TheSamba.com :: View topic - Question on Leak Down Test vs Compression Test Results

Thu Oct 06, 2016 5:52 am

I'm getting less than impressive compression results on my 1776 engine, and yet the leak down test seems fine. Anyway, here are the results:

Compression conducted with a Snap-On gauge, while engine was warm, carbs open, valve lash checked, timing set with strobe:
Cylinder 1 = 100, 2 = 123, 3 = 150, 4 = 110

Leak down done at 90psi for each cylinder:
Cylinder 1 - 86, 2 = 87, 3 = 86, 4 = 86

Not sure why I'm getting these confusing results but any help would be appreciated. Added info: engine does not smoke nor burn oil but does not idle smoothly, almost sounds like it's running on 3 cylinders at low rpms. It does act like it has tons of power when up and going. Carbs are Weber 44idf, which i believe to be a tad oversized for the engine - 40's would suit better.

Here's the build sheet for the engine:
• 1776 with 69mm stroke
• Counter weighted Gene Berge balanced crankshaft
• Lightened 8 dowel pinned Flywheel 12.5 to 13 lbs
• 90.5mm Cyma forged pistons and cylinders
• Heads & manifolds ported by IMS .40mm inlet valves and 35.5mm exhaust valves
• Chambers are Hemi cut
• Cam Engle W-110 228 Dur .425 lift (?)
• Rockers reworked with shims and swivel adjusters
• Gene berg C/M push rods
• TWR high volume oil pump with full flow filter system
• Doghouse sheet metal and oil cooler
• High capacity fuel pump
• Additional oil sump adds 1.5 extra quarters – total 4.5 quarters
• Valve lash .004 intake and .005 to .006 exhaust
• Weber 44 IDF with Air = 50’s and Mains 135’s (changed from 55’s and 140’s due to richness)

Thanks
Silvano

Thu Oct 06, 2016 7:00 am

So your leak test shows they are at 95% yet your compression test show one cylinder with higher compression.

I would do the compression test over again to verify results. Do it on a warm engine normal and wet (teaspoon of oil in each hole) and see if there is a difference.

The idle issue? Engle 110 has enough lift and duration to give a rolling rough, uneven idle. Carbs need to be balanced well and verify cylinder contribution is even (idle drop with plug lead pulled and shorted for each cylinder the same).

Gordo.

Thu Oct 06, 2016 7:04 am

flemcadiddlehopper wrote: So your leak test shows they are at 95% yet your compression test show one cylinder with higher compression.

I would do the compression test over again to verify results. Do it on a warm engine normal and wet (teaspoon of oil in each hole) and see if there is a difference.
....

Gordo.

that is what i would do.

Has this engine idled better before or is this a new engine?

Thu Oct 06, 2016 8:04 am

Thanks for the replies. So I did the 10cc of oil in each cylinder and the compression did increase for each cylinder. I did this test prior to doing the leak down test, (got this tip off of Rob and Dave's), which made me think it might be rings so did the leak down expecting to see poor results, which did not happen. BTW, testing of compression was done on a warm engine.

This is not a new engine, but not a high miler either - engine sounds strong, seems to have plenty of power and has perfect end-play no rattles etc. I have in the past, been able to tune this engine to run extremely smooth (usually it's short lived) so the carbs are a concern to me. Putting the carb issue aside, I think I can smooth things out with a different set of carbs or messing with jetting, but I am still confused about the leak down vs compression numbers. Am I over concerned with these numbers? I have the engine out of the vehicle on a stand that I can run the engine on.

Thanks
Silvano

Thu Oct 06, 2016 8:12 am

Bad compression gauge? Try a different one to rule it out.

Thu Oct 06, 2016 8:32 am

Did you record the difference between dry and wet compression testing?

my feeling is that you have 2 cylinders that are "normal", one that is "low" or worn, and one that is unusually high.

i have always been a stock engine guy, so i am working off of stock numbers, but 150 seems high

Thu Oct 06, 2016 8:57 am

You need to repeat your compression and leak down tests. You need consistent results to answer your question

Thu Oct 06, 2016 9:01 am

Will try another gauge to rule out but pretty sure the gauge is fine.

Uli: I agree that 150 seems high and the 100 seems low. If the cylinder that is 100 is worn, as you mention, wouldn't I catch that with a leak down?

Very confusing.

Thanks for the replies
Silvano

Thu Oct 06, 2016 9:05 am

Silvano Angelone wrote:
Uli: I agree that 150 seems high and the 100 seems low. If the cylinder that is 100 is worn, as you mention, wouldn't I catch that with a leak down?

Are you setting it to TDC when you do the leakdown test?

If only part of the cylinder was worn and the piston was rotated to an unworn part (rocker assembly off) when you did the leakdown test you may see inconsistent results.

Thu Oct 06, 2016 1:59 pm

campingbox wrote: Silvano Angelone wrote:
Uli: I agree that 150 seems high and the 100 seems low. If the cylinder that is 100 is worn, as you mention, wouldn't I catch that with a leak down?

Are you setting it to TDC when you do the leakdown test?

If only part of the cylinder was worn and the piston was rotated to an unworn part (rocker assembly off) when you did the leakdown test you may see inconsistent results.

I did the compression test with another gauge and no luck - number was even a little lower than 100 but could be difference in both gauges.

When I did the leak down I did it at TDC, so will now take the rocker assembly off and try leak down test on the entire cylinder. At that point I should probably take the head off and have a peak.

Thanks All

Sun Oct 09, 2016 8:05 am

Update on test:

So took the rocker assembly off and tested the leak down at Bottom Dead Center - and the numbers dropped drastically for cylinder 1 (see attached pic). Due to this, i suspect that there's an issue with the rings/cylinder, so pulled the heads off, jugs, pistons for further inspection. To the eye, Im not seeing much wrong, but plan on replacing all cylinders, pistons and jugs.

BTW, I think the pistons and jugs look like Daleks - exterminate!

Silvano

Mon Oct 10, 2016 8:19 pm

Unless the other side of the piston is worn the side in the picture looks close to new did you see if the ring gaps were staggered