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Post by humanshield on May 18, 2019 22:15:37 GMT -5
I have a 200cc single cylinder motor that has LOW compression (40 psi)
The motor has about 6 hours total run time.
The motor was disassembled and carefully inspected. NOTHING was found. The valve seats were fine, tight, flush, secure, no evidence of gas blow by underneath
The valves were fine, no burning, not bent, no binding
The valve seating was superb (checked with Prussian Blue and double checked) The guides are fine, side to side play at minimum OEM specs
No cracks can be found using an aluminum stain and developer system to find cracks in Aluminum heads The piston was fine The cylinder walls were perfect The rings are installed correctly and working properly. Leak Down Test did not indicate ring issues.
The loose head was inverted, spark-plug installed and combustion squish area filled with water. The intake port was then pressurized with compressed air. No bubbles were observed.
With NO ROCKERS INSTALLED, The engine was tested with a Snap On Cylinder Leak Down Tester.
No leak at the rings was indicated. No exhaust valve leak was indicated.
A significant leak presented at the Intake port. We cannot yet determine the source of this leak. Obviously, there is a path for air to escape somewhere in the combustion chamber area to the intake port between the intake valve seat and the carburetor mounting surface. But we can't find it.
Anyone seen this and know of anything else that can cause this? Usually the cause of such leaks is found fairly quickly. Not this time.
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Post by jackrides on May 18, 2019 22:30:21 GMT -5
Obviously this can't happen, so how about doing the leak down test with some kind of dye (Dykem?) in the liquid filling the combustion chamber, pressurizing thru the spark plug hole, then seeing where the dye comes thru? Staring down an intake port may require support only avail. in certain states but ya gotta do what ya gotta do.
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Post by SMALL CC TEK on May 19, 2019 0:15:38 GMT -5
wow 40psi dang... 200 gy6 or a bike style motor ? So are all the surfaces square cylinder head base? Did you put a flash lite on the valves and look in the intake and exhaust to see if you had light? Do you have a crack between the valves ? Like jackrides said leak down test and little soapy water in a spray bottle for exterior of cylinder and head visual .. The rings cracked or piston crack .. If it's a single bike style does it have a compression release ? Need a visual lol Does it start , yeah sounds laim but you would be surprised at how some stuff works that should not !
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Post by FrankenMech on May 19, 2019 0:36:24 GMT -5
Can you just get a new head?
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Post by humanshield on May 19, 2019 12:04:37 GMT -5
Thanks guys. I've done a LOT of heads, but this one.....never encountered anything like it. UPDATE. I was able to resolve the Intake valve leak. I lapped the Intake valve a 2nd time and now I get practically ZERO leak at the intake valve, a very mild leak at the exhaust valve and the expected leak through the breather (rings) (although I'm suspicious that the 2nd lapping actually was not the reason for the improvement).
So you'd expect the compression to be up right? Nope. Exact same. 30-40psi on the gauges. I again tested the leak down with my SnapOn MT324 and it reads at 9%, which indicates a good healthy motor. But.... 3 different Compression gauges all agree on 30-40psi max. That's REALLY low compression. There are absolutely no compression relief mechanisms. The engine is about 7 years old (but only 5-6 hours runtime) so I can't find a replacement head. This is the oddest thing I've ever seen on a small engine. My suspicion is possibly a valve seat or seats that is/are shifting so that one moment it looks good and the next not so good. even though every Leak Down test has been consistent. So to recap...... The head , valves and rings are reported in excellent condition by the SnapOn Cylinder Leak Tool, but awful by the compression gauges (all three agree). The motor is almost impossible to start, "feels" like it has little or no compression and seems to develop very little power. All symptoms of low compression. Jackrides, I like your response "This Can't Happen"......lol. I totally agree. But it is.
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Post by jackrides on May 19, 2019 14:35:52 GMT -5
Time to measure combustion chamber volume, should be around 24cc with the piston at TDC. Divide whatever cc it is into the combined volume of displacement (200cc) plus combustion chamber volume, and that is the compression ratio. 90GTVert recently did a Great post on this. With your assembled engine, it will be less effort other than turning the engine so the spark plug hole faces directly up. Comparing that ratio with your 30-40 psi will direct you to the answer.
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Post by FrankenMech on May 19, 2019 16:22:08 GMT -5
A valve seat shifting around should show up with the dye tests.
Physical measurements of displacement; deck height, combustion chamber volume, and cylinder volume etc. may help.
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lupo76
Scoot Enthusiast
Posts: 157
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Post by lupo76 on May 20, 2019 2:04:50 GMT -5
No offending, but is your compression gauge broken? Can you test it on another engine?
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Post by Happypancake! on May 20, 2019 4:50:44 GMT -5
Make sure you open the throttle 100% when cranking over the engine .
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Post by houseoffubar on Jul 18, 2019 4:14:51 GMT -5
So you had run this motor before and it was fine? Any chance at all of a bent rod, causing excessive piston to head clearance? Water intrusion? You are for sure on the cam timing, it could not have possibly jumped?
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Post by fugaziiv on Jul 18, 2019 8:33:11 GMT -5
The the carb a CVK and if so did you take the carb off when doing the compression test?
Matt
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Post by GrumpyUnk on Jul 23, 2019 12:09:34 GMT -5
I had similar with a brand new compression gauge. My old one, from Warshawsky/JC Whitney decades ago, registered a lot more compression. The new, around 30-40psi. I figured the mechanism was not accurate with the small volume displacement. A tiny piston going up and down did not generate enough volume to move the gauge properly. I used the new gauge adapter with the old gauge(bought new because no 10mm adapter) and got good results. Maybe you are fine, and need a better gauge. Hos does it feel when kicked? Have you used the kick starter by hand to feel the compression or lack thereof? tom
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