Help! Uneven Compression on New Motor

badvinny

badvinny

Location
Saskatoon,SK
///M5 said:
I adjusted the ring gaps, but they were pretty much spot on to spec from the box. I think that the glare from the WD on the walls is throwing the photos off. The final hone was to 320.
Click to expand...
320 is fine, it should hav been bored from about. 0025" undersize, if it was accidentally bored too large it ends up not getting enough honing , and won't clean up the machining marks. Now this only give low compression, not high. Is one cylinder 25 high, or the other one 25 low? I am guessing high, so forget about the honing/cylinder boring. The squish is correct an even on both so you don't have one incorrect piston? Did you swap those domes yet?
 
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///M5

///M5

Location
Guam
badvinny said:
320 is fine, it should hav been bored from about. 0025" undersize, if it was accidentally bored too large it ends up not getting enough honing , and won't clean up the machining marks. Now this only give low compression, not high. Is one cylinder 25 high, or the other one 25 low? I am guessing high, so forget about the honing/cylinder boring. The squish is correct an even on both so you don't have one incorrect piston? Did you swap those domes yet?
Click to expand...
Domes have been swapped, and made no change. I installed a set of 37cc domes, and the problem persisted of the PTO Cyl reading 25psi higher.

I measured the exhaust port heights from the deck, and got the same numbers on both cyls.

After I picked up my cyls from the machine shop, I measured the clearances and they are both within spec. I am totally stumped on this...

ba2f7700638dcc3374d22fddbac20c67.jpg
19a2102c29f1ddf441a5d702d00c6e06.jpg


With 37cc Dome
98ca55445ca1b8a469d5aa793b6921c8.jpg
 
JetManiac

JetManiac

Stoked
Site Supporter
Vendor Account
Location
orlando
"After I picked up my cyls from the machine shop, I measured the clearances and they are both within spec. I am totally stumped on this..."

Measured in multiple directions and places with a bore guage?
 
badvinny

badvinny

Location
Saskatoon,SK
So the pto side is 25 high? You have a problem that everybody wants and I have no idea how to create high compression, it's always the other way around? The scuffing is odd on that cylinder also?
Thinking out of the box, I can only think the ring on that cylinder is over sealing, a tapered ring installed upside down. ' might' do that but usually would act like a stuck ring with lower compression.
If the ring groove is not deep enough in the piston may be another possibility? I have never seen it but it's possible.
One I have seen is the piston pin bore cut 1 mm low, but your squish check would rule that out.
 
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///M5

///M5

Location
Guam
JetManiac said:
"After I picked up my cyls from the machine shop, I measured the clearances and they are both within spec. I am totally stumped on this..."

Measured in multiple directions and places with a bore guage?
Click to expand...
I took 3 measurements from each piston and cylinder using a telescoping bore gauge and Starrett Micrometer.
 
john zigler

john zigler

Vendor Account
Location
wisconsin
Pull the cyl off, and inspect. This is all speculation, but I think your gauge is actually reading high. You have smeared / stuck a ring, and are low on the one side. You cannot always see until the cyl is off.
 
Harbor

Harbor

Location
Stockton
Next time you do a compression test, listen for compression leaking out on the low cylinder, (@ the seal on the compression testers hose tip)..... It's hard to hear but is faintly audible and leaves a small amount of blackish oil behind. My Snap On hose tip seal is currently leaking (only leaks on one of the cylinders domes) and is taking 200 psi and making it 175. After the compression test the seal rolls sorta and gets pushed out some
 
///M5

///M5

Location
Guam
Harbor said:
Next time you do a compression test, listen for compression leaking out on the low cylinder, (@ the seal on the compression testers hose tip)..... It's hard to hear but is faintly audible and leaves a small amount of blackish oil behind. My Snap On hose tip seal is currently leaking (only leaks on one of the cylinders domes) and is taking 200 psi and making it 175. After the compression test the seal rolls sorta and gets pushed out some
Click to expand...
Interesting, I will check that out. How did you correct it?
 
///M5

///M5

Location
Guam
john zigler said:
Pull the cyl off, and inspect. This is all speculation, but I think your gauge is actually reading high. You have smeared / stuck a ring, and are low on the one side. You cannot always see until the cyl is off.
Click to expand...
Looks like removal is definitely next. What do you mean by smeared/ stuck ring?

As for the gauge, I checked it on other motors and it has always read even. After saying that, I will go borrow my buddy's gauge to verify.
 
Harbor

Harbor

Location
Stockton
My gauge always reads even too until it didn't on Saturday. Had that seal leak 2 years ago but I swapped the seal and it's been fine since, even on this engine. Snap on comes today so I'll buy a new seal and see if that helps. I have not looked closely at the surface of the spark plug seat in the dome yet. Funny thing is it seals correctly on the my front dome but leaks on the rear dome. Tried multiple times with different amounts of tightness to the hose but still leaks past the seal and deforms it's shape.
In my case it creates a false low reading.

I also have similare wash as you, my cleaner piston is wet as well and its from my carb dripping fuel out the high jet thru the center Venturi. Ran the engine with the filters off and found it dripping.
 
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