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Discussion Starter #1
My motor is a newer alum head 455 combo with ill admit it, a cheaper china hei. It does has a high quality pre stretched and heat treated double roller timing chain in it. 3000 miles on the clock. Here is my issue.

When I look at the balancer with my timing light i get a few degrees of fluctuation at idle. +/- 4 degrees in either direction at complete random. Also when holding the throttle at about 3000rpm when my timing is all in it is steady but then at random ill see a quick flash of the other side of the balancer then back to steady at 32 degrees. No rhythm or reason to it just quick blips every few seconds. I do hear the motor react when it happens too.

Dizzy specs: China HEI, BOP polymer gear for my roller cam, 14 initial, 18 mechanical, vac can adds 5 degrees, 364 center plate, 139 weights, lightest gold springs from mr gasket curve kit. End play never addressed.

I have done plugs, wires, cap and rotor recently. Bought a new timing light to make sure the light was good too.

Any thoughts? Endplay? Module? Chewed dizzy gear?
 

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Discussion Starter #3
It sounds like a bad chain. Pull the dissy cap off and with a socket on the balancer move the crank till the rotor stars to turn then stop. Then move in the other direction till the rotor starts to move making note of about how may degrees the crank is moving.
I will surely give that a go. For what its worth I have a very nice, new top of the line timing chain on it. Less than 1500 miles new. I cant imagine its bad already?!
 

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Good tip by Rukee, also what about that vac can? Is it dithering, blipping in and out? Is it an adjustable can? You can change that can easy, and they cost 15 bucks. Does it knock over 32 degrees total? and why only 5 degrees on the vac can?...

You may have an answer there Ben......
 

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Discussion Starter #5
Good tip by Rukee, also what about that vac can? Is it dithering, blipping in and out? Is it an adjustable can? You can change that can easy, and they cost 15 bucks. Does it knock over 32 degrees total? and why only 5 degrees on the vac can?...

You may have an answer there Ben......
I never check timing with the vac can plugged in its always capped so its definitely a mechanical issue. It is an adjustable can though, the Accell type, im just going by their instructions in guessing its adding 5 degrees from the setting im on at cruise. The car pings at cruise if its 42 degrees or more with the can. So High 30s is about the max I ever wanted to give it with vac and mech. At least I think i hear it pinging I honestly dont know i hear ticking under low rpm load but the car is kinda loud. I hear something. 32 total was the recommendation from Dave @ SD Perf. He built my heads and designed the cam and i gotta say that 32 total figure must be good cause from 3k rpm to 6k rpm the car is an absolute tire roasting terror but down low im having issues.

Im only 373 geared in a 66 Le Mans so its a kinda heavy car and doesnt have a lot of gear. In hindsight I should have bought the AED750HO instead of AED850HO, because of those gears and the 850 I think im losing out on lower end pull in leui of more top end hp. Im toying with accelerator pump cams, squirters and vac timing trying to get some lower end back. Thats when I found my timing wobble. Cruise 4th or 5th gear at 2000rpm and give it gas it stumbles bad and then cleans up real good around 3200. I know gear would help but it has to be some timing and fuel too. My advance is full in by 3200 but you know hei's, they tend to creep another little 4 degrees or so up by 4000rpm+. Are you saying bump the vac can up a tad? After speaking with Jim Butler he said just measure the chain slack and see if an undersize .005 will fit it. That would be good news in my opinion, $89.95 timing chain fix and be done. Free fix would be the best news!
 

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I gotta say, timing bouncing around sounds more like a badly bushed breaker plate in the distributor. If the weights on the plate move around or the springs are weak, it would have a tendency to do what you describe. Also, when you put the distributor in did you notice any side to side movement in the shaft? I think I'd be more inclined to suspect the distributor than anything else.

Personally, whenever I've deviated from what GM did, I took a step backwards in reliability.

Chuck
 

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Discussion Starter #8
Could your harmonic balancer have a problem with the rubber slipping on the outer ring?

Just a thought.
Possibly but it too is a newer power bond unit so doubtful. I think my line hone for ARP studs caused the issue. My chain might be a little loose cause it is a stock size chain in line honed block. After talking to Jim Butler im going to measure to see if i can get a .005 smaller chain on there. If I can not he said itll most likely be fine with a little wander at idle. The high rpm blip he mentioned could be my chinese module going in and out. Might through a new one in to check but "checking to see if this fixes it" usually costs $100 a crack! Chain, module on and on! That shit is old hahaha!

Im working toward a GM assembled and warrantied ZZ502 crate when I sell my 455 but that isnt the end game. Everything always needs work and fine tuning. This hobby isnt cheap or forgiving with time spent.
 

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Discussion Starter #9
I gotta say, timing bouncing around sounds more like a badly bushed breaker plate in the distributor. If the weights on the plate move around or the springs are weak, it would have a tendency to do what you describe. Also, when you put the distributor in did you notice any side to side movement in the shaft? I think I'd be more inclined to suspect the distributor than anything else.

Personally, whenever I've deviated from what GM did, I took a step backwards in reliability.

Chuck
Im pulling it out and inspecting play side to side and end for sure. Great advice. I do have gold "loose" Mr Gasket springs in my dizzy and Butler also mentioned that could cause the bouncing too. I need them though cause with the mediums my car is a dog midrange. Pop those light gold ones in and holy hell its like an extra 75hp. Maybe ill put the stiffer ones in and try the timing light again to eliminate it being the springs before i pull the dizzy. Shit now were cooking with gas yee haw!! lol
 

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I've never liked the gold springs for EXACTLY this reason. I chased my tail for awhile on my friend's Chevelle, until I looked at the curve kit he put in. Damn light springs.
 

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Ben...Dave than built the motor and with those heads and compression if he says 32 total ...mechanical that is solid. You don't get any better than Jim Butler....so his timing chain advice I would follow....

chuck is right two as light dizzy springs will cause wander... that adjustable vac can is suspect as well.....32 total advance is for wide open throttle...idle and cruise are different.....have you asked dave what vacumn can they run?

an AR-31 for an HEI pulls 8 dist degrees, 16 at crank and at low vac...

Is that too much? or is your timing too retarded with 32 mechanical and 5 vacumn....

which is 37 for light throttle cruise seems way to low....

52 is normal...that motor maybe 50 48, Dave should have a recommendation......

anyway, ported or full manifold vac to the can matters as well

you try an Ar-31 from Napa...to full manifold vac and take a drive..

15 bucks....those adjustable cans can be a little tricky to dial in

the carne needs a limiter plate to regulate the vac advance...the allen wrench only adjusts the rate of vac..have not used the accel..

maybe a little different
 

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Discussion Starter #12
Ill try a little more vac timing and see if that helps lower end. First i need to time it with the heavy springs in it to see if it still jumps. If not i know its the springs. If it moves still with the heavy springs its custom shorter timing chain time. More vac pull might help the lower end stumble out its worth a shot for sure. Its going to have to sit for a few days as were going on a four wheeling trip this weekend but ill get at it Monday and update this Tuesday. Luckily I have Summit Racing about 25 mins from my house so everything is in stock and right there whenever i need it so its easy to tinker. I love it and so does my bank issuing VISA card im sure.
 

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My 4.3L GMC truck bounces around about 2* at idle. Timing chain has about 2* slop doing Rukee's test, so it makes sense (has about 150K miles on it - all probably original). Hopefully this not the case with yours.

Would be nice if you had an original distributor to throw in... to isolate distributor from everything else.

Hopefully it's the springs since it's easy to check.

I've seen flashes of different areas of balancer before - wonder if it's not minor crossfire, but mine never seemed to affect engine running - thought maybe enough to trigger the light, but not crossfire the plug. See if it shows when light's hooked up to a different wire (won't see marks, but can usually see if same place on balancer [dirt, imperfections, etc] gets lit up).

Watch out with detonation especially if engine is loud. I swapped a 75 400's HEI into 10.25:1 428 and it pinged a lot either off the line or highway acceleration (never could get it fixed). Dropped a valve/tore it down and was surprised to see how much damage that "crackling sound" did to my rod bearings. I've learned more since then but still scared of detonation.

Sounds like you've got a nice ride set up.
 

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X3 on Rukee's test: easily done and very accurate. I have also had poor results from the gold colored light mechanical advance springs. I've had better luck using one light spring and one stock spring, or just making sure everything was moving easily, the limiter bushing was still in place, and using the stock springs.
 

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Since the issue seems to be somewhat intermittent the mechanical advance mechanisms are suspect. The springs might be an issue or just part of the symptom. I would take the distributor apart and check every moving surface for any drag, burrs etc. If you go to heavier springs and then aren't getting your advance in soon enough heavier weights could be needed.
 

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I had the same problem with the gold springs so change them out before spending any more money.
I was all in at around 17 with the gold, 27 with the black and it runs a lot better now.
 
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