Erratic Timing at 3800+ ('67 427/390)

[Owen L.]

Copy/pasted from https://www.forums.ncrs.org/showthre...hat-the-hell-)

I always recommend the OE replacement truck double roller chain. It's available in a number of different brands, but all are made by Cloyes and you should be able to find the complete set for about 25 bucks.

This is NOT the Cloyes aftermarket timing set that sells for close to a hundred bucks. The OE truck replacement chain has 0.200" diameter rollers. The Cloyes aftermarket rollers are 0.250" diameter, and you need to check carefully for interference with the block. Unless you are doing 9000 rev clutch drops the truck replacement chain in typical vintage car service will outlive all of us.

Duke

Duke------


You're right; the double roller truck chain will last longer than any of us are going to need it. Without a doubt. However, I still only use and recommend the Cloyes 9-3100 True Roller timing set for small blocks and 9-3110 for big blocks (with flat tappet cams).

James------

A "non-invasive" "clue" to indicate deterioration of the cam sprocket is to observe the timing mark illuminated by a timing light. If the mark "jumps around", even a little, the timing sprocket is worn or deteriorated. If it "jumps around" a lot, you're "living on borrowed time".

My '67 427/390 shows very erratic timing starting at 3800 rpm. I've gone through the entire ignition system and replaced the maintenance items with quality parts, and rebuilt the distributor (which showed no slop in the bearings or end-play). Joe's last comment above made my heart stop a bit...

When checking timing, the balancer line is pretty stable with only minor degree or so fluctuations every now and then when increasing engine revs all the way to 3800. Once it hits this threshold the mark begins swinging wildly off the timing tab in both directions. Does this sound like a timing sprocket and chain issue? Engine is completely original and never been out of the car. 65k miles and owned for 43 years.

I was looking at the Cloyes chain sets, and heeding Duke's comment about the 0.200" rollers, found #C3024 @$45; Duke, is this the one you are referring to? Joe's comment about #9-3110 shows 0.250" rollers @$170(!) so I'd much rather save the $125 if the 3024 will work fine with no issues.

Thanks all!
Owen

[Joe L.]

Re: Erratic Timing at 3800+ ('67 427/390)

Owen------


Usually, the cam timing sprocket problem will be seen as the timing mark "jumping around" at idle. However, I suppose it's possible that, depending on the particular wear situation, it might not be evidenced at idle but at higher RPM. With the age and mileage of your engine the cam timing sprocket is highly suspect and due for replacement whether or not it's the source of your particular problem. I can absolutely guarantee you that if the engine has never been apart it has the nylon tooth cam sprocket and early link chain.

The less expensive timing set will work fine for you. However, consider that even if you were to replace the timing set with a stock early link type timing set with nylon tooth cam sprocket (assuming you could find one), it would also work just fine for as long as you're going to need it.

The purpose of the stock type timing set is to reduce engine noise. For MD/HD trucks noise control is not an issue so the more durable but noisier "roller" type timing set is used. However, the "rollers" in this chain do not roll. With the Cloyes "Tru-Roller" sets, the rollers do roll. In fact, that's why they are called "Tru-Roller" and why they cost a lot more. In addition to durability, strength and precise timing, they operate at about the same noise level as the stock-type early link type with nylon tooth cam sprocket. So, they represent the "best of both worlds".

[Duke W.]

Re: Erratic Timing at 3800+ ('67 427/390)

I'm pretty sure your engine was built with the nylon tooth cam sprocket, and I'm sure it's quite deteriorated. See the recent LT-1 thread for some photos of what these things look like after decades.

I believe the C-3024K is the OE replacement big block double row timing chain. The C-3024X has two additional notches in the crank sprocket to allow 4 degrees advance or retard, which you do not need

I think if you shop around you can find the K for less than 30 bucks. I believe this Cloyes manufactured chain is available in many brands, so if you have a favorite local parts store, give them the Cloyes number and they cross it to whatever brand they sell, and you can order it and inspect before you pay and take delivery.

Duke

[Duke W.]

Re: Erratic Timing at 3800+ ('67 427/390)

Owen------


With the Cloyes "Tru-Roller" sets, the rollers do roll. In fact, that's why they are called "Tru-Roller" and why they cost a lot more. In addition to durability, strength and precise timing, they operate at about the same noise level as the stock-type early link type with nylon tooth cam sprocket. So, they represent the "best of both worlds".

We've had this conversation before and a roller chain is a roller chain, like any bicycle or motorcycle chain. The roller locks to the sprocket and rolls about the pin. If the roller didn't do this and had to rotate relative to the sprocket the sprocket would wear very rapidly.

Most motorcycles now have "O-ring" roller chains. They are manufactured with grease between the roller and pin sealed with O-rings and theoretically require no periodic lubrication. Engine timing chains don't need the grease or O-rings because there is enough splash oil inside the timing cover to maintain an oil film between the pin and roller for long service life.

Cloyies Tru-roller name is just marketing hype.

Duke

[Joe L.]

Re: Erratic Timing at 3800+ ('67 427/390)

Duke-----

Yes, you're right; we've had this conversation before. Nothing has changed, though.

Attached are photos of a Cloyes C-163 timing chain. This is the "HD" "roller" timing chain. This is the chain supplied with the Cloyes timing set C-3023. This chain and set have been around for 60+ years. The chain is equivalent to the GM #3735411 which dates back to about 1957 and is discontinued.

The rollers on this chain DO NOT ROTATE. They don't rotate by hand and they don't rotate using the pliars seen in one of the photos. They are stationary. No doubt, at all, about it.

As I said, that's why the Cloyes Tru-Roller is called Tru-Roller because the rollers actually rotate.

DSCN3979.jpgDSCN3982.jpgDSCN3984.jpg

[Owen L.]

The .200" number is speaking to the diameter of the roller or bushing of the chain. Our higher quality chains will have a .250" or 1/4" diameter roller.

There are generally 2 different types of double row chains. A double row fixed bushing (C-3024X), or a true double roller (9-1110). The fixed bushing chain looks the same as a typical double roller, but the "rollers" do not spin freely on the chain pins, instead they are fixed bushings between the links. On a true double roller chain, the rollers spin freely on the pins resulting in lower friction and wear. The double row fixed bushing chain will be an upgrade over a factory inverted tooth chain, but a true roller chain will ultimately be the best quality replacement.

Thanks,
____________________________
CODY SMITH
Technical Support Manager

Given the equal pricing, I guess I'll go with the 9-1110. I absolutely know that, even going with the $24 #3011 direct replacement chain, they will all outlast me. I guess I'm caught in the trap of "high performance" and "best quality".

[Joe L.]

Re: Erratic Timing at 3800+ ('67 427/390)


Owen------


Yes, the Cloyes and GM part numbers I provided are for small block chains. I guess I had forgotten that the original post was inquiring about big blocks.

For Mark IV big blocks the basic Cloyes "roller" timing set is C-3024 and the upgraded "True Roller" is 9-1110 just as you have been advised. The best "True Roller" is 9-3110. That's the one I recommend regardless of the fact that ANY of the aforementioned will work just fine and last longer than you're ever going to need them.

[David H.]

Re: Erratic Timing at 3800+ ('67 427/390)

... My '67 427/390 shows very erratic timing starting at 3800 rpm. I've gone through the entire ignition system and replaced the maintenance items with quality parts, and rebuilt the distributor ...

Owen

How far was this distributor torn down/tested during rebuild?

Link below is to a Sun Distributor Testing Machine pdf. Don't expect you have a Sun Machine or access to one, however pdf includes several tests that don't really require machine operation. For example, Breaker Point and Breaker Plate checks where problems would result in erratic timing.

Might give you some ideas of areas to check.

Dave


sun manual.pdf (starchak.ca)

[Owen L.]

Re: Erratic Timing at 3800+ ('67 427/390)

Owen

How far was this distributor torn down/tested during rebuild?

Link below is to a Sun Distributor Testing Machine pdf. Don't expect you have a Sun Machine or access to one, however pdf includes several tests that don't really require machine operation. For example, Breaker Point and Breaker Plate checks where problems would result in erratic timing.

Might give you some ideas of areas to check.

Dave


sun manual.pdf (starchak.ca)

Thank you for the Sun file - I wish I had easy access to a machine. I tore the distributor down to all its component parts and did my best to measure wear in the bushings and amount of end play, but did not really look into the breaker plate. I'll investigate further with the Sun information.

[Owen L.]

Re: Erratic Timing at 3800+ ('67 427/390)

Owen

How far was this distributor torn down/tested during rebuild?

Link below is to a Sun Distributor Testing Machine pdf. Don't expect you have a Sun Machine or access to one, however pdf includes several tests that don't really require machine operation. For example, Breaker Point and Breaker Plate checks where problems would result in erratic timing.

Might give you some ideas of areas to check.

Dave


sun manual.pdf (starchak.ca)

I'm not totally convinced my erratic timing is due to the timing chain. Not that it doesn't need to be changed for peace of mind given the nylon gear coating, but it doesn't make sense to me that stretch and looseness would only appear under the highest rpm and not at lower rpms as Joe alluded to above. I'll wait until Spring or Summer before I pull the engine apart to tackle the chain set but am still noodling the timing cause being something else.

The distributor was completely disassembled, cleaned, and reassembled. I didn't feel any side-to-side looseness of the shaft in the housing and the end-play for the camshaft gear was right around .006" (as I recall) when I pulled it from the car. I didn't measure with a dial indicator for bushing wear but they certainly felt firm. If I don't come up with other things to try in my questions below, I'll pull the distributor again and do real measurements of the shaft play.

I need some affirmation on my knowledge of point gap and dwell: The gap and dwell are both adjusted with the hex-screw through the cap's window; meaning an adjustment for one changes the other and there is no separate way to set gap and then independently change dwell. Yes?

In reading through the Sun pdf, is this section:
Screen Shot 2022-02-04 at 10.49.16 AM.jpg

1. By ignition failure at high speed, could this be what I'm seeing on the timing tab? If a greater dwell angle solves it, is the "only" downside quicker wearing points? And, what might be the reason a particular distributor may be outside the recommended 28-32 setting? I always set dwell in the middle at 30. (I've always been curious about the range setting - why choose 28, 29, 30, etc?)
2. Lastly, I'm wondering about the advance weight springs. Can I test the total advance for timing stability with no springs attached at all or should I at least have the lightest Mr. Gasket springs on it?

[David H.]

Re: Erratic Timing at 3800+ ('67 427/390)

... I need some affirmation on my knowledge of point gap and dwell: The gap and dwell are both adjusted with the hex-screw through the cap's window; meaning an adjustment for one changes the other and there is no separate way to set gap and then independently change dwell. Yes?
...

1. By ignition failure at high speed, could this be what I'm seeing on the timing tab? If a greater dwell angle solves it, is the "only" downside quicker wearing points? And, what might be the reason a particular distributor may be outside the recommended 28-32 setting? I always set dwell in the middle at 30. (I've always been curious about the range setting - why choose 28, 29, 30, etc?)
2. Lastly, I'm wondering about the advance weight springs. Can I test the total advance for timing stability with no springs attached at all or should I at least have the lightest Mr. Gasket springs on it?

Ultimate "dwell", leave key on (ignition on), but engine off - if points are closed (66% chance), then ignition current will burn up points and fry coil.

Timing does not effect dwell. Set dwell with your adjustment of point gap.

On the other hand, Dwell can effect timing. Ignition spark is triggered when points open. Points open later - spark comes later - retarding timing. Points open sooner - spark comes sooner - advances timing. Common effect, as points rubbing block wears, point gap diminishes retarding spark.

Advance weights come into effect at higher rpm. Weights determine how quickly mechanical advance comes on - no weights would negate mechanical advance.

Duke has numerous posts and pdf re timing that are very helpful as to setting initial, vacuum, and mechanical timing.

If you can carefully examine/measure lobes of distributor cam for wear that might identify a cause your erratic timing.

Dave

[David M.]

Re: Erratic Timing at 3800+ ('67 427/390)

Owen,
What are the circumstances around this problem? Did it start out of the blue or after maintenance?

Have you checked the cranking and running VDC to the coil?
Have you checked the primary & secondary coil winding resistance?
Have you verified the negative wire to the points is intact? If the wire has a break in it, it will show up when the breaker plate moves around. Years of stress on that wire will break it down. Usually right at the point it enters the distributor base.

Do you know what the timing curve is...Base+Vac+Mechanical, all in at what RPM? = total timing.

Things to check:
-Read the spark plugs. I've seen plugs visually inspect ok yet act silly under higher RPM. Especially the cheap offshore chineasium junk. NGK is a great plug brand. A/C, Autolite & Champion are no longer the great plugs they once were.
-With the ignition shielding removed run the engine in the dark. Look for voltage leakage (arching) on the cap and wires. Pinched plug wire(s)?
-Re-inspect centrifugal advance mechanism...the stops the weights the springs.
-Test each plug wire resistance. New means nothing today.
-Engine grounds
-Rotor tab loose at the rivet?
-Fuel system...if its starving for fuel at higher RPM of course you are going to see the timing mark bouncing around.

[Owen L.]

Re: Erratic Timing at 3800+ ('67 427/390)

Thank you David for the clarification on my understanding of dwell/point gap.

Duke has numerous posts and pdf re timing that are very helpful as to setting initial, vacuum, and mechanical timing.
If you can carefully examine/measure lobes of distributor cam for wear that might identify a cause your erratic timing.

I have read Duke's and Lars' methods and info over and over. That's how I came to identify the erratic spark at higher rpms -- trying to see full mechanical advance.

Yesterday I decided to pull the distributor apart again to check the shaft to bearing play along with other things. It's #1111247, with football #168, and advance/cam plate #732 CCW. (I don't recall if, in the last 40 years, I changed out the main shaft due to a destroyed tach drive gear or not.)

• Using a dial indicator and mag base, the most I can get the shaft to move against the bottom bushing is .0025"-.003"; measured in a couple of different locations of the housing. The shaft to top bushing is .0015" to .002". In an article by Dave Fiedler on restoring these distributors he advises .002" clearance of the shaft and bearings.
• End play is right around .008".
• The rubber stop bushing on the underside of the football is gone - actually found in pieces under the breaker plate. My hope that it would survive when I reassembled last time is dashed! So, I installed the brass bushing that came in the Mr. Gasket advance spring kit.
• The breaker plate was a touch sloppy on the vacuum can's arm, so I tightened that up a bit. Ground is solidly attached.
• The points cam lobes look good with barely any rounding. When measured, they all had seemed to be within .001" on the flats of each other and the lobes were all pretty consistent at about .024" height above the flats.
• Tach drive gears look fine and undamaged.
• I misplaced one of the stock advance springs, so installed the middle weight Mr. G spring pair. The weights move smoothly on the posts.
• Vacuum can is original (can't recall the #s but holds vacuum and advances and releases the breaker plate as it should).
• Coil wire and breaker plate grounding wires are in good condition, flexible, with no insulation damage. Terminals are firmly attached.

Reinstalled, re-timed, and still have the fluttering at higher rpm. Now I'm thinking that whatever the issue is, it's outside of the distributor. On to David Morland's reply...

[Duke W.]

Re: Erratic Timing at 3800+ ('67 427/390)

One thing that can cause ignition problems is a wobbly breaker plate. It was a problem on my SWC from day one and took me a couple of years to figure out. The dealer was worthless.

The breaker plate should fit very snuggly over the distributor housing. When you push down on the edge is should not dip. The force of the breaker arm spring places a significant downward moment on the breaker plate and, if it's not snug it can wobble, which reduces dwell that can result big spark scatter and misfiring.

Free rev it to 5000. Dwell should not fall by more than two degrees.

The standard 19-23 oz. breaker points should be satisfactory given the 5300 redline. What make/model did you install and what is the breaker arm tension?

How much does the timing vary up and down at various engine speeds (specify), and under load does the engine misfire?

Duke

[Owen L.]

Re: Erratic Timing at 3800+ ('67 427/390)

Owen,
What are the circumstances around this problem? Did it start out of the blue or after maintenance?

Thanks David for the detailed questioning. Honestly, I have no idea when it started. I've had the car since I was 18 way back in '79 but never really knew about timing it beyond base idle until a couple years ago when I got my focus back into my Corvettes with the engine rebuild of my '72. I discovered this issue a couple summers ago but didn't fully tackle it because of other engine performance issues from disuse.

I'm not great when it comes to electricals, so you if you can walk me through some of it I'd appreciate it.

Have you checked the cranking and running VDC to the coil?
Have you checked the primary & secondary coil winding resistance?
Have you verified the negative wire to the points is intact? If the wire has a break in it, it will show up when the breaker plate moves around. Years of stress on that wire will break it down. Usually right at the point it enters the distributor base.

In the process of measuring volts and ohms I cleaned up all spade terminals and the contact screws.

• Battery= 12.5v after sitting overnight; while running= 13.8v;
• Alternator output on +/- terminals while running= 13.98v;
• The two coils I've tried measure 0.9 & 8.80 and 1.1 & 9.02. I also have a new (just installed this a.m.) NGK that measures 1.6 & 10.42. The NGK specifically says for external resistor, nothing specified on the previous 2, so that's why I got the NGK;
• Coil cranking volts = 13.7; running volts= 5.24 (I think); For cranking, I pulled the coil wire and put the meter's red in the socket, black to ground. How to do running measurement hookup?
• The ballast resistor on the firewall measures 2.3 ohms; when running 13.68v on lower terminal, 9.87v on upper;
• The coil to distributor wire looks fine with no hard or cracked insulation; the terminals are firmly soldered and crimped on.

Do you know what the timing curve is...Base+Vac+Mechanical, all in at what RPM? = total timing.

Things to check:
-Read the spark plugs. I've seen plugs visually inspect ok yet act silly under higher RPM. Especially the cheap offshore chineasium junk. NGK is a great plug brand. A/C, Autolite & Champion are no longer the great plugs they once were.
-With the ignition shielding removed run the engine in the dark. Look for voltage leakage (arching) on the cap and wires. Pinched plug wire(s)?
-Re-inspect centrifugal advance mechanism...the stops the weights the springs.
-Test each plug wire resistance. New means nothing today.
-Engine grounds
-Rotor tab loose at the rivet?
-Fuel system...if its starving for fuel at higher RPM of course you are going to see the timing mark bouncing around.

The issue has been there through two sets of plug wires - new 8.5mm currently installed; new BPR4ES NGK plugs; centrifugal advance weights and springs move easily; rotor tab is firmly attached. I've not measured wire resistance or fuel pressure or run it in the dark to look for spark jump. I pulled 6 plugs, 5 were similar to the first 3 photos, the darkest one is from #7. I didn't pull #6 or #8 because of the battery and hot manifolds.

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