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Misfire Teaser - straight flathead 6


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My new 1950 Nash Rambler is only firing on 5 cylinders. Confirmed by when you remove the lead to cylinder 1 it makes no difference. By contrast, remove any of the others and there is a noticeable difference.

 
There is a good spark at cylinder one - tested with inline tester and the more traditional way of holding plug to earth (really strong Blue spark).
 
Looking into the spark plug hole, the valves seem to be opening when the engine is running.
 
The real mystery, the internet tells me the firing order should be 153624. However, it's set-up as 142635. I did try and put the leads as they should be, but the car would not even start. The fact is the car runs quite sweetly with the incorrect firing order, save for missing on one cylinder.
 
The engine is the correct 1950 Nash Rambler straight 6.
 
I performed a Compression test (I only tested 3 cylinders)
1.   105 (the offending cylinder)
2.    90
6.   90
 
More mystery, the offending cylinder has good compression.
 
I have tried to test the timing, but I could not get my hands on a timing gun that would work with 6 volts, but seeing as she runs pretty well apart from a slight misfire I don't think the timing has anything to do with it.
 
When you remove the plug after running the car for a short-while, the plug is wet so there is fuel getting to the cylinder and you can see it down the plug hole.
 
Yes, I know I should take the head off and see what's happening, but I have just bought this car and I am not near my workshop and hoping I have missed something really obvious.
 
All suggestions welcome.
 
Mike
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Is there any sort of vacuum port along the intake trough near cylinder #1? (It would probably look like a fitting right into the head.)

 

Regarding the firing order, crank the engine with the cap off and see which way the rotor turns. The firing order will go around the cap in the same direction the rotor turns..

 

Edited by Bloo (see edit history)
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Interesting point---somewhere online is a fascinating piece about possible alternative firing orders, but all I find in my notes is one small note, that late 1920s Buick 6s were 1-4-2-6-3-5, and that Wisconsin water cooled truck/ind'l 6s were the same...

Can't comment re' the Buick, but have a little Thonpson "manual" (more like a pamphlet) that verifies the Wisconsins...

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The same web site that gave me the firing order also stated the direction of the dizzy was CW, but I should have checked, which I will do tomorrow. I really don’t think the leads are a problem given I am getting a really strong spark.

 

i don’t remember a vacuum port, but I will look tomorrow also.

 

it would appear based on the replies so far there is nothing obvious for the cylinder not to be firing.

 

mike

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Verify the engine's firing order with lose plugs and that compression weeps out in the correct order. Maybe someone put the plug wires into the cap in the wrong order. With all the references to the ignition firing order mentioned here and yours doesn't match seems to be a root cause. Is this an original engine or was it rebuilt at some time? Could the distributor be installed a few teeth off? #1 plug is wet because its not firing at the right time (during compression stroke). Go back to basics.

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4 hours ago, strictlyballroom said:

 

 

The engine is original, but recently rebuilt.

 

 

 

This should be a HUGE contributing factor.

As suggested, back to basics.

My money is on a valve problem.

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The plug and lead you say are OK. You say you have a good spark. Hopefully, it is equally as strong as the other five sparks. You have checked them 3 x so any further checks on them must be in a completely different (independent) way to any done up to now.

 

Back to static timing. Make sure it is set up correctly, as @Friartuck said. Can you can try another distributor cap - you might have a track or other electrical leakage in the cap disturbing spark timing to number 1.

 

Is it possible there is fuel in the cylinder but not enough? e.g. a partial blockage of the port, such as at the manifold gasket? There has already been mention of a vacuum offtake on that leg of the manifold that might be taking fuel. Just an off the wall thought - did it take more strokes to work up the compression pressure in that cylinder than the others? This might happen if the inlet is blocked, meaning the volumetric efficiency of that cylinder was reduced.

 

Is it possible the spark to number 1 is leaking out of the cable somewhere? i.e. a short. When you remove the plug or lead to test, you move the lead away from any possible short.

Edited by Spinneyhill (see edit history)
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 A professional engine builder friend of mine tells me that, these days, he prefers using a "leak-down" tester instead of a compression tester, because it can reveal more problems. 

 

Personally, I would clamp the plug from the non-firing cylinder to the head somewhere with a  good ground connection, attach the spark plug wire to it as normal, turn off most shop lights so I could readily see the spark when it happens, and cover the spark plug hole with my finger or thumb. Then have someone crank it over with a ratchet or breaker bar, and see if the spark occurs just as my finger gets pushed out of the hole by compression stroke. That will isolate answers about timing issues to just that one problematic cylinder. 

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You said that the plug on the duff cylinder was wet. That suggests that it is not sparking in the cylinder. Can you not confirm sparking in cylinder using a strobe light? I know they need 12v, but you could use a spare battery beside the car to power the lamp and put the inductive clamp onto each lead in turn whilst the engine is running. The flashes will soon tell you if a spark is occurring at each cylinder. Definitely worth a try before you lift the cylinder head. Also, look for escaping sparks in a darkened garage with the engine running (but mind the rotating fan!!!). 

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I would think that if either valve is left open during the compression/spark sequence, the OP would experience a backfire, either through the carb or the exhaust, depending on which valve was hanging open.

 

On the other hand, perhaps a cam lobe was wiped out when the motor was first run after the rebuild? We've all heard of that happening, (often when not enough break-in lube was used on new camshaft lobes). if that intake cam lobe is wiped out and the intake valve is not opening, then I would expect the plug to remain dry (unless it is getting soaked in oil somehow). But if the exhaust valve was not opening (or not opening fully), then the plug might indeed get fouled. But then I would expect it to be hard to crank over? 

 

Obviously, the same symptoms could occur if hydraulic lifter(s) on that cylinder are collapsed. Yet the compression tests have been good. Maybe THAT'S why the compression was even higher on this cylinder? Maybe that exhaust valve is not opening all the way? 

 

Interesting conundrum. This is the type of problem that can drive an old car restorer crazy, yet give a great feeling of satisfaction when the problem is finally solved.

 

Looking forward to learning the final answer. ?

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There are a few checks you can perform. Remove the distributor cap. With the rotor on have some one turn the crank pulley clock wise. That will tell the direction of rotor and will help to determine #1 plug. Rock the crank pulley back and forth an look at the movement of rotor. ( delay ?)That will give an indication of wear on distributor gears and timing gears. Remove all spark plugs and stuff all holes with paper or cork. Turn the engine slowly clockwise by hand . The corks will pop out according to the firing order.  Shadow install the distributor cap. turn the crank shaft until the rotor points to the wire and terminal  that goes to the # 1 plug . Interchange #1 spark plug with another. Sometimes sparkplug fires outside.By using metal brush to clean spark plugs causes metal tracking on the ceramics.

I may be wrong . I am subject to correction.

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  • 1 month later...

I can now advise what the cause of the problem. In brief, the cylinder head gasket was leaking and coolant was getting into cylinder one preventing the plug from firing. The engine had been rebuilt, but from what I can tell, the head had not been torqued properly.

 

Thanks to all those for your helpful suggestions.

 

Mike

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