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Old 19 May 2022, 18:56   #1
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Suzuki DF50 HP problem

Hi all, took my RIB out on the Solent yesterday for the first time. Bought earlier this year boat and Suzuki DF50 HP about twenty years old and recently serviced.



Had a blast for about 20 minutes at 6000rpm and when I slowed down outside Cowes to about 3000rpm I noticed the outboard was vibrating like it was misfiring and some sort of alarm was sounding from the throttle mounting. I stopped, left the engine on tick over and checked for anything obvious but nothing looked out of place and eventually the alarm stopped. So on the return journey I took it slower apart from the last half mile and as I throttled back to about 3000rpm it all started again. Again I put it in tick over and waited for the alarm to stop but this time it kept going for another 5 minutes as I cruised along at 2000rpm.



So just wondering what this situation could be? thanks in advance for any input.
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Old 19 May 2022, 20:37   #2
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Sounds like overheat warning?
Restricted water intakes, Worn impeller, leaking water tube, stuck / restricted thermostat, restricted cooling passages??
All worth looking at....
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Old 19 May 2022, 20:46   #3
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Hi Jon


Thanks for your reply.


The engine has just had annual service and replacement thermostat fitted.


But I'll be asking the question of the guy who did the work tomorrow when I see him.


cheers Brian
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Old 19 May 2022, 21:13   #4
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Could well be overheating.

May be lack of water flow or the engine is running lean. First two things you can do yourself is check the telltale for a good flow but also whip the plugs out and have a look at them.
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Old 19 May 2022, 21:19   #5
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The telltail was looking good when the engine was running yesterday so I'll check the plugs tomorrow....thanks Tm
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Old 19 May 2022, 21:39   #6
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If it was first time out are you sure the leg was deep enough when you were on the plane to pick up water? Sounds like your engine may have vern a little high
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Old 19 May 2022, 22:17   #7
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Thanks for your input Neil.... yes I think the leg was probably deep enough as the telltail was strong and a near vertical stream of water..... However I must admit when testing its temperature it appeared to be vaguely luke warm
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Old 20 May 2022, 06:26   #8
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Would it seem right that the alarm, as far as you can tell, is coming on when the engine is powering down from wot? In other words, you don't think it is coming on while at wot? And it then goes off after a few minutes of low rpm?

It has to first be worth checking the manual to establish that it is the overheating alarm that we've assumed. I guess there could be other alarm/warning features.

Assuming it is overheating, I'm not wholly sold on it being a lack of water flow. There is clearly flow, it's warm which implies it's being released by the thermostat to flow round the jacket and if it was being restricted I think I would expect the alarm to be going off on WOT as it wouldn't be coping with the peak heat during peak flow.

But I think the spark plugs may give the best clue. If it's not a water flow issue then the next port of call is a fuel flow issue. Probably the most common cause of overheating is a fuel mix that's too lean.

It could be something as simple as the carb idle having been set on 100:1 mix and you're running 50:1. Or the idle of one carb is just too lean.

If you go out again I think you need to be careful re the alarm. I'm not sure keeping the engine running is the best course of action. Two schools of thought there, turn it off to start letting the heat dissipate but have that spike due to water flow stopping or run on idle, out of gear to keep water flow to help with cooling. Either way, if it were me I'd be putting it out of gear, removing the cowl and lowering the aux in.

But I'd just check the plugs. If they give no clues and both look nice and brown and dry then it's back to the drawing board a little and my next step would be to remove the thermostat and test it in boiling water just to check it's opening fully although I wouldn't be expecting to find anything. Next would probably be to tune the carb on the idles myself and then dial in a little bit of rich, flush the water ways with cleaning chemicals and stick a thermometer gun on the cylinders to see if both are same temp.

After that it's time for a cup of tea and having a ponder of deeper issues but starting simple is always best as it often is something simple.
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Old 20 May 2022, 06:47   #9
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Hi Tm
Sorry there has been a bit of confusion regarding the engine, it is a four stroke three cylinder outboard.

Yes the alarm appears only to sound when powering down from WOT and then goes after a few minutes but was also vibrating badly as well.
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Old 20 May 2022, 07:06   #10
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The vibrating can be metal parts seizing but in this case it's probably just one cylinder or more with an issue as the problem goes away.

Is it EFI or carb? If a 4s EFI then for me it's heading towards voodoo land as I'm most comfortable with engines that can be fixed with a hammer not a laptop but I'd check the plugs for clues and also do a compression test to rule out those mechanical basics and then contemplate plugging it in and running diagnostics.

If it's EFI then along with checking all the fuel lines are clear, checking filters etc then I'd also blow some carb cleaner through the injectors as it could be as simple as a bit of muck in one of those.
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Old 20 May 2022, 08:51   #11
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If you intend keeping the engine for any length of time, it’s well worth investing £60 or so for a diagnostic kit off eBay. Removes all the guessing & head scratching.
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Old 23 May 2022, 19:40   #12
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Thanks to everyone for your input regarding my engine problem.


My mechanic is pretty sure it isn't an overheating problem due to recent major service and nature of the alarm, which was a beep every 4 seconds rather than continuous. However there are some aftermarket cavitation plates fitted which I will remove to see if this resolves the problem.


I'm taking the boat out again on the bank holiday weekend and if there any problems I'll go down the diagnostics route......
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Old 23 May 2022, 20:57   #13
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I don't know that model of engine but even at 20yrs old, assuming it's fuel injected I'd assume it has some ability to plug in diagnostics - which your mechanic should have if he's any good.
Plug in and see if it gives you any clues - an alarm should be setting a code.....?
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