Topic: Someone smarter then I please interpret my leakdown and compression test

[ncstatecamp]

Result:

Front of bike
1 left to right 4
Compression
Low is starter motor only
High is starter motor first and then kick start, or kick starter only

Cylinder 1: lowest 90 - highest 130
Cylinder 2: lowest 120- highest 145
Cylinder 3: lowest 110- highest 130
Cylinder 4: lowest 120- highest 130

Leakdown results (air hissing from)
Cylinder 1: moderate exhaust
Cylinder 2: very very slightly from breather hose
Cylinder 3: very slightly exhaust and breather hose
Cylinder 4: no leak

Back story:
1972 cb750k
rebuilt top end January - march
700-1000 miles since rebuilt
standard flakes in first oil change at 75 miles, no flakes after that at 500 oil change
New: gaskets, rings, valve seals
1 newer sleeve (number 2 I think)
honed cylinders
original pistons
lapped original valves
4 to 1 exhaust
1 step down (richer) on carbs
before rebuild I was leaking from pucks and getting about 80-90 psi across the board.
Recently I did a cold leak down (pre-valve adjustment, 500-600 mile mark) and got 90-100's psi, and was told I needed to ride the piss out her, so I have been.
difficult to start cold
starter perfectly when warm

Tested:
Slightly warm engine (not hot or would have burnt fingers,sorry)
Valve clearances set just before compression/leak down
Timing perfectly set


ok so my questions/concerns:
I've always had a difficult time starting the beast when shes cold. My procedure to get it to catch is: Neutral, Choke close/up/on, spark off, full open throttle, turn over 4-5 times, stop, turn spark on, open/down/no choke, starter while feather slightly throttle till she catches. My theory is clears out old air, builds pressure with gas fumed air. Any advice to this?

The fact that solely starter motor gets me low numbers in my compression test I find strange. Also when I pressed it till the number stopped climbing, stopped for a second and pressed it again I would get a higher number, why is this?

The breather hose hiss on 2 and 3 means rings right?

How am I getting a hiss on 2 but not on 4 when 2 has higher numbers?

[DavePhipps]

I  hate to ask something silly but did you make sure the there were no valves in an open position when doing the leakdown test?

[ncstatecamp]

I  hate to ask something silly but did you make sure the there were no valves in an open position when doing the leakdown test?

Yeah one of the first ones I did on the 1-4 stroke it was tdc on the wrong side (180) of what it shouldve been and I freaked out, realized what was up and rolled the crank 180 and it read better. For each measurement I did it at tdc, and again at tdc 180 degrees just to make sure. Its very apparent whens it on the wrong stroke.

[RJ CB450]

I  hate to ask something silly but did you make sure the there were no valves in an open position when doing the leakdown test?

Mostly it is TDC.  Nice thing about TDC, is that if you have good seal, and it is off at all, then the air pressure often will rotate your engine for you.  Proper leakdown test is in neutral, so if your engine doesn't turn, or you need very little holding with the wrench, you are good.

My interpretation of the results?

First is how did you do the leakdown test.  The thing about a leakdown is it isn't uncommon to get slight leakage.  An engine isn't a perfect seal.  What is important is the percentage leakage.  The following image is basically how it works.



So you have your set pressure, and a small orifice leading into it with a post restrictor pressure gauge.  You compare the two in order to get a result.  So if you had 100 psi in the line, but showed 85 psi in the cylinder, you would have 15% leakage.  Even freshly rebuilt engines will have a few percent leakage.  Low compression pressure might not be from leakage.  It could be poor draw into the cylinder as well, something restricting.

[Duanob]

I think what Dave Phipps is saying is; are you valves adjusted correctly and not too tight? If they are too tight then they will hold the valve open even during comp stroke. Even if ever so slightly. When the pistion is on it's comp stroke, wiggle the tappet and to make sure it's has play. If not then it's too tight. If the valve is open due to the wrong stroke you won't have any compression or pressure for a leak down test because the valve will be wide open.

Just curious why you're doing a comp and leak down test? Is the bike running bad?

[TwoTired]

Leakdown tests are reported with the pressure supplied over the pressure held.    Like 100/80.   That is the "what".
"Where" and "why" are then discerned from where the hissing is heard, all when at TDC.

By the way, you always look for the highest value when rocking the crank about TDC, so the rings will settle on the bottom lands of the pistons where maximum seal is required.  This mimics running conditions.

[ncstatecamp]

I think what Dave Phipps is saying is; are you valves adjusted correctly and not too tight? If they are too tight then they will hold the valve open even during comp stroke. Even if ever so slightly. When the pistion is on it's comp stroke, wiggle the tappet and to make sure it's has play. If not then it's too tight. If the valve is open due to the wrong stroke you won't have any compression or pressure for a leak down test because the valve will be wide open.

Just curious why you're doing a comp and leak down test? Is the bike running bad?

Did you read my opening message?

[ncstatecamp]

I  hate to ask something silly but did you make sure the there were no valves in an open position when doing the leakdown test?

Mostly it is TDC.  Nice thing about TDC, is that if you have good seal, and it is off at all, then the air pressure often will rotate your engine for you.  Proper leakdown test is in neutral, so if your engine doesn't turn, or you need very little holding with the wrench, you are good.

My interpretation of the results?

First is how did you do the leakdown test.  The thing about a leakdown is it isn't uncommon to get slight leakage.  An engine isn't a perfect seal.  What is important is the percentage leakage.  The following image is basically how it works.



So you have your set pressure, and a small orifice leading into it with a post restrictor pressure gauge.  You compare the two in order to get a result.  So if you had 100 psi in the line, but showed 85 psi in the cylinder, you would have 15% leakage.  Even freshly rebuilt engines will have a few percent leakage.  Low compression pressure might not be from leakage.  It could be poor draw into the cylinder as well, something restricting.

My leak down test wasn't really with an actual tester. It was in neutral, and tdc, but my method was just using the hose from my compression tester  hooked up to my air compressor running a pressure of 90psi, falling to 45psi. I don't have a leak percentage or anything just knowledge of where things are leaking.

Is it standard for the kick starter to product higher compression like it was?

[RJ CB450]

The higher compression, best I can think of is the speed of it cranking over.  My poor 450 has been out for a few years so cannot remember if kickstart spins it faster or my starter.

If you have a good seal, and spin slower, you have more time to draw air in and be close to or at atmospheric resulting in a "Higher" compression reading.

If you turn slow and have a leak, then more pressure will escape on compression.  So if starter spins it faster, then higher at kickstart is good.  If kickstart is turning it over faster, then you have loss.

This is personal deduction, no source on others verifying this.

[Duanob]

I think what Dave Phipps is saying is; are you valves adjusted correctly and not too tight? If they are too tight then they will hold the valve open even during comp stroke. Even if ever so slightly. When the pistion is on it's comp stroke, wiggle the tappet and to make sure it's has play. If not then it's too tight. If the valve is open due to the wrong stroke you won't have any compression or pressure for a leak down test because the valve will be wide open.

Just curious why you're doing a comp and leak down test? Is the bike running bad?

Did you read my opening message?

Yes. Setting them and double checking or two different things. But if you have 100% confidence in your work the first time then I commend you. I usually double and triple check my work before putting the caps back on.

[ncstatecamp]

I think what Dave Phipps is saying is; are you valves adjusted correctly and not too tight? If they are too tight then they will hold the valve open even during comp stroke. Even if ever so slightly. When the pistion is on it's comp stroke, wiggle the tappet and to make sure it's has play. If not then it's too tight. If the valve is open due to the wrong stroke you won't have any compression or pressure for a leak down test because the valve will be wide open.

Just curious why you're doing a comp and leak down test? Is the bike running bad?

Did you read my opening message?

Yes. Setting them and double checking or two different things. But if you have 100% confidence in your work the first time then I commend you. I usually double and triple check my work before putting the caps back on.

O I triple checked everything, and I it was running better as they do after a proper valve job.

The reason I'm doing it is the hard starting and checking my rebuild/seeing if it's higher compression then before. O and it seems to be puffing a bit of oil on occasion...

[iron_worker]

Cylinder 1 seems to be leaking past the exhaust valve.

IW

[RJ CB450]

All cylinders or just a couple.  What puffs can tell alot