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Old 17-07-2012, 10:12   #1
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Problems After New Tranny Install

I'm scratching my head to understand a problem that arose after installing a new ZF Hurth transmission.

Background: I have a Universal M50 with a Hurth transmission. The old transmission has a 1.90:1 reduction. The transmission seized with the engine in neutral. No transmission fluid. Presumably, it leaked from a seal after I checked it last Fall. The transmission was replaced with the current comparable ZF Hurth transmission which has a 1.95:1 reduction. I have a three blade Maxprop which was serviced in the Spring of 2011.

The problem: The engine runs fine in neutral and revs to about 3400 RPM. No smoke. In gear, it runs to 2K normally but only gets to about 2200 at full throttle. Same in forward and reverse. The ramp up of RPM is normal to about 2100 and then just goes flat with the extra throttle. I saw some black smoke when I tried to push it up, but the mechanic didn't notice any. No vibrations in neutral or under load. I cleaned modest growth off the prop, the shaft and the strut. No change. The yard is suggesting that it may be that the Maxprop is worn and has loosened so that the prop is now overpitched and the modest change in reduction exposed the issue. While that's an option, and the black smoke was probably unburned fuel, I would have expected the graph of RPM vs throttle to be more gradual. This behaves almost like it has a governor -- normal increase and then none at all.

So, while we talk to PYI about the Maxprop, what else are we missing?
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Old 17-07-2012, 12:05   #2
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Re: Problems After New Tranny Install

Just my opinion is that the old trans was always slipping, allowing for a greater pitch of the max prop (I'm assuming that the prop is manually set and not self adjusting). My feeling is that you will need to haul in the slings and adjust the prop. Hope this helps.
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Old 17-07-2012, 13:02   #3
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Re: Problems After New Tranny Install

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Originally Posted by Celestialsailor View Post
Just my opinion is that the old trans was always slipping, allowing for a greater pitch of the max prop (I'm assuming that the prop is manually set and not self adjusting). My feeling is that you will need to haul in the slings and adjust the prop. Hope this helps.
Thanks. That's an intriguing hypothesis. It explains the problem appearing now better than the relatively minimal change in the reduction. Given our short season and having missed the past month, we're going to sail the boat for a while and just keep the RPM's down below the threshold where it seems to have problems.
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Old 27-07-2012, 21:53   #4
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I'm interested in knowing what happened with your problem. Did you find what was causing it???
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Old 28-07-2012, 07:22   #5
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Re: Problems After New Tranny Install

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I'm interested in knowing what happened with your problem. Did you find what was causing it???
We don't have a definitive answer yet and probably won't until we haul in October. The engine still runs fine with a normal RPM to throttle curve up to 2000 RPM's but maxes out at about 2300 RPM. The hypothesis is that the prop is over-pitched. One reason that it might be when it wasn't initially is that the MaxProp is worn and slides past the stops it was set to. The plan is to check the prop for play more fully when it's out of the water. If there is any question, we'll pull it and see if it can be rehabbed.

Celestial's explanation for why this is happening after the new transmission, but not before is as good as any other. I'll post any new information that we get, but I'm still interested in other things to consider.
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Old 28-07-2012, 13:53   #6
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It's weird... The old tranmission wasn't slipping all the time. It's mechanical, so once the pinions are engaged, is impossible that it slips. Old Hurth transmissions have different ratio than what the tag says. For example, the tag on a friend's HBW100 that says 2:1, is n fact a 1.89:1. He found out when bought a new prop and it didn't worked, so, the manufacturer told him to check it manually and that's how we found out about the difference. That will make for 1" down on pitch adjustment... Did you manually checked the old tranny ratio?? Your case might be the opposite to my friend's problem (your ratio was higher the new one)
In my opinion, there might be a slightly bigger ratio difference, so your prop pitch needs some adjustment.
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