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

[ronchamblin]

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.

[chap52]

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

[Capt Kirk]

Originally Posted by ronchamblin

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.

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

[chap52]

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.

[flatjack9]

Originally Posted by ronchamblin

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.

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?

[rotorwrench]

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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.

[flatheadmurre]

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....

[Ronnie]

Originally Posted by flatjack9

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?

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

[Jack E/NJ]

>>>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

[Capt Kirk]

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.

[40cpe]

Originally Posted by flatjack9

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?

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.

[Jack E/NJ]

Huh? Jack E/NJ

[Ggmac]

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

[flatjack9]

Originally Posted by 40cpe

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.

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?

[Jack E/NJ]

>>>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

[40cpe]

Originally Posted by flatjack9

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?

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.

[37 Cab]

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.

[Mike in AZ]

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

[Jack E/NJ]

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

[37 Cab]

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