I'm trying to get to the bottom of my unstable dwell as i can't progress to setting anything else up until this is sorted. The reading is bouncing around all over the place on both my timing gun and dwell meter. When i get a moment i'll get a video of it

Someone else had suggested it could be the timing chain. The timing looks pretty steady when checked, but doing the chain slack test (turn to TDC then turn back until dist rotor just starts to move) i get about 8 degrees. Consensus on the interweb seem to be if this is above 7 degrees it needs to be done. Not sure if this is affecting my dwell or if the relatively steady timing mark would indicate its OK (but the chain would need replacing very soon).


Its got a new coil, condenser, points, rotor and cap. leads are all new, although i didn't put enough grease on the plugs and pulled two of the leads out of their plugs , so have some new leads coming. I think this might be why my light is erratic as the light is way more consistent on the #2 lead. Will know for sure when i get the new leads. Distributor was rebuilt with new shaft and bushings are good, no play at top or bottom. Shaft end play was brought down to 0.07. Holding the rotor i can't feel any noticeable play in the shaft in any direction.

Its got new plugs all round, gapped correctly before installation. I've temporarily run a separate earth wire from one of the screws holding the points down to the breaker plate to rule out a bad ground.

Are there any electrical issues that could cause the dwell reading to be unstable? Anything else that i can check to rule out an electrical issue? My gut says there's something electrical to find and fix, but i could be wrong

Any help greatly appreciated as i don't want to do the timing chain right now unless i've ruled out everything else as it could take me quite a while to complete with life getting in the way

 

Since dwell is solely determined by the screw on the points set, I'd pull out the points and make sure the spring and arm are not binding in some way. Look at the rub block on the points to see if there's any looseness in the arm. Dwell shouldn't vary much at all at idle.

Do you recall the brand and type of points set you put in?
Do you still have the old points you could reinstall to see if the problem exists with those too?

 

If the timing chain has enough slop in it to affect your dwell then I might consider getting the parts together in preparation. It can be done fairly easily on a small block as I did mine in a day, IF you have all the parts and tools and a few hours of time.

Has anything happened to the distributor while you were working on it? After being overhauled it should be fine but weird things do happen. I am not familiar with broken spark plug wires affecting the dwell because the #1 cylinder's spark plug wire is the only one that matters. For a timing light at least.

If your distributor has a built-in Centrifugal Advance or even a Functioning/ non-functioning Vacuum Advance mechanism would be capable of a fluctuating dwell I might think so they are worth a good look.

It is always wise to grease the inside of the Spark plug wire's cap as it makes them easier to remove without breaking another expensive wire. Too much lubricant (which does not conduct electricity) is not a good thing if it gets between the wire and the spark plug. There was one gentleman who used it between the spark plugs and the wire caps, then he put it between the battery leads and the battery. He used it often until he started having problems. Just remember "a Little Dab will do ya" with die-electric grease. Battery grease is good to have and use on the terminals to keep oxidation at bay.

Grounding your points plate may be a bit of overkill. I would check the engine's ground to be sure it has a ground strap somewhere. Measure your Battery Positive at the Bat + terminal at the starter solenoid or even just at the alternator output post. Then check the ground and verify that you see full battery voltage between the Positive point and the ground. I should have suggested checking the battery first to know that you are looking for voltage wise. If the engine is not grounded then I would seriously think about a nice ground strap connecting the frame to your engine.

I hope that someone chimes in that knows the problem better. If I listen to my gut feeling after hearing about the bouncing around on your timing light. I am going to agree with the timing chain diagnosis is a clear possibility.

Best regards,
Chris

 

Are we getting dwell vs timing confused here?
I’m with 67:72 on his post with respect to dwell.

 

Dwell is entirely the points. Years ago I had an erratic dwell reading similar to what your describing. The condenser was bad and so the points would arc across the contacts when the points were slightly open. Dwell would erratically fluctuate 5 degrees.

 

You can't trust your timing light to indicate firing / not firing of a certain cylinder.

Go to Harbor Freight and buy a sparkplug test light ($7).
Hook that up to number one cyl instead of the timing light.
Monitor the strobe. Is it the same misfire?

Your dwell meter. Hooked up to the correct post on the coil?
Some dwell meters have the 4, 6, 8 cyl option switch too.

 

The points are Standard T-series DR2270PT. The spring feels strong, the contact surfaces line up nicely.


The mechanical advance is good, just had to change the springs back to stock as the Mr Gasket springs were too loose and the advance wasn't snapping back if i turned the rotor. I'll worry about that another day since i don't need advance at the moment. Vacuum is disconnected and plugged.

Brand new battery, its an optima red top. Had to spend quite a bit of time cleaning up the positive battery cable end, it was all gunged up with acid/oxidation. Used bicarbonate of soda mixed into a paste with water to clean it up and its come up pretty nicely. I'll check the engine ground and battery voltage etc tomorrow

Unless the timing is jumping in the gaps where the light isn't firing, the timing mark looks pretty steady. When i tried the #2 wire, i was getting a much better flash with no significant gaps. Once i replace the #1 lead i'll hopefully have a good flash. The RPM reading on the timing gun was also jumping around when connected to the #1 lead, but was pretty stable on #2, so i suspect that lead is bad. Was also getting some arcing off the sheilding on that lead when it made contact with surrounding metalwork as i haven't connected them up yet. All points to a bad lead but not a dwell fix.

 

New leads will hopefully be here Friday. Was planning on using my spark test light all round to see if i'm getting consistent results or different results across the cylinders.

Dwell meter doesn't have different settings for cylinders, just have to multiply/divide the reading to get the correct value for the number of cylinders. My dwell meter reads while the engine is cranking but not running. Timing gun will measure when running, connected to the negative/output side of the coil. Both give the same jumpy reading

 

I will throw my $.02 in. How old is the distributor? Are you sure the cam lobes on the dist shaft that the points contact are not worn? If all else fails try another set of points. Maybe they are sticking somewhere. As Mr 67 suggested, did you have this issue with the old set.

 

vaux_2,

Something to consider- Tested a NEW, right of the box, AutoZone distributor on a Sun Machine. Same thing as you see - unstable dwell. Turns out lobes on shaft were not spaced uniform. (If it was a circle, you'd say it was out of round.).

Sun machine to test. Or borrow distributor that has a known stable dwell.

Where was that NEW distributor shaft made?

Mapman

 

Worn distributor bearings will cause erratic dwell.

Jason

 

Somethings fishy here:

Timing Light works when hooked-up to NEG of coil?

Dwell Meter will not work while engine running? Only cranking?

Should be three wires / alligator clips on the Timing Light.
1. Positive, such as off of the ALT.
2. Any GRD source.
3. #1 plug wire

You have two different voltages. Cranking voltage & running voltage.
I believe cranking is 12v and running is 9.6v.
Your dwell meter is either incorrectly hooked up, or its time to buy a new one.
I bet the distributor is fine. Its your tools or application that is questionable.

Dwell by definition is, the time accumulated when the points contact close until they open.
This duration allows the coil to recharge for the next firing of the plug.
I don't see how a timing chain would affect dwell. Timing? Yes. Dwell no.
The points, condenser & coil don't care what the timing chain is doing or how much slack it has.

 

I have an old Sears engine analyzer and a newer Innova timing light, both instruct to read and adjust dwell with the engine running. (Anyone else ever get shocked when you're hunting for the screw with the Allen key? Shazzzam!)

The Innova 5568 has 4 connections:

• Pos spring clamp - since battery is so far away, I clip positive to the alternator
• Neg spring clamp - ^ditto^, I clip negative to the frame below the master cylinder
• Magnetic pickup - keep this away from the exhaust manifold! (BTDT) I now clip to the #1 wire at the distributor cap (because I have the braided shielding on my wires)
• Dwell (green roach clip) - to be connected to negative terminal on the coil

Maybe the OP's light has the same green clip as mine above.

 

My brand new Innova timing light acted like that when I tried putting the clamp probe on #1 up by the distributor to get it away from the searing heat of the headers. (My wires come up from underneath) When I moved it down close to the plug, the flash got steady. I chalked it up to my cheap wires. I didn’t have any misfires, though. I made a protective cover for the probe from a welding sleeve so I could confidently use it down by the plug.

 

The cam/ weight base isn't part of the shaft, it slides onto the shaft and sits directly below the top. I assume mine is original.

I've pulled the distributor, going to take it to work tomorrow and get a micrometer on the cam to measure the face to face values and the apex to apex values. See if i get even measurements or not.

Bearings are good, can't feel any lateral play at the top or bottom

Like 67:72, my timing light has 4 connections so it can measure timing and dwell/voltage etc, its a green clip too

If the distributor is being driven off the back of the cam shaft, which itself is being driven by the crank via the timing chain, then if there is slack in the chain i assume it can cause the ratio between the crank and cam to vary slightly, causing the opening/closing of the valves to be out and the opening/closing of the points in the distributor to be out as well? Presumably it would be more prominent when coming on and off the gas and less prominent at steady speed? Sounds logical but hopefully someone will correct me if i'm wrong.

Yes, its a weird dwell meter. Not much choice over here in the UK! At least the dwell function on my timing gun agrees with the dwell meter.

Hadn't thought to try connecting the gun at the dist end, i'm the same, the braiding on the leads get in the way. Not sure how much its affecting the timing guns ability to read either as i assume its getting in the way of some of the EMF the gun's trying to read if the clamp is too close to the braiding?

Someone in another post i'd read on the forum said that changing the timing chain fixed their dwell issue. Just want to make sure i've checked everything else before i go to the trouble of doing the chain.

 

My '67 still had the original GM nylon gear in it until a year or two ago. The slack was close to 16° - just about off the tab. With the new chain, the slack is only about 2°. I don't know if/how this will affect the distributor shaft and varying dwell but I'd think it wouldn't help it any.

 

Not sure if I read all the posts, but have you considered the possibility of just a bad dwell meter? It sounds like the engine is idling smoothly granted it seems pretty high. Since dwell angle is associated with point gap, my suggestion is to manually set the points with a feeler gauge, plug the vacuum line coming from the manifold that feeds all the vacuum driven stuff, and proceed with the tune up, to see if she's running good, until you verify your meter is good. Newly replaced electrical parts can always be suspect too. I never throw out the old parts until everything checks ok, and try to avoid changing too many parts in 1 shot.

 

So i got the cams measured across the apex's, all measurements except 1 were within 0.04mm of each other, the outlier was about 0.08mm. That certainly wouldn't translate into the wild bouncing i'm see on the dwell meter or timing gun readings. While the distributor was out, i used a feeler gauge to check the points gap on all of the lobes (new points set at 0.019"gap), no noticeable difference between them. I've checked for play again, i'm 110% i have no play in the shaft, no lateral movement in the top bearing, bottom bearing or rotational or lateral play on the breaker plate. When i rebuilt the distributor, i closed up the vertical play on the shaft from 0.13" to 0.07". I did find that the cross gear for the tach was ever so slightly too tight and causing very slight binding within a 20-30 degree segment of the shaft rotation. That was me turning it by hand, doubt that would cause anything when it was being turned by the engine. I think the problem lies outside of the distributor, i may not be an expert but i can't find anything else wrong with it.

Forgot to add in my original post, the vacuum advance is disconnected and plugged, not reconnecting it until i've got my dwell/base timing sorted.

I checked the engine ground from the carb to the wiper motor housing on the firewall, 0.03 ohms so that looks fine.

The replacement spark plug leads arrived at the weekend so i'll get the offending leads swapped out and run again and see if my light's performance improves on #1 with a new lead. I'll also use my spark plug light to see if i'm getting a consistent spark on all cylinders. If all of that checks out, I'll then start on the electrical system upstream of the distributor and see if i can find anything there.

If the electrics upstream of the distributor are all working correctly, what should i see on my voltmeter at the coil? a steady 12v on the input when cranking and steady 9.6v when running (from HeadsUP post above), what about the output side to points? Or does the condensor charging cause volt drop and make measurements tricky? Anything i can check on the king lead?
I'll check coil too 1.5 to 1.7 ohms on the primary side, 11Kilohms or higher on the secondary side?

67:72, with your timing chain slack at 16 degrees, what was your dwell reading doing? I'll get a video of what i'm seeing on both my dwell meter and timing gun.

J-vette1, your suggestion of just running is very appealing!!! I've still got all of my old parts so if everything else checks out, i can swap them out 1 by 1 and see if anything changes. I'll need to double check tomorrow when i run it, but i think when i tried to turn the distributor down to the 10 degree base timing (you can see how high it was in the video above) it started to stall before i got there. Could just be because my dwell was wrong though?

Forgetting about the meters and just using the 'adjust the points until the engine starts to stumble/stall then back off half a turn is starting to sound pretty good just now!!!

 

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Is your points plate (that the points screw to) firm and not wobbly? It should have practically zero vertical movement, just rotational when the vacuum can pulls on it. If the points plate isn't firm, it will cause spark stability issues.

I was seeing the timing mark on my 427 fluttering about 6° to 8° at higher rpms (3000+); one of the suggestions to investigate was chain & cam gear slop. Another suggestion was to see how variable the dwell reading was through the rpm range. Both before and after the timing set change, dwell didn't fluctuate much at all. The new chain settled down the timing flutter but it still swings ~2° at higher rpm though I don't feel misfires or anything affecting performance.