Some timing questions (experiencing some spark knock)

Dutch Rutter

Full Access Member
Joined
Aug 13, 2013
Posts
1,027
Reaction score
1,086
Location
Independence, Or
First Name
Joshua
Truck Year
1984
Truck Model
K20
Engine Size
5.7
I'm feeling pretty dumb here so please forgive my ignorance.

A short recent backstory:
I just had my 700r4 rebuilt again. The builder wired my Torque converter lockup to my 4th gear solenoid. So whenever the truck is in 4th gear, the converter locks up. It would also appear as though he disabled or removed the stock configuration. Because above 45MPH and in D the transmission will NOT automatically downshift nor will it unlock the torque converter. I can shift it down to 3 and everything works, then once back to cruising speed back to D. (I plan on starting this in a separate transmission thread separately, after calling the shop next week and seeing what he says).

The timing problem/question:
I was driving in D about 55-60mph, very low RPM (probably around 1500) 4th gear, torque converter locked. When adding any more throttle than part cruising throttle (no down shifting) I can feel the engine bog and then the pre-detonation spark knock starts. If returning to cruise throttle it goes away, If manually shifting to 3rd it went away, and I can not replicate the knock at WOT in a normal driving RPM range and gear.

I always run Premium 92 octane and never had this problem before the transmission changes, or at least I never noticed it because it would downshift and I would be at a higher RPM.

ANYWAY, while checking things this evening. I put my timing light on it, unplugged and capped the vac advanced (I have had it going to the ported port on my Edelbrock 1405) at idle I was seeing 30°BTDC.... I checked everything and tried it again same result.
Plugged the vac advanced back in and raised the RPM with the throttle linkage and I was seeing the same 30°BTDC.

Is it just my thinking is wrong or is that kind of high?
The truck idles fine, starts well (not a slow crank), has no run-on dieseling when shutting down, and otherwise does well.
Also is this worth chasing or should I just do the easy fix and keep it in 3 until I'm traveling 65mph+?
 

C10MixMaster

Full Access Member
Joined
Dec 6, 2016
Posts
735
Reaction score
1,306
Location
Kingman AZ
First Name
Ben
Truck Year
1977
Truck Model
C10 BIG10
Engine Size
ZZ4 350
that doesn't sounds right. 30 is a lot for initial timing. If you rev the engine how much advance do you get? 36 should be about max for initial and mechanical not including vacuum advance. Things I would check advance springs Missing broken?, advance seized , advance weights badly worn, vacuum advance stuck?
 

75gmck25

Full Access Member
Joined
Oct 13, 2016
Posts
1,539
Reaction score
1,138
Location
Northern Virginia
First Name
Bruce
Truck Year
1975
Truck Model
K25 Camper Special TH350 NP203
Engine Size
5.7
Your explanation of whet you checked is not clear.

- at idle of about 650-700 rpm, with vacuum advance disconnected and line plugged, base timing should be between 4-16 degrees BTDC. Original GM spec was probably 4 or 8 BTDC, but most engines run better with a little more base timing.
- then run rpm up to about 3k and use an adjustable timing light to see how much timing advances. It should advance about 18-20 degrees by no more than 3k rpm. If someone has recurved the distributor it might reach 20 degrees as low as 2000-2500 rpm, but a stock distributor curve will take more rpm to max out.
- now go back to idle and reconnect the vacuum line. If you connect it to full vacuum You will see idle timing advance 18-20 degrees because of the vacuum advance.
However, if you are connected to a timed vacuum port on the carburetor you may have no idle vacuum, so you will have to then use a vacuum pump to pull vacuum on the line, or connect it temporarily to a full vacuum port.
Total timing (base plus mechanical) should not exceed about 34-36 degrees, and adding in the vacuum advance will take it to 52-54.
 

Dutch Rutter

Full Access Member
Joined
Aug 13, 2013
Posts
1,027
Reaction score
1,086
Location
Independence, Or
First Name
Joshua
Truck Year
1984
Truck Model
K20
Engine Size
5.7
Big thanks to you both!
I did leave my initial post a little vague mainly because I don't know what specific questions to ask, all of this has me second guessing my wrenching/tinkering abilities and knowledge.

The engine is not stock, I built it and installed it October 2019. I'm not sure if it matters but I'll list the specs here.
bored .30 over
stock deck
4 relief cut piston tops
Cam is a speed pro cs-1013
Heads are Pro Comp PC3001-060-0026 aluminum which measure out to 64cc chamber and 190cc runners
Edelbrock RPM Performer 7101 intake
(older) Accel HEI
Carb is a Edelbrock 1405 carb.

When I stabbed that thing in there I do remember setting it to around 12° initial. but that was a while ago now, and its been in and out of various shops for other things (not engine related).
I checked where the dizzy is pointing and it looks to be the same as I remember it being, so I don't think that has been messed with.

@C10MixMaster It idles about 650-700 RPM, when I raised it by hand it was probably 1500RPM. I did not notice any change in the timing. Might be onto it here.
@75gmck25 I am pretty sure I was at 12-18 initial. I never dug into the distributor and changed anything but it is probably a little old now. I also have no problem putting that vac advanced over to the manifold port just to see what its doing.

I'll have a helper tomorrow to hold RPM and check things over closer. I am a little hesitant to just start moving the distributor around because other then that slight knock its running well enough.

My plan for tomorrow:
First and foremost I am going to make sure that my timing marks on my harmonic balancer still zero out on TDC.
like 75GMCk25 said, with the engine warm, at idle plug the Vac adv and see what my timing is showing.
Then have my helper raise the RPM to 3k and see what timing shows
Then at idle plug the vac advanced into the manifold port and see what it shows

With that all done I'll probably pull my dizzy cap (since its too easy not to do) and make check things out in there. Probably snap a few pictures to post up and let you guys know what I find.
 

Dutch Rutter

Full Access Member
Joined
Aug 13, 2013
Posts
1,027
Reaction score
1,086
Location
Independence, Or
First Name
Joshua
Truck Year
1984
Truck Model
K20
Engine Size
5.7
@C10MixMaster and @75gmck25 I got some solid numbers on this thing.

Numbers on the balancer are accurate.
Idle 700rpm with vac adv plugged = 30°
2500 and 3000 rpm with vac adv plugged = 30°
Idle with vac adv plugged into the manifold port = 39°

By these numbers I'm thinking weights and springs under the cap. Pulled the cap and I found that I am able to move the weights freely to what is pictured with no spring tension. This can't be correct can it? By what I'm seeing at least I believe that the springs are not holding any tension which means when started, I'm getting the full centrifugal advanced plus my intial.

I also found some metallic buildup on the contacts on the actual cap. Also pictured.

I figure I might as well replace the cap and rotor. But the spring and weights have me a bit confused.
 

Attachments

  • 20220417_174622.jpg
    20220417_174622.jpg
    84.1 KB · Views: 30
  • 20220417_174635.jpg
    20220417_174635.jpg
    72.7 KB · Views: 30
  • 20220417_174439.jpg
    20220417_174439.jpg
    73.6 KB · Views: 33

C10MixMaster

Full Access Member
Joined
Dec 6, 2016
Posts
735
Reaction score
1,306
Location
Kingman AZ
First Name
Ben
Truck Year
1977
Truck Model
C10 BIG10
Engine Size
ZZ4 350
@C10MixMaster and @75gmck25 I got some solid numbers on this thing.

Numbers on the balancer are accurate.
Idle 700rpm with vac adv plugged = 30°
2500 and 3000 rpm with vac adv plugged = 30°
Idle with vac adv plugged into the manifold port = 39°

By these numbers I'm thinking weights and springs under the cap. Pulled the cap and I found that I am able to move the weights freely to what is pictured with no spring tension. This can't be correct can it? By what I'm seeing at least I believe that the springs are not holding any tension which means when started, I'm getting the full centrifugal advanced plus my intial.

I also found some metallic buildup on the contacts on the actual cap. Also pictured.

I figure I might as well replace the cap and rotor. But the spring and weights have me a bit confused.
vacuum advance is working. the fact you can advance the mechanical advance and take a a pic of it advanced shows there is something wrong there. it should spring back on its own. You might have to remove the distributor and disassemble the shaft, I think these 2 parts are rusted or gummed up. You might get lucky and dribble some oil in it work it back and forth and loosen it up.
You must be registered for see images attach
 

75gmck25

Full Access Member
Joined
Oct 13, 2016
Posts
1,539
Reaction score
1,138
Location
Northern Virginia
First Name
Bruce
Truck Year
1975
Truck Model
K25 Camper Special TH350 NP203
Engine Size
5.7
I’ve never completely disassembled my mechanical advance, so I don’t know how hard it is to remove it. However, yours is definitely screwed up if it does not spring back when released.

I did a google search and found several articles about how to take it all apart. Apparently there were also different advance mechanisms over the years for stock vs. performance applications.

I have aluminum heads and I use a about 16 degrees base timing and 20 more from mechanical advance. I was concerned about too much vacuum advance, so I switched to a can that maxed at 15 instead of the stock 20. You can also find kits that “adjust” vacuum advance, but most adjust only the rate (the vacuum where it starts working) and not the max. Max advance is usually set by moving the stop or adding a bushing over the pin.
 

Dutch Rutter

Full Access Member
Joined
Aug 13, 2013
Posts
1,027
Reaction score
1,086
Location
Independence, Or
First Name
Joshua
Truck Year
1984
Truck Model
K20
Engine Size
5.7
@C10MixMaster That's exactly what I'm looking for. By the looks of the internals on this one I would not be surprised if those shafts are seized up in some fashion. I did try twisting things in there and got nothing to move free.

@75gmck25 I did manage to snag the part number of the distributor. It is an Accel 59107 which has an advance curve of
6 degrees at 900 rpm
8 to 10 at 1,200 rpm
10 to 11 at 1,500 rpm
and they suggest 2-4 of initial over factory.
With that and assuming I'm stuck at full advanced my initial is around 19.

I found a new cap and rotor for it for $40, but I also found an entire new Accel 59107 for $167 (without coil). I'm thinking go that route and once I get the truck dialed in with the new Distributor. I can tear the old one apart. If I'm able to get it freed up, then I'll have a spare on the shelf.

I'll get things ordered up and report back.
 

Dutch Rutter

Full Access Member
Joined
Aug 13, 2013
Posts
1,027
Reaction score
1,086
Location
Independence, Or
First Name
Joshua
Truck Year
1984
Truck Model
K20
Engine Size
5.7
New Dizzy should be arriving this Friday.

I ended up going down the timing rabbit hole when I couldn't sleep last night and found this "How To Set Timing".
1. Plug vac. adv. line.
2. Hook up timing light
3. Read the timing while you increase RPM's until it stops advancing...record that RPM.
4. Observe the total advance and adjust to 32-34 degrees (keep the RPM's up while doing this).
5. Only mess with the advance springs if it took more than about 2,500-3,000 RPM to come in fully.
6. Don't worry about your timing at idle. Just hook up the vac. advance when you're finished and that's it.

Obviously, the idle on the carb would need adjusted some after that.
but is this seen as the "right" way to dial in ones timing on a not-stock engine or is there a better way?
In the past I've always focused on setting the timing based off the startup behavior and throttle response which probably was not the correct way.
 

Matt69olds

Full Access Member
Joined
Jan 8, 2018
Posts
1,757
Reaction score
2,296
Location
Central Indiana
First Name
Matt
Truck Year
81
Truck Model
GMC 1/2 ton
Engine Size
455 Olds
Read this.

 

Dutch Rutter

Full Access Member
Joined
Aug 13, 2013
Posts
1,027
Reaction score
1,086
Location
Independence, Or
First Name
Joshua
Truck Year
1984
Truck Model
K20
Engine Size
5.7
@Matt69olds thanks for that awesome link! That explained a lot. You sir are a fountain of knowledge. :favorites13:

@75gmck25 and @C10MixMaster
I am happy to say that I have the new distributor in, and dialed in quite well. At idle with the vac advanced plugged off I'm seeing 18°, then 39-40 at 2500rpm. I put it all back together and took it for a drive. Pinging is all gone (even running regular pump) at too low of RPM and adding slight power. (where the trans should down shift but doesent right now) I'm hoping to get the new transmission shop to fix that.

Whatever the cause. It's fixed now and running better then ever. Minus the transmission issue. Hopefully, it's just the TV cable like the trans builder thinks, that will be up to him to fix.
 
Last edited:

78C10BigTen

Full Access Member
Joined
Apr 12, 2017
Posts
13,240
Reaction score
15,607
Location
pennsylvannia
First Name
Ted
Truck Year
1978
Truck Model
C10 BIG TEN
Engine Size
350
@Matt69olds thanks for that awesome link! That explained a lot. You sir are a fountain of knowledge. :favorites13:

@75gmck25 and @C10MixMaster
I am happy to say that I have the new distributor in, and dialed in quite well. At idle with the vac advanced plugged off I'm seeing 18°, then 39-40 at 2500rpm. I put it all back together and took it for a drive. Pinging is all gone (even running regular pump) at too low of RPM and adding slight power. (where the trans should down shift but doesent right now) I'm hoping to get the new transmission shop to fix that.

I also dug into my old distributor and found exactly what was causing it to stay advanced.. I believe the old transmission shop (the sketchy one that went out of business) was trying to avoid fixing their POS transmission and pulled some sh!t... the stud under the centrifugal advanced portion of the distributor was welded into the advanced position. I have no idea why ANYONE would do something like that, but thats the only thing I can think would have happened... see the attached pic of the perfect spot welds.

Whatever the cause. It's fixed now and running better then ever. Minus the transmission issue. Hopefully, it's just the TV cable like the trans builder thinks, that will be up to him to fix.
Wow! But that looks way too perfect to be a human weld. At least to my eye. But yes i understand that shouldnt be welded. Glad to see you got it fixed easy.
 

Dutch Rutter

Full Access Member
Joined
Aug 13, 2013
Posts
1,027
Reaction score
1,086
Location
Independence, Or
First Name
Joshua
Truck Year
1984
Truck Model
K20
Engine Size
5.7
Wow! But that looks way too perfect to be a human weld. At least to my eye. But yes i understand that shouldnt be welded. Glad to see you got it fixed easy.
I was thinking that too and they are very small and tight. Not like any spot weld I've ever done. I do remember it working when I first plugged it into this engine. I do hate assuming the worst of that garage. I'll just go with, it broke and its fixed and working now.

I suppose it could be possible something shorted and it welded itself?
 

75gmck25

Full Access Member
Joined
Oct 13, 2016
Posts
1,539
Reaction score
1,138
Location
Northern Virginia
First Name
Bruce
Truck Year
1975
Truck Model
K25 Camper Special TH350 NP203
Engine Size
5.7
The article you found on tuning that used a maximum advance of 34-36 degrees is generally correct, but a little backward from the way most folks think. Most think base + mechanical = total, but you can also start out with total and work backwards.

The mechanical advance in my Pace distributor adds 20 degrees by 2000 RPM (a fairly fast curve) so with my 15-16 degrees base timing I have 35-36 degrees total. The original vacuum advance added 22 degrees, so I swapped it for one that only adds 15 degrees. I have aluminum heads, which tolerate more timing than stock iron heads, but 22 degrees from vacuum advance just seemed like a lot.
 

RaisedK5

Junior Member
Joined
Sep 12, 2021
Posts
17
Reaction score
4
Location
Long Beach
First Name
Leland
Truck Year
1975
Truck Model
Blazer
Engine Size
350
That plate is affixed to the shaft. Look at c10mixmasters photo above. The welds are not wrong. The rest of the assembly floats around the plate to allow for the advance.
 

Latest posts

Forum statistics

Threads
36,195
Posts
764,801
Members
25,352
Latest member
El82Blazer
Top