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Old 03-31-2017, 08:28 AM   #1
ronchamblin
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Default No. 7 Cyl never gives evidence of a power stroke on '46 Flathead

I'm a decent troubleshooter but this confounds me. A just rebuilt 239 cu. in. runs on 7 cylinders, but No. 7 seems to have never fired in spite of changing the plug, the wire from the crab distributor, and ensuring that compression equals the other cylinders ... all being around 100 to 110 at good cranking speed. And when I set at idle and remove the plug on No. 6, and then No. 7 to check the compression reading, they both read 45 lbs. ... indicating a normal valve activity. Clearly, when it is idling, each of other cylinders drops the RPM slightly when their spark plug wires are removed ... but No. 7 cylinder wire removal makes absolutely no difference in the RPM. When the plug on No. 7 is removed and set upon the head, it has a good spark. Does anyone have any ideas? I've been on this problem for a week.
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Old 03-31-2017, 08:43 AM   #2
Ken/Alabama
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Default Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead

Have you tried hooking up a vacuum gauge while its idling? Needle should be steady if there is no vacuum leak.
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Old 03-31-2017, 08:45 AM   #3
Capt Kirk
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Default Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead

Try hooking up a vacuum gauge. You can tell a lot by doing that.
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Old 03-31-2017, 08:56 AM   #4
J Witt
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Default Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead

That's peculiar alright. I remember hearing of a race engine (not mine) once that had a shop rag in one of the ports. If you pull the intake manifold off, you should be able to feel the suction as the piston goes down on intake. Also can check for obstructions in the intake path. However, an obstruction doesn't seem too likely if #7 will make the same compression as the other cylinders.

I assume you don't have any fluid leaks into that cylinder, not using up coolant, etc. If it has air, fuel and spark, it should fire, assuming spark is occurring at the right time. I guess I might have to vote for something causing the timing to be off for that cylinder. If you have the firing order off it would have to be for two cylinders, not just one, but perhaps some issue with the dist cam is causing that one to fire very late or early.

Keep us posted, this is better than playing "Clue".
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Old 03-31-2017, 09:02 AM   #5
ronchamblin
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Default Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead

Actually I forgot to convey this in my post. I've a dash mounted gauge. At idle, there is a continuous and distinct flick to the left .. from about 16 inches to 15 inches ... or occasionally to 14.5. I've noticed that when the engine is cold, the gauge settles pretty smoothly at idle at 15 or 16. However, even then, the No. 7 is clearly not contributing to the engine ... never causing a drop in RPM when the wire is pulled. At 45 mph, the gauge is steady at 15 or 16 inches, depending on the wind and any incline.
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Old 03-31-2017, 09:22 AM   #6
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Default Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead

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45 lbs compression is very low. May be why the cylinder is not firing. A just rebuilt engine should have at least 100 lb compression.
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Old 03-31-2017, 10:16 AM   #7
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Default Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead

I would check the valve adjustment, especially the intake.
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Old 03-31-2017, 10:19 AM   #8
19Fordy
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Default Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead

Have you tried installing a new plug in #7?
I bet the plug is bad.
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Old 03-31-2017, 10:49 AM   #9
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Default Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead

Think everyone needs to re-read the original post.
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Old 03-31-2017, 11:02 AM   #10
Andy
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Default Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead

Just guessing, I think the head gasket may be leaking between 6 and 7 and as 6 fires it puts burnt gasses into 7. 7's mixture may be so bad it does not fire.
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Old 03-31-2017, 07:10 PM   #11
Tony, NY
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Default Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead

I think something in your troubleshooting is off. How can the compression all be good and then go to 45? What did you do different?
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Old 03-31-2017, 07:46 PM   #12
ronchamblin
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Default Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead

Andy ... that's a possibility. Tony ... you are correct. I've received some ambiguous info about the cranking speed pressures on the No. 7, as compared to the others. The battery is on charge overnight. I drove the Ford ('34 w/46 engine) to work today, with cylinder No. 7 not contributing at all to the engine. I also sprayed mist from a can of starting fluid near any intake manifold protrusions and gasket areas near No. 7 to see if there would be an increase in RPM. There was no change. Nor was there any change in the RPM when the wire was removed. When wires were pulled from all other cylinders, the RPM dropped noticeably. Given that the spark is high, the changed plugs make no difference, and that a manifold leak has not been found near No. 7, I suspect a valve problem. Upon checking the compression again in the morning, if I discover that No. 7 is low, then I will remove the intake manifold and inspect the valves.
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Old 03-31-2017, 08:11 PM   #13
Tony, NY
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Default Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead

Have to ask. How do you know #7 isn't contributing while your driving? Is there a performance problem?
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Old 03-31-2017, 08:13 PM   #14
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Default Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead

To positively determine that it is not ignition, swap plugs and plug wires between 7 and 8, including at the distributor (bottom and top on drivers side of crab distributor cap). See if the problem moves to 8 or stays at 7.

Long shot: Be sure the both idle jets are set correctly and slow the idle if you close it down. If so, open the left jet another half turn and see if #7 starts hitting.

With the engine idling, take a very long screwdriver or socket extension and hold one end to your ear while touching the block on either side of each intake manifold port. If the #7 valves are sticking or have too much or too little gap, you will hear the difference between cylinders.

How does it run at higher RPM? Smooth? A little rough?
Any hesitation or backfiring on throttle burst?
Is the plug wet when you take it out; or just black? How do the other plugs look in comparison after running a bit at higher RPM?
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Old 03-31-2017, 08:19 PM   #15
Jack E/NJ
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Default Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead

Maybe you checked this already. But does the number #7 plug seem wet or dry after a ride? Jack E/NJ
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Old 03-31-2017, 08:20 PM   #16
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Default Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead

when next to cylinders show low compression its a very good bet its the head gasket leaking into the adjoining cylinder, I doubt its a vale problem because both cylinders show the same comression
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Old 03-31-2017, 08:29 PM   #17
Jack E/NJ
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Default Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead

A-hem. >>>all [cyls] being around 100 to 110 at good cranking speed. >>>

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Old 03-31-2017, 10:06 PM   #18
WestCoast
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Default Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead

he also said two adjoining cylinders read 45 #
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Old 03-31-2017, 10:27 PM   #19
Mike in AZ
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Default Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead

have you checked other cylinders while running to see if you have 45psi on them, too? at least, if i'm understanding, you checked the 2 cylinders while engine running....and welcome to the 'Barn....got pics??....Mike
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Old 04-01-2017, 06:16 AM   #20
Frank Miller
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Default Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead

A friend's Harley stopped firing on one cylinder riding to NY. After much fiddling around we noticed the intake pushrod lock nut came loose not letting the valve open. That seems to be the logical place to look. I am not sure how a compression tester would work but if you hold your finger over the spark plug hole while cranking you will fell a difference from a healthy cylinder. If you can determine the crank is in a position where the #7 intake is open or should be, blowing air into the cylinder should let you hear it coming through the carb.
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