Vacuum advance

[Rooney77]

Verify all plug wires are correct, anyone can accidentally cross a couple...

Verify the mechanical advance is working. If it is stuck, or inop it'll do that.

If you have a mechanical engine(valvetrain) issue, it'll do that as well.
Bent pushrod, really badly burned valve, flat cam lobe(s).

Have you done a compression test?

Yes I'd like the vacuum pod rang a stang, I'll pm you and thank you.

Wires confirmed.

How do I check mechanical advance? When rpms are up I checked the timing and it was a 25 degrees from 10 base. No idea what rpms were.

No way to compression test, I lack the tools. Also the engine really seems to run spot on unless you blip the throttle. It's barely got 67k miles on it so I'd hope it's not a mechanical failure. I know it's possible but I'm the eternal optimist.

While checking the wires I double checked (for the 15th time) my vaccum lines and noticed a drip of fuel from the port for the power valve when I pulled the line off. I'm gonna assume that means it needs replacing. I have a kit sitting right here so it's no biggie but I'm just curious could a bad power valve cause this?

I'm relatively inexperienced with carbs outside the vw world. Most of my builds have either been fuel injected or diesel.

[letank]

noticed a drip of fuel from the port for the power valve when I pulled the line off. I'm gonna assume that means it needs replacing. I have a kit sitting right here so it's no biggie but I'm just curious could a bad power valve cause this?

running rich, which the dripping fuel indicates, would not create backfire, unless something else is off. It will make it very rich at idle and your idle is fine as we can read.

Start by simply disconnecting and plugging the vacuum advance hose at the engine side, and test....


Then if you have an EGR valve plug its hose -for testing purposes- and see what happen?

Can you measure intake manifold at idle, it should be about 20" at sea level


Also, make sure that you air cleaner is fully functional, there are a couple of flappers that are vacuum controlled, and if one of them is non op, you won't have enough air, which will make the engine run rough... you can always pop the air cleaner cover off and check the engine behavior

[babywag]

Yes I'd like the vacuum pod rang a stang, I'll pm you and thank you.

Wires confirmed.

How do I check mechanical advance? When rpms are up I checked the timing and it was a 25 degrees from 10 base. No idea what rpms were.
Then mechanical advance is working
No way to compression test, I lack the tools. Also the engine really seems to run spot on unless you blip the throttle. It's barely got 67k miles on it so I'd hope it's not a mechanical failure. I know it's possible but I'm the eternal optimist.
Parts stores rent tools for free, Harbor Freight or others sell compression testers very cheap.
While checking the wires I double checked (for the 15th time) my vaccum lines and noticed a drip of fuel from the port for the power valve when I pulled the line off. I'm gonna assume that means it needs replacing. I have a kit sitting right here so it's no biggie but I'm just curious could a bad power valve cause this?
A bad power valve will just make it run rich/poorly
I'm relatively inexperienced with carbs outside the vw world. Most of my builds have either been fuel injected or diesel.

Ristow has a good rebuild write up in archives.

http://www.ifsja.org/forums/vb/showthread.php?t=101921

To me, it sounds like you have a mechanical problem. I doubt replacing the distributor will resolve.

[nograin]

Well maybe I should explain what's happening then.

Runs great at idle but as soon as it's under load it coughs and backfires and wants to die. When not under load if I run the rpms up slowly it's fine but if I blip it it'll backfire/bog down. I have set initial timing at 10 degrees and no amount of playing with the timing has resolved this.

This is pretty unusual:
a. If its not right with no load, it will certainly not be right with load.
b. On the fueling side, the difference between slow opening and quick opening is accelerator pump. No shot is needed for slow opening.
c. Backfire into the carb means the combustion was occuring off before the intake valve closed. If it's 'coughing' that's a bit different.

New plugs, cap, rotor. I replaced a leaking accelerator pump and all the vacuum lines have been replaced or capped off, depending on need. I'm non smog around here.

What did the spark plugs look like?
Have you made a drawing of what you've plugged? There's a pretty good semi-demogged page by one of members - troubledchild? But regardless, based on what you've written, I don't see this as related to the problem other than what the other guys have already mentioned.

I know my vacuum advance is dead. Whether or not that's my problem is the question then.

a. How do you know this? (My apologies if this and other questions come across poorly in text. Not meant in any way other than to understand how it was tested.)
b. As mentioned earlier, no. There's no vac advance used at idle, and just a little in off idle. It's used to increase timing lead with the less dense, slower burning, fuel mixtures for better efficiency.

As for the advance curve, skip white is saying this is for an AMC 360 so would it not be the right curve?

Many different curves may have been used. I can show you a half dozen curves for any given Chrysler motor of the late 60s (because I service manuals). Here's some of the factors that effect the timing with rpm.
Compression, cam (valve timing), load, heat.
An engine with good low rpm efficiency will run best with less initial and a steady increase in timing lead with rpm.
An engine with poor low rpm, but good high rpm efficiency often needs more initial and quick off idle advance, but then very slow advance in the mid to upper rpms.
In these two generalization, efficiency refers to power. Timing can also be used to help create different percentages of exhaust gases (HC, CO, CO2, NOx, O2 and H2O). For example, one way to raise the burn enviroment's temperatures is to have a slightly later timing than ideal for power. This puts more heat into the cylinder walls and exhaust.
Someone with some AMC books may be able to be more specific on the various curves used with their 360 over the years and in different vehicles.

It's like the timing can't keep up with fuel delivery.

Nah. The beauty of carburation. It only supplies what the engine asks for. It responds directly to the manifold pressure or airspeed.

I was hoping the vacuum advance being dead was the cause and I wouldn't have to rebuild the carb since it idles so well and fires right up.

I forgot to mention I set the mixture screws to 2.5 turns out as per ristows carb rebuild tread.

Well considering the power valve is leaking. I'd say its time to remove the carb, clean it, and replace all the leaky gaskets and diaphrams. Good chance there's dirt, or maybe even charcoal, in the passages.
Set the mixture screws at whatever the engine runs best. 2.5 turns out is just a starting point. Turning the screws in should have a noticible effect. As soon as the engine speed drops, turn them back out 1/4 turn. Turning out is usually more subtle effect. (The engine must be fully warmed and choke open)

How do I check mechanical advance? When rpms are up I checked the timing and it was a 25 degrees from 10 base. No idea what rpms were.

Invest in a tach or tach/dwell. Shouldn't be too hard to find one at a garage sale or just being given away. You'll also need an alligator clip or probe assuming your engine still has the Ford style coil connector.
Timing should begin to advance quickly around 850 rpm, and certainly by 1000 rpm. Then advance very slowly after 1200 or 1400 rpm.
This is '79. '84-'85 is similar. So yours will be something close to the chart here.http://oljeep.com/gw/vac/79/79-FSJ-360_DistAdv.jpg
If its 10* below 800 rpm, then I would not expect it to be at 25* until 3200 rpm. BTW, How are you measuring 25* BTDC? I can barely see best the 10* timing mark on my engine.

No way to compression test, I lack the tools. Also the engine really seems to run spot on unless you blip the throttle. It's barely got 67k miles on it so I'd hope it's not a mechanical failure. I know it's possible but I'm the eternal optimist.

Buy, borrow or rent tools!
I'm happy to spend your money there
It's always good to do a compresion check anyway so you have a baseline.
I'd put a vacuum gage before a compression tester. (for example. jumpy vacuum at idle is an indicator of valves not closing or seating well)
Tach or Tach/dwell.
Mity-vac vacuum pump is also handy tool to have.
Compression tester, screw in type is better.

[Rooney77]

This is pretty unusual:
a. If its not right with no load, it will certainly not be right with load.
b. On the fueling side, the difference between slow opening and quick opening is accelerator pump. No shot is needed for slow opening.
c. Backfire into the carb means the combustion was occuring off before the intake valve closed. If it's 'coughing' that's a bit different.


I replaced the accelerator pump but not the red plunger thing. I'm starting to think it's time I bite the bullet and rebuild it.


What did the spark plugs look like?
Have you made a drawing of what you've plugged? There's a pretty good semi-demogged page by one of members - troubledchild? But regardless, based on what you've written, I don't see this as related to the problem other than what the other guys have already mentioned.


Spark plugs were covered in carbon. It was running rich for sure. I didn't make a drawing but basically I ran the power valve to the manifold vacuum line just in front of it. The vacuum advance line runs straight from carb to dizzy. I'll have to make a drawing to capture the rest tomorrow. I too don't think vacuum lines are my issue but who knows.


a. How do you know this? (My apologies if this and other questions come across poorly in text. Not meant in any way other than to understand how it was tested.) I used a mityvac and not only did it not budge, it wouldn't build vacuum at all.
b. As mentioned earlier, no. There's no vac advance used at idle, and just a little in off idle. It's used to increase timing lead with the less dense, slower burning, fuel mixtures for better efficiency.

Many different curves may have been used. I can show you a half dozen curves for any given Chrysler motor of the late 60s (because I service manuals). Here's some of the factors that effect the timing with rpm.
Compression, cam (valve timing), load, heat.
An engine with good low rpm efficiency will run best with less initial and a steady increase in timing lead with rpm.
An engine with poor low rpm, but good high rpm efficiency often needs more initial and quick off idle advance, but then very slow advance in the mid to upper rpms.
In these two generalization, efficiency refers to power. Timing can also be used to help create different percentages of exhaust gases (HC, CO, CO2, NOx, O2 and H2O). For example, one way to raise the burn enviroment's temperatures is to have a slightly later timing than ideal for power. This puts more heat into the cylinder walls and exhaust.
Someone with some AMC books may be able to be more specific on the various curves used with their 360 over the years and in different vehicles.


Nah. The beauty of carburation. It only supplies what the engine asks for. It responds directly to the manifold pressure or airspeed.


Great information. I always appreciation the sharing of knowledge.


Well considering the power valve is leaking. I'd say its time to remove the carb, clean it, and replace all the leaky gaskets and diaphrams. Good chance there's dirt, or maybe even charcoal, in the passages.
Set the mixture screws at whatever the engine runs best. 2.5 turns out is just a starting point. Turning the screws in should have a noticible effect. As soon as the engine speed drops, turn them back out 1/4 turn. Turning out is usually more subtle effect. (The engine must be fully warmed and choke open)


I'm debating rebuilding it in the morning. I have Fridays off so why not.


Invest in a tach or tach/dwell. Shouldn't be too hard to find one at a garage sale or just being given away. You'll also need an alligator clip or probe assuming your engine still has the Ford style coil connector.
Timing should begin to advance quickly around 850 rpm, and certainly by 1000 rpm. Then advance very slowly after 1200 or 1400 rpm.
This is '79. '84-'85 is similar. So yours will be something close to the chart here.http://oljeep.com/gw/vac/79/79-FSJ-360_DistAdv.jpg
If its 10* below 800 rpm, then I would not expect it to be at 25* until 3200 rpm. BTW, How are you measuring 25* BTDC? I can barely see best the 10* timing mark on my engine.


Tach is on the list of things for sure. I despise not having a tach. I knew it was 25* by adjusting the timing light until the crank mark aligned with TDC then reading the knob on the timing light. I don't know RPM because I had a dumbell on the accelerator pedal. Nice and ghetto.



Buy, borrow or rent tools!
I'm happy to spend your money there
It's always good to do a compresion check anyway so you have a baseline.
I'd put a vacuum gage before a compression tester. (for example. jumpy vacuum at idle is an indicator of valves not closing or seating well)
Tach or Tach/dwell.
Mity-vac vacuum pump is also handy tool to have.
Compression tester, screw in type is better.

See my responses in red.

[nograin]

See my responses in red.

I replaced the accelerator pump but not the red plunger thing.

If by that you mean the umbrella like seal, that's the check valve. It should seal when the pump is actuated and let fuel into the pump when its released.

Spark plugs were covered in carbon. It was running rich for sure.
Clean 'em or replace. When its heavy enough the carbon will cause misfires. IIRC you mentioned replacing the coil, cap etc.

I ran the power valve to the manifold vacuum line just in front of it.
OK
The vacuum advance line runs straight from carb to dizzy.
From either side port should be OK as well

I used a mityvac and not only did it not budge, it wouldn't build vacuum at all.
Best way! Sorry its got a bad seal.


I'm debating rebuilding it in the morning. I have Fridays off so why not.

Take your time on the removal.
Note/photo where the springs attach.
a. Throttle return
b. Transmission link (aka kickdown, or throttle pressure linkage)
There's a couple of hair pin clips. One for the cruise control chain.

When removing he nuts from the carb base, if the stud comes out with one, thats fine. If the plastic spacer beneath is warped, it may hang up on the studs. Remove one or just leave it.

Tach is on the list of things for sure. I despise not having a tach. I knew it was 25* by adjusting the timing light until the crank mark aligned with TDC then reading the knob on the timing light. I don't know RPM because I had a dumbell on the accelerator pedal. Nice and ghetto.

LOL
But you have a dial back light! Nice combo.

[gophman]

With regards to HEI, don't pay a fortune.... Proform dizzies are good quality, adjustable vacuum can and a pretty good curve out the box imo, just swap your cam gear. I went hei from a large cap with a Jacobs mileage master unit and coil (to enable easier setup for dual fuel).

[acct21]

Runs rich, has an off-idle stumble, and fuel comes out of the PV vacuum port.

Sounds like a blown power valve.

[Rooney77]

Runs rich, has an off-idle stumble, and fuel comes out of the PV vacuum port.

Sounds like a blown power valve.

Thanks for the info. I'll swap it here soon.

I've been focusing on body work this weekend. I pulled the tcase to swap it for a 231 and I need to get a part machined before I can complete the swap. In the meantime my trans puked all over the shop floor so it's empty and I don't wanna keep tinkering with the motor till I can refill the trans. So the mechanical stuff is waiting till I get the seal sleeve made.