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Will not crank. Need suggestions


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I am reaching the point of being completely stumped over this one. My 51 Pontiac (straight 8) fails to crank. I've only owned the car for a little over a month, and thus far have been able to keep it running only two times (about 15 min each time). Other times I get a brief firing for a few revs and then it fails to catch and run. I replaced points, condenser, and have gone back to verify that the points are opening and closing properly. Checked wiring inside the distributor, replacing one frayed wire which was needed. Continuity in and around distributor is good, with no shorts to ground. I've set and reset the point gap (as well as plug gaps). I have spark at the plug, and the spark looks pretty healthy to me, bright, jumping about a 1/4 inch gap. I have fuel. In fact, the rear two plugs (7,8) are often wet with unburned gasoline, which oughta tell me something, but I don't know what. The front six plugs are usually full of black soot, as you see at the exhaust when the engine runs too rich. I've temporarily bypassed the original fuel pump with an electric one, as I suspected insufficient fuel delivery. I'm keeping the electric one in place until I uncover the problem. I have compression averaging around 110-120 for cylinders 1-6, and averaging around 90 psi for the rear two cylinders. I installed a new coil with internal resistor (in fact, I've tried four different coils from my stockpile of old parts). I bought a carb from ebay and installed it, and it seems to be working well. The old carb was suspect, having bent metering rods. I've peaked down thru some of the spark plug holes and can see one valve moving on each, so I assume valves are not stuck. Besides, I would get no compression if they were stuck. By this afternoon, I will have replaced the plugs, although the existing plugs were all checked with an ohm meter for shorts or opens. I have a good battery (thankfully). Otherwise I would be continually having to recharge. The previous owner changed it over to 12 volts, but drove it that way for a couple of years. I verified that the car ran well, while he owned it, and he has no offering of anything that could be wrong, as he had good experience with it. Any ideas will be investigated. Thanks, Bob

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I forgot to add that I also bypassed the wiring to the ignition switch, by connecting the positive side of the battery to the + side of the coil. That ruled out a bad connection at the switch. I get an occasional "bump" when a cylinder fires and tries to start, but that has become a rare event. The only major discrepancy so far is the difference in the unbalanced compression. Cyl. 7&8 are the ones with wet plugs, and also the ones with the lowest compression. Could I get compression if the intake valve isn't fully closed? And, wouldn't the car try to start anyway, even with only six of the eight cylinders working? Not having a manual yet, I'm unsure about the timing marks on the balancer on the front of the engine. There are three marks there, located about 3/8" apart. Should I use the center mark for setting the timing? So far, I've not changed the timing either way because I don't want to introduce any more variables and compound the problem.

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the three marks or lines on the harmonic balancer are as the balancer rotates with the crankshaft, the first mark to pass the pointer is six degrees advance timing, the second mark is three degrees advance timing, the third and last mark is zero degrees or what's known as "top dead center". you can try anyone of the three marks to try and get the engine to fire. are you putting the ignition system back to being six volts ?, if you are, the ignition system and the coil needs no resistors. have you check the rotor and dist. cap for corroded contact area's ?, Charles Coker, 1953 Pontiac tech advisor.

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Charles,

After spending a considerable amount of time on this problem for the past month, I finally discovered the answer. By the way, thanks for explaining the timing marks to me. I will make a note of that in my paperwork. I am on the lookout for a manual on the car, one that won't cost a fortune. I'm sure that info is in a manual.

The problem was this: Someone must have left the hood open during rainy weather in the not-too-distant past. Rainwater puddled in the cups at each spark plug, and apparently must have stayed there until it dried. That created a fair amount of flaky rust in the cast iron, and around the base of each spark plug. I had previously scraped some of that rust away and used a shop vac to clean it (but not too good). At that time, I was trying to prevent flaky rust from dropping down into the hole when I removed the plugs.

Today, I used a small diameter stiff wire brush that mounts to the chuck on a drill motor. The diameter was just right for going down into the cup and cleaning the metal until it was shiny. I polished every spark plug hole and installed new spark plugs. As soon as I finished that task and hit the switch key, it fired right up. It was rough going for a while because there was a lot of liquid gasoline down in the intake manifold, the result of countless hours trying to get the thing to start.

My theory is that the rust prevented the plugs from grounding to the motor, and thereby not allowing for a spark inside the cylinder. I could remove the plugs and put them near a sharp metal point on the engine block to test whether I had a spark, and it would work. But apparently there was no ground, or not enough ground on enough spark plugs to fire off the engine. I was in disbelief when the engine cranked. I really didn't think it was possible to screw a plug into a hole, tighten it down, and still not have a good grounded connection. But I can't think of any other answer. I tried to keep all my troubleshooting as methodical as possible, not changing more than one thing at the time. And by the time I decided to clean the plug holes, I had exhausted every idea I could come up with, which is where I was when I wrote this thread on the forum.

For the first time, I drove my "new" Pontiac, staying within a half mile of my home. I shut if off after about 45 minutes, and will allow it to cool overnight. We have a rather hard freeze predicted tonight, so it'll be interesting to see how well it cranks tomorrow morning.

Thanks to the replies, suggestions, etc., and the info on the timing marks.

Bob

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Thanks, Charles. I ordered a reprint version of the 49-54 shop manual last week. Should be here any day. Found one for only $22, which was a lot better than the previous $70 manual I found on eBay. Of course the $70 one was an original manual. I don't need an original, especially. Reprints can get dirty and take some abuse without worrying about them.

It appears that I've solved most if not all my ignition problems, and is engine running smoothly. I'm happy to report that, because it's been a long, deductive process getting to where I am now. Had more than one problem, which made it difficult to troubleshoot. The failure of the spark plugs was the last thing I would suspect, but finding that solved the problem of not cranking at all. Then there were lots of issues with bad ground inside distributor, which would not let it run without misfiring. Oh, and a previous owner installed a real Rube-Goldberg set of spark plug wires which would actually vibrate out of the distributor. I don't know how the thing ever managed to run at all, considering there were a total of five somewhat serious problems occurring all at one time. I've only had the car for about six weeks, and the process of eliminating one problem at the time has given me a good short course on the operation of the Straight 8. While I was itching to hit the road for some joy riding, I am also glad I've been forced to go through all these problems. Frustrating, but a real learning experience. Thanks for your replies. Bob

Bob

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