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Author Topic: leak down test readings  (Read 559 times)

Offline 88LT500

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leak down test readings
« on: August 28, 2018, 08:52:11 am »
Hey Guys, i completely rebuilt my 88 LT500 engine.  I did a leak down test.  I started at 6psi and it dropped to 3psi after 6 minutes.  Ive read that a 2 stroke engine will NOT hold air for ever, but its supposed to hold 6psi for 6 minutes.  Well if it can do that with out dropping at all, then it should hold forever (technically).  I guess im asking "what is a legitimate reading in this case".  How much can it drop after 6 minutes and still be OK? 1 psi? 3 psi?  I realize the less the better, but trying to be realistic.  Also, i have checked everywhere (case halves, base of cylinder, PV, gaskets, spark plug, test equipment) and cannot find any leaks.  I know its somewhere, but want to know if i need to keep looking or if its OK the way it is.  I really dont want to pull the engine back apart.  Thank you for the help guys!

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Offline ZillaFreak

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Re: leak down test readings
« Reply #1 on: August 28, 2018, 10:43:57 am »
has it been ported? maybe blew through into a water jacket?
head gasket is another place. check coolant for bubbles while testing.

Offline MotorGeek - Jerry Hall

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Re: leak down test readings
« Reply #2 on: August 28, 2018, 10:01:44 pm »
An air leak is never good theoretically but very small air leaks will not appreciably affect main jet size or peak power but may affect idle mixture and a lean condition under closed throttle deceleration. 

Has the engine experienced a broken piston skirt or broken thrust washer? Has the engine had a connecting rod bearing fail where the big end of the con-rod rubbed the engine case?  If the engine has experienced any one of these 3 types of failures, you cases may have damage causing the cases not to seal together at their mating surfaces

Have you run the engine yet?  If so, does it seem to smoke excessively?  If so, does the exhaust smoke smell like some other type of oil other than your two stroke oil you are putting in the fuel?  Is the transmission oil level going down?     

1.  Does the engine rpm return to an idle slowly with the ring ding dings after blipping the throttle?

2.  If it does return to an idle very slowly, is the throttle slide completely closing?  If the slide is not completely closing adjust the throttle stop screw so it will close.

3.  If the throttle is completely closed and the carburetor is jetted properly and the engine still wants to idle excessively high, the air leak is large enough to cause a  problem.

The Power Valve design for the LT 250s and LT 500s makes the engine very difficult to accurately pressure check.  If the engine shows some leakage during a pressure check, the power valve may be responsible. 

If all of the critical machined surfaces were checked for flatness and scratches and the proper sealers were used on these surfaces, pressure checking should not be necessary.

I have been finding a lot of leaking cylinder base gaskets and reed gaskets on cylinders that have had the ears welded the last year or so.  These two gasket surfaces were not machined after welding on the cylinder and should have been machined!!!!!

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Offline 88LT500

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Re: leak down test readings
« Reply #3 on: September 01, 2018, 06:22:47 am »
Thanks guys for the info.  The engine is still on the bench.  I have completely restored the frame and everything else.  Im doing final checks on the engine before i put it back into the frame.  This is where i came up with the pressure readings.  It has not been ported.  It has a duncan pyramid reed valve with brand new petals.  Jerry, you rebuilt my carbueartor last fall.  Which looks amazing by the way!!!!!!!  I have not even installed it yet.  The engine has not had any of those issues that you asked about.  I cannot find any bubbles anywhere, not even the tiniest ones?  I will REcheck the coolant hose ports to see if there is leaking, maybe put a balloon over the ports to help diagnose.  What are your thoughts on what my PSI readings were???  Thanks guys.  Jerry, its an honor to have you help me

Offline MotorGeek - Jerry Hall

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Re: leak down test readings
« Reply #4 on: September 02, 2018, 09:42:06 pm »
When you have the engine pressurized put the transmission vent line in a bottle of water.  If you see bubbles it means the leak is some where that is connected to the internal space where the transmission lives or is remotely connected to the vent line ( use your imagination  to construct of all of the possible surfaces internally that share the volume that is connected to the vent line).  This space is connected to the case halves parting line,  the right crank seal,  the three 3mm holes that support the iron main bearing sleeve during the casting of the cases but are fill with epoxy from the factory, and the seals on the power valve that are suppose to prevent exhaust from getting into the transmissions living space.

It would probably take a drop of more than 3 psi per minute to lean it enough to make the idle hang at 2000 rpm or so...........or make it act real lean at an idle.


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