Another Reason to Pressure Test Motors

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Another Reason to Pressure Test Motors

Postby Schoobytwo (Bruce) » Mon Dec 06, 2010 10:47 pm

Hi all,

We have lots of discussions about carbs and adjustments on the Hirths used in our Diablos. Getting them to run well sometimes is more good luck than anything. For over a year now, I've been pressure testing my motors by blanking off the intake and exhaust ports and pumping air into them. I've been pretty confident that a leaky motor not only runs lean, but is hard to adjust the carb and get it to run right.

This past week, I had an opportunity to work on a 503 with the Hirth 190 in it. Prior to pulling the motor out to recondition the mechanical components on the chassis, the motor was running. It started fine and revved up good, but no matter how I adjusted the mixture screws or spring tension within the carb, I could not get it to idle.

So before I put the motor back in, I pressure tested it. I was surprised to find the motor would not hold any air. Looking down the exhaust port, there was a little scoring on the piston, but not enough that I would pull the motor apart for. But not holding air is another issue. So apart it came. I found the top edges of the piston showing significant signs of wear which I was not expecting. After sealing the housing and putting in new seals and a gasket kit, the motor pressure tested good holding 4 PSI of pressure.

I put the motor back in, finished up the repairs, and reinstalled the same carb it had on before. Without adjusting anything, it started up and idled fine. To me, that is proof that if you have a motor that cannot be adjusted right, there is a good chance more than a carb kit is needed to fix it right.

Bruce
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Too many Diablos and parts to count.
Yamaha Apex (it outruns the Diablo for sure)
Yamaha 600 SX
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Re: Another Reason to Pressure Test Motors

Postby Barrie2777 » Tue Dec 07, 2010 7:42 am

How long should it hold the 4psi?
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Re: Another Reason to Pressure Test Motors

Postby Schoobytwo (Bruce) » Tue Dec 07, 2010 12:02 pm

Good question. The motors are not air tight. But if I can pump them up to 4 psi and it takes maybe a minute or so to bleed down to 3 psi, then I know it is pretty air tight. When the seals leak or the case has a leak, typically you cannot pump them up at all with the little hand pump I use.
Too many Diablos and parts to count.
Yamaha Apex (it outruns the Diablo for sure)
Yamaha 600 SX
User avatar
Schoobytwo (Bruce)
Site Admin
 
Posts: 1716
Joined: Fri Feb 22, 2008 12:47 am
Location: Sobieski, Wisconsin


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