Strange running behavior/erratic timing

[bhobbs]

Hey all. Since the last time I posted, I managed to solve my alternator bracket problem on my '68 327. Turns out Autozone has an entire aisle of things that fit. I ran across a hot rod show in Granbury, TX on the 4th of July, and saw everyone with a 327 that had a single wire alternator was using the same generic chrome bracket from Autozone. Who knew.

So now, with new water pump and radiator installed, new plugs, new wires, and a new coil, it starts reliably, and will run like a banshee so long as you're giving it gas. If you leave it at a specific rpm, it just bogs, and stumbles, usually backfiring and blowing flames out both ends(intake and exhaust). At idle, it just stumbles and dies. I've not rebuilt the carbs(reman. 4bbl of indeterminate lineage, but pretty standard-looking), but as it is(no adjustments at all), it used to run and idle, if poorly due to the previously poor spark. Dwell is a consistent 32degrees at all rpms, so a little long, but again, it was running well at this setting prior to the rewiring, new Hooker headers(which are still open, I haven't put the exhaust on since I'm still having fun annoying the neighbors), new plugs/wires, and new coil. The previous headers had blown the seals at the down pipe, so they were effectively open.

Checking the timing, it's all over the map. From a little retarded to way advanced. I replaced the hose from the distributor to the carb, and have tried crimping it off to get the thing to calm down, but it seems insistent on not running smoothly. I also put the original air filter assembly back on, and hooked up all the hoses. Just seemed to muffle the noise from the carb was about it. Behavior all around is the same.

Spraying carb cleaner around the manifold didn't seem to make a difference in how it ran either. Any ideas?

Thanks,
Barry
'68 GMC Custom LWB, 327

[Cruiser71]

my guess would be the timming gear and chain.. Most likely worn out.

[Grim Reaper]

I agree with Cruiser. Probably timing chain. The quick check is turn the motor clock wise till you get to TDC. Take the distributor cap off and then turn the motor backwards till you see the rotor turn. Look at your timing tab and see how many degrees it turned before the rotor turned. 5 and under is probably just al ittle slip in the worm drive at the distributor. over that its starting to sound like the chain is loose and as the lifters crest on the timing lobes it will jerk around and give an eratic timing.

Check the mechanical advance as well under the rotor. Somethimes the weights and inner shaft will get gummed up and cause simular problems. If the weights feel slugish at all and doesnt snap back it's gummed up.

To clean it pull the distributor. Remove the weights. Drive the roll pin out of the gear. Slide the inner shaft out and clean it. Use a good high temp THIN LIGHT grease. I have been using white lithum grease and it seems to work well in this application. Reassemble and check it.

[MandarinSixSeven]

Maybe the spark plug wires are crossed somewhere...I know I did that once, so that sounds familiar

[ChevLoRay]

I don't know, Mandarin, about the crossed wires. He says it runs well as long as you're givin' it the gas. I have also seen crossed wires, but it seems like there was always something "not quite right". It would pop and sputter, and it did backfire out both ends, so maybe I'm forgetting something. Heck it's only been since '76, that I had that problem, so I coulda forgot something.

If it isn't the timing chain, or crossed wires, and the distributor checks out, I'd have a hard time believing that the carb could be the only problem. Usually, if it's the carburetor, nah, it ain't the carb. Here I go, thinkin' and typin' at the same time.

I'd have to second the emotions of the others, for diagnosis, especially checking the timing chain for wear. But, I'd sure check the firing order on the plug wires, first. Then, if that didn't fix it, check the chain for wear.

I recall setting the dwell angle at 30 degrees, whenever I installed a set of points, but the book called for 28-32. As the rubbing block wears, the points close up, widening the angle, right? So, setting them at 32 degrees, may be a little bit wide...maybe .018? Isn't .016 the recommended setting? Don't matter. If you you adjust them until the dwell angle is right, the gap will be just right, too. I don't think that the dwell is the problem, but you can play with it when you have time. Check all of the vacuum hoses. Make sure none are split at their ends.

[bhobbs]

Yerg. I'll definitely check the spark plug connections again. The manual I was using to hook up the plugs had two illustrations, one for HEI, and one for non-HEI. I've got an old-school non-HEI, so I would THINK I used that one, but maybe I misread/plugged. I seem to remember it just had to do with which position was #1 on the distributor cap.

The cam chain seems very likely though, and now that I've got good spark, maybe it's showing itself. And I just got the front of the engine back together darn it I was reading something about having to drop the oil pan just to change the timing chain. That sounds like fun. I'll check the free-play though. I'd be willing to guess that it's WAY out. Probably original equipment. Any suggestions on chains/sprockets/procedures if I find that's a likely problem?

I've been avoiding installation of a new Mallory distributor until I had it sort of running well, but maybe I'll give that a try before I tear into the timing cover. Removing the crankshaft pulley doesn't side like a lot of fun either.

Barry

[ChevLoRay]

Just remember the firing order: 1 - 8 - 4 - 3 - 6 - 5 - 7 - 2. Find TDC for #1, and see where the rotor is pointing. That will be your #1 terminal. Just remember, tho, that #1 piston will come up top twice, in a cycle. It's only when both valves are closed, that it is at TDC. Look at the timing pointer, too. It should be somewhere close to the mark on the crankshaft pulley. It may not be exactly, but it should be visible (right folks?).

[franko72]

check the firing order again, try different set of points and condenser, check lash on timing chain ,pull out dist. and visually inspect. for wear and tear before buying a carb. that might not be needed

[grayw0lf]

Be sure to drink beer while working on it. But, check the plug wire order Before you drink much beer. Otherwise, that "other" logic kicks in.

[flip66]

I would check the fireing order I had them wrong a v6 blazer and it did the exate same them.

[GMC AMI]

The contact points control the timing,when I had a truck that the timing was
jumping around I found the problem was the points. The backfiring may be the wires being crossed,double check for the rotation. I also would set the dwell at 30 degrees. Bon chance!

[Harold Shepard]

Just a thought but when I first started my truck after putting the body back on, engine ran fine before I put body on, My timing went everywhere, one day it would crank one day no crank. Pulled the dist. cap off, and rotor was destroyed, ok replace rotor, as I'm tightening the new rotor, the dist shaft moves, not a little twist but really moves, start watching and the stupid shaft is turning in the weights retaining plates, spot weld was boken and shaft rotated freely. This was a new dis.too, maybe give your a check, and has anyone else ever had this happen? Replaced the shaft with one from another dis. an no more problems.

[RON WOODGEARD]

all kinds of good info here, and the accumulation of problems.. One other is the distributor has worn where the points contact it. This can also be checked but it's tedious.. If you plan on changing distributors anyway, do it now and try it. Also, The easiest way to check this problem is put the distributor on a distributor machine.

I do have a tendency to "agree" on the timing chain. The factory gears have a nylon coating on them. it's very thick and when it starts to wear it breaks. That would/could be your irratic timing.
You have a few choices, Depends on what you are after. If you want it quiet, and the motor is stock, You can just replace the gears and chain with a factory one. the noise is why the factory went with the Nylon coverage. If the noise isn't an issue with you, I would go to a single or double roller timing set. they are all metal, they can stretch but no wear. Then you have a Steel GEAR set, Mine is Pete Jackson, They are noisy as all hell... They do make a quiet one, But I have never heard it. These are as accurate as you'll get. your timing will stay Rock solid as far as the Gear is concerned. The last and most expensive, is a rubber belt chain set up. I have known guys that run them, but they are very expensive.

[bhobbs]

So, it turns out, that it's the simplest of all problems. After checking the wiring thoroughly, the voltage drop across everything in the system, spark-plug color/condition(fluffy black deposits on nearly new plugs), and confirming that there weren't leaks around the carb or manifold, and that I'd successfully found TDC on cyl 1 on the compression stroke the first time through, and I had tolerable compression for such an old engine, I decided to really carefully inspect my hoses. I'd replaced several already that were obviously cracked. I discovered that the one running from the driver's side rocker cover(with the valve in it, which I'd confirmed was working), had a lengthwise crack in it that I'd never noticed before. I think that I'd managed to break it even further when I reassembled it the last time, since it looked freshly split all the way up to the carb.

Replace tube, and voila, it runs just a little bit better than it did before I fried the wiring a few years ago. Timing is reasonable(I can make it better, but for right now, it's close enough), and it actually idles pretty well.

What's the function of that tube wrt a typical 4bbl carburetor operation? It's fascinating that I had such an odd behavior for something so simple. I'm guessing that it was generating sufficient vacuum during acceleration that it could suck in fuel and run, but at idle/constant rpm, it was using a different set of jets that had no vacuum. That's just postulation though.

Thanks for all the suggestions guys. It gave me enough ideas to go through and at least be more methodical about getting it going. Now I can upgrade/replace individual pieces and know that it was working tolerably prior to replacement, and the city will get off my back about having a non-working truck in my driveway(despite my custom-fit cover and lack of HOA for my neighborhood).

Barry