Leak down test

[Taylorman]

How do you do a leak down test?

[Rexone]

Step one.. you need a leak down tester... duh.
Step two.. warm the motor up.
Step three.. remove spark plugs
Step four.. Calibrate (set) tester to zero with your air pressure source.
Step five.. get #1 on tdc firing and screw tester in #1
Step six.. plug tester in and read % leakage on #1
Step seven.. rotate motor 90º and repeat step six for each cylinder in the firing order rotation.
When you are testing "listen" to where the leakage is occuring... carburetor, exhaust, crankcase (breathers). This tells you where the problems are. If you have trouble hearing, use a piece of vacuum hose up to your ear to isolate the sound at different locations on the engine.
If you have carb noise you likely have an intake valve issue.
Exhaust noise likely is exhaust valve issue.
Crankcase noise is likely rings, piston damage, crack cyl, etc.
Look for wide variance between cylinders. This is usually the biggest indicator of a problem.
However if they're universally abnormally low readings the motor may just be worn out evenly.

[Taylorman]

Ok maybe thats not what i need to do. Heres the deal. I have a newly rebuilt, about 9 months ago, 455. No thats not the problem. Ran good all last summer up to the last time I ran it which was on new years day. Last weekend, i took it out and it ran like shit. Problem is that the engine pops when Im running. I thought i may have a bent valve or a problem with my heads, so i just did a compression test and it seems ok
1 145
2 160
3 150
4 160
5 145
6 150
7 145
8 165
The only thing I changed on the boat was a new water injection system that I rigged up while running it in my driveway. I noticed a couple times that it got water at idle but no water in the oil is noticeable. I cant imagine that could have hurt anything. Ive had my old tvalve get stuck open before and i had no problems. Any ideas what this popping could be caused by?

[Rexone]

popping - exhaust or carburetor?
And at what rpm, load no load etc.
more info = better answers
Also, a leak down test will tell you things the compression test misses. It is not tough to do and you can likely rent or borrow a tester. You do need a compressed air source to do it.

[Taylorman]

Popping is from the exhaust. Starts at about 2500 rpms under a load. Its popped like this once before when I ran it in about 50 degree weather. Could running it to rich cause popping. I looked at my plugs when i did the compression test and they are dark brown.

[Wet Dream]

I had a similar thing happen to my 455. Idle and partial throttle were good. When I started getting into it it would have a popping sound (out the exhaust). Not necessarily a backfire, but a popping, especially on acceleration and hard turns. Turns out the carb needed a minor going through. A good cleaning, new gaskets and float adjustments and she hasn't done it since. The old gaskets were paper and probably developed a leak. I like the blue gaskets and coat them with cherry chapstick if you ever need to pull them apart...they come right off without having to replace them. When you first stated that you had a popping, I was thinking cam or lifters, but if you're good to a certain rpm or throttle, I would look into a simple carb clean-up.

[Fiat48]

Here comes Rexone...and he's gonna ask about that Cherry Chapstick!

[GofastRacer]

Those compression numbers look more like unequal chamber volumes, do a leakdown on it!. All this talk about popping and nobody mentioned anything about ignition!..

[Taylorman]

What about ignition?

[Snowboat]

I would disconnect the water injection system for a few minutes. Just to be sure that you have not created the problem. I'd go with the carbs. Are you sure you haven't moved your timing.