Timing Curve Advice

[RotaryRocketeer]

Noticed lately that the truck had been running progressively poorer and fuel "economy" was falling off as well. Did a pretty solid tune up, rebuilt a carb I had on the shelf and installed it, did plugs, wires, inspected HEI for wear and degradation, and hunted for vacuum leaks.

During the process, I found that my adjustable vacuum advance can wouldn't hold any vacuum at all. Replaced that with another adjustable unit, reset my initial (to the same value I had set it to after purchasing it) to make sure it was dialed, and the truck is running very well overall. I suspect the new carburetor needs dialed in a bit, but I also don't think that's contributing to the issue.

Problem is, it seems to get a slight and short-lived ping when accelerating from cruise under certain conditions. I am of the opinion that I need to tweak the mechanical/centrifugal advance to come in a bit later and leave the vacuum advance tuning alone. It's running very well as is, and the detonation is very slight, but ideally it wouldn't be there at all.

Opinions?

[Gregski]

please provide more information what are we working with here, 6 banger or a 350, manual or automatic transmission, what year truck? what brand carb, what intake manifold, what exhaust, headers? what spark plugs and what gap? please check your plug wires for resistance. What initial timing did you set? What is your total timing? What vacuum are you pulling at idle? Where did you connect the vacuum advance can to ported or manifold vacuum source? what's your RPM at idle? what octane fuel are you using? not being a jerk just we need to know specifics, and I super highly recommend the Innovate dual Air Fuel Ratio gauge, see my super detailed post here please

INNOVATE DLG1 Wideband O2 Oxygen Sensor Tuning Diagnostics Logging Air Fuel Ratio etc

just trying to help, have fun, take your time, and enjoy the process, these trucks are awesome

[RotaryRocketeer]

1976 GMC (the truck in my profile photo.)
350 w/Vortec 062 heads
Unknown towing/RV cam
Auto trans with stock stall
Edelbrock 1406 (previously a 1407, but it's off for a rebuild/refresh)
Performer RPM intake
Unknown brand longtube headers
2.5" dual exhaust
DUI cap, rotor, module, and coil
Brand new plug wires that ohm out fine
Delco plugs with .055 gap
Initial timing 18 deg
Full manifold vac advance
15" vacuum at idle with advance unplugged and capped
Idle in gear: 600-650
Always running 91 octane, due to chamber volume on the 062s.
Adding a wideband is on my to-do list

Owned the truck for over a year and it sees daily use most of the time, so it isn't like I'm setting up something totally new. Has run very well over period of ownership.

[Gregski]

lower initial timing by 2° and test, then repeat till pinging goes away

[SunSoaked]

I'm still tuning my new 350 with Vortec heads. From what I've researched, vortecs don't like alot of timing advance due to the effeciency of the chamber design. It runs good with 12 degrees static. 055 plug gap is impressive, that DUI must have some power behind it? I had to use accell "shorty" plugs to clear the headers on a few tubes. I also am pulling 15" vac with a mild (256 comp cam).

[Dead Parrot]

Most likely the new vacuum unit is either coming in at a slightly lower vacuum or has a bit more total advance or both. Unlikely that any two will behave exactly the same out of the box due to manf tolerances.

[RotaryRocketeer]

Originally Posted by SunSoaked

I'm still tuning my new 350 with Vortec heads. From what I've researched, vortecs don't like alot of timing advance due to the effeciency of the chamber design. It runs good with 12 degrees static. 055 plug gap is impressive, that DUI must have some power behind it? I had to use accell "shorty" plugs to clear the headers on a few tubes. I also am pulling 15" vac with a mild (256 comp cam).

When I tried running it with less initial (and advance plugged off) it didn't want to maintain an idle that I found acceptable, but I'll toy with it and see what it'll do. I had heard that about these heads too. The DUI spec is .055 and, yes, their ignition setups are pretty gnarly. I was running an Accel Super Coil before. Night and day difference. No plug clearance issues with exhaust. These headers are pretty generous on clearance.

[RotaryRocketeer]

Originally Posted by Dead Parrot

Most likely the new vacuum unit is either coming in at a slightly lower vacuum or has a bit more total advance or both. Unlikely that any two will behave exactly the same out of the box due to manf tolerances.

Certainly. The previous advance unit was an Accel adjustable and this is a Proform. Accel was adjustable for both total increase and rate of increase. The one I just added is only able to be adjusted for rate of increase. Features of the Accel unit were nice, but I've sworn off their products after this latest sudden failure. Not even close to the first time they've failed prematurely, on this vehicle or several others.

[RotaryRocketeer]

Originally Posted by Gregski

lower initial timing by 2° and test, then repeat till pinging goes away

It's definitely worth a shot. I know everything else, component wise, is up to snuff. But like I mentioned in another reply, this cam just doesn't seem to like less initial than what it's currently set at. More fine tuning to come haha. At least I enjoy dealing with these kinds of hassles.

Impressive thread on the wideband tuning, by the way. A wealth of info that I'm gonna put to use. Might have to write your username under the hood when done lol.

[geezer#99]

650 rpm in gear.
What rpm in park?
What’s the rpm that you check the timing at?
Your 18 initial might be inaccurate.

[RotaryRocketeer]

I always set timing at idle in gear. Come on man, give me a little credit now.

Sits around 950-1000 in park, but I'm fairly certain I know what's happening there. My centrifugal advance springs are pretty soft and spindly, so it's almost guaranteed that as the revs rise it's kicking in a bit of mechanical advance. Not to mention how suspect the weights are. Not sure how they return at all, judging by the state of them. I've got a mechanical curve kit coming in tomorrow for testing/repair.

[RotaryRocketeer]

Also FWIW, I got the truck with this cam already installed. If it's been degreed at all or anything like that, I wouldn't know. Just to be clear.

[geezer#99]

Ok, a little credit!
Most people don’t do it that way.
I’ve set it like that a few times with the bumper tied to a power pole when the emergency brake cable was broke.
I’ve used a vac advance limiter like in this link to control total vac advance.
http://www.crankshaftcoalition.com/w...EI_distributor

Your motor likes lots of initial. Likely your vac pot needs a few tweaks.

[SunSoaked]

Ok, so I've learned something here; I've never set timing in gear. Makes sense now that I think about it. The last time I adjusted anything in gear it fell off the Jack's and those 4:56 gears and lockers almost resulted in the K20 crawling over my wife's Volvo!

[RotaryRocketeer]

Originally Posted by geezer#99

Ok, a little credit!
Most people don’t do it that way.
I’ve set it like that a few times with the bumper tied to a power pole when the emergency brake cable was broke.
I’ve used a vac advance limiter like in this link to control total vac advance.
http://www.crankshaftcoalition.com/w...EI_distributor

Your motor likes lots of initial. Likely your vac pot needs a few tweaks.

I grew up as tool assistant in my dad's shop where he built performance cars, bikes, and boats, so there are a FEW things I do know haha. He told me a story about a friend he used to have that would pull the bumper up to a telephone pole and advance initial until the tires spun and then call it tuned. You old dudes are wild haha.

Thanks for the link. I'll definitely look into it. Last night and today have been the first drives since making changes, so it may take a bit to get it perfect.

[RotaryRocketeer]

Originally Posted by SunSoaked

Ok, so I've learned something here; I've never set timing in gear. Makes sense now that I think about it. The last time I adjusted anything in gear it fell off the Jack's and those 4:56 gears and lockers almost resulted in the K20 crawling over my wife's Volvo!

I don't really trust the parking brake to hold while I'm fiddling around trying to make the idle stronger, but the jackstands are a clever idea. My method is to purchase beer, send picture of said beer to a buddy, and tell him it's free as long as he holds a foot on the brake and tells me idle RPM.

[Jake Wade]

https://www.summitracing.com/parts/c...00-1/overview/

I recommend one of these. Use the two blue springs along with the factory HEI weights. It is best to limit the travel of the advance arm to around 1/8 of an inch. I do this by tack welding a plate that acts as a stop. This will give you about 12 degrees vacuum advance. Your HEI should provide 20-22 degrees mechanical advance assuming it hasn't been jacked with. Set initial timing to 12-14 degrees at low idle in gear. experiment hooking vacuum advance to full vacuum(only if you limit the travel) or timed(when vacuum is only applied after the butterflies are cracked open. 32 degrees total (initial+mechanical/all in around 3200) is all you want/need with the combustion chamber shape of the Vortecs.

Been setting them up a long time like this and they haul if everything else is right

[Gregski]

Originally Posted by RotaryRocketeer

Also FWIW, I got the truck with this cam already installed. If it's been degreed at all or anything like that, I wouldn't know. Just to be clear.

Brother is that cam installed straight up, or did some knucklehead install it 4* off thinking if it's a 4* grind you don't install it straight up, so now 4+4=8* out, it's a lot of work but you may need to pull the timing cover off and check the marks, otherwise you are going to drive yourself a crazy.

[Gregski]

Originally Posted by RotaryRocketeer

1976 GMC (the truck in my profile photo.)

I'm sorry I have the signature feature turned off as some people like to share their life story in there, ha ha

Originally Posted by RotaryRocketeer

15" vacuum at idle with advance unplugged and capped

I would suggest you hook up a big analog vacuum gauge (big 4" ones I found to be more stable and reliable than those pesky 2" ones where the needle never stops jumping around) to your intake manifold and hunt around for better vacuum that cam should pull more than 15". 19-21 would rock your world

you'll need to mess with the idle mixture screws a bit and the idle speed screw a bit and go back and forth and back and forth

don't mean to insult your intelligence or micromanage you at all, I type and share a lot of detail in case others are reading this thread and I don't want to assume or insult anybody because I sense you know a lot already

just take your time and always ONLY CHANGE ONE THING AT A TIME and test

[RotaryRocketeer]

Originally Posted by Jake Wade

https://www.summitracing.com/parts/c...00-1/overview/

I recommend one of these. Use the two blue springs along with the factory HEI weights. It is best to limit the travel of the advance arm to around 1/8 of an inch. I do this by tack welding a plate that acts as a stop. This will give you about 12 degrees vacuum advance. Your HEI should provide 20-22 degrees mechanical advance assuming it hasn't been jacked with. Set initial timing to 12-14 degrees at low idle in gear. experiment hooking vacuum advance to full vacuum(only if you limit the travel) or timed(when vacuum is only applied after the butterflies are cracked open. 32 degrees total (initial+mechanical/all in around 3200) is all you want/need with the combustion chamber shape of the Vortecs.

Been setting them up a long time like this and they haul if everything else is right

I got my MSD advance curve kit over the weekend, so I'll be doing a little experimenting with springs coming up soon. My weights are a little more worn than I'm comfortable with, and I'm gonna be putting the MSD weights and bushings on. I'm thinking one light and one medium to get all in a bit quicker, due to how quickly this cam outruns it lungs on the top. Makes great power to about 4200 rpm, and nothing more.

[RotaryRocketeer]

Originally Posted by Gregski

I'm sorry I have the signature feature turned off as some people like to share their life story in there, ha ha



I would suggest you hook up a big analog vacuum gauge (big 4" ones I found to be more stable and reliable than those pesky 2" ones where the needle never stops jumping around) to your intake manifold and hunt around for better vacuum that cam should pull more than 15". 19-21 would rock your world

you'll need to mess with the idle mixture screws a bit and the idle speed screw a bit and go back and forth and back and forth

don't mean to insult your intelligence or micromanage you at all, I type and share a lot of detail in case others are reading this thread and I don't want to assume or insult anybody because I sense you know a lot already

just take your time and always ONLY CHANGE ONE THING AT A TIME and test

The big vac gauge/fuel pressure tester is what I use, I don't even have a small one. Are you saying that I might be able to find a higher vac reading by reading directly from the manifold, instead of using the full manifold port on the carb?

You're not hurting my feelings at all, amigo. I'm the one that asked for advice, so you all just keep it coming.

[Dead Parrot]

Might do a test drive with the vacuum gauge in the cab and see what the reading is when the pings happen. If you give more throttle when it starts pinging, does the ping get worse or go away?

Thinking if the pings start before the vacuum drops low enough to deactivate the vacuum advance and quit when it falls out, you might solve the pinging with a pot that needs a bit more vacuum to operate.

More throttle = less vacuum = less or no vac advance.

Further complicated by what the rpms are as they influence the mechanical advance.

Checking for a difference between the manifold port reading and the carb port reading might uncover a problem with improper gasket installation.

[RotaryRocketeer]

Pinging does go away as more throttle is applied, so I was thinking the same thing. Although the truck didn't ping once on my drive this morning in various traffic conditions. I think a pot that drops out quicker may be a good solution. Problem is, so many of them don't list any specs. Most just say adjustable or non adjustable. I'll check vacuum in both spots and keep an eye out for measurable difference.

[Gregski]

Originally Posted by RotaryRocketeer

Delco plugs with .055 gap

this is a rather big gap, I tried a .050 with my Vortec heads and she was running very hot and very lean, I would recommend just for exercise regapping to .040 or .045 and seeing how she behaves

I have gone through half a dozen junkyard Vortec heads and they were all cracked in one spot or another so I really hope you bought brand new ones, otherwise I will bet you a case of 805 that at least one of yours is cracked

check out a section of my build thread

http://67-72chevytrucks.com/vboard/s...645440&page=75

[RotaryRocketeer]

Originally Posted by Gregski

this is a rather big gap, I tried a .050 with my Vortec heads and she was running very hot and very lean, I would recommend just for exercise regapping to .040 or .045 and seeing how she behaves

I have gone through half a dozen junkyard Vortec heads and they were all cracked in one spot or another so I really hope you bought brand new ones, otherwise I will bet you a case of 805 that at least one of yours is cracked

check out a section of my build thread

http://67-72chevytrucks.com/vboard/s...645440&page=75

I've been thinking of closing the plug gap up 5 thousandths or so, just to see, but that's what the DUI spec calls for with their ignition. Heads definitely weren't cracked when they went on, and I doubt anything has changed. No problems that would indicate any issues with the heads.

Going to dial back the initial a bit today and see what happens. Been pretty busy with job interviews these last few days. Thread will be updated with results.