Cylinder compression test - failed ??? :(

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Re: Cylinder compression test - failed ??? :(

21 Jul 2012 23:59 - 22 Jul 2012 00:02
#537566
After giving it considerable thought, the only possibility I could envision is a piece of carbon got stuck on the intake valve and is holding it open a tiny bit.Possibly caused by air getting by the rings when the piston was on the intake stroke and you had your hand over the intake creating a vacuum. Take the valve cover off and check the clearance between each cam lobe and shim/bucket, if that cylinder's intake valve has excessive clearance, then my guess is correct.

If you didn't rotate the motor one complete revolution between testing number one and testing number two, then one of them was on the exhaust stroke and the intake valve was open. Number one the way it sounds. :)
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Last edit: 22 Jul 2012 00:02 by steell.
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  • 650ed
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Re: Cylinder compression test - failed ??? :(

21 Jul 2012 23:59
#537567
Bunty27 wrote: OK, so I did the poor man's compression test with cylinder 1 that I managed to mess up with my hand trick and viola, I could clearly feel a lot of air coming out of the open carb intake (I took the carbs are out). I repeated the test on cylinder 4 which I did not mess with and I couldn't feel air escaping out of anything, especially not out of the carb intake for sure.

So it's certain that by putting my hand over the carb intake while doing the compression test, I managed to mess things up. But the question now is what is it that I did to that one cylinder and what can I do to fix it? Gurus, please help!!!!!!

Not possible unless you dropped something inside the intake port. Consider this - when the carbs are on and the throttle is closed there is only a very small opening for air to pass through, not unlike having your hand over the intake. If you are feeling a lot of air coming out of the intake I would guess your intake valve is not closing all the way. Is it possible you had a piece of rag or paper towel or something else in your hand that got sucked into the intake? Can you peek in there through the port to see if any obstruction can be seen? Ed
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Re: Cylinder compression test - failed ??? :(

22 Jul 2012 00:39
#537575
I'm certain there was nothing else in my hand and nothing else go sucked into the intake. Maybe something was there before and got pulled in because of the vacuum created by me putting my hand, kinda like steell suggested. What's the best way to get to the bottom of this? Can I do it by just opening the valve covers and looking around?
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Re: Cylinder compression test - failed ??? :(

22 Jul 2012 00:45
#537576
Bent valve or out of adjustment, start with a valve adjustment, its for sure hanging open.
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Re: Cylinder compression test - failed ??? :(

22 Jul 2012 01:52
#537582
If something is indeed holding the valve from closing, how do I get to it? Can I remove the valve from the top after taking out the valve cover? I've "never" opened up an engine and so I'm a bit nervous.
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Re: Cylinder compression test - failed ??? :(

22 Jul 2012 02:26
#537586
With the carb off, look into the intake port, use a mirror. See if the old choke flap is there holding the valve open.
No you can't remove the valve, by only removing the valve cover. The cylinder head would need to be removed to change valves. Look in your port, of nothing there, check valve clearances. I would also verify correct cam timing while your at it.
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Re: Cylinder compression test - failed ??? :(

22 Jul 2012 03:57
#537595
I was reffering to the initial choke flaps that was missing. Not the ones that you made afterwards :(
And no you cannot remove a valve by removing the valve cover off and if your not familiar with rebuilding a cylinder head including valve clearance and cam timing then you better think a couple of times before diving into it. It is perfectly doable if you have little more then basic mech skills and a complete rebuild manual. I think the leak down test is a bit over the top at this time for you. The important thing is that you get a regular compression test done - after those number are in we can take it from there. And again - it IS impossible to ruin something by holding the hand over the intake so you can put that to rest.
I am more worried about your pistons and that´s why you have to get a reliable compression test.

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  • Patton
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Re: Cylinder compression test - failed ??? :(

22 Jul 2012 08:53
#537611
Would again measure the intake valve clearance in #1 cylinder to confirm that at least some minimal clearance exists when the cam lobe is pointed away from the valve. The intake valve is the valve closest to to the carburetor.

The poor man's leak down test (introducing compressed air into a spark plug hole and listening to where it escapes from the combustion clamber) may be done at TDC (top dead center) or any other cam positions where both valves are fully closed and supposedly not leaking.

Top dead center (TDC) is when the piston is at its highest position. An easy way to tell TDC is by alignment of the T mark with the case mark when viewed through the timing window.





Would then repeat the poor man's leak down test in #1 cylinder after first being positive that the piston is at top dead center (where both the intake and exhaust valves are supposed to be fully closed against their seats).

The leak down test helps determine whether fully closed valves are leaking (for whatever reason, such as zero clearance, burnt/damaged valve or seat, blockage between valve and seat, bent valve, incorrect cam timing, weak/broken valve spring, etc.)

Good Fortune! :)
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Re: Cylinder compression test - failed ??? :(

22 Jul 2012 10:15
#537620
Make sure your on the compression stroke when doing the Leak Test. Turn the motor, with the 17mm nut, and have either a finger over the top of the plug hole or listen to when the air is forced out when the piston comes up. Be sure to be turning the motor in the correct direction, which for the 4 cylinder is clockwise when looking at the right end of the crankshaft, thats where the Ignition cover and 17mm nut is. Get the 1-4 "T" aligned then, when the air is being pushed out the plug hole. As the camshafts turn once while the crankshaft turns twice, a 2 to 1 gearing. So you could be on the correct 't' mark but 180 degrees out for the cams, which is 360 at the crank.
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Re: Cylinder compression test - failed ??? :(

22 Jul 2012 11:40
#537645
Thanks much Patton. I followed your directions and aligned the external mark to the T mark and repeated the poor man's air compression test. And thankfully, the air escaping from the carb intake does NOT happen any longer. :)

I did look through the carb intake valves (using my iphone instead of the camera) and I did not find any choke flaps inside in any of the cylinders. In fact, all the valves looked the same to me. I'll try and share a picture soon.

I need to retry the compression test with a good compression gauge. Meanwhile, I drained the batteries out in the process of all this compression testing. I'll get the battery re-charged, retry the compression tests and share the results.

Thank you very much for all your advice here. You guys are awesome. I've decided to limit my work to the chassis & carbeurator. If there is something wrong with engine, I'll let some knowledgeable mechanic take care of it. Please stay tuned for my compression results. :)
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Re: Cylinder compression test - failed ??? :(

22 Jul 2012 11:48
#537647
Be sure to charge the battery using a LOW amperage (1.5 - 2.0 amps) charger. Charging it using a higher amperage charger will shorten its life expectancy. Ed
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Re: Cylinder compression test - failed ??? :(

22 Jul 2012 12:09
#537650
Bunty27 wrote: Thanks much Patton. I followed your directions and aligned the external mark to the T mark and repeated the poor man's air compression test. And thankfully, the air escaping from the carb intake does NOT happen any longer. :)

I did look through the carb intake valves (using my iphone instead of the camera) and I did not find any choke flaps inside in any of the cylinders. In fact, all the valves looked the same to me. I'll try and share a picture soon.

I need to retry the compression test with a good compression gauge. Meanwhile, I drained the batteries out in the process of all this compression testing. I'll get the battery re-charged, retry the compression tests and share the results.

Thank you very much for all your advice here. You guys are awesome. I've decided to limit my work to the chassis & carbeurator. If there is something wrong with engine, I'll let some knowledgeable mechanic take care of it. Please stay tuned for my compression results. :)


And make sure throttle is fully open and spark plugs removed.
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