low-speed & idle roughness

[Guest 53and61]

This is a modified version of a posting on the BCA site; I doubt that my engine problem is specific to Buick, so maybe someone here can help. <P>My recently acquired 401 V8 will not idle smoothly below about 650 rpm (Buick specifies 525 rpm). Low-speed acceleration is rough, but the engine performs like a dream at moderate and high speeds. The engine was rebuilt 4000 miles ago. <P>I can't find a vacuum leak. The manifold vacuum is about 16" and steady at 650 rpm (at 5000ft altitude), and about 15" but with some bouncing at 525 rpm. Buick specifies at least 14" at 525 rpm at sea level, so I conclude that there is no vacuum leak. <P>Cylinder compression is 130 to 145 psi with about 100 psi on first compression stroke on all cylinders; injecting oil causes the final pressures to go up only about 5 psi. Buick specifies a minimum of 180 psi at sea level, which translates to 165 psi at 5000ft. <P>When I got the car, the base ignition timing was about 35 deg BTDC, and the engine did not ping at all! Setting the timing to the recommended 12 deg BTDC made the symptoms worse. The crank pulley timing mark is exactly correct relative to #1 piston TDC.<P>Dwell is on the money. Plugs, points, condenser, and plug wires are all new. Distributor bearings have no detectable slop. Vacuum and centrifugal advance are functional, although I have no means to test these precisely. I carefully rebuilt the carburetor; the accelerator pump works fine. The plugs become sooty (not oily), but this isn't uncommon at 5000 ft with carbs set up for sea level as mine is. The low-speed roughness wouldn't be explained by the rich mixture of altiutde/carb descrepancy in my experience.<P>Extrapolating from Buick straight-8 data, I checked valve timing at the rockers and believe that it is correct; to check this correctly would require disassembly of the timing chain cover/water pump. <P>I've run out of ideas, and this problem is casting a pall over the restoration project. Any ideas? Would a "performance" cam cause the symptoms? Is 4000 miles enough to break in even hard-alloy rings? What am I overlooking?

[Guest stude8]

Hello 53:<BR>I am not familiar with Buick engines by years but expect a 401 is mid 70's vintage, (you didn't state year we're talking about)<P>You may have a problem called "Mutual Inductance Misfire" it happens when consecutive cylinders in firing order have their spark plug wiring running PARALLEL next to each other. (Some guys think all the wires in neat rows looks professional)<BR> <BR>The spark pulse for the cylinder ahead of the offending cylinder "Induces" enough energy in the wire for the next cylinder in firing order, which is on compression but not yet at TDC. It tries to fire weakly BTDC and the result is a rough idle.<P>Studebaker had this problem with their 1960's V8's and had to issue a service bulletin to revise the plug wire order to cause the two interferance wires to cross each other at 90 degrees to prevent the misfire.<P>Stude8

[Guest 53and61]

Thanks, Stude8. You might be on to something. The car is a '61. The shop manual does indeed caution against plug wire arrangements that lead to crossfiring, but it is skimpy on exactly how to position the wires to prevent the crossfiring. The wire alignment grommets are missing, and the wires are running pretty much higgledy-piggledy. I found that a 'scope monitoring any wire shows some induction from every single other wire, but I have no idea if the magnitudes are enough to misfire the plugs. The firing order is 12784563; which wires should be separated? The manual mentions only 1 and 3--are there other critical pairs? Thanks again.

[Guest 53and61]

Stude8,<P>Whoa! It just hit me. Do you mean that the critical pairs are 3&1, 1&2, 2&7, 7&8, 8&4, etc?

[Guest]

the critical pairs will be any pair of cylinders that r side-by-side on the SAME side of the block AND that fire directly one-after-the-other. e.g. Chev and Packard V8's fire 18436572. NOTICE that 5 and 7 are in the same bank and rite next to each other. These 2 cylinders are the most susceptible to 'cross-firing'. Your 12784563 place 1&3 next to each other (assuming all odds r in on one side). So 1& 3 r the most suseptible.<P>My above explanation is a good rule of thumb but not entirely accurate. It has more to do with ANY 2 cylinders that mite have plug wires routed closely to each other and that fire 90 degrees apart.

[Guest]

correction: ....that fire LESS than 90 degrees apart

[Guest]

in any case, plugg wires should not be routed with less than 1/2 inch clearance for more than 3 inches. Sometimes, under difficult routing conditions, the wires can cross over each other at one point and then crossed over again at a more distant point in the opposite direction. This will cancel out the inductance effect.

[Guest]

the best policy is to keep them seperated. NOONE is more guilty than I of sloppy spark plug wire routing.

[Guest]

r u sure about that 12784563 firing order???<P>Very odd that 3-1-2 would fire that close together. Especialy 3 and 1 that close to each other and so far from the flywheel unless some off-beat numbering arrangement is used for the cylinders like ford used on a few V8's. Usually the left bank is odd w/1 at the front and rite bank even with 2 at the front.

[Guest 53and61]

Thanks, Packard. You've given me something to think about and a direction to take in plug wire routing. I double-checked the firing order; it's correct. Odd that the straigh-8 bunches the wires up together without a crossfiring problem. Are V8s more susceptible for some reason?

[Guest]

don't mean to butt in but chevrolets in the sixties used wire supports with rubber grommets that were numbered for wires in specific order. hope your on to something.

[Guest]

v8's r no more suseptible than any other engine configuration. Its just that the routing of wires on V8's tends to cause wires to get bunched together more. Its odd that i have seen V12 jags or other engines run MANY wires tightly packed together in a steel tube with no problem but that mite have something to do wioth grounding to the tube also. It also depends greatly on wheather u r using solid core or suppression wires and the quality of the wires. Someone in here with more expertise on this matter mite be able to shed more lite on the subject than i can.

[Guest 53and61]

Nearchocolatetown--I welcome butt-ins; this thing isn't going to run right until someone identifies the problem. Every junkyard nailhead that I've found has had the wire grommets stripped off. A photo or diagram of the original routing would be a big help.<P>BTW, I forgot to mention that a new coil didn't help.<p>[ 06-22-2002: Message edited by: 53and61 ]

[Guest]

I'm with stude 8, my 32 Nash twin ignition 8 {16 spark plugs!} suffered from the same problem, misfiring at low speed AND high gear after awhile.<P>Because the wires are really bunched together with no means to separate them and are subject to twisting with the spark lever the only thing that worked was to replace them with new wires, took me a year and 2 carb rebuilds later to figure it out.

[Guest]

Buick started to have "Induction firing"problems with the 53 V8's.When two cylinders fire consecutively have wires that lie in each others magnetic field.The first plug of the two has high voltage(due to high compression), and wire # 2 is exposed to low compression and a load of fuel,so "induced" voltage can ignite it. There is a pair of wires on each bank so affected, Get the wires to the opposite end of the separaters(near distributor and at the back of the plug covers 53-56)then where the wires lay inside the cover make certain the affected wires criss-cross each other.57 up the wires are exposed,but please notice the grommets are held in place with NUMBERED brackets and in each case,the grommets have a WIDE spacing to accomodatethe affected wires. No problem with straight eights, the distributor fires one cylinder forward then one rearward. No problem. Buick has a BPS on this in 1953,(V8's)also I have written it up in the "Bugle"

[Dave@Moon]

I bought a set of plastic reproduction wire looms/grommets from C.A.R.S out of New Jersey for my 1960 LeSabre about 6 years ago. If you still have the little metal "stands" {brackets that attach to the rear valve cover bolts on both sides} that they go on you can use these to fix any wire routing problems. <P>They look pretty neat too! Sort of like tiny interstate bridge supports or telephone poles.