360 Questions.. pardon my ignorance.

[CoolDave]

Alright I’m trying to properly nail down how my 360 runs in my ‘73 D150 so I can actually start to make runs to the store with it. I feel like it’s about 90% there..

So essentially the whole ignition system is replaced already with the exception of the pigtail to the ignition module. I also went to a 4-pin module in place of the 5-pin. The main symptom I’m fighting is a bounce in the timing reading with my timing light, I’m talking like a 5-10 degree swing for a couple of flashes of the light. The weird thing is yesterday I added some additional grounds to the system just to eliminate variables and the bounce dropped to less than 1 degree so I thought it was fixed. Tonight I went to confirm my actual timing and vacuum at idle and it’s back to doing the same thing.. starting to lose my mind on it.. ideas of what else to check that may cause that issue?

My second problem that I will, I’m sure, rectify after I nail down the timing bounce is actually timing the motor. So with the vacuum advance kinked off I fired it up and set my initial at 1000rpm, right around 13-14 degrees, idled nice, all in was right around 35 or so. Then I added back in the vacuum can and it’s pulling around 30 at idle and 40+ when throttled up. Motor still idles well, runs cool, doesn’t stall in gear and runs down the road, the only sign it has of anything off is a miniscule stumble if you try to drive just barely off idle. Should I attempt to back the timing off just a few degrees and shoot for the all in number with the can? I’d like to keep the vacuum advance in the system as it does seem to help overall.

Any and all input is appreciated!

 

[jazak5]

that stumble might not be ignition /might be the carb

 

[volaredon]

Might be the timing chain causing that bounce

 

[Dana67Dart]

Setting timing at 1000 RPM might have some mechanical advance in play.

I would back it down to 600 to 700 rpm

 

[Oldmanmopar]

Many times there is a stumble from the primary throttle blade open to far for the idle speed to be correct. If that makes sense.

Drilling a hole in the throttle blades was old school. Or carbs having 4 corner idle circuits and a secondary idle screw. Just a thought you may want to check if you have a larger cam that would need to have more fuel at idle. You should never be above square on the transfer slots.

 

[Bewy]

Is the cam larger than stock? What carb?

 

[CoolDave]

that stumble might not be ignition /might be the carb

Good point, I may be too focused purely on the ignition side of things.

Might be the timing chain causing that bounce

I’ve worried about that, it’s just odd to me it was holding correctly for a period and the problem is intermittent. I’d like to think a mechanical problem like the chain would be more consistent.. or maybe that’s just what I’m telling myself.

Setting timing at 1000 RPM might have some mechanical advance in play.

I would back it down to 600 to 700 rpm

Ok, good to know. I’ll tinker with that and see.

Is the cam larger than stock? What carb?

Stock to my knowledge cam, carb is the factory 2-barrel. Nothing fancy.

 

[Uncle Bob]

So with the vacuum advance kinked off I fired it up and set my initial at 1000rpm, right around 13-14 degrees, idled nice, all in was right around 35 or so. Then I added back in the vacuum can and it’s pulling around 30 at idle and 40+ when throttled up.

Use ported vacuum for the advance, not manifold vacuum.

 

[CoolDave]

Use ported vacuum for the advance, not manifold vacuum.

It’s hooked to what I’m pretty confident is the correct port on the carb, there’s a vacuum tree on the manifold that’s used exclusively for the brake booster and vacuum gauge when I’m using it.

 

[Uncle Bob]

It’s hooked to what I’m pretty confident is the correct port on the carb, there’s a vacuum tree on the manifold that’s used exclusively for the brake booster and vacuum gauge when I’m using it.

Not according to what you wrote earlier - timing at idle speed increased when you reconnected the vac line to the distributor. You will have manifold vac at idle, not at WOT, and ported vacuum works the opposite, so it pulls in more timing when vacuum drops because some lead foot just hit the floorboard.

One more thing I just thought of - I have a Brawler carb that has two vac ports out the front and one is ported, the other is not. That about drove me nuts until I figured that out. IIRC the ported connector is the larger of the two, which is even weirder because it's too big for a typical vac hose to the distributor. I was connecting my vac guage to the larger port thinking it was like the one on the other side for the PCV. Imagine my surprise when my engine would run with zero vacuum.

 

[CoolDave]

Not according to what you wrote earlier - timing at idle speed increased when you reconnected the vac line to the distributor. You will have manifold vac at idle, not at WOT, and ported vacuum works the opposite, so it pulls in more timing when vacuum drops because some lead foot just hit the floorboard.

One more thing I just thought of - I have a Brawler carb that has two vac ports out the front and one is ported, the other is not. That about drove me nuts until I figured that out. IIRC the ported connector is the larger of the two, which is even weirder because it's too big for a typical vac hose to the distributor. I was connecting my vac guage to the larger port thinking it was like the one on the other side for the PCV. Imagine my surprise when my engine would run with zero vacuum.

Makes sense.. but all the diagrams I’ve seen and threads I’ve read show it to be correct. It’s the standard Holley 2245, the vacuum advance is hooked to the small port that comes out of the passenger side of the carb.

 

[Uncle Bob]

Makes sense.. but all the diagrams I’ve seen and threads I’ve read show it to be correct. It’s the standard Holley 2245, the vacuum advance is hooked to the small port that comes out of the passenger side of the carb.

I found this but it doesn't help much if at all. I've got 99 vacuum ports but none of them are labeled for the vac advance. Unless they're calling it an OSAC valve in this diagram.

 

[TT5.9mag]

Not according to what you wrote earlier - timing at idle speed increased when you reconnected the vac line to the distributor. You will have manifold vac at idle, not at WOT, and ported vacuum works the opposite, so it pulls in more timing when vacuum drops because some lead foot just hit the floorboard.

One more thing I just thought of - I have a Brawler carb that has two vac ports out the front and one is ported, the other is not. That about drove me nuts until I figured that out. IIRC the ported connector is the larger of the two, which is even weirder because it's too big for a typical vac hose to the distributor. I was connecting my vac guage to the larger port thinking it was like the one on the other side for the PCV. Imagine my surprise when my engine would run with zero vacuum.

Not to be an *** because you’re trying to help but you need to rethink how ported vacuum works, your description is very wrong. It is similar to manifold vacuum except for at idle.

 

[AJ/FormS]

This happened to me once and it ended up being a reverse polarity at the pick-up coil. This sent the ECU into a dither and it couldn't figure out exactly when to fire. Funny thing is it would idle just fine. The strobe only went whacky when the rpm began to rise.
Quick question;
Why are you trying to idle a pure-stock 360 at 1000 rpm, and 14* Idle-Timing. If you're having issues at 600rpm, figure that out first.
**********************************
Here's a tip:
24 years ago I had a 292/292/108 cam in my Hi-compression 367, and that beast was able to idle down to 550 in gear (manual trans) and pulling itself with just 5* advance. The point is; that what you think is making the engine happy, may not be.
A miniscule stumble, such as you describe, is usually called a Tip-In sag, or a momentary hesitation. This is almost universally caused by the the Transfer Slot circuit having been put into sleep mode, by the throttle being too far closed; and the closed throttle is almost universally adjusted so, because the timing is overly advanced.
So Try;
backing the timing up to maybe 6*(VA disconnected), resetting the Idle speed to 600 in gear, and twiddle the mixture screws for best quality idle. Then connect the VA to the sparkport. This assumes the fuel-level is set correctly, and that ALL the air that the engine is getting, besides the PCV, is coming in past the throttle blades. If your intake is sucking air from somewhere else, all bets are off.
Now, once you have the engine idling nice and the tip-in sag cured, only then can you try more Idle-Timing, say 2 or 3 degrees atta time, until the tip-in sag returns, then back it up .
If you try to set the Idle timing
by highest vacuum, your timing will go into the twenties and/or thirties, and your idle rpm will rise way to high, resulting in the trans banging into gear, and a mighty Tip-in bog. Forget this method.
AFTER
you have got rid of the tip-in sag, and have sneaked up the timing as described above,
THEN you can install the vacuum gauge and twiddle the MIXTURE screws for highest vacuum at that idle speed and at that timing COMBO; But DO NOT TOUCH THE SPEED SCREW.
Be advised that,
with the factory carb, distributor, and the factory stall convertor, it is as good as impossible to satisfy the engines appetite for Idle-Timing and and and, still be able to put it into gear without a mighty bang from the transmission and most likely it will stall as soon as you touch the throttle, if it hasn't already.
Whereas, retarding the Idle-timing only makes it idle slower and slower until idle vacuum is too low to support proper fuel delivery.
@Oldmanmopar provided an excellent image of the transfer slot exposure, that if you set your throttle like that, it will solve your stumble, after which, DO NOT READJUST THE SPEEDSCREW. To adjust your Idle speed, you will now change your Idle-Timing. Do not care about the actual timing number, your idling engine does not care. Job #1 is to get rid of the Tip-in hesitation.
IN ALL CASES,
after the idle-timing has been chosen, the POWER TIMING has to be revisited and modified accordingly.
Can you run 13/14 degrees at idle?
IDK;
maybe yes /maybe no
For sure NO!, if the Transfer Slot gets closed off while reducing the Idle-speed back to a sane number like 600/650 in gear.
*************************************
BTW,
when you get done balancing the T-Slot fuel, your Mixture screws should be close to in the center of their range, which on a Carter 2bbl is about 2.5 turns out.
You can use that information like this;
Set the mixture screws to 2.5 out and leave them there. Reset your Idle-timing to ~6*. reset your idle-rpm to 600/650 in gear.
Now, twiddle the screws for best quality idle. Then count the turns out to achieve best idle.
What to do;
1) If your mixture screws are out to 3.0 turns or more, then you need MORE fuel from the T-slot exposure, so increase the speed screw, then put the mixture screws back to 2.5. If the Idle-speed goes too high, then retard the timing, then verify best quality idle.
2) If your mixture screws are 2.0 or less, then you need LESS T-Slot exposure. so reduce the Idle speed, and crank the Mixture screws back to 2.5. If the idle speed is too slow, add timing, then verify the twiddle.
3) Whatever the Idle Timing ends up at, that is the limitation of your current SYSTEM. If you want to change it, then you will have to modify it.
But let me re-iterate,
the automatic transmission equipped engine does not care about Idle-Timing beyond what it takes to get rid of the Tip-in sag, and not bang the trans going into gear.
The First time your engine cares about timing is at WOT stall.
Then again at WOT after ~3600, where detonation has to be avoided at all costs.
A manual-transmission car has different needs.
Ok so
Happy HotRodding

PS
Yes, keep the VA.
AFTER the Idle timing is bugged out,
AND the stall-timing has been established,
AND the Power-timing is fixed,
AND the dots have all been connected by rate(s) of advance;
THEN comes tuning the VA for Part Throttle Performance,
and setting of the cruize-timing, which can greatly increase your fuel-economy; "greatly" being relative.
Honestly, you're looking at a percentage increase of maybe 10 to 20 percent which will depend greatly on your current combo. If you're only getting 10 mpgs without VA, then 10% is just one mpg.
But if yur getting 20 mpgs, then 10% is 2 mpgs. and that can grow to say 15% by reducing your cruize-rpm, or even to 20% by optimizing your chassis, and/but this does not take into account driving style, nor towing.

But, if you have a Holley 2bbl on it, all bets are off. That POS carb is just way too lean. I only ever got to work on one of those, for my younger brother, in an old Van. Good Lord that thing cost me hours of modifications. I thought I would come out on the losing end of that deal. But in the end of course, with timing changes I performed, she was running like a champ on 87E10, and bonus is that she was now using way less fuel on the hiway. My brother paid me for every hour I put into that thing and left me a nice tip as well. I loved that guy(RIP) . If you have one of those Holleys, and it runs well, Thank the Good Lord. If not, put a 4bbl on it and call it done.

 

[CoolDave]

I found this but it doesn't help much if at all. I've got 99 vacuum ports but none of them are labeled for the vac advance. Unless they're calling it an OSAC valve in this diagram.

View attachment 1716304914

That “OSAC” port is the one I’m using for the advance cannister.

This happened to me once and it ended up being a reverse polarity at the pick-up coil. This sent the ECU into a dither and it couldn't figure out exactly when to fire. Funny thing is it would idle just fine. The strobe only went whacky when the throttle was opened.

Quick question;
Why are you trying to idle a pure-stock 360 at 1000 rpm, and 14* Idle-Timing. If you're having issues at 600, figure that out first.

Here's a tip:
24 years ago I had a 292/292/108 cam in my Hi-compression 367, and that beast was able to idle down to 550 in gear (manual trans) and pulling itself with just 5* advance. The point is; that what you think is making the engine happy, may not be.
Try
backing the timing up to maybe 8*(VA disconnected), resetting the Idle speed to 600 in gear, and twiddle the mixture screws for best quality idle. Then connect the VA to the sparkport. This assumes the fuel-level is set correctly, and that ALL the air that the engine is getting, besides the PCV, is coming in past the throttle blades. If your intake is sucking air from somewheres else, all bets are off.

I guess at this point I’ve read so many varying ideas, numbers, methods and adjustments that I’m probably chasing my tail. The vague consensus I came to was 12- 14 degrees initial and all in timing around 32-34 seemed to be about the average people suggested.

It’s set at 1000rpm because it drops to 550-600 in gear (auto). In gear it idles fine and will pull itself along without issue until the tiniest part of throttle is applied where it stumbles a little. I guess I will start by backing off the initial timing and make sure it’s adjusted while idled down.

 

[AJ/FormS]

It’s set at 1000rpm because it drops to 550-600 in gear (auto). In gear it idles fine and will pull itself along without issue until the tiniest part of throttle is applied where it stumbles a little. I guess I will start by backing off the initial timing and make sure it’s adjusted while idled down.

I knew you would bring that to the table, lol!
Lemmee explain.
With so much Idle-timing, your engine it is nearly impossible to bring the rpm down without completely closing off the Transfer slot which is your main fuel system. The mixture screws are just there to richen up the "curb-idle", which is your basic throttle opening.
The image that @Oldmanmopar provided shows you almost exactly what your Transfer slot exposure should look like, if you take the carb off and flip it upside down. Personally, in your case, I would make it square. This method is not the easiest, but it is pretty accurate.
The method I detailed is easier and faster, once you understand what needs to be done, but perhaps not quite as accurate for a newbe.
Either method will get you into the ballpark.
The freedom from tip-in sag is what you want ...... together with an rpm drop from N>in gear of NOT more than 100>150 rpm. This can easily be achieved with the stock engine, assuming no vacuum leaks, and an accurate/stable fuel level. The trick is to keep the Transfer Slot from drying up, and the combination of fuel-level and exposure underneath the throttles is what controls most of that.
Let idle-timing be what it will be for now; whether 8* or TDC, just get rid of the Tip-in sag. Earlier I suggested a starting point of 6*, and Ima sticking to it.

I guess I will start by backing off the initial timing and make sure it’s adjusted while idled down.

Good decision.

That “OSAC” port is the one I’m using for the advance cannister.

I'm not sure that's the one you want. I think it comes in too early and quits too soon.
The one you want is in the vicinity of the front edge of the passenger side, closed throttle valve, outboard of the Transfer slot. It should be slightly ABOVE the valve at idle, and be progressively opened by the progressively opening throttle.
On the idling engine, in Neutral, the sparkport might not begin to open until around 1600>1800, but must be supplying full vacuum while cruizing.
When it truly begins to pass vacuum, while driving, is somewhat dependent on the Idle-timing. The more you start with, the more power the engine will have, and the less throttle it will take to deliver a given roadspeed, and so, the VA will come in later. That is working backwards. You want the VA to come in as early as possible, cuz it can supply way more advance than the centrifugal advance can, over a much smaller rpm range, at part throttle.
Here's what I mean;
Say your idle-timing is 14* and say you have 20* in the mechanical that begins coming in at 1000, and is all-in by 3400. Your rate of advance is thus 20* per 2400rpm, which converts to .833 degrees per 100rpm.
So say your VA begins at 1800rpm and you are cruising at 1800rpm. Your timing will be, 14* from the initial, plus 8x .833, plus zero from the VA=20.66

Now, lets change the idle timing to 6*. The engine has thus lost a lot of idle power and will continue to lag. But to get to 34* @3400rpm, the rate of advance will have to be changed to 26* over 2400rpm which is now
1.083 degrees per 100 rpm. Therefore, at 1800, your total mechanical timing is 8 x1.083 plus 6= just 14.66, but say your VA began working at 1600 now, because of the less power the engine has, that requires a larger throttle opening; and lets say the VA has 22* capacity and Say it's all-in by 1800, Ok so 14.66 +22=36.66 ...... compared to 20.66
I rest my case.

Obviously I am making up numbers and terminology to prove my point., and some things do not work exactly as I described, HOWEVER, the point remains the same, which is to, at Part Throttle, bring the VA in as soon as possible, as fast as possible, and as much as possible, until it's too much, then take a lil out, bit by bit, until it's drivable without detonation.
If you try to sneak up on it from the bottom, it'll take you all summer to find the sweets spot. Maybe two summers.
There is no 22* VA available over the counter.
and yours appears to be a real loser supplying only 40 less 34=6degrees. I gotta wonder if the diaphragm is perforated.
But I have not seen a VA that can not be modified to at least 22* simply by filing the stops down. IDK how much your engine can tolerate. or should I say wants., and that is gonna change as your Power and Stall-timings get changed. The point is, to hold off modifying the VA until last, to avoid back-tracking, which will require a second VA unit, cuz once the stops are filed off, there's no going back.
You can monitor your sparkport operation by Tee'ing into it with your vacuum gauge.

 

[CoolDave]

The plot thickens! Found the passenger side of the intake damp with fuel when I went to pull the carb.. turns out the screws in the bottom of it were not tight at all. So I snugged those, confirmed the ones up top, set the idle screws to 2.5 turns out and the idle transfer slot to a square.

Got the carb remounted, kinked off the vacuum advance, fired it up and adjusted the initial timing down to 5 degrees BTDC at 650rpm. The ignition bounce appears to be gone so I’m hoping it was a minor vacuum leak in the carb where it was loose, that said I thought I fixed it with the grounds so we’ll see. I unkinked the vacuum advance and it’s now pulling around 20-22 degrees at idle and it progressively increases with throttle added.

So now! Next steps.. once it’s warm it wants to stall out, so do I simply adjust the idle screw even though it’s going to tweak the transfer slot relationship with the throttle plates? If I hold just the weight of my foot on the pedal and bring it up to 650-750 it idles great.

Then after that it’s confirming the tip-in stumble is gone and the overall timing is safe?


EDIT: Oh and as an aside, I kept the vacuum advance on the OSAC port. From my understanding this went, originally, to a valve that would delay the timing advance. The consensus was bypassing the valve like my motor already is so there is no delay and the advance happens immediately.

 

[Dana67Dart]

Not according to what you wrote earlier - timing at idle speed increased when you reconnected the vac line to the distributor

Idle is too high at 1000 rpm so throttle is open too much and providing vac to the ported port

 

[Dana67Dart]

. From my understanding this went, originally, to a valve that would delay the timing advance

Are you talking the green capped valve?

 

[volaredon]

Most trucks I've seen didn't have that osac thing.
Mostly cars over about 2-3 years in the mid 70s. It was gray/black, sometimes on the air filter housing sometimes on the firewall and the VA line went from carb to it, then to the VA. Best thing to do if there is one would be to run a VAC line right from that same port in carb straight to the VA and forget that valve is even there.

 

[RustyRatRod]

Alright I’m trying to properly nail down how my 360 runs in my ‘73 D150 so I can actually start to make runs to the store with it. I feel like it’s about 90% there..

So essentially the whole ignition system is replaced already with the exception of the pigtail to the ignition module. I also went to a 4-pin module in place of the 5-pin. The main symptom I’m fighting is a bounce in the timing reading with my timing light, I’m talking like a 5-10 degree swing for a couple of flashes of the light. The weird thing is yesterday I added some additional grounds to the system just to eliminate variables and the bounce dropped to less than 1 degree so I thought it was fixed. Tonight I went to confirm my actual timing and vacuum at idle and it’s back to doing the same thing.. starting to lose my mind on it.. ideas of what else to check that may cause that issue?

My second problem that I will, I’m sure, rectify after I nail down the timing bounce is actually timing the motor. So with the vacuum advance kinked off I fired it up and set my initial at 1000rpm, right around 13-14 degrees, idled nice, all in was right around 35 or so. Then I added back in the vacuum can and it’s pulling around 30 at idle and 40+ when throttled up. Motor still idles well, runs cool, doesn’t stall in gear and runs down the road, the only sign it has of anything off is a miniscule stumble if you try to drive just barely off idle. Should I attempt to back the timing off just a few degrees and shoot for the all in number with the can? I’d like to keep the vacuum advance in the system as it does seem to help overall.

Any and all input is appreciated!

First, are you checking the timing with the engine completely warmed up? If not, this can affect how the timing is read from a light. There are other variables such as timing chain wear, slop between the distributor and camshaft gears. You also need to know that "some" bounce is normal. In an engine with a lumpy camshaft, sometimes you get a little movement that goes along with the camshaft lope. No big deal. None of them are rock solid, unless you have a gear drive.

Problem #2 is that you have the vacuum advance connected to manifold vacuum.......which you CAN do and tune for, but FOR NOW, get it on a ported source. Ported vacuum has zero vacuum at idle and increases as the throttle is opened. Lastly, your idle should be about 650 "or so".

 

[RustyRatRod]

Idle is too high at 1000 rpm so throttle is open too much and providing vac to the ported port

This ^^^^ is also another possibility. You need a vacuum gauge to discern which port is providing WHAT. You also need a vacuum gauge for general tuning purposes.

 

[CoolDave]

Are you talking the green capped valve?

Honestly no idea what it looks like, I’ve just read a few threads on it.

Most trucks I've seen didn't have that osac thing.
Mostly cars over about 2-3 years in the mid 70s. It was gray/black, sometimes on the air filter housing sometimes on the firewall and the VA line went from carb to it, then to the VA. Best thing to do if there is one would be to run a VAC line right from that same port in carb straight to the VA and forget that valve is even there.

I don’t believe my truck ever had one but at some point in its life it gained a replacement 360 that I have to assume came out of something that did. Vac advance is hooked directly to that port.

First, are you checking the timing with the engine completely warmed up? If not, this can affect how the timing is read from a light. There are other variables such as timing chain wear, slop between the distributor and camshaft gears. You also need to know that "some" bounce is normal. In an engine with a lumpy camshaft, sometimes you get a little movement that goes along with the camshaft lope. No big deal. None of them are rock solid, unless you have a gear drive.

Problem #2 is that you have the vacuum advance connected to manifold vacuum.......which you CAN do and tune for, but FOR NOW, get it on a ported source. Ported vacuum has zero vacuum at idle and increases as the throttle is opened. Lastly, your idle should be about 650 "or so".

I expect some bounce in it but it was dramatic, now it’s back to only wavering a degree or so. Definite improvement.

My advance is not hooked to direct manifold vacuum, it’s on the “OSAC” port which according to other threads is where it should be. I can confirm if my carb has other ported sources but I don’t believe it does.

This ^^^^ is also another possibility. You need a vacuum gauge to discern which port is providing WHAT. You also need a vacuum gauge for general tuning purposes.

I have a cheap vacuum gauge on hand. I will try hooking it to the ports on the carb and see if any are ported or if the “OSAC” port is my best option.

 

[RustyRatRod]

Honestly no idea what it looks like, I’ve just read a few threads on it.


I don’t believe my truck ever had one but at some point in its life it gained a replacement 360 that I have to assume came out of something that did. Vac advance is hooked directly to that port.


I expect some bounce in it but it was dramatic, now it’s back to only wavering a degree or so. Definite improvement.

My advance is not hooked to direct manifold vacuum, it’s on the “OSAC” port which according to other threads is where it should be. I can confirm if my carb has other ported sources but I don’t believe it does.


I have a cheap vacuum gauge on hand. I will try hooking it to the ports on the carb and see if any are ported or if the “OSAC” port is my best option.

Gotchya. Just my personal advice, but I would disable and remove the OSAC valve.

 

[emmett442]

Came here because I have pretty much the same truck, same setup. '74 D200 truck, 360, Holley 2210. My setup runs PERFECT, so I wanted to let you in on where I ended up with it, starting with square transfer slots:

10 degrees initial, limited to 33 all in
Ported vacuum advance
mix screws 2.25 turns out.

This equals a 700rpm idle.

For what it's worth.