sure that you are not trying to cruise with too much advance. So with the car in park and fully warmed up, rev it up to the rpm that you noticed the problem. Keep it there and check the timing. Then disconnect the Vcan, and recheck the timing at that same rpm.
Let's say you have 3.23s, 25.5" tires and had a problem at 45mph. The rpm would be, I'll guess 1800ish.At 1800 your 360 with low compression will not like too much more than 35/40*.And the way it complains is often mistaken for a lean misfire.It will like 30 to 35 degrees tho. With higher compression it might be OK with 40*, or even 50*.
So if you are between 30 and 40 with the Vcan included, you are probably ok.If you are less than 30*, then the Vcan may not be working. To prove it; with the engine idling, and the timing lite hooked up, just suck on the hose and watch the timing change.Or hook the hose to manifold vacuum and again watch for the timing change. No change means the can is junk.
So after you get that ballparked, you will need to check the fuel. And the way I ballpark that is to engage the fast idle cam to put the rpm back into the trouble zone. Then reset the mixture screws for smoothest/ highest rpm. Then richen them up 1/8 turn. Return the throttle to idle. If it runs like crap, your T-port sync is out.(See below.)
But first take it for a cruise,even if it idles like crap.
The PV should be totally dead under cruise situation. You could be running a plug in there. However, if it had a perforated diaphragm, it would be dribbling fuel out the main nozzles,any time the engine is running, and perhaps after shut-down too.

But here's the thing, if you have advanced the idle timing, then you have also advanced the power-timing, and all the timing in-between.The power-timing has to be limited, to prevent engine damage. So stay out of WOT until you get the bottom end figured out.
And IMHO, if you can idle your 360 at 18inches of vacuum at 700 with just 10* of timing, the Bit of a cam would have to be a wee,wee,bit of a cam.

I would start by proving that neither Vcan nor the PV, is perforated.And then I would set the T-port sync. And then roadtest!

 

[Belvedere66]

At 50 mph or less in top gear,with 3.23s your carb will be working on the low-speed circuit only. You could take the main jets out, and it wouldn't make a difference.
So what to do?
Well firstly you need to be sure that you are not trying to cruise with too much advance. So with the car in park and fully warmed up, rev it up to the rpm that you noticed the problem. Keep it there and check the timing. Then disconnect the Vcan, and recheck the timing at that same rpm.
Let's say you have 3.23s, 25.5" tires and had a problem at 45mph. The rpm would be, I'll guess 1800ish.At 1800 your 360 with low compression will not like too much more than 35/40*.And the way it complains is often mistaken for a lean misfire.It will like 30 to 35 degrees tho. With higher compression it might be OK with 40*, or even 50*.
So if you are between 30 and 40 with the Vcan included, you are probably ok.If you are less than 30*, then the Vcan may not be working. To prove it; with the engine idling, and the timing lite hooked up, just suck on the hose and watch the timing change.Or hook the hose to manifold vacuum and again watch for the timing change. No change means the can is junk.
So after you get that ballparked, you will need to check the fuel. And the way I ballpark that is to engage the fast idle cam to put the rpm back into the trouble zone. Then reset the mixture screws for smoothest/ highest rpm. Then richen them up 1/8 turn. Return the throttle to idle. If it runs like crap, your T-port sync is out.(See below.)
But first take it for a cruise,even if it idles like crap.
The PV should be totally dead under cruise situation. You could be running a plug in there. However, if it had a perforated diaphragm, it would be dribbling fuel out the main nozzles,any time the engine is running, and perhaps after shut-down too.

But here's the thing, if you have advanced the idle timing, then you have also advanced the power-timing, and all the timing in-between.The power-timing has to be limited, to prevent engine damage. So stay out of WOT until you get the bottom end figured out.
And IMHO, if you can idle your 360 at 18inches of vacuum at 700 with just 10* of timing, the Bit of a cam would have to be a wee,wee,bit of a cam.

I would start by proving that neither Vcan nor the PV, is perforated.And then I would set the T-port sync. And then roadtest!

That's a lot of information to take in, wish I had you as a next door neighbor lol. And yes the cam is a summit racing cam, don't have the tag on hand. 442 lift is what I was told by PO.

 

[AJ/FormS]

I went to Summit but couldn't find a .442 lift cam.
What I did find was this; #6901; 276/288/114&.441/.441 lift. The .050 numbers are 218/228.
If that is what you have;How that monster idles at 18" is beyond me.

EDIT; lol

 

[Belvedere66]

I went to Summit but couldn't find a .442 lift cam.
What I did find was this; #6901; 276/288/114&.441/.441 lift. The .050 numbers are 218/228.
If that is what you have;How that monster idles at 18" is beyond me.

Yes that is what I have, that list number looks familiar. So i talked to a local guy who uses to work for Holley for many years and he had me reset the mixture screws, check vacuum in drive with brake pressed(which is still about 18-19), and advance initial to 12. Still runs about the same and still has that surging under load around 45+ mph (1800/2000rpm). I also hooked up the Vcan up to manifold and I couldn't tell if timing changed because I don't have an advance light but idle very very slightly changed. So it should be ok? I will check condition of the power valve tomorrow and also figure out which one it is. Some of this is above my head still.

 

[Belvedere66]

Well I think I may have found what has been causing the car to run a bit lean......so rewinding to a couple days ago, I had to reset the initial timing to 8 BTDC (I think maybe the mechanical is higher than I anticipated possibly) to get the car to run correctly. Got rid of about 98% of the hesitation and I had zero pinging as well so I figured whatever, car runs good I shouldn't mess with it again. Forward to yesterday.....went to start the car to go out for the 4th. Crank crank crank....nothing. Checked the glass filter and it was bone dry, hmmmm weird. I am thinking that the fuel pump has been wearing out without me noticing which would make sense with the car running lean if it isn't pumping enough fuel. Since the car was running well enough and I always had fuel to the carb I never thought to check the pressure/flow. I will be taking the pump off either today or tomorrow too make sure it isn't a blockage or the sending unit, etc. Just my luck!

 

[TrailBeast]

Well I think I may have found what has been causing the car to run a bit lean......so rewinding to a couple days ago, I had to reset the initial timing to 8 BTDC (I think maybe the mechanical is higher than I anticipated possibly) to get the car to run correctly. Got rid of about 98% of the hesitation and I had zero pinging as well so I figured whatever, car runs good I shouldn't mess with it again. Forward to yesterday.....went to start the car to go out for the 4th. Crank crank crank....nothing. Checked the glass filter and it was bone dry, hmmmm weird. I am thinking that the fuel pump has been wearing out without me noticing which would make sense with the car running lean if it isn't pumping enough fuel. Since the car was running well enough and I always had fuel to the carb I never thought to check the pressure/flow. I will be taking the pump off either today or tomorrow too make sure it isn't a blockage or the sending unit, etc. Just my luck!

I was driving down the road at a totally casual pace one day and my engine hiccupped.
A quick glance at the A/F gauge told me it went way lean for a few seconds, so I stopped and got another fuel pump and tossed it in the trunk.
Next day, guess what died?
I realize we can't all run out and buy everything we want when we want it, and I had some serious reservations about the 150 bucks for the kit but man it has been soooo nice to have so many times I can't really imagine not having it.

 

[Belvedere66]

I was driving down the road at a totally casual pace one day and my engine hiccupped.
A quick glance at the A/F gauge told me it went way lean for a few seconds, so I stopped and got another fuel pump and tossed it in the trunk.
Next day, guess what died?
I realize we can't all run out and buy everything we want when we want it, and I had some serious reservations about the 150 bucks for the kit but man it has been soooo nice to have so many times I can't really imagine not having it.

I haven't even looked up the light for my oil pressure gauge. It's usually laziness on my part lol.

 

[Belvedere66]

The weird thing to me is that it died while sitting in my garage, not while driving like I figured it would have.

 

[TrailBeast]

The weird thing to me is that it died while sitting in my garage, not while driving like I figured it would have.

And this is why my reasoning was the fuel pump.
With fuel restrictions like a sock or a filter they start cutting out at higher RPM's and progressively get worse, and not just suddenly die from lack of fuel sitting idling.

 

[Belvedere66]

And this is why my reasoning was the fuel pump.
With fuel restrictions like a sock or a filter they start cutting out at higher RPM's and progressively get worse, and not just suddenly die from lack of fuel sitting idling.

So I took it off and compared it to one my dad had that was newer. I was able to pump by hand pretty easily, it hadn't busted yet but it was worn out a good amount. Replaced the rubber inlet hose as well....it was very soft.