Board index Flathead Power-Technical Questions, Answers, and Suggestions Big Twin Flatties intake leak check

intake leak check

Post Tue Apr 07, 2015 9:37 pm

Posts: 109
even,'n folks, i have been getting my flathead ready for the new season, one item was the dreaded intake leak test....well i started with 15 lbs and no bubbles, could not believe it was sealed i upped it to 20lbs...still no up to 25 lbs and no bubbles.....i was using a hardware store brand leak solution and i thought this stuff might not be up to snuff so i rigged up an air leak with my compressor hose and it bubbled up just fine. after 3 years and nearly 30k miles i finally got a lasting seal on the intake. while i did the test i had no exhaust pipes on the motor and some air was leaking out the front cylinder exhaust port, i tried several times to rotate the motor but the leak was constant......leaking past the intake and then the exhaust....i set my valves the evening before and to my surprise i barely had .002 thou clearance! last winter i had the valves ground and lapped....i can only remember adjusting the tappets once last season and all was well, what would cause a loss of clearance? tappets were tight. the front cylinder is down in compression. i looked at the ex valve and seat thru the port, very hard to see much seat wise but i could see a small section and seat and valve looked very good no pitting, burning with a light rust on the seat. there is no carbon build up just a light sooty coating on the port wall. i am hoping the loss of clearance let carbon coat the seat and valve......if this is the case what do you think the odds are that running the bike will clean up and seal with some miles? bike fires up easily and runs fine, any thoughts would be helpful. i would like to run it this season and this winter pull the top end and do a proper valve job again.

Post Wed Apr 08, 2015 8:35 am

Posts: 3159
Location: Central Illinois, USA

There is no need to go over 15 psi on a bubble-test.
Absolute vacuum at sealevel is ~14.7 psi, so 15 is overkill enough.

Over that, it just starts opening the intake valve.

If you wish to test the combustion chamber, headgasket, etc. through the sparkplug hole, then you can bump the pressure up.


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