WTF would cause this

Torque Junkie · October 25, 2002

The truck's been running funny lately so I went and hit it with a timing light again and it's fine at idle, about 24 deg. BTDC advanced w/ the vacuum advance on, 12 w/ it off. But, as soon as I rev the engine, the timing retards back to between TDC and 4deg ATDC! ??? ??? ??? I don't like the idea of revving it higher than 2500 rpms or so while My head's the engine so I don't rev it way up w/ the light, but it's definatley hurting my performance. When driving the power seems to come back in about 2500 or so, which is, I assume, when whatever's wrong w/ the timing system corrects itself partially and comes back.

What would cause such a wierd thing?

I think I'm gonna put the old weights back in there and try it again. I wonder if it has anything to do w/ the new mechanical advance weights I put in when recurving the whole thing after swapping the cap, rotor, coil & plug wires.

Any help would be appreciated

-Mike

76BBSub · October 25, 2002

Hmmm...almost sounds like the mechanical advance weights are swinging backwards and taking timing out instead of adding advance. I don't know if that's possible though...

Is the timing light dependable? I have had trouble with dial-back timing lights from Sears.

Jeff

Torque Junkie · October 25, 2002

Hmmm...almost sounds like the mechanical advance weights are swinging backwards and taking timing out instead of adding advance. I don't know if that's possible though...
Is the timing light dependable? I have had trouble with dial-back timing lights from Sears.

Jeff

The light's just a $12 jobber from a pawn shop. No dial or anything, just an inductive pickup and a +12v and GND connection.

I dont think it's the light because, when I let off the throttle after it retards back, it stays back for a few seconds, and when it does that, the engine idles rough for a couple seconds, and as it starts to advance back to where it's actually set at idle, the idle smooths out.

I wonder if I put the new weight system in upside down or somethign? I mean really, that is so wierd. What i"m gonna do is check to see if the weights are retarding the timing instead of advancing becuase of maybe being upside down or something, if that's the case fix it. If not the case, I'll yank those weights out and put the old ones back on, becuase I know the old weights advanced the timing w/ rmps.

Thanks for the input. Any other suggestions?

-Mike

Torque Junkie · November 3, 2002

Well I think I've narrowed it down to a "sloppy distributor shaft" There is excessive slop in the rotor, and I'm almost positive thats what's causing it. It's really frustrating.

So I'm gonna hit the Junk Yard on my day off on Tuesday and try to pick up a decent used HEI Distributor bottom for cheap. I've already replaced the whole top end of the distributor w/ my ignition upgrade last month.

And If this doesn't fix this timing gremlin I'm gonna go down to the boat ramp down at the lake, put it in neutral and kick its rear bumper as it rolls into the lake.

Torque Junkie · November 5, 2002

OK, the junk yards wanted 50-75 for a used one, and only 2 of the 5 junk yards within 30 miles even had one, and those 2 only had 1 each, one for $50, one for $75. So I hit eBay, and won this bad boy!!!!

http://cgi.ebay.com/ebaymot....1829052

That module is worth the $40 in itself, so if it's as nice as he claims it is, it's a killer deal!

I'll let yall know how it turns out when I get it installed

MarkD · November 5, 2002

OK, the junk yards wanted 50-75 for a used one, and only 2 of the 5 junk yards within 30 miles even had one, and those 2 only had 1 each, one for $50, one for $75

Yeah, junk yards are no longer the bargain store they used to be. The biggest local yard here charges 50% of RETAIL NEW!!!! No matter what shape it is in!

76BBSub · November 8, 2002

Well I think I've narrowed it down to a "sloppy distributor shaft" There is excessive slop in the rotor, and I'm almost positive thats what's causing it. It's really frustrating.

So I'm gonna hit the Junk Yard on my day off on Tuesday and try to pick up a decent used HEI Distributor bottom for cheap. I've already replaced the whole top end of the distributor w/ my ignition upgrade last month.

And If this doesn't fix this timing gremlin I'm gonna go down to the boat ramp down at the lake, put it in neutral and kick its rear bumper as it rolls into the lake.

yep, slop in the distributor shaft will mess with your timing and performance...shame that the junkyards are getting lame, but it's true. I ended up buying a brand new Accel HEI distributor for my '76 Suburban, it was $115 with everything new except the coil. Luckily I had bought an Accel HEI SuperCoil a few weeks earlier so I just swapped it over. HUGE difference in performance and gas mileage.

Hope the new distributor works out for ya!

Jeff

Torque Junkie · November 12, 2002

Well I got the new distributor in today. WOW!!! it idles so smooth, and has alot more mid & high range power..... BUT the timing problem just isn't as bad, but it didn't completely fix it. The timing still comes back between 1000 and 1500. After that though, it starts to climb up earlier

So it's got me thinking..... I wonder if it's the timing chain? If there's alot of slop in the timing chain would it possibly cause it to do funky stuff like that?

But the new distributor did make a big difference in how long it came back for, and how much, but it's still doing it

Any other suggestions? Could it be the Timing Chain do you think?

Thanks!

-Mike

Torque Junkie · November 13, 2002

Maybe it's just time for an engine rebuild

76BBSub · November 19, 2002

You may be on to something with the timing chain idea. If the chain was slacked it could cause a change in the advance of the distributor firing relative to the angular position of the crank. I don't know how much effect this could cause before you had other issues. Also, if your truck has a nylon cam gear like my Suburban it could be ready to give up on you. Mine just suddenly stripped half it's gears off on the way to work one morning...new cam, lifters and timing set and I was back on the road, not fun though...

I am assuming that you replaced the complete distributor and all it's guts with new/different stuff?

Jeff

Torque Junkie · November 20, 2002

You may be on to something with the timing chain idea. If the chain was slacked it could cause a change in the advance of the distributor firing relative to the angular position of the crank. I don't know how much effect this could cause before you had other issues. Also, if your truck has a nylon cam gear like my Suburban it could be ready to give up on you. Mine just suddenly stripped half it's gears off on the way to work one morning...new cam, lifters and timing set and I was back on the road, not fun though...
I am assuming that you replaced the complete distributor and all it's guts with new/different stuff?

Jeff

Every part of the igntion system has been changed....... the entire distributor unit, the module, the weights & springs, the rotor, cap, coil, wires and plugs. The only items left to be changed that are in the "chain of events" from the crank to the spark plug is the cam itself, and the timing chain.

A timing chain change wouldn't be something I could very easily do in my apt. parking lot either, seing as how the oil pan will need to be dropped. Although, it'd be a nice chance to get those front and rear main seal oil leaks fixed. Not that I couldn't do it or wouldn't tackle it, but it wouldn't exactly be convenient.

Plus, I figure since already halfway in there, I'll do a cam swap too so I know exactly what I've got in there. I dont know what is in there now, but I suspect it's the Edelbrock Performer series which specs out like:

Specifications:

* Advertised duration: 278 intake/288 exhaust

* Duration at .050 in.: 204 intake/214 exhaust

* Gross valve lift: 420 in. intake/442 in. exhaust

* Lobe separation: 112 degrees

* RPM range: Idle to 5,500

* Designed for optimum torque from low end to the mid range. Smooth idle for daily driven vehicles.

I'm thinking of going with the Comp Cam Extreme Truck cam, which specs out like:

Specifications:

* Advertised duration: 270 intake/278 exhaust

* Duration at .050 in. cam lift: 226 intake/234 exhaust

* Gross valve lift: .480 in. intake/.498 in. exhaust

* Lobe separation: 111 degrees

* RPM range: 1,800-6,000

* Big off-idle torque, strong mid-range and great throttle response

Which seems to have a steeper lift rate and a higher lift, which will compliment the other mods.

Right now I've got the timing adjusted enough advanced to compensate for the slack to where my midrange power doesn't suffer as much as it could, so it should last me until I can afford and can find the time and place to do a half of an engine rebuild.

If you have any other ideas of what might cause that, please let me know One other symptom you may want to note too is if I give it a quick rev, up to say 2000 and then let off the throttle abruptly, the timing drops WAAAAAAAYYYYYY back, we're talking like 30-40 deg ATDC retarded, to where it barely idles, for like 2-5 seconds before advancing back up to where I have the distributor set.

I'm sure you can immagine just how frustrating that is :angry:

Thanks for your help.

Technicrat · January 3, 2003

I know this may sound funny but check your vacuum.

I had an issue with a rebuilt Q-jet that Really messed with my driveability. Turns out the rebuild (from auto-zone) was dorked up. I had 2 vacume ports that were not working.

The port that was supposed to be for the distributer turned out to be SOMEHOW reacting to ported vacuum instead of manifold vacume. Found the issue with a $20 vacuum gage.

I finnaly got fed up with the rebuilds and bought an Edelbrock q-jet 4mv from Summit racing. Edlbrock is manufacturing BRAND NEW q-jets. I guess they bought the plans from GM.

At any rate it did wonders for my power and I even pass the emissions tests without having to fight the carb.

ClipperSkipper · January 21, 2003

Your Distributor is quite confused and your parts compatabilty doesn't quite jive. Do you want to run 180 mph or pull down a building, or run 7.87 @178 mph at Englishtown? This, is a strange cobination you cannot hurl components together and expect them to work their own corridor, without thinking wo is next door.

Have a great one!