Curing a miss

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Curing a miss

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Curing a miss

Postby Bucolic » December 16th, 2017, 11:49 pm

What will cause a miss? My jeep starts every time with some choke and as soon as it's running I push the choke back in. I do, however, have a miss. It is there regardless of my speed or what gear I'm in.
I cleaned my plugs and checked or regaped them. I had my carb rebuilt by Billy in Florida. I checked and/or cleaned everything inside the distributor, which is brand new. I am running points and not an electronic ignition. The plug wires metered at or very close to zero on my ohmeter.
How do I cure the miss? Could I have yet another coil going bad?
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Re: Curing a miss

Postby rickf » December 17th, 2017, 7:28 am

Would not be a coil, that would involve all cylinders. When was the last time you adjusted the valves? Have you done a compression test? What does the plug look like on the missing cylinder? Have you checked the intake manifold for a vacuum leak? There are many things that can cause a miss but need some more information on what the plugs are telling you before I could say what may be the cause.
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Re: Curing a miss

Postby Bucolic » December 17th, 2017, 11:36 pm

The valves were adjusted about a year ago. As far as the compression, I'll check it. However, I should first check for a vacuum leak.
How can I tell which cylinder is the one missing? It's not a steady miss. I mean it has a miss all of the time but at 3600rpm, each plug should be firing about 450 times a minute. I'm not getting 450 "stumbles" per minute. (3600/2=1800, 1800/4=450). Did I calculate that correctly?
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Re: Curing a miss

Postby Mark » December 18th, 2017, 1:52 am

How about have it running, then pull off each spark plug wire.Seems to me, the cylinder that is missing, would have no effect on engine performance when you pull that plug wire
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Re: Curing a miss

Postby rickf » December 18th, 2017, 9:15 am

What Mark said. Your calculations are off a little bit. At 3600 rpm each cylinder is firing 1800 times a minute. Do you have a dead miss in one cylinder or is it sputtering around on different cylinders at idle? Can you hear/feel the miss while driving?

Here is your statement that is very confusing to me. "It's not a steady miss. I mean it has a miss all of the time "
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Re: Curing a miss

Postby Horst » December 18th, 2017, 9:25 am

on my jeep engine (Go Devil motor), when it idles, it misses one cylinder every 30 seconds or so. I have never understood why that is the case.
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Re: Curing a miss

Postby rickf » December 18th, 2017, 9:31 am

I had an old 1960 Jeep pickup with a flat head 6 and it did the same thing, it idled on five cylinders. Everything checked out fine but that was how it liked to run and as soon as it came off idle it ran great. These should not do that though so check it out.
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Re: Curing a miss

Postby m3a1 » December 20th, 2017, 5:01 pm

I don't know if you have a mechanical or electronic ignition but I once had an experience with a rotor that was loose - just a little sloppy on the shaft. I tore my hair out on that one until I found it. Because there are an infinite number of degrees of looseness, such a problem could produce the minor symptoms you are describing to the other end of the scale; an engine that will not start or run just awful. It might also be right one moment, and wrong the next which is a lot like what you are describing. This also brings us to the possibility that your whole distributor assembly might be the culprit. If not properly tightened down, or if terribly worn it would have the same effect as a bum rotor, regardless of what type of ignition is in there. Just something to think about.

Cheers,
TJ
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Re: Curing a miss

Postby muttguru » December 20th, 2017, 6:46 pm

Bucolic wrote:What will cause a miss? My jeep starts every time with some choke and as soon as it's running I push the choke back in. I do, however, have a miss. It is there regardless of my speed or what gear I'm in.


Does the water level in your radiator drop? Even slightly?
Ken
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Re: Curing a miss

Postby Bucolic » December 20th, 2017, 10:45 pm

No, the water level is right up where it belongs. Found a lot of rust on the underside of the radiator cap, so I probably should flush the block and radiator. The distributor is about a year old, it was a NOS points type when I bought it. I will check the rotor though.
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Re: Curing a miss

Postby rickf » December 21st, 2017, 9:05 am

Look at the inside of the cap real close for anything like a pencil line from one of the contacts down the side of the cap, that is a carbon track and would be the cause of the miss. You will need a new cap. You can try to clean it off with carb cleaner but it is most likely etched into the cap. That would be a best case scenario if that was your problem. Since you have points be sure to check the point gap while you are in there and then check the timing again since changing gap will change timing. If it is one cylinder with a dead miss then it is not timing, it is either something in the ignition like a bad plug, wire or cap or it is mechanical like a burnt valve. If it were missing on two adjacent cylinders I would say a head gasket. That will happen with no inclusion into the water jacket. It can also blow a head gasket out the back and not get into the water, mine did that and I found that the rear head bolt holes were not properly tapped all the way down. I found that on three motors so far.
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Re: Curing a miss

Postby Bucolic » March 5th, 2018, 1:24 am

Well I think I found the cause of my miss.After doing everything I could, including spraying carb cleaner just about everywhere and replacing the fuel filter, I took my jeep to my mechanic and had him run it down. He checked compression, points, coil, distributor cap and rotor. He thinks it's one or more bad plug wires. He suggested I get a new set and replace my rotor and distributor cap. He said that should cure it.
I did, however, notice the miss was much more noticeable in the higher RPM range, especially while accelerating just before shifting.
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Re: Curing a miss

Postby rickf » March 5th, 2018, 8:49 am

A bad plug wire is easily found with an ohms check. His suggestion is basically telling you to do a tune up. I don't want to be critical of your mechanic but that is not exactly an advanced diagnostic call after all that you have already tried. Didn't you already check all of those things? A new set of plug wires is going to set you back 150.00 and they seldom go bad on these, especially with the low mileage that these vehicles usually have. Don't get me wrong, I have seen a bad wire here and there but it readily showed up on a simple ohms test. I have seen bad spark plugs cause a miss and not show any outward appearance of a problem, that would be one thing I would invest in since eventually you will need them anyway so if it does not solve the problem it is not wasted money. Cap and rotor are maintenance items and again, if they don't solve the problem they are still needed in the future so money not wasted. If you have a steady miss it should be showing up on the spark plug of the cylinder that is missing. That will lead you to the source of the problem. If the plugs all look the same then the miss is random and more then likely it could be a burned through rotor but usually is a vacuum leak or a carburetor issue. Have you ever checked the float level in the carb?

Did the mechanic do a leak down test? That will show up a LOT of things that a compression test will not show. Sorry, but I am of the class of mechanics that thinks that you don't take someones car for testing and hand it back to them and tell them to throw (expensive) parts at it and "I think this may be the problem and this "might" fix it"".
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Re: Curing a miss

Postby Mark » March 5th, 2018, 9:24 am

Now Ken's reply has me wondering what that might entail? Head gasket, cracked head?
mark


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Re: Curing a miss

Postby Fil Bonica » March 5th, 2018, 10:02 am

Two things come to mind:
Sticking valve
IntermittNt ignition wire.
Run the engine in the dark and carefully look for any flashing around an ignition wire.
Had one like that and it was the culprit.
For what it’s worth!

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