Pressure test fail

[wolfgangh]

Hello, today I did a pressure test on my freshly rebuilt GT750 engine. Crank rebuilt with all new seals, new cylinder base and head gasket, crankcases sealed with grey THREEBOND.
I sealed the exhausts and put a plug into carb manifolds with attached pressure gauge and pump. Target is to hold 5 psi for 5 minutes.

The left and the right cylinder worked fine, no leaks. The middle cylinder though drops more than 1 psi per minute. Spraying soap water did not reveal the source of the leak, and I cannot hear it.
Do you have any idea what the likely cause could be?

It could leak into the gearbox at the primary drive between middle and right cylinder, but how could I confirm that? Do you think it would suck gearbox oil then, and could it eventually also draw air from the gearbox?
Or could the head gasket leak from cylinderhead into water jacket? Is this why SUZUKI recommends to add some bar’s leak into the coolant?

And oh, I replaced the SRIS valves with solid studs. They dont leak either.

I rebuilt quite a few of other 2T engines and I was always able to easily identify and fix any leaks. Here I dont know what to do. Leave it as is?

[karl pa]

Do you think it would suck gearbox oil then, and could it eventually also draw air from the gearbox?

The answer is yes, is the gearbox closed up, if so can you detect leak at gearbox vent

Or could the head gasket leak from cylinderhead into water jacket? Is this why SUZUKI recommends to add some bar’s leak into the coolant?

I would think no, bars stop leak is for water jacket leak, not really for compression leak.

[Vintageman]

That's a slow leak for it sounds like you almost made spec at 5 psi over 5 minutes. I wonder if that would be the cases for you after a few miles of running to let things break in?


If Coolant:
Empty pressure from radiator cap and put back on. Redo test and let pressure leak out. After it leaked down remove radiator cap slowly to hear if any pressure escapes. Just a thought

If the leak is into the liquid cooling via head or base gasket leak Bars stop leak could work.

I think I would loosen all head bolts just enough to allow you to re torque out any creases that may have occurred so to speak and then re torque in the proper sequence. I would apply most all of the torque to a bolt before moving to the next bolt in sequence to work out any warps Maybe you did it that way already.

If Crankcase:
By the oil pump the crankcase vent exist. Can you slip a balloon over this for example to see if air is leaking into crankcase from you leak test pressure applied.

If Seal: Maybe there was a tiny rust divot from moisture or slight rotary wear from the old crank seal lip wearing as the crankshaft spun over time for example. The seals may fit better once broken in.

Good luck

[Vintageman]

That's a slow leak for it sounds like you almost made spec at 5 psi over 5 minutes. I wonder if that would be the cases for you after a few miles of running to let things break in?


If Coolant:
Empty pressure from radiator cap and put back on. Redo test and let pressure leak out. After it leaked down remove radiator cap slowly to hear if any pressure escapes. Just a thought

If the leak is into the liquid cooling via head or base gasket leak Bars stop leak could work.

I think I would loosen all head bolts just enough to allow you to re torque out any creases that may have occurred so to speak and then re torque in the proper sequence. I would apply most all of the torque to a bolt before moving to the next bolt in sequence to work out any warps Maybe you did it that way already.

If Crankcase:
By the oil pump the crankcase vent exist. Can you slip a balloon over this for example to see if air is leaking into crankcase from you leak test pressure applied.

If Seal: Maybe there was a tiny rust divot from moisture or slight rotary wear from the old crank seal lip wearing as the crankshaft spun over time for example. The seals may fit better once broken in.

Good luck

[wolfgangh]

Thanks alot Vintageman for your detailled reply, appreciated. I already tried most of your recommendations during the weekend, but still was not able to find the cause of the leak. I already rebuilt many twostroke engines like RD500, RD350, NS400 etc. and always easily identified and solved any leaks, with soap water or by simply listening.

I also removed the CCI hoses of the middle cylinder and blocked the holes to exclude possibilty of air pushing back into the hoses towards the oil pump.

I decided to disassemble the engine again to check for any obvious damages. Do you think I can re-use my head gasket ( it is the thicker 1,7mm one original SUZUKI) or would this likely not seal completely any more? The gasket was brand new, just installed before doing leak down test.

I believe your theory of light corrosion or run in of the collar of the seal to the primary drive is the most likely one.

Thanks again!

[sportston]

Do you think I can re-use my head gasket ( it is the thicker 1,7mm one original SUZUKI) or would this likely not seal completely any more? The gasket was brand new, just installed before doing leak down test.

Officially no. Unofficially, you might get away with reusing it if you put a thin smear of Wellseal on it, but only do that if you have no choice. And don't blame me if it doesn't work.
It would be much safer if you use a new gasket as the old/new gasket has now been compressed and will not have much squish left in it so it might leak.