Buick 455 idle questions

I have a bit of a rough idle on my afaik stock Buick 455 sitting in my Electra 225 Custom Convertible 1970. I can see the car body rock slightly and the sound is not perfectly right. Low throttle performance can feel a bit mediocre.

I have carefully looked through https://www.v8buick.com/index.php?threads/455-vacuum-idle.293589/ but there are still some stones to turn. Rough idle can stem from many things I understand, so I'll try to narrow down the current state into a few specific questions.

Current questions

1. Base ignition on cylinder 1 reads 6 deg consistently but cylinder 6, that should fire a crankshaft revolution after cylinder 1, reads 0 deg! I guess this should be more than enough to contribute to a rough idle and other issues? I mean unless the engine was designed to be asymmetrical in some senses, but that sounds unlikely? Any input on this before I spend a few hundred dollars on a new distributor? One possible root cause could be the 8-sided cam that used to actuate the mechanical points historically. I think this is used as a trigger for the electronic replacement system present day. From how inductive sensors work I can see it being hard to get distinct and precise sensor readouts, given that the flat faces of the cam does not provide very distinct features. Also, the cam is worn asymmetrically it seems. Maybe a new cam that is shaped more like a gear? Where would I find that though...
2. Should I be able to kill the engine by disengaging the idle adjustment screw completely? In my world RPM should keep dropping until the engine dies while screwing it out, but mine just stabilizes on a low RPM. I am pretty sure the high idle settings did not interfere when I did the experiment. I guess that if it is supposed to die, I have a leak, a poorly matched PCV valve or maybe throttle valves not closing well enough? I did search for leaks using a propane torch and carburetor cleaner but could not find anything.
3. How long do I have to run the spark plugs to infer information from their appearance? The new spark plugs still looked shining white and new after around 25 miles of mixed driving. I know this is an indicator of lean mixture among other things but thought it might be soon to tell?

General information

• Engine is as far as I can tell stock
• Vaccum reads around 15 in-hg at idle, I have no tachometer yet so unsure about the exact RPM
• Ignition
  • Looks stock, except that the condenser and breakers have an electronic replacement
  • Base ignition set to factory 6 degrees before TDC. I did play around with base ignition from 6-20 degrees, idle would subjectively improve at more advance but still persist
  • New spark plugs new vacuum advance, checked all ignition cables and the distributor cap
  • I did check spark by making sure the timing light would trigger on all ignition cables while running
  • Connection of cables double checked
  • Verified that the marker on the harmonic balancer is correct using a piston stop
  • The mechanical advance is probably alright idle-wise, it has no influence on total timing until the RPM is way above idle
• Fuel
  • The Quadrajet carburetor was just rebuilt by me
  • Float level, air doors and choke set according to the Buick Chassis Service Manual
  • Front vacuum break new, rear one checked
  • Replacement of all gaskets, the float needle and the rubber for the accelerator pump.
  • Bushings installed for both the primary and the secondary throttle shaft. Play determined to be absolute max 0.1 m.m. / 0.004 in determined using an indicator dial at the shaft ends
  • All vacuum hoses as well as the PCV valve are replaced
  • Brake- and HVAC vaccum disconnected during tests
• Mechanical parts
  • Valve lifters on the driver side make a very slight noise but you basically have to lay your ear on the valve cover to hear it
  • Did not determine the slack in the cam chain, but as ignition seems consistent maybe that is not a biggie?
  • With a cold engine lowest reading was 107 PSI and the ratio lowest/highest pressure determined to 79%. Compression is not great but probably alright?

 

Probably has an aftermarket cam

 

Yes, true that I cannot know if the cam is stock. But since I have potential issues with both fuel and ignition I want to rule out issues regarding these systems first.

 

Aaaa to stall! That is the verb I was needing

I can make it stall by bottoming the idle mixture screws. However, should I be able to stall this engine with the idle level adjustment screw as well?

 

Yes

 

A leaking carb (air horn/metering etc) can cause running with curb idle screw backed off, and also "run on" when switched off.

You may see fuel dripping inside the bores while idling.

 

I'm by no means as knowledgeable as some of these guys here but I'm dealing with a similar issue I've been researching so I'll chime in. You said you did a dry pressure test on a cold engine and got 107psi as your lowest reading.. but the percentage difference between lowest and highest was 79%? Is that right? I think the acceptable tolerance for this is only supposed to be 20% difference. Sounds like there could be a big difference in pressures that are causing it to bounce around.

 

Thanks for the replies!

Thanks for the double check. I meant that the fraction [lowest pressure]/[highest pressure] * 100 = 79%. So highest pressure was then 135 psi.

Good point! Actually did not consider the possibility of internal fuel leakages. Will try to get a look in there.

I also realized that I forgot to leak test the transmission vacuum modulator. Also I will take the distributor apart tomorrow and check clearances etc. They did connect the 12V feed to the electronic ignition system directly to the positive side of the coil, and I understand that can be an issue depending on what points replacement system I have, and if I have a series resistor or not on the ignition coil.

 

1. Your cranking compression test results stink. Depending on elevation, I'd expect more like 150 psi. GM has said that 100 psi is a minimum; and 25% difference between highest and lowest. Their specs have changed over the years--I don't know what they are for your particular vehicle. I've seen cylinders run at 80 psi cranking compression, and I've seen cylinders with 50 psi that were dead at idle but ran at higher speed. As the pressure gets lower, the engine gets "lazy"--poor power, poor fuel economy, poor response.

Was your pressure gauge tested for accuracy? The gauge itself can easily be 20+ psi "off" from correct.

2. I have no use for aftermarket "electronic conversion" ignition systems. They tend to be unreliable and when they do fail, you need to mail-order parts to repair them. I'll take a used-but-usable (or rebuilt) Genuine GM "HEI", thank you very much. 6 degrees of timing variation from cylinder-to-cylinder is insane. My standard is ONE degree, and I hope for less than that.

3. Consider performing a cylinder-balance test to find weak/dead cylinders. Connect a vacuum gauge to manifold vacuum. (A tach can also be used, but the vacuum gauge is more sensitive.) Short out the spark to each cylinder, one at a time, 5 seconds at a time, with a pause so all cylinders run for 5 seconds before moving to the next cylinder. Record the amount that the vacuum gauge drops for each cylinder. Ideally, they all drop the same amount. A cylinder that drops the vacuum less than others is weak. A cylinder that doesn't drop the vacuum at all is dead.

When it's me, I blunt the sharp points of eight small, "finishing nails", apply a hint of silicone dielectric grease to them, and SLIDE them between the plug wire and distributor boot. Then touch a grounded 12V test light or grounded jumper wire to each nail in turn to short the spark. The test light will not light up. DO NOT PUNCTURE THE INSULATION, just get between the wire and the boot--you'll feel when the nail touches the metal end of the wire at the distributor end.

4. Is the manifold vacuum level steady at idle, or does the gauge needle jump around? Should be steady. 15 inches of vacuum at idle is low. Has the gauge been tested for accuracy?

5. "Good luck" getting anything useful from inspecting spark plugs on a street engine running unleaded gasoline, in 25 miles--aside from massive oil fouling or massive fuel fouling. If the plugs were run for several thousand miles, you'd MAYBE get a hint of the operating conditions of a normal "good condition" cylinder. Unleaded fuel just doesn't color the plugs--they're always white which folks think means the engine is lean.

 

Yes, I might revisit compression later on depending on how well I can get the engine to run as it is. I was using a rather cheap compression tester that I have not calibrated. I did some partial measurement series first just for a try out where I got values up to 145 psi. Which might be related to maybe to the battery state, cylinder lubrication state or maybe something else I guess. My 1970's Chassis Service Manual does not specify a lower value for pressure, but they present pressure values with similar magnitudes in some examples.

I took the distributor off today and it was in a terrible state. Which is both good and bad news, now I at least know what to fix. I could now see that I have some Pertronix systems on, some of the ones with a disk with magnets. One magnet had popped out so it had probably been running on at most 7 cylinders for a while A bit weird as the timing light triggered on all plug wires, should check spark with some other method next time. I read in the Pertronix manual that the magnets should have some protective tape on them and this one has fallen off. I must say quality seems mediocre on this unit considering the price point! Solid state electronics in this price range should last very long.

Furthermore, the distributor shaft seemed to be shimmed incorrectly for the Pertronix unit and the mechanical advance assembly were the magnet disk is mounted is a bit wind. And this should be enough to explain the timing inconsistency between the cylinders. But then, the worst part, the lower bronze bushing for the distributor is extremely worn and unevenly so. The shaft could be moved several millimeters radially. The distributor shaft itself was around 0.06 m.m. thinner around the bushing according to my micrometer. The gear looked fine and no debris though, but otherwise it seems completely shot. I am so happy I discovered this before potentially the gears down there would blow up completely! Given how many aftermarket and refurbished OEM products there are around I do not think I will go through the hassle to rebuild this distributor.

I have considered performing such a test. The practical solution you use for grounding out one spark plug when running seems useful, thanks! I have been thinking about how to perform this test practically without having to rip the ignition caps on and off all the time.

The vacuum is not steady and the gauge is not calibrated. Lets see what the vacuum looks like when I have sorted out the ignition.

Ooh really? Well good news I might not be running lean at least then.

 

I can see how a warm engine should be more representative. Have to figure out a way to connect the compression gauge without the o-ring seal that would melt though. While not toasting my fingertips for that matter

Now I have to figure out what to do with the distributor. The HEI solution with an integrated coil seem practical but I want my engine to look as stock as possible on the other hand... Afaik everything visible on the engine is stock, would be a bit sad to break that. This one looks fairly stock but I wonder how it will be finding parts to it the day it starts to break down: http://www.taperformance.com/proddetail.asp?prod=TA_724B
Fixing the marred distributor shaft I have would be challenging least to say. And then all the bushings on top of that. But if I invested in a lathe and some reamers... Hmm...

 

Not a concern. Even a fully-warm engine wouldn't have the exposed metal of the cylinder head much above 250 degrees.

My experience is that having the throttle open, and battery charged makes way more difference than the temperature of the engine.

The better compression tester gauge set-ups have a quick-coupler in the hose (generally "Industrial Interchange"; Milton "M" style) so that the partial hose can be threaded into the plug hole, then coupled to the rest of the gauge assembly. Your fingers stay 6" or a foot away from the engine. Again, not an issue for heat except maybe with Chevys and some Mopars where the plug holes are buried under the exhaust manifolds.

 

Warm is fine doesn’t need to be HOT.

 

You NEED that rubber O ring on the fitting, without it you’ll get more whacky readings.
Also, remove your converted distributor and throw it out, put in a GOOD working GM HEI.
I’d recommend an MSD distributor, but I haven’t heard good things about them lately, go GM HEI

 

I have a gauge with a quick coupling fortunately. Well, with so many people saying it is possible to to this without getting charred I guess I have to give it a go some time

Yes, the o-ring makes it more convenient. Else I would need some sort of adapter resembling the conical seat of the head and torque it down like a spark plug, which would be inconvenient. Where to find these mysterious GM HEI distributors people talk about? Or do people mean aftermarket HEI distributors in general, not OEM ones?

Thanks, appreciate it! I found the full manual here some time ago: https://www.teambuick.com/reference/library/70_chassis/index.php. I liked the Chassis Service Manual so much I bought a 150 USD-ish paper reproduction of it last month. Best money I spent related to the car so far!

Thoughts on rebuilding
It haven't found an affordable (<800$) Buick distributor in Sweden yet. And it has gotten harder to order from the US Few US part suppliers ship internationally and when they do (e.g. Summit) shipping is like 90 USD, and to that 25% tax is added when it arrives here. On top of this I would need female connector ignition cables for ~70$. Spending 400$+ when I might get away with just replacing a bushing and a magnet feels a bit out of proportion. It is just the lower end of the distributor shaft that is badly scored. What I could to is putting another bushing a bit deeper in the housing where the distributor shaft is still fresh. Around 20 m.m. more of the lower shaft would not be fully supported radially by a bushing anymore then though. Which probably is not that big of a deal? Before bringing out any equations, lets see if this information can be extrapolated from a similar system. I see that many mopars have a distributor shaft that protrudes quiet a bit from the distributor housing, but maybe those are further supported by the engine block though? Else I guess the situation should be somewhat comparable?

 

https://www.teambuick.com/reference/distributor_buildup.php