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MW dyno surging

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ToddF

09-24-2002 05:15:49

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We borrowed an M&W dyno that can theoretically measure up to 190 Hp. On the first pull, with the dyno oil at room temp we got a reading of around 87 Hp. Then after warming the dyno oil up to the operating temp we were pulling around 80 Hp when the dyno started surging. By surging I mean it appeared to unload the oil pressure, then load up, unload, load...at roughly one second on, one second off. My first thought was that the inlet to the pump was restricted somehow, or that there might be a pressure relief that is blowing off. Well with a quick look inside I didn't see a pressure relief, and the oil is murky enough that I couldn't see the inlet to see if there is a filter or screen over it. If any of you are familiar with these dyno's I would appreciate your input before I start to get my hands dirty.
Thanks--ToddF

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SEAMO

09-24-2002 19:51:51

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Re: MW dyno surging in reply to ToddF, 09-24-2002 05:15:49

Call M&W GEAR CO Gibson City ,Illinois they have a service dept.I dont have there phone number but it is 217 area code. Try www.Anywho.com they will have address and phone number. Earl

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bigmotors

09-24-2002 07:14:51

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Re: MW dyno surging in reply to ToddF, 09-24-2002 05:15:49

Look for air bubbles in the oil tank, or foam. You don't have to heat up the temp. before using it. The oil also helps in the cooling of the dyno. Make sure it's got the right kind of oil, I've been told this might cause some of the surging. Make sure everything is lined up, and that the tractor is in a fixed postion.(Does not move)

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ToddF

09-24-2002 07:25:52

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Re: Re: MW dyno surging in reply to bigmotors, 09-24-2002 07:14:51

What is the right kind of oil? It currently has hydraulic oil in it. I looked maybe 3-4 minutes after the pull in the reservoir and the oil appeared cloudy-ish, would that have been air in the oil? I'll look again tonight and see if it is still cloudy.
Thank you--Todd

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bigmotors

09-24-2002 13:14:18

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Re: Re: Re: MW dyno surging in reply to ToddF, 09-24-2002 07:25:52

Hydraulic oil is fine. If the oil is cloudy, you might be getting water in the oil. That's common with the copper tubbing. The only way to fix it, is to drain it, and fix the tubbing. Some have, what looks like a small radiator. Again, you have to drain, and take it out to fix. This should not cause the surging problem. This is just another problem to bother with. Make sure the pull down wheel, the wheel that puts pressure to messure hp. works smooth. It could be slipping or grabbing. Two Bangers sometime act funny on a dyno. but, I don't think this is your problem. Good Luck!

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baystatebinder

09-24-2002 05:50:17

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Re: MW dyno surging in reply to ToddF, 09-24-2002 05:15:49

You have to make sure there is water running through the dyno.

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ToddF

09-24-2002 06:04:37

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Re: Re: MW dyno surging in reply to baystatebinder, 09-24-2002 05:50:17

baystatebinder,
I had the water turned on, and the oil temp was in the operating range on the second pull. The first pull the water was on but the temp had not come up yet, seemed to work fine then. The second pull I slowed the water down some so the oil would warm up. Do you suppose I was overheating the oil directly in the pump? Thanks--Todd

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Bill

09-24-2002 05:24:46

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Re: MW dyno surging in reply to ToddF, 09-24-2002 05:15:49

Sounds like the tractors clutch or pto clutch is slipping. Have had the exact same symptoms before and that is what it was.

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ToddF

09-24-2002 05:55:58

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Re: Re: MW dyno surging in reply to Bill, 09-24-2002 05:24:46

Bill,
I talked to the kid I borrowed it from and he said "Oh yeah, it did that on the last tractor I dyno'd also." So I'm inclined to think that it is something to do with the dyno. My tractor has a transmission driven PTO and I have not experienced any other problems with the clutch slipping. I will double check that because I had the clutch apart during the rebuild. Thanks--Todd

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TimC

09-24-2002 06:07:54

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Re: Re: Re: MW dyno surging in reply to ToddF, 09-24-2002 05:55:58

Sounds like cavitation. Any air entering the system?

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ToddF

09-24-2002 06:44:30

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Re: Re: Re: Re: MW dyno surging in reply to TimC, 09-24-2002 06:07:54

Could be, how do you tell where it might be entering? The oil level is right to the top. I can't see the inlet to know if it is near the bottom or top of the tank. It may be possible if it is close enough to the top for a vortex to form and drop to the point where the inlet gets air. Maybe there is a screen on the inlet that is partially plugged and when the oil warms up and thins down some the oil cavitates. I guess that is the first thing to do is drain the oil and get a look at the inlet. Thanks--Todd

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Goldsburg

09-24-2002 09:30:58

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Re: Re: Re: Re: Re: MW dyno surging in reply to ToddF, 09-24-2002 06:44:30

Todd -

The inlet to the pump is actually less likely to cavitate with thinner (warm) oil. There is less of a delta P (pressure drop) to pull the thinner oil into the (restricted?) inlet. The pickup does not necessarily have to be "leaking air" to cause cavitation. A severe restriction at the inlet to a pump can be enough to cause such a huge pressure drop that any normally entrained air can come out of suspension and form bubbles, aka cavitation.

I would suggest that the inlet screen became increasingly plugged as you used the dyno and the oil circulated crap in the tank around, hence the cavitation got worse or noticeable. The fact that the oil looked murky would lead me to believe that this dyno has not had proper maintenance and that the tank is dirty. I would try to drain the oil and clean out the tank, pickup, and inlet screen.

Sorry!

Goldsburg

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ToddF

09-24-2002 09:46:30

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Re: Re: Re: Re: Re: Re: MW dyno surging in reply to Goldsburg, 09-24-2002 09:30:58

Thanks Goldsburg.
Todd

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