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.

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.

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.

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".

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.

Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead
 

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.

Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead
 

I would check the valve adjustment, especially the intake.

Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead
 

Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead
 

Think everyone needs to re-read the original post.

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.

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?

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.

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?

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?

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

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

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. >>>

Jack E/NJ

Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead
 

he also said two adjoining cylinders read 45 #

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

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.

Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead
 

I sense some ambiguous data offered by me. Am working today, but tomorrow I shall do some "solid" testing, including a "leak down test on No. 7". BTW, the 45 lbs referred to was an experiment at idle speed. All cylinders seem to have read about 45 lbs at idle, including No. 7. This morning, while cold all cylinders on the left side read from 90 to 110, the number 7 being the 90. But, I shall return tomorrow with clearer date, as I think the situation requires it. At this point, I've not seen any power strokes from No. 7, as when the wire is removed, there is no change to RPM, or vibration. Also, last night, while idling, I sprayed "starting fluid" all around the intake manifold on the left side to see if RPM increased, indicating a leak. There was no change in RPM. As I said, I will do better test data for Sunday.

Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead
 

I am leaning towards the head gasket leak theory, but....
Ron, don't get discouraged with all the minds churning on this site you will get it figured out.
Here's another idea for you to think about.
I would remove #7 plug, crank engine with your thumb over open spark plug hole and when you feel the "puff" you will be on compression stroke for that cylinder. Now using the proper fitting (probably from leak down kit) inject a little bit of air into the cylinder. If you have the crank in the right position, both #7 valves closed and piston on compression upstroke, listen closely. You may hear air coming out through the carb (Intake valve issue) or the tail pipe (exhaust valve issue). Now if not one of those remove the adjacent spark plus, add air to #7, do you hear air coming through the adjacent cylinder spark plug hole? If so, leak is probably a head gasket issue?
Please keep us posted. Chap

Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead
 

You're vacuum low...should be around 20 and be steady unless you're moving the throttle.

Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead
 

Another thought...If an adjacent cylinder is either on intake or power stroke when #7 is on compression stroke and there is a slight head gasket leak-over to one of those cylinders it would impact #7 and cause it not to fully compress & detonate properly.

Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead
 

I have never taken a compression reading with the engine idling nor have I ever heard of it being done. I don't see how you can get 45 at idle and 90 - 100 when cranking. Am I missing something?

Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead
 

I only do differential compression tests if I find a problem like this. Some folks call them a leak down test. The tester has two gauges and a regulator with a calibrated orifice in the pressure manifold in between the two gauges. I put just enough air into the cylinder so that the engine can still be rotated though. By watching the gauge and feeling the pressure on the crank, a person can find top dead center on compression. Once it's at TDC, I then increase pressure in the cylinder to 80 PSI. If the other gauge reads between 65 and 79 PSI then there may be no problem there. You can listen at the carb or exhaust to check for valve leakage and you can check at the breather for excessive ring leakage. If it checks out to be 70 to 79 PSI then I would pull the intake a check for obstructions.

We have mud daubers here in Texas and they can build a nest overnight that would plug an intake manifold or even an intake port. A blocked exhaust port can happen too but it depends on what type of manifolds it has or whether someone put a port divider in wrong or something of that nature. If #7 gets blocked, it would likely be blocking #6 too with the siamese ports in there.

Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead
 

Is there fuel coming into the cylinder...crank it plugwire off and check the plug ?
If a horseshoeclip is missing you get issues like this....

Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead
 

Yep (WTF) the new age tech's seem to do why I have no clue but they do !!!!

U tube it and see for yourself. Us old farts have been written off when it comes to technology "LOL" I would have done a leak down and then a scotch on rocks to determine what the next move would be.He stated more than once compression was low on #7.

R

Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead
 

>>>all being around 100 to 110 >>>
>>>while cold all cylinders on the left side read from 90 to 110, the number 7 being the 90.>>>

??? And I'd still like to see if the plug's wet. Jack E/NJ

Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead
 

I've never heard of taking a compression test with the engine running and don't know a reason to. If a head gasket was leaking between cylinders, it would show up with a crank test. Vac is low and fluctuating...valves or ignition.

Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead
 

I'm not an old hand at compression checking, but doesn't it take 2-3 revolutions before the compression gets to max reading on the gauge? While running, the cylinder only gets one pump before it is bled off.

Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead
 

Huh? Jack E/NJ

Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead
 

Remove all plugs , hook up a compressor to #7 set the pressure less than 100 , listen at exhaust pipe , carb adjacent cyl , oil galley .

Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead
 

I don't know about your compression tester, but mine has a check valve that allows the cylinder to pump to max pressure. You then hit the release button to zero the gauge again. And wouldn't it bleed off after one revolution when you are cranking it without the check valve?

Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead
 

>>>mine has a check valve >>>

Well maybe if it's revved fast enough your check valve will float due to a weak spring. 8^) Jack E/NJ

Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead
 

Jack, I'm saying that the check valve lets the cylinder build pressure on the gauge until you get to the max reading. It takes more than one revolution to build it. When the engine is running, the exhaust stroke bleeds off the previous compression stroke's pressure so it can't build like it does with the check valve on the gauge.

Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead
 

Retired Ford dealer guy here. Ford would ask for a running compression test in situations where a misfire concerns were not pinpointed by the usual tests. Just gives you an idea of what is going on but probably not something a vacuum gauge would not tell you. We looked for an average of 90 psi running on a modern engine that would make 170 cranking. The check valve is removed for a running test.

Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead
 

intake valve not opening??....no gas mixture to fire.....Mike

Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead
 

40cpe>>>When the engine is running, the exhaust stroke bleeds off the previous compression stroke's pressure so it can't build like it does with the check valve on the gauge. >>>

I can definitely see where that's possible. But again, if the check valve spring in the pressure guage is strong and responds fast enough, the reading should be about the same as cranking it with the starter a half dozen or so times. Jack E/NJ

Re: No. 7 Cyl never gives evidence of a power stroke on '46 Flathead
 

That is why you remove the check valve. But like I said.. probably won`t see anything a vacuum gauge will not show you.