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  1. #1

    1390 Leak Down Test

    Just finished building my lowell 1390. Pressure test and it leaks about .25psi per minute. Small leak from the drive end crank seal at the center on the crank. I can put some pressure on it with my finger and it nearly stops. OEM crank seals and used marine grease on the surfaces. Should I be worried or do you think it will run in with heat?


  2. #2
    Spun the crank a few times by hand and checked it again. leak stopped at crank but still loses 3psi in 10min. Pumping up to 9psi. I know oside bills thrasher thread he only had 1.1 psi loss in 10 min. Not sure the tolerance here.

  3. #3
    I am getting ready to venture in your shoes once my parts come in , have you located the leak ? If not at .25 psi a min you should be able to see some bubbles from seal area leak with a safe liquid spray kinda like finding.pin hole leak in a tire ??? At that point you will prolly have to take off that part and use a safe
    (Gasket seal high temp ) possible to fill the void if there is one , bad gasket ???



    Quote Originally Posted by Roberd42 View Post
    Spun the crank a few times by hand and checked it again. leak stopped at crank but still loses 3psi in 10min. Pumping up to 9psi. I know oside bills thrasher thread he only had 1.1 psi loss in 10 min. Not sure the tolerance here.

  4. #4
    Haven't had time to mess with it this afternoon. I will check it again tomorrow more thoroughly and let you know what I come up with.

  5. #5
    Well it's still slow leaking at the nose of the crank on the drive end. My rubber autozone carb block caps for the other two pulse ports I noticed were cracked. Put some black rtv on those and now it only loses two pounds in 10 mins. I think I'm gonna go with it and keep an eye on the rear cylinder for a while, maybe it'll run in under vacuum and pressure cycles over time.

  6. #6

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    You may want to double check your intake/reed seals, especially if testing with a block off plate. Sometimes its hard to get it sealed at the intake or reed area. A mixture of Joy dish soap and water will go a great ways to finding the leak. Spray that stuff everywhere, cylinder bases, head gasket, exhaust. I would want less leakage than 2 psi in 10, especially on a 1390. A 1390 has a much stronger vacuum signal to suck in air from leaks and will run lean easier than a everyday 1200.

  7. #7
    I've got cut rubber under the stock reed plate. I soaked it down good this morning with soapy water, no bubbles anywhere but at the rear crank seal at the crank. Sprayed it again with wd-40. Same thing. I agree, 2lb isn't real comfortable but I have a feeling most of it, if not all of it, is at the crank.

  8. #8
    Re torqued all the head, cylinder, and exhaust bolts. Ratcheted a zip tie on the crank seal and put a little heat on it. Removed the zip tie and tested it again at 7psi this time. Dropped 1 psi in 10 min, 2psi in 20 min.

  9. #9
    Ive always used high temp rtv both sides of gaskets on previous engine builds just for that piece of mind.....some say its not needed and its overkill but never had an engine loose any pressure during leak down tests....its better to ues it vs blowing up an engine that you just rebuilt...

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