Performed a leak down test on my newly rebuilt motor tonite, not sure what values are acceptable but I was a little disappointed to see the results. Cyl #1 was about 25%, cyl #2 was about 20%. These are in the green zone on the gauge, but seem high to me. Test was done with 15 PSI input. Sounds like the leakage is on the intake side.

The motor has heads decked .045, shaved pistons, EVT cam. New stock springs shimmed .060. MM exhaust. Mikuni HSR42 carb, freshly rebuilt, with K&N filter and EVT stainless intake. I had lapped all valves prior to assembly, compression is 175 & 178 PSI. Valves have been set both for .005" at both intake and exhaust, and also .004 intake, .006 exhaust, 1/4" past TDC. Coil gap set to .006.

This motor is just not performing as I hoped it would after mods. Right now, in addition to not producing much more power than stock, the #1 cyl is running so rich it's fouling the plug and the #2 cyl is running lean enough to cause the exhaust to glow on that side. I originally had major intake air leaks, I cut my own intake gaskets and sealed with Permatex aircraft gasket dressing, still seeing red exhaust #2 side only. I installed new coils the end of last season, and a spark checker indicates spark is healthy thru the operating range. I just replaced the #1 spark plug but haven't run it since.

Man, I sure could use some advice at this point. What else can I check? Any thoughts about what's wrong?

Was #1 oil or gas fouled? Bad head gasket or piston rings ?

Piston rings may be in upside down. Some rings are have a taper they recently switched to square edge ring. May still have an air leak. From what I was told and read those leak down numbers are not that bad ,typical for a stock motor

not sure how big of a deal it is but from what ive read i thought a leak down test was done with 90-100 psi

15 psi for a leak down test is quite low. Most people use 80-100 psi for a leak down test. Did you remove loosen the rocker arms when performing the leak down test? With more air pressure it will be easier to hear where leak is coming from.

With one cylinder being rich and the other lean it could possibly still be an intake leak. An intake leak will affect the air/fuel mixture.

Did I read that you have shaved piston, longer rods, Heads Decked .045, and your compression is 175? Something is not right if this is the results.

^^. The masters eye. My heads are decked .015 with the pistons and I'm at 240psi ,maybe the rods are stock length.

240!?

Why would you install shaved pistons with stock length rods?

I wouldn't

Maybe I shouldn't of assumed.

I was about to ask how in the hell it went up 20 PSI.lol

Are you holding the throttle wide open when performing the comp test? The stock gaskets are fine, but not the problem. Depending on you block measurement you should be around 235-250 with those mods, unless some how you got the stock length billet connecting rods with stock pistons, It could happen.

Yes, throttle is WO when checking compression.

Not sure about rod length, but if I recall the top of the piston pretty much comes flush with the face of the cylinder. Not sure what that means, or if the difference in lengths would be obvious?

Hmmmmmmm??

To me without measuring and putting my hands on the motor, those numbers that is stock length rods with stock pistons. Now, to fix you power problem I read that you used cork gaskets from the intake to head? The cork gasket will burn rather quickly and develop a leak at the intake

Try a different compression tester. I had on quit on me the other day


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With out measuring the rod it self there is no way that I know of telling what rods you have.

Can you take a picture of your sparks plugs after you run them?

When you installed them, did the depth in the dish of the pistons look the same as the stock dish?

Jamie let me know what the id is on the rods for the crank and then I can tell y'all if the rods are the same length from the picture

Jamie, what's the deck height on a stock 31? If its like the 35, stock deck height is around 60 thousandths I think. Him running shaved pistons and aftermarket rods and the piston coming flush to the deck, it's gotta be longer rods or the pistons have a taller compression height, which should in the end end up with more than 180
Psi compression right?

Unless the stock deck height on the 31 is different, I know the stroke is. Just trying to brainstorm.

I thought shooters motor was a 32 it never specified above tho

Just going out on a limb here, but wouldn't the rods for a short stroke motor be longer than those for a motor with a longer stroke? So going to a longer rod for a long stroke might still be too short for the short stroke.

It makes sense in my head.

Same block, same bore, different stroke. I know that for sure. The 31/32 is a short stroke motor.

If that was the case, the pistons wouldn't be close to flush with the deck like he said, right? 3.07 is the 32 stroke and 3.41 is the 35 stroke. A .120 longer rod for the 35 in a 31 doesn't seem like it'd be close to coming flush with the deck, would it?

I don't know, but sure seems like a good way to loose 70 psi.

Wurd!

The deck height on the 31 I have was a lot closer to 0 deck height than a 35( i don't remember exact measurements). This would make sense if he had a 32 HP motor with stock length rods and pistons. If this is the case his compression numbers are pretty much dead on at .045 head decking. The 31/32 has the same block and shorter stroke on the crank, the means the the rods are longer on the shorter stroke( Reason the 31with .080 oversize bore was one of Drake 1 favorite motors). The 35/36 has a longer stroke crank with the shorter rod.

SHIT!

Once again, the obvious bites me in the ass. I swear, some times I get so caught up in details I miss the big picture. Yes, this is a 32 motor. Yes, the parts are from a 36 motor. Yes, I am a dumbass.

So... can I just get some aftermarket rods for a 32 and get right?

Or get a crank for a 36. If I'm correct