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Newly rebuilt L36 (427/390), all stock configuration with the exception of forged pistons instead of cast, 0.030" overbore. Compression should be right around 10.0-10.25.
I have a mid-range knocking/rattling that happens around 60mph if I try to accelerate hard in 4th gear (~3500rpm?). The engine sounds fine when accelerating hard in 1st through 3rd and pulls well. I am using the original 360-12 vacuum can which is adding 14°, engine is pulling ~19mm Hg at 800rpm. Dwell was checked at 30°.
My timing is set up this way:
I'm thinking that lowering the "all in" to 36° mechanical is what I should be aiming for.
To do this, check for what bushing (if any) is on the underside of the football; install variations to get max 36°?
Is my thinking incorrect that an "all in" mechanical reading at 3600 is a less aggressive set up than at 3000?
For total timing, I'm thinking that I need to do something similar to my original vac can that's adding 14° instead of 12°. Yes?
Is this still a GM distributor? I like a lot more initial and limiting the total. 39* might be what it needs at higher RPM.....but is probably too much at hard throttle in high gear.
Not sure what rear gears you have......but making a hard pull at 3500 RPM without a downshift from an auto or dropping a gear in a 4 speed is tough on stuff. If you want to keep driving that way, you're going to need to slow down the total advance so it comes at a higher RPM.
Dropping a gear if you have to punch it is an easier plan. But either way...ao re-curve of the distributor will help everything. More initial...limiting total to keep it at 36-38 or so. Sometimes they can take 40-42* total (no vacuum)...but that takes good tuning and fuel.
Yes, you need as big a bushing as possible on the advance pin to limit maximum centrifugal advance. That will probably limit it to around 24 degrees and you can then try 12 degrees initial timing. As you shorten the advance curve with a bigger bushing, the pin will not travel as far and that will tend to bring all centrifugal advance in sooner so if would leave the springs alone and see what the bushing does. Odds are it will be OK.
What fuel are you using? The 427/390 with its milder cam may be more demanding of good fuel at low to mid rpm due to its tendency to have strong cylinder pressures at low rpm compared to an L72. Total advance of around 36 may end up working well in it. Or you may have to cut another degree or two out of it or it may be able to take a bit more timing depending on fuel and engine condition. But with the increased initial advance setting it should remain feeling strong and responsive.
Is this still a GM distributor? I like a lot more initial and limiting the total. 39* might be what it needs at higher RPM.....but is probably too much at hard throttle in high gear.
Not sure what rear gears you have......but making a hard pull at 3500 RPM without a downshift from an auto or dropping a gear in a 4 speed is tough on stuff. If you want to keep driving that way, you're going to need to slow down the total advance so it comes at a higher RPM.
Dropping a gear if you have to punch it is an easier plan. But either way...ao re-curve of the distributor will help everything. More initial...limiting total to keep it at 36-38 or so. Sometimes they can take 40-42* total (no vacuum)...but that takes good tuning and fuel.
JIM
Yes, original points/condenser, which has been rebuilt with shims limiting up/down movement in the last 4-5 years.
The car has 3.08 gears, Muncie 4-speed. I can't imagine downshifting to 3rd at 60mph! Seems I'd be way out of the power band and in the 5000 rpm area. The need for acceleration at this speed doesn't seem like it's out of the ordinary or would be hard on an engine -- like to get out of the way of a merging semi or open spacing in a crowd.
* I am just guessing at the 3500 rpm where this happens - thinking on it this morning it's probably lower than that given the 3.08 rear. I didn't look at the tach but we're talking highways speeds and just putting your foot in it to jump maybe 10mph.
Last edited by barkingrats; Sep 21, 2025 at 11:15 AM.
Yes, you need as big a bushing as possible on the advance pin to limit maximum centrifugal advance. That will probably limit it to around 24 degrees and you can then try 12 degrees initial timing. As you shorten the advance curve with a bigger bushing, the pin will not travel as far and that will tend to bring all centrifugal advance in sooner so if would leave the springs alone and see what the bushing does. Odds are it will be OK.
What fuel are you using? The 427/390 with its milder cam may be more demanding of good fuel at low to mid rpm due to its tendency to have strong cylinder pressures at low rpm compared to an L72. Total advance of around 36 may end up working well in it. Or you may have to cut another degree or two out of it or it may be able to take a bit more timing depending on fuel and engine condition. But with the increased initial advance setting it should remain feeling strong and responsive.
Thanks for the confirmation on the bushing path. The fuel is Texaco non-ethanol premium (93).
Could this have anything to do with a lean secondary circuit in the Holley?
I would try to limit the V/A to around 10* and see if the ping stops. How much idle vacuum does the engine have warmed up at idle?
It's pulling about 19mm Hg at about 800 rpm.
My plan is to check for and make sure it's got a mechanical advance bushing to get total ≤ 36° and then reset the "all in". (With 2 very similar BB distributors, it's hard to remember what I've done to each).
I've been thinking on the vacuum advance @ 14° where it was originally supposed to be spec'd at 12°. I think that at 60mph where I am trying to push it suddenly harder, the vacuum should be dropping out so that the engine is relying way more heavily on mechanical... yes?
That sounds like pretty good fuel for the car. Not sure if pinging is very related to a lean mixture. A lean mixture would heat things up which certainly could contribute to pinging. But I doubt it would cause it immediately upon getting into the throttle. Maybe on a long pull in 4th gear up to high speed. Vacuum advance, on the other hand, should fall out very quickly upon flooring the throttle - unless the advance plate is dragging or something. But Tbarb know his stuff so if he suggests a trial at limiting the amount, I would give it a shot.
Here is a somewhat old and blurry photo of a stop I made for my vacuum advance out of sheet metal. it has a bent tang that extends into the slot the advance pin rides in. Bending the tang varies the amount of vacuum advance. Looks like I also put a bushing on the advance pin first to cut it down. You kind of have to play around with it to get to the desired point.
Go to your local general aviation airport and put in a tank of 100LL. Enjoy it for a while. Should be great.Then go back to absolute factory specs as a baseline. Adjust one thing at a time. Change to pump gas. Adjust one thing at a time. Test.
I have a mid-range knocking/rattling that happens around 60mph if I try to accelerate hard in 4th gear (~3500rpm?). The engine sounds fine when accelerating hard in 1st through 3rd and pulls well. I am using the original 360-12 vacuum can which is adding 14°, engine is pulling ~19mm Hg at 800rpm. Dwell was checked at 30°
Originally Posted by barkingrats
It's pulling about 19mm Hg at about 800 rpm.
My plan is to check for and make sure it's got a mechanical advance bushing to get total ≤ 36° and then reset the "all in". (With 2 very similar BB distributors, it's hard to remember what I've done to each).
I've been thinking on the vacuum advance @ 14° where it was originally supposed to be spec'd at 12°. I think that at 60mph where I am trying to push it suddenly harder, the vacuum should be dropping out so that the engine is relying way more heavily on mechanical... yes?
When trying to accelerate hard in 4th gear you should have less than 8" vacuum, with no vacuum advance. Confirm/ make test runs with a vacuum gauge.
It could be going lean at that point. Turn your idle screws out a turn and see if it helps. Maybe block off the PCV and see if that will richen the mixture a bit too. I eliminated a slight ping when I stepped up my primary jets from 65 to 69 on a 67 L79.
Yes, original points/condenser, which has been rebuilt with shims limiting up/down movement in the last 4-5 years.
The car has 3.08 gears, Muncie 4-speed. I can't imagine downshifting to 3rd at 60mph! Seems I'd be way out of the power band and in the 5000 rpm area. The need for acceleration at this speed doesn't seem like it's out of the ordinary or would be hard on an engine -- like to get out of the way of a merging semi or open spacing in a crowd.
* I am just guessing at the 3500 rpm where this happens - thinking on it this morning it's probably lower than that given the 3.08 rear. I didn't look at the tach but we're talking highways speeds and just putting your foot in it to jump maybe 10mph.
With your gearing and I'm guessing a 26" tall rear tire....you're only turning 2389 RPM at 60 MPH. It's only 2300 RPM if you have a 27" tire. Dropping to 3rd gear with a wide ratio Muncie (1.46 ratio) only puts you at 3358 RPM with those 27" tires.
You'll be going 89 MPH in 3rd gear at 5000 RPM with 27" tires.
Heck...you could drop to 2nd gear at 60 MPH and still only be spinning 4325 RPM!!
You're asking a lot from that engine to take total timing at way under 3000 RPM at WOT.
WHAT AN IDJIT I AM!
<ahem>... i found the problem...
I had very muchly the wrong springs in the distributor. When looking for the advance bushing (it was there), I noticed the black springs had awfully fine coils - yep, the lightest springs in a Mr. Gasket kit! In my distributor parts baggie I found a GM original, which is also very dark in tone, though not black. I must have swapped in the soft springs for the GM without realizing it. The full timing must have been coming in way down in the rpm range.
After putting in heavier springs, I measured the timing curve over a range and did notice a "flat spot" right around 2000 to 2500 where the timing doesn't advance but by 2750 it's back on track. I took it out this afternoon and put it through its paces. (It's nice living in the country.) There might be the slightest ping at times, but it's hard to hear with the side pipes; it's definitely not the knocking rattle from before. I'll pull it back another degree or two to be sure.
Thanks everyone for listening and offering suggestions on what to look for.
I always avoid the finest/weakest springs in those kits. Full advance in by 1500-2000 rpm is fine for the drag strip but most of these cars live moderately sedate lives on the street now with pump gas.
I have been experimenting with my distributor after finding the mechanical advance binding. I used A dial back timming light and after much reading and expermiting I have found the further up you take your base timming the less hesitation there is when you stomp the accelerator at lower RPM which means you have got to limit mechanical advance numbers by the bushing to get the 35-38 total without vaccume advance. I also shimmed the vaccume advance² to where it only pulls 8 degrees advance.
Right now iam running 12 base 26 mechanical =38 total there and 8 vaccume for A total of 47. Iam going back in with A larger bushing for the mechanical advance, thanks Lowes for a neoprene bushing, to try to get my base up to 16.
I found if you have alot of vaccume advance at lower steady RPM and stomp the accelerator there is too much going on with that sudden drop in vaccume advance and my motor hesitates/ stumbles.
I have one of the lightest springs and one middle spring on my weights now from A MOROSO kit and my advance starts at 900 and all in by 2800.
I’m not sure everyone agrees with this but I have 4 old cars and I have reworked the distributor in 3 of them to allow 15-18 degrees initial timing and 33 to 36 degrees total depending on the engine. The 4th distributor is a Ford dual point from a 427 so it was already pretty racy.
On my Corvette distributor I got to 24 degrees mechanical advance with the biggest bushing that fit in the slot. To improve on that I took a larger MSD bushing and filed down 2 opposing sides until it fit in the slot and would travel smoothly in the slot. The longer length limited my mechanical advance to 18 degrees I believe without looking it up. That allowed me to move up my initial timing and still keep my total down so it will run on 91 non-ethanol. and don’t forget that the shorter travel with the larger bushing will speed up your advance since it travels a shorter distance and doesn’t stretch the springs as much.
I’m not sure everyone agrees with this but I have 4 old cars and I have reworked the distributor in 3 of them to allow 15-18 degrees initial timing and 33 to 36 degrees total depending on the engine. The 4th distributor is a Ford dual point from a 427 so it was already pretty racy.
On my Corvette distributor I got to 24 degrees mechanical advance with the biggest bushing that fit in the slot. To improve on that I took a larger MSD bushing and filed down 2 opposing sides until it fit in the slot and would travel smoothly in the slot. The longer length limited my mechanical advance to 18 degrees I believe without looking it up. That allowed me to move up my initial timing and still keep my total down so it will run on 91 non-ethanol. and don’t forget that the shorter travel with the larger bushing will speed up your advance since it travels a shorter distance and doesn’t stretch the springs as much.
Do the above to eliminate the, "flat spot" right around 2000 to 2500 where the timing doesn't advance."
Keep the stiffer springs that work.
I run 12* at 650rpm + 22* mechanical, for 34* total by 3200rpm (aluminum GM Phase 6 heads).
WHAT AN IDJIT I AM!
<ahem>... i found the problem...
I had very muchly the wrong springs in the distributor. When looking for the advance bushing (it was there), I noticed the black springs had awfully fine coils - yep, the lightest springs in a Mr. Gasket kit! In my distributor parts baggie I found a GM original, which is also very dark in tone, though not black. I must have swapped in the soft springs for the GM without realizing it. The full timing must have been coming in way down in the rpm range.
After putting in heavier springs, I measured the timing curve over a range and did notice a "flat spot" right around 2000 to 2500 where the timing doesn't advance but by 2750 it's back on track. I took it out this afternoon and put it through its paces. (It's nice living in the country.) There might be the slightest ping at times, but it's hard to hear with the side pipes; it's definitely not the knocking rattle from before. I'll pull it back another degree or two to be sure.
Thanks everyone for listening and offering suggestions on what to look for.
If you are so inclined try the 201/15 or a B22 vacuum advance and see if the remaining rattle goes away.
Mid-range pinging
