‘72 combat leakdown & compression reading

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Yeah, re-do the leak-down; I'm sure you either misread the gauge or the piston wasn't at TDC. FWIW, if you have the piston precisely at TDC, you don't need to put a wrench on anything to prevent it from rotating. Admittedly, using a stock pointer/marks on the damper (or, in this case, the alternator rotor) to place the piston at TDC usually doesn't work! You need to have found true TDC with a piston stop/degree wheel at some point and appropriately marked the relevant components.

As noted, the camshaft makes a big difference in compression numbers. A stock cam usually has higher numbers than a performance cam. It seems odd but a 9:1 compression ratio engine will usually have a higher compression test PSI than a 12:5:1 competition engine due to the camshaft.
 
maylar said:
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.
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That's how my low pressure set works.
It operates at about 30psi after regulation, but calls for 100psi from the compressor.
 
MexicoMike said:
As noted, the camshaft makes a big difference in compression numbers. A stock cam usually has higher numbers than a performance cam. It seems odd but a 9:1 compression ratio engine will usually have a higher compression test PSI than a 12:5:1 competition engine due to the camshaft.
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Interesting, I hadn't thought about that.
 
worntorn said:
That's how my low pressure set works.
It operates at about 30psi after regulation, but calls for 100psi from the compressor.
Click to expand...
My tester is obviously the low pressure type. Instructions say adjust the tester regulator knob so that the leakage gauge shows zero leakage then connect to the cylinder and read leakage. When I do that the air pressure is 20psi and I get 95% leakage reading. Setting the knob on the tester to 50 or 70 psi and the leakage gauge is pegged beyond zero. Think this isn’t a good tester for a Norton. At 70psi I can hear air escaping at the oil tank filler. How much noise there is acceptable?
 
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A leak down tester is a leak down tester - type of engine doesn't matter.

Your compression and your leak down numbers don't correlate. The hissing in the oil tank and the leak down numbers you mention indicate serious leakage past the rings. BUT...your compression numbers indicate the opposite and, for that matter, abnormally high! ASSUMING the comp gauge reading is correct and the leak down test reading is also correct then the only explanation I can come up with is that the bike is occupying a space-time warp into an alternate universe where the laws of physics as we know them do not apply.

OK, I'm reasonably confident that's not the case. It's a stumper but I suspect the explanation will end up being one of those "DOH!" moments. :)

Re camshaft vs compression - generally, the higher "performance" the cam, the lower the compression test numbers will be on the same engine with no other changes. We built some Mopar Hemi's with 13:1 compression that had lower compression test numbers than a grandma's grocery car. But when grandma's car hit 5000 RPM and 250 HP, it was done (unless she was the Little Old Lady from Pasadena. The Hemi was putting out 1000HP+ at 9000+ RPM. But you would have absolutely hated the engine if you had to use it to drive to the grocery store!
 
It is a stumper. I trust the compression guage. I don’t necessarily understand the leakdown tester. This is a combat engine that seems to be down on power. It pulls very well in 1st through 3rd gears, but doesn’t seem to pull all the way in 4th. 21 tooth gearbox sprocket, but that should not be a problem. Regardless of the leakdown results, I’m leaning toward pistons as the problem.
 
illf8ed said:
It is a stumper. I trust the compression guage. I don’t necessarily understand the leakdown tester. This is a combat engine that seems to be down on power. It pulls very well in 1st through 3rd gears, but doesn’t seem to pull all the way in 4th. 21 tooth gearbox sprocket, but that should not be a problem. Regardless of the leakdown results, I’m leaning toward pistons as the problem.
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FWIW, we always considered a compression check to be 1/3 of an engine check. The other 2/3 was the leak down test.
 
Cam advanced 1 roller? or mismatch at crank/intermediate gear?
intake closes early,
pressure indicates high.

Would also have very loud exhaust bark since ex also opens early and lets out a lot of residual pressure meant to push the bike.

2S cam ..right?
 
Have the carbs been rebuilt lately? The needle jets may well be worn significantly if not. This would cause it to be rich in the 1/3rd to 3/4 range. Might be making it feel a bit "boggy". Most pronounced would be opening throttle in 4th gear. If the needle jets have quite a few miles on them, I'd drop the float bowls and replace them. May not help but quick and cheap and just part of routine maintaince.
 
Be careful doing a leak down. When I did mine the rings were leaking and the timed breather was closed, the air had to go somewhere so it found its way between the case halves.
 
dynodave said:
Cam advanced 1 roller? or mismatch at crank/intermediate gear?
intake closes early,
pressure indicates high.

Would also have very loud exhaust bark since ex also opens early and lets out a lot of residual pressure meant to push the bike.

2S cam ..right?
Click to expand...
Yes it is mismatched at the intermediate by one tooth compared to the dots. That’s how I corrected the 15 degrees retarded cam timing previously. Confirmed current cam timing with degree wheel and travel dial a couple of years ago.
 
Mwood7800 said:
Be careful doing a leak down. When I did mine the rings were leaking and the timed breather was closed, the air had to go somewhere so it found its way between the case halves.
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mine is a ‘72, no timed breather.
 
htown16 said:
Have the carbs been rebuilt lately? The needle jets may well be worn significantly if not. This would cause it to be rich in the 1/3rd to 3/4 range. Might be making it feel a bit "boggy". Most pronounced would be opening throttle in 4th gear. If the needle jets have quite a few miles on them, I'd drop the float bowls and replace them. May not help but quick and cheap and just part of routine maintaince.
Click to expand...
I routinely replace jet needles and needle jets. If you run your thumbnail along the needles you can feel the notches on a worn one.
 
Thanks everyone for the replies. My conclusion is to pull the cylinder. Will measure cylinder and piston wear. Dave, I will check again the cam timing. Lastly worntorn appreciate the info on low pressure leak down tester which is what I have.
 
Well...you can't beat Snap-On stuff...though the price can be a bit off-putting! ;) Harbor Freight has some excellent tools for some applications and well worth the (minimal) cost, especially if you are a DIY-er as opposed to a commercial user. But for things like leak down testers, torque wrenches, good multimeters, etc, I'd go for a well-known manufacturer.


In my serious engine-building days, the Snap-On truck was a valuable asset! :)
 
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