deceleration backfire

[Bubstr]

Do a plug check. Not one of them, they look light brown, they are ok checks. Google plug check fire ring. they are checked with good light and a magnifying glass. You should find pictures. It will tell you if your plug got hot. If your plug got hot from ignition timing, good chance your piston top did too. If the piston top did, your rings did too. They loose spring and that causes compression loss. Double check with a leak down test and stethoscope probe. Hear where the air is leaking from, crankcase means rings. Carb means intake valve. Header means exhaust valve. Hope this helps.

If you don't have these tools, an old plug drilled out with a fitting welded on can suppy airr to cylinder and a lone 3/8ths extension to your ear will give some results.

You want to check on Top Dead Center for each piston. Valves need to be closed and air will turn engine over if not at the top.

[Duckhnter83]

I borrowed a friends harbor freight leak down gauge I know its not very nice gauge and who knows how accurate it only uses about 15psi to test. he has never used it. And i am using it until I get a better one. It looks like I have leak down numbers up around 40% and you can hear air coming out of the valve cover breathers. I know an engine won't be 100% sealed but how much air would be to much? I ask so I can compare the gauge to the air loss.

[let-r-eat]

On every cylinder?

[Duckhnter83]

Yeah on every cylinder

[Bubstr]

As I said every gauge reads different. But that sounds like a lot. If it wasn't all of the cylinders, I'd say pull it down and take a look. But to be on the safe side, check something you know is good. That will give you a base line of good and bad. These machines are pretty simple. air going in and a restrictive orifice going out. It just measures the difference of what air is lost as compared to what it should be if there was no leak. As air flows threw all the time and only has a restriction orifice the flow of that orifice and input of air are important to get comparable readings.

It is important to test at TDC, this insures both of your valves are closed and piston won't be driven down the cylinder by the air and the top part of your cylinder is the most effected by heat. 40% leak down on my Snap On Gauge would be a holy chit moment.

If you mark your vibration damper into 4 equal marks 90 degrees from each other each mark will put you on TDC for the next piston in the firing order. This simplifies things and makes the test go quicker. And double check calibration each time before test.

If that test is accurate I'd be checking some compression ring lands and rings. Detonation from erroneous timing can screw things up fast.

[Duckhnter83]

I'm trying to come up with someone else to borrow another leak down gauge to double check. I also ran the car up to 180 this morning and tested it again same results on the leak down percentage gauge with a noticeable air leak from the valve covers. My gauge only reads at 15 psi on the left gauge and the right gauge will measure set. The thing that bothers me about my gauge is when I click the regulator so it cant be moved the set gauge will climb. I think my gauge is screwed up?? So I tried it by using the reading on psi gauge. I set the pressure to 100 psi on the gauge with 120 psi constant air source. When I hook up to the cylinder the psi gauge reads 90 psi so I take that as a 10% leak correct? The right gauge on my tester is pegged out and doesn't move.

[Bubstr]

I don't know the Harbor freight leak down, but what your doing don't sound correct. Read the instructions. You should have a gauge that you can calibrate to zero when not hooked to the engine. You then plug it in and read percentages. If it's defective or your not doing it right, it isn't going to do you much good.

[Duckhnter83]

Here is the way I did the first leak down test. Top dead center #1 cylPlug air into gauge (120psi supply)Set the right guage to the highest point that says set with the regulatorPushed regulator adjuster closedHooked up to engine Read leak down Rocked it slightly back and forth from tdc to see if it changes

[Bubstr]

Here is the way I did the first leak down test. Top dead center #1 cylPlug air into gauge (120psi supply)Set the right guage to the highest point that says set with the regulatorPushed regulator adjuster closedHooked up to engine Read leak down Rocked it slightly back and forth from tdc to see if it changes

If you did it right, wrong or indifferent, it makes no difference if you have nothing to compare the test to. If you believe the 40% leak down numbers, you better tear that bad boy down, If you don't believe the leak down test, you better test one that you know is good. Having 40% leak down on every cylinder is not believable. One, two or even 4 would be believable not all of them. If they are, you have a worn out piece of junk. You need to find out if your doing it right. Then you need to find out if the machine is working right, by checking something that you know has no problems.

Unless someone on here has one, your the one with the machine and hopefully instructions.