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Stumbling under load - it's fixed!
https://www.slantsix.org/forum/viewtopic.php?t=57550		 Page 2 of 4
Author:  sargentrs [ Wed Apr 22, 2015 4:06 pm ]
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My pointer only goes to 10*. I'll have to get some timing tape tomorrow if the parts store has any.
Author:  SlantSixDan [ Wed Apr 22, 2015 9:37 pm ]
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There is no timing tape available that will work on a slant-6 engine. If you want to plot the total advance you're getting, you'll need to make more marks on the crank pulley (move the original mark to 5 BTDC, make a new mark at 0, move that mark to 5 BTDC, make a new mark at 0, etc. If you want to get fancy about it, use a different color of paint for each mark.

Unstable timing such as you describe can be a flopping distributor advance weight (broken or stretched spring), a slack timing chain, or crossfire (faulty cap/rotor/plug wires).
Author:  sargentrs [ Thu Apr 23, 2015 2:22 am ]
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I've ordered new plug wires from Aurora (thanks for the tip, Dan!). I guess I'll wait and put those on to eliminate that variable. Would plotting the advance curve eliminate the distributor as a problem? I'd hate to spend the money on a new distributor unnecessarily.
Author:  Nicademas [ Thu Apr 23, 2015 2:48 am ]
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Make sure your valves are adjusted to. Mine were to tight causing a vacuum reading of 15Hg and iradic timing jumps.
Author:  sargentrs [ Thu Apr 23, 2015 7:11 am ]
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Thanks for the head's up Nicademus!
Author:  nm9stheham [ Thu Apr 23, 2015 7:13 am ]
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Timing chain slack is pretty easy to find..... grab the crank pulley and try to turn it back and forth. When the chain slack pulls taut, the force needed to rotate the crank will go way up. Any significant amount of rotation of the crank pulley back and forth with little resistance will be timing chain slack. You can put a mark on the crank pulley and read the crank degrees of chain slack by reading against the timing marks. If you get much over 4 crank degrees, you are reaching the service limit for the chain. A tight timing chain will do very good things for this engine; if nothing else, it corrects retarded timing of the cam, and the engine will be noticeably more responsive at low to mid range throttle.

Pull the rotor button in the distributor and see if there is felt pad down in the hole in the end of the shaft. That is a point for lubing with a light oil every now and then to keep one part the advance mechanism working freely.

I migth be looking at distributor and chain at this point but there are a number of things and you are just going to have to go through them thoroughly one at a time. And the changes you make by pulling the vacuum advance off an on may just be amplifying another problem elsewhere; the initial symptoms would make me suspect a weak ignition system (coil, ballast, etc.); this is because when you change engine load conditions, the energy required to fire the compressed fuel/air mixture changes.

What ignition system do you have? If a coil and ballast system, what coil and what ballast PN/brand?
Author:  sargentrs [ Thu Apr 23, 2015 9:33 am ]
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Thanks for the reply, nm9stheham. I do believe I have excess slack in my timing chain as well as my valves needing adjusting. I installed the HEI ignition system per the instructions here. http://www.slantsix.org/forum/viewtopic.php?t=15779 Even with the potential timing chain and valve adjustment issues, she's been running great until Monday morning, half way to work. Ran great again this morning, even kicking in the passing gear and passing cars on the highway with no stuttering. However, when I went to lunch, she ran fine under moderate acceleration through all 3 gears and didn't start stuttering until I accelerated heavily from 50 mph - 60 mph when I backed off. It all appears to be worse after she warms up good. At this point, I'm still trying to isolate the problem before I start throwing money/labor at it.
Author:  nm9stheham [ Thu Apr 23, 2015 11:31 am ]
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Quote:
Thanks for the reply, nm9stheham. I do believe I have excess slack in my timing chain as well as my valves needing adjusting. I installed the HEI ignition system per the instructions here. http://www.slantsix.org/forum/viewtopic.php?t=15779 Even with the potential timing chain and valve adjustment issues, she's been running great until Monday morning, half way to work. Ran great again this morning, even kicking in the passing gear and passing cars on the highway with no stuttering. However, when I went to lunch, she ran fine under moderate acceleration through all 3 gears and didn't start stuttering until I accelerated heavily from 50 mph - 60 mph when I backed off. It all appears to be worse after she warms up good. At this point, I'm still trying to isolate the problem before I start throwing money/labor at it.
I was gonna ask, after reading today's symptoms, if the temps went up from the morning drive to the lunch time one. I would agree that it is temp related. That would make me look towards ignition and carb/fuel supply first.

So confirm that you got rid of the ballast? And what coil is being used? And spark plug wires? Have you checked the plug colors to see if things are normal or abnormal, or if 1-2 plugs looks way different from the others?
Author:  SlantSixDan [ Thu Apr 23, 2015 12:43 pm ]
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Quote:
Timing chain slack is pretty easy to find..... grab the crank pulley and try to turn it back and forth. When the chain slack pulls taut, the force needed to rotate the crank will go way up.
...except the force needed to rotate the crank will also go way up on each and every compression stroke, making it more or less impossible to tell anything this way. An easier, more accurate way to check for timing chain slack is to remove the distributor cap, turn the engine by hand using the fan and fan belt in its normal direction of rotation until the distributor rotor starts to move and the timing mark on the crank pulley is at the lowermost mark on the timing tab. Then rotate the engine by hand the other direction until the distributor rotor once again begins to move. The timing marks will then give a direct reading of how many crank degrees of slop there are in your timing chain, i.e., how far your camshaft is retarded due to timing chain slack.
Author:  sargentrs [ Thu Apr 23, 2015 12:47 pm ]
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So confirm that you got rid of the ballast? And what coil is being used? And spark plug wires? Have you checked the plug colors to see if things are normal or abnormal, or if 1-2 plugs looks way different from the others?
Yes, the ballast resistor and old ignition box are disconnected. I'm using the Standard-BlueStreak #UC16 coil. Currently I'm using the 6mm wires that always been on it since I got her, not sure of the brand, and I have some new wires ordered. All the plugs look pretty much the same and appear to be firing appropriately. Light brown insulator, clean, dry barrel, and the electrodes look like it's been firing with little to no crud on them. No noticeable pitting or burn marks. All are dry, free of oil or gas. I was thinking it was temp related too. Appears to be worse the longer she runs or the hotter the day is. Temps only increased from the low 50's to the mid 60's by lunch and she didn't do badly. I was thinking it was possibly the timing chain stretching more as heat built up or distributor pick up/reluctor getting sloppy with additional heat. I'll feel more comfortable once I get the new plug wires in and can put the new cap/rotor/wires in place and eliminate that variable. Thanks!
Author:  sargentrs [ Thu Apr 23, 2015 12:50 pm ]
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Quote:
Quote:
Timing chain slack is pretty easy to find..... grab the crank pulley and try to turn it back and forth. When the chain slack pulls taut, the force needed to rotate the crank will go way up.
...except the force needed to rotate the crank will also go way up on each and every compression stroke, making it more or less impossible to tell anything this way. An easier, more accurate way to check for timing chain slack is to remove the distributor cap, turn the engine by hand using the fan and fan belt in its normal direction of rotation until the distributor rotor starts to move and the timing mark on the crank pulley is at the lowermost mark on the timing tab. Then rotate the engine by hand the other direction until the distributor rotor once again begins to move. The timing marks will then give a direct reading of how many crank degrees of slop there are in your timing chain, i.e., how far your camshaft is retarded due to timing chain slack.
Thanks, Dan! I'll check that out.
Author:  sargentrs [ Thu Apr 23, 2015 2:54 pm ]
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Quote:
The timing marks will then give a direct reading of how many crank degrees of slop there are in your timing chain, i.e., how far your camshaft is retarded due to timing chain slack.
Thanks for teaching me that, Dan. By this method I get 9-10 degrees total slack between rotor tip movements. Is that bad?
Author:  SlantSixDan [ Thu Apr 23, 2015 3:21 pm ]
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That's extreme, alright! You're definitely (over)due for a timing chain and sprockets. Installing them carefully will net you a big improvement in drivability, performance, and fuel economy.
Author:  nm9stheham [ Thu Apr 23, 2015 4:00 pm ]
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Quote:
Quote:
Timing chain slack is pretty easy to find..... grab the crank pulley and try to turn it back and forth. When the chain slack pulls taut, the force needed to rotate the crank will go way up.
...except the force needed to rotate the crank will also go way up on each and every compression stroke, making it more or less impossible to tell anything this way.
Uuuuh you just don't turn it to make any compression.....or pull the plugs......whatever...

Agreed, that chain is pretty bad. The OP will be very pleased with the engine after a new chain. Not sure it is the cause of the current stumbling problem however. With misfiring under varying loads, I always think of mixture and ignition first. BTW, I would not expect the timing chain length to change to any degree with temp; everything esle is expanding at about the same rate with temp. I would expect it to be a large part of the timing jumps that you have observed based on similar experience.
Author:  sargentrs [ Thu Apr 23, 2015 4:24 pm ]
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Thanks guys! Last October I pulled the engine and tranny and replaced all the seals and gaskets to stop her bleeding. Told myself then that it needed replacing but couldn't't afford to wait on parts. Did it all in a weekend because it's my only ride. Looks like that's on the agenda now. Plug wires shipped Tuesday so, hopefully, they'll be here and can swap them out this weekend.
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