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Gearbox rattle


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I have a PRM 150 driving an Izuzu 38. As I engage the forward gear there's quite a rattle. With an increase in revs and the gear engaged the rattle stops. There's no rattle when I engage reverse.  I was convinced it was my drive plate but when I asked my marina engineers to look at it they assured me it wasn't the drive plate, although they didn't say exactly what the rattle would be. It doesn't affect the performance of the engine and even at tick over there's no rattle which I know is characteristic of a drive plate problem.  Any thoughts or suggestions would be much appreciated. 

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25 minutes ago, Paolo Reale said:

I have a PRM 150 driving an Izuzu 38. As I engage the forward gear there's quite a rattle. With an increase in revs and the gear engaged the rattle stops. There's no rattle when I engage reverse.  I was convinced it was my drive plate but when I asked my marina engineers to look at it they assured me it wasn't the drive plate, although they didn't say exactly what the rattle would be. It doesn't affect the performance of the engine and even at tick over there's no rattle which I know is characteristic of a drive plate problem.  Any thoughts or suggestions would be much appreciated. 

Although the hydraulic gear system tends to servo itself on, I would check that you are getting the correct travel on the gear lever (the one on the side of the gearbox). Maybe disconnect the cable at the gearbox end and see what happens when you manually move it into gear. Presumably you have already checked the gearbox oil level?

There is a bit more load from the prop in fwds vs reverse, so as Tony says could it just be that the idle revs are too slow? Not certain of the minimum idle figure for an Isuzu but it’s probably around 850rpm.

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How long is it since you changed the Drive Plate, It sounds like yours may be starting to disintegrate. It is no big deal, they go on like that for years, and changing it takes less than a couple of hours.

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Thanks for all your thoughts, I'll certainly check the throttle cable as there does seem to be quite a distance between idle and engaging the gear. Just changed the gear box oil. Just to check - this should be right at the maximum mark of the gear stick - unlike the engine oil level which is half way between max and minimum?

Quick addition question  - how to adjust the level of idle revs?

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Locate the lever on the engine that is moved by the throttle cable. You should find that lever is stopped from moving in either direction by two "bolts" with lock nuts. Close the  throttle and the bolt it stops against needs screwing in slightly to speed the idle up. You may then find that you need to adjust the cable a little.

 

Most single lever controls are designed to allow the gear to be engaged before the throttle start to open.

 

 

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As Tracy D'Arth says, check all the drive line bolts and also worth looking at engine mounts. Also try grasping the top of the engine to steady it just in case there is some sort of harmonic vibration that has developed with some other part, air filter lid, exhaust joint or something. Unlikely but you never know.

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Often a rubber engine mounting softens allowing the vertical pin to drop through until it touches the engine bearer, the noise it makes is very rattly and often only when the engine is pulled back in forward gear.  Does your drive shaft have a thrust bearing after the coupling bolted to the hull?

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