advice wanted ( BMC Thornycroft 1.5 + PRM g/box)

[galeus]

Been running all day on our little Colvic 26 (Northerner) today on a perfect day from Ramsgate to Tichmarsh , but in the last 10 miles I seemed to develop a fault which I guess is gearbox related, but don't know how serious it is. What do you guys think?.
Cruising along at about 2000rpm everything is fine until the engine races up to 23/2400 rpm for a split second before resuming where it was, This happened 3 times in 30 mins so I throttled back to 1800 rpm and all was OK until we arrived at the marina.
If it was a car you would have said the clutch was slipping. I don't think it was the engine surging, definitely the gearbox slipping.
It has a BMC 1500 diesel driving a PRM gearbox,

 

[QBhoy]

Worth checking the gear oil for water Perhaps. Only thing I’d ask was if when the rpms increased…did it feel like your speed did too ? If it did…it ain’t the gearbox…it’s a fuel delivery issue or fuel pump/throttle assembly issue.

 

[galeus]

Worth checking the gear oil for water Perhaps. Only thing I’d ask was if when the rpms increased…did it feel like your speed did too ? If it did…it ain’t the gearbox…it’s a fuel delivery issue or fuel pump/throttle assembly issue.

No the speed did not increase, the engine revs did for a split second so thats why I'm thinking in the drive.

 

[QBhoy]

No the speed did not increase, the engine revs did for a split second so thats why I'm thinking in the drive.

Think you’re likely right

 

[oldharry]

Dont assme its a slipping clutch just because it feels like that in a car!

Could be a slight air leak in the inlet manifold upsetting the mixture. Is it starting normally? If not (and they arent easy to start cold without some sort of manifod heater, specially in cold weather) If not there may be a slight air leak in the fuel system.

Can also be wear in the injector pump control rack. With a distributor type pump this means professional attention.

Check the gearbox control linkage. Wear in the linkage can prevent the gearchange lever on the box going fully home, causing the clutch to slip. If not, check the gearbox oil level. Check the oil, is there water in it? If so your gearbox is a fair way to being knackered. The PRM 160 had a fibre clutch. The others had bronze clutches. All require the correct level and quality of oil to function properly.

 

[SimonA]

I've got the same engine and have had that issue, it is caused by air in the fuel. I changed my primary and secondary fuel filters and the problem went away.

 

[scottie]

First assuming that it’s a standard setup there is no car type clutch the engine is in constant drive with the input side of the gearbox
So slippage we’re it to occur is more likely to be noticed on initial drive take up and can be internal to the box and if larger prm oil pump related or slippage between coupling and shaft or propeller and shaft again more likely on initial engagement both these possibilities are unlikely to rectify themselves
The fuel issue is worth checking

 

[galeus]

I've listened to all your advice and changed the fuel filter and cleaned the screen in the injection pump, which was absolutely rammed with filth, I can't believe how it had kept running. I had done both these jobs in July, so felt it unlikely to be these, but we have been caught out in some very rough seas this summer and really taken a pounding so i'm sure everything from the bottom of the tanks has been disturbed, On the move again tomorrow, but feeling confident that this is the problem. I'm filling up with diesel tomorrow and dosing up with Grotamar (noticed that there is a use by date on the grotamar, which is well passed ) and only about 10 hours running left before liftout. It will be interesting when I do these jobs at the start of next season to see what else has come through.