Dandy Dave

1940 Olds got my head spinning....

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Helping a fellow with a 1940 Oldsmobile series 70  with an inline 6-1947- 238 CID engine that was installed over 30 years ago. Dang thing losses power and skips over 40-45 miles per hour. Below 30- 35 it runs as it should and pulls a hill well. Idles like it should. He has had a lot of mechanics look it over and no one has quite pinned down just why it breaks up at highway speeds. Things that have been done. Carburetor rebuilt, installed a newly rebuilt distributor. (The old one had worn bushings.) New manifold gaskets. Checked for vacuum leaks. New coil installed, wires, plugs, points, condenser. Fuel pump and fuel delivery seems fine. Does not seem to be vapor locking. Compression is equally around 80 pounds on all cylinders. Thinking maybe weak or broken valve springs floating at high speeds? Even jumped out the ignition switch thinking there may be a bad connection but it did not make a difference. Not sure what else to look at. Any thoughts?  

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Have you confirmed that the vacuum advance is working properly? If it is not that, I would probably try swapping out, one at a time, a known good condenser, and a known good coil, to see if anything changes. Just because they have been changed does not mean that the new parts were not bad too.

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Every time I've had a high-speed breakup like that, it's been the coil. I didn't see that on your list of replaced parts.

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Don't forget to check the polarity of the coil. Its doesn't make any difference as far as voltage is concerned but reversed polarity makes the plugs fire "backwards"....... we forget things like that when we've been dealing with newer cars for awhile. Also check out the ground connections (all of them). 

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I recently had a new coil that was defective. Took a long time to find it because I kept checking everything else.

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MCHinson, You have me thinking. Maybe the coil is one with a built in radio resistor. I know they will not work on something like an older Kohler lawn mower engine. Automatically retards the spark enough so it changes the timing. 

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OK, Sounds like the coil could be the problem. Even though it is new, we will try a different one, check polarity, and double check that grounds look ok and see what happens. I'll be back. Thanks All. Dandy Dave! 

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I like the coil theory. If it isn't that, ohm test all your plug wires, inspect the distributor cap under a bright light for carbon tracking, and put in a new ignition rotor.

 

How sure are you about the fuel delivery? Mid to late 30s GM sending units (at least on Buick and Pontiac) have a copper pickup tube, and it can crack where the strengthening rib is crimped around it on the top of the sending unit. I am not sure if Olds used this design.

 

Could the exhaust be plugged? Mice? I would go drive with a vacuum gauge attached to the intake. It probably wont be definitive, but might provide a clue or two.

 

Is the heat riser working or at least open?

 

Will it screw up if you rev it high in the shop with the engine unloaded?. If so, disconnect the vacuum advance, and if it still does it try backing off the timing at the engine speed in question, and see if it smooths out. Most people today find that they need more timing on todays fuel, but on my 1936 Pontiac, I found I needed less, a lot less. I have three distributors, and according to my Sun 404, they all met factory spec for advance. I had to shorten the curve up, a lot. I took 10 degrees out of the vacuum advance, and some out of the centrifugal as well. I cannot get my head around how it can be that it will not run right with the factory curve, but it wont. It runs terrific now. That Olds engine isn't the same, but is a fairly similar design.

 

One more thing, Does it use the 2-piece points? If so, look your points over really closely. I had some new production Standard or BorgWarner points that fit very loosely around the pivot pin. They were floating at high speed, and the timing was jumping around about 10 degrees. NORS points from Ebay in some off brand solved that.

 

If you have a moving breaker plate, make sure it has a good flexible ground wire to the distributor case.

Edited by Bloo (see edit history)

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Is it speed related or engine RPM related?

In 2nd gear does it do the same (same rev's)?

As above, blocked muffler is another remote possibility.

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Check the plug gap also. The old gap recommendations were for crappy gas. If your running a replacement OEM plug you might find it needs to be opened up some. On my 31 Chevy we go from the original .028 to .040-.045. What a difference in how that old stovebolt runs. Bigger hotter spark. We also advance the timing 6 degrees from the original setting.

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Breaks up at high RPM's in the shop as well as on the road. I know what you folks are saying about inferior parts coming from places in far away corners of the earth. Sometimes it seems like their getting us one cheesy part at a time.

  

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I can't ad anything over what has been said, but just want to say what a pleasure it is to see all the constructive comments. The AACA Forum is such a wonderful thing, where people are so helpful in situations like this. Makes me proud to be an AACA member and see the great fellowship and helpfulness in today's world where most of the time most of the people are so self centered . Not my question, but thanks to all who give sage advice.

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Just on the long shot, you have a flexible fuel line on the suction side of the fuel pump and, maybe, one at the sending unit on the tank. I had one that would pull air under vacuum but didn't leak out. I saw traces of bubbles in the fuel filter bowl.

 

That car was a '58 Cadillac convertible that belonged to one of Grimy's buddies. He came to my house and said "My car won't go over 100 any more." Then took me down my road to show me. I am a poor rider to begin with. He just took me out and scared the hell out of me. I fixed the car with a new hose and let him test it himself!

Bernie

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I have seen this problem in different engines before; and a few previous posters have mentioned ed the coil.  The coil, breaking down, is a very likely reason for your problem.  one of the first things I would have checked.  The vacuum advance may cause similar problems, but not really a skip or a miss.

 

Along with the coil; I have found condensers to act your problem.  Sometimes a condenser, will go bad quickly and prevent starting; but other times the condenser can act like a faulty coil also.   May or not be heat related, such as some coil problems.  Condensers are not expensive, slip a known Good one in for a try.  While there, check those points again; for pitting and proper adjustment.

 

It is worth a try, I have had bad condenser as well as bad coils.  

 

intimeold   

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Also check the breaker points tension, too. A weak points spring will cause point bounce as RPMs increase, but otherwise it runs well at lower RPMS,.... and it has no relationship to engine load like a going-bad coil often does, so it will breakup at higher RPM's no matter if it's in the shop, or out on the road.

 

Paul

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Rebuilt or remanufactured distributor? Did they replace the centrifigal springs in the distributor or reuse old weak ones? Not even sure if yours has them but just a thought. 

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Dave

When testing in the shop under load, are you able to warch the fuel filter?  Maybe has enough fuel for lower speeds, but runs out at higher rpm’s.  Does it flatten out over 50-55? Or stay the same?  A dead spot is one thing,....  makes me think is loss of sufficient fuel.

 

Matt

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Years ago, when I was working as a used car mechanic at a Volvo dealership, we had the same problem with, I think, and Olds station wagon. Everything worked perfectly but it broke down as soon as the revs got above a certain point. We tested it by disconnecting the exhaust from the manifold... then it revved up perfectly - if rather noisy. It turns out it has some sort of double wall exhaust pipe. Apparently, some unburned gas got between the walls and detonated. The inner layer of pipe was blown in, partly obstructing the exhaust but this did not show on the outside. After we'd fixed it, we saved a piece of the pipe as a souvenir.

 

I'd try disconnecting the exhaust from the manifold... if it responds, you just have to work down the line until you find what is blocked.

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Coil  6 or 12v ? If here I'd put it on the scope (have a portable Heath).

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7 hours ago, Dandy Dave said:

Maybe the coil is one with a built in radio resistor.

 

Dave, the resistor inside or external to a coil has nothing to do with a radio! It has everything to do with the voltage needed for the coil to work properly in the circuit it is designed for.

 

7 hours ago, Dandy Dave said:

Automatically retards the spark enough so it changes the timing. 

 

What is this in reference to? A resistor does not do this..... 

 

Vermont boy has a good suggestion. Have you checked coil polarity?

 

GM double wall exhaust pipes are legendary in the repair field, but there should not be one on this car. I've seen big engine Buicks and Oldsmobiles only do 15 mph!

Edited by Frank DuVal (see edit history)

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4 hours ago, 60FlatTop said:

 

That car was a '58 Cadillac convertible that belonged to one of Grimy's buddies.

The Godfather didn't tell me about that one....you could have said it was the factory governor.

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I have seen many  coils with  no voltage or polarity markings on them. 

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 I have a car that would run fine at low speeds but at high speeds it flattened right out.

 It turned out that I had a "ball" of crud in the fuel line and it passed gas until it got sucked up in a restriction  in the fuel line and shut down the flow.

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I had an old Jeep wagon once that was very dependable. It would start on the cold days when none of my other cars would.

It developed a similar problem.

After driving me crazy for a couple of weeks I took all of the fuel lines apart, along the frame there was a coupler that had a small pebble in it that was acting like a valve.

At speed it would pull that pebble up into that coupling and wouldn't pass thus shutting off the fuel flow.

One of the oddest failures I ever found.

I found this when I was blowing thru the line.

 

 

Edited by JACK M (see edit history)

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