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Leak down tests doen't always show problems...

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Leak down tests doen't always show problems...

Postby KE4AVB » Sat Dec 15, 2018 1:23 pm

Well here is an experience with a Tecumseh OHH60 using a lead down tester.

The engine comes in with a bad carburetor (water damage) and busted fuel tank. Test fired the engine and did a leak down test with the leak down indicating only 20% leakage pass the rings, no other leaks indicated. So I order a new carburetor and fuel tank. Once I got it running it was smoking a like freight train. I found oil in the intake after the carburetor indicating a bad intake valve stem seal. No biggie just install a new seal I thought.

Well that only reduce the smoking. Btw the valve stem seal did fix the intake oil problem. Normally we do leak down tests at TDC compression. I decided to disable the valve train and test BDC. Boy what a difference in leakage rate as it went to over 60%. Well I got a new engine installed in it place so I decided just to see what actually was wrong the Tecumseh. The cylinder is so worn that it is already at 0.020 over with the piston easily moved side to side in the cylinder. Too bad only .010 piston and ring are available as I would need do 0.030 over for this one.

Just proves even with good test results using test tools as indicated still will not find all the problems with a cylinder. I probably would had been still scratching my head if I didn't try a secondary leak down test.
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Re: Leak down tests doen't always show problems...

Postby 38racing » Sat Dec 15, 2018 3:51 pm

I've just been working on a 5 hp tec on an ariens snowblower dumpster drop. Carb was messed up but I ran it through the USC. Seemed to run good so I proceeded to make some wheels for it. Got that done along new idler and grease and went to try it. Just wouldn't run right. Rough or falter and no carb adjustment would fix it. Then I realized that the old cj8 plug was in it. Replaced with a br2lm and seems to run great. Just have to wait for snow. Rained yesterday.
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Re: Leak down tests doen't always show problems...

Postby wristpin » Wed Dec 19, 2018 6:30 pm

Several (many!?) years ago Briggs majored on crankcase vacuum in one of the annual update booklets and at regional meetings in the UK. Lots on making oil filler adaptors and the introduction of a digital measuring device. Then a couple of years later it was the turn of leak down testing with the appropriate Briggs banded gauge.
Nothing really replaces experience (aka inspired guess work??) and the application of the right test at the right time!
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Re: Leak down tests doen't always show problems...

Postby KE4AVB » Thu Dec 20, 2018 2:51 am

Yes it does take thinking outside the box at times and experience does help a lot too. Sometime you learn something new. Anyway that go cart is out of here with a new HF engine with minor mods. I don't like having only 1/4" of crankshaft for the outside part of the Comet clutch but a tech that does this regularly says it is no problem. Anyways I replaced the grade 5 retainer screw with a grade 8 screw.

As for the leak down tester I was using a homemade one until this year for the simple tests for blown head gasket and valve problems. But I do like the low pressure version I got from Harbor Freight where I can use only 10-15 psi to test. At this pressure you can actually hold the flywheel by hand and cycle the engine. Only one draw back to the tester you must start initially at zero pressure at setup or you will damage the percentage gauge. Always zero the regulator when storing the test gauge assembly and reset per the manual when testing. It might be because I am running 130 psi as a line pressure here but I did blowup the first tester because I didn't go back zero on storage and it pegged the percentage gauge severely damaging the internal pressure tube for the gauge.

Now this weekend I get to test my new master fuel pressure test kit on a Bobcat that has a FI setup. I got the front end basically done other than waiting on a RH hub. You be surprised had much wobble you get with .020 of journal wear. The customer thought it was the bearing but it just as bad as before. I caught it before I installed the bearing and showed the customer the wear before trying. Being a mechanic himself he to try it anyway as he was cutting corners. It was not fun having cut 4 A frame bolts out to replace the bushings. It took a half day just to do the A frames; grade 10.9 screws but only took 15 minutes to replace the bushings once the frames were freed. I had spider webs all over the place from those nylon bushings as I cut thru them.

Boy the customer going to flip as I got 8 hrs of labor in this project already. This time I knew what I was doing so no give on the labor time. I think I have done most of the work now. Rebuilt front end, replace CV and steering boots RH side, adjusted the parking brake, and rebuilt the fuel pump assembly, just awaiting the hub.
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