‘72 combat leakdown & compression reading

Status
Not open for further replies.
ILLF8ED

ILLF8ED

Joined
Dec 30, 2003
Messages
3,689
Country flag
I’ve been riding my ‘72 combat for 22 years and my sense of diagnosis isn’t that good anymore. Just did a leakdown and compression this morning.
Leakdown show 95% leakage both sides
Compression 180lbs both sides after six kick through s with throttle wide open and cold engine.
Rabers did black diamond valves, springs seats 8,000 miles ago.
.040” overbore on the cylinders. 8,000 miles on the rings and 38,000 miles on the pistons.

Conclusion?
 
Some thing isn't matching. You shouldn't have that much compression with 95% leak down. Is the crank turning slightly and opening a valve when doing the leak down? I've found locking the rear brake isn't enough to keep the engine from turning. You need to hold it with a wrench on the crank nut. If I had that compression, I wouldn't worry about the leak down test and start looking elsewhere.
 
Leak down is tough to use unless you are experienced with it. The leakage display of my snap-on guage is based on pressure differential across a calibrated oriface. The readout is a "relative" # and will vary a lot based on the instrument.
My MKIII I assembled showed the lowest leakage I ever saw. One side was 2% the other was 3%. I had even reused the rings. If the same gauge was used on a chain saw it might only show 1/2 of 1%.
If you had 95% leakage did you hear it hissing out the carbs? exhaust pipes? or out the oil tank breather? Where did the leakage go?
 
Had a wrench on the alternator nut. I moved the crank forward and back a little to see if the pressure would come up. It didn’t. Will go back and and check again were the sound is.
 
‘72 combat leakdown & compression reading ‘72 combat leakdown & compression reading Just finished. Both sides have no leak sound at exhaust or carbs, but at 10psi connected directly into the plug plug hole with piston at TDC I can hear a leak at the oil tank. Ran the engine for a bit to heat it up then ran a compression check throttle wide open kicking through six compression strokes.
Here is right side and left side gauge readings.
 

Attachments

  • ‘72 combat leakdown & compression reading
    2C879EEE-8A22-4536-B18A-02B30F64F8A7.webp
    75.4 KB · Views: 230
Are you sure you performed the leak down correctly? 95% leak down is beyond terrible; an engine with no piston rings could probably manage that! ;)
 
MexicoMike said:
Are you sure you performed the leak down correctly? 95% leak down is beyond terrible; an engine with no piston rings could probably manage that! ;)
Click to expand...
No not sure. Leakdown is a PITA. Instructions say increase air pressure until the second gauge shows zero then connect to the cylinder. That was at 20psi.
 
at 10psi connected directly into the plug plug hole with piston at TDC I can hear a leak at the oil tank.
Click to expand...

I know nothing about leakdown tests, but it must take more than 10 psi to press the piston ring out against the bore, to make it seal.

How well does the bike run?
 
Illf8ted, you’ve had the bike 22 years, so I’m fairly sure you know how it should feel.

If it feels normal on the kickstart and if it goes well / normal, then surely there’s no need to worry?

If it does not feel normal on either of the above, a strip down is required anyway.

What I’m getting at, is I don’t see how the compression test is helping you here, it’s either normal = leave it, or it’s not = strip it.

Testing it like this may simply be causing you to fret about non existent issues?
 
Plus 1 with what FE writes. 180 compression is fantastic IMO; so I wouldn't worry about the leak down. I admire your zeal, but perhaps your just being overly paranoid? Cheers, Pat
 
Do you have a photo of the leakdown gauge reading?
I do leakdown tests now and then, mainly to establish a baseline for the engine and to watch for change.
They are an excellent test.
I'm wondering if you are reading the gauge/ gauges properly?



Glen
 
Normal procedure is first the compression test, then if the readings are low or more than 10% different side to side you do the leakdown test. Your compression test is good, no need to progress to a leak test. The hissing from the oil tank is the compression leaking through the ring gaps, that is normal unless you have gapless rings.
 
Fast Eddie said:
What I’m getting at, is I don’t see how the compression test is helping you here, it’s either normal = leave it, or it’s not = strip it.
Click to expand...
Strip it ? what to look for? cam? pistons/rings/ head gasket?

My Snap-On leak down gauge runs at 70 psi. So 70 psi out of the regulator, then if 70 psi on engine side of calibrated orifice you have 0 leak down.
There is no way to hold the piston, if off of TDC, it would almost break your arm/hand trying to hold it.
You get very good at finding TDC +/- 1 degree.
IMO A leak down is the first most important test, which tests only the sealing surfaces.

A secondary test is a compression test since it ADDS the camshaft to the dynamic test. If it is low you still have no idea why... unless you first did a leak down.
 
Last edited:
It's impossible to have 175 psi compression and 95 % leakdown. Perhaps you're reading 5% as 95%?

Leakdown is typically done at 100 psi, and you need to have the valve covers off. Rotate the crankshaft until both valves are closed (slack in the rockers). It's usually a 2 man job with a Norton because air pressure wants to push the piston down and someone has to hold the wrench to prevent that.
 
maylar said:
Leakdown is typically done at 100 psi
Click to expand...
The designer of the instrument uses what ever they want for test pressure. Curious to see the results of a test equipment survey showing any "typical" pressure...Snap-On must be an outlier?
I always (99%) do leakdown by myself
 
dynodave said:
The designer of the instrument uses what ever they want for test pressure. Curious to see the results of a test equipment survey showing any "typical" pressure...Snap-On must be an outlier?
I always (99%) do leakdown by myself
Click to expand...

Now that I think about it, you're right. My tester says to set the compressor to 100 psi then adjust the tester's regulator for 100%. I dunno what pressure is actually applied to the cylinder.
 
Sounds like I’m doing the leakdown wrong, need to use more air pressure. I’ll try again. After riding this bike for 22 years, yes it isn’t running right. I corrected cam timing a while back and that improved it, but still seems to be down on power. The testing I’m doing is to see where to go correcting this. Valve lift and cam timing are correct.
 
dynodave said:
Strip it ? what to look for? cam? pistons/rings/ head gasket?

My Snap-On leak down gauge runs at 70 psi. So 70 psi out of the regulator, then if 70 psi on engine side of calibrated orifice you have 0 leak down.
There is no way to hold the piston, if off of TDC, it would almost break your arm/hand trying to hold it.
You get very good at finding TDC +/- 1 degree.
IMO A leak down is the first most important test, which tests only the sealing surfaces.

A secondary test is a compression test since it ADDS the camshaft to the dynamic test. If it is low you still have no idea why... unless you first did a leak down.
Click to expand...


The leakdown test can be done at any point where both valves are shut.
Put the bike in top gear and use the rear brake to hold the engine. I use a 2' long 2x2 to wedge the brake on.
Its good to check leak down near the top and down in the bore a bit as well.

There are low pressure and high pressure leakdown testers.
The lower ones work at about 30 psi.
I've used a higher set that worked at up to 100 psi but also gave consistent readings at 70 psi.

Glen
 
Status
Not open for further replies.
Back
Top