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Old 25 June 2015, 16:52   #1
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Impeller Change Mystery

If you, after some months of poor water flow in a 90hp engine, (without overheating) rectified that by changing impeller, gaskets and cover plate found that instead of revving to 4600 rpm you were only making 4000 rpm, what would you think had happened?

The mystery currently occupying me........
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Old 25 June 2015, 16:59   #2
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4000rpm under load? i.e in the water in use?

you shouldn't be revving the nuts out it on land hence the question.

first thing i would do is check compression, 4600rpm is too low for most outboards, should be circa 5800-6k for most motors. you don't mention the make to confirm though?

if you have only ever got 4600rpm then you may have ruined the engine by running it while over propped for long periods.

can you trust the rev counter? have you lost any speed?

need more info really to point you in a direction, but i would start with a compression check, spark then fuel etc.

poor water flow and over propped is probably the worst thing you could do to a motor i'm afraid.

cheers
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Old 25 June 2015, 17:07   #3
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Quote:
Originally Posted by dubrus View Post
4000rpm under load? i.e in the water in use?

you shouldn't be revving the nuts out it on land hence the question.

first thing i would do is check compression, 4600rpm is too low for most outboards, should be circa 5800-6k for most motors.

if you have only ever got 4600rpm then you may have ruined the engine by running it while over propped for long periods.

need more info really to point you in a direction, but i would start with a compression check, then spark, fuel etc.

cheers
Hi, thanks for taking the time to look. I may have been a bit deliberately vague too see if I was missing anything really simple!

The engine is an aged Honda BF90 (carbs) the manual says it should max out at 5000rpm (presume off load) When I bought it I think it was somewhat over propped and a change down moved me from 3800 max to 4600 which I reckoned was about right.

Ran like that for a couple of years but as I said, spring this year, serviced the raw water pump and dropped a load of revs.

Not bogging down just not making revs......

So have done compression test and serviced carbs, running smoother now but still lacking RPM. (when say done, I do mean have had done. I am not a spanner spinner)

Make any more sense with more info? I am at a a loss!
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Old 25 June 2015, 17:22   #4
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i'm surprised it is only 5000rpm....live and learn.

i was going on this- Honda BF75/BF90 Specifications - Honda.com.

what was the compression numbers and when did you last do it?

have you unscrewed the tank breather?

so many things it could be here.

cheers
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Old 25 June 2015, 17:24   #5
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I have your engine and its brill! Might not set the world alight but bullit proof. Two things. My honda tacho is not accurate and I have a separate much more accurate on instead. Secondly their top revs is 6000 not 5000. obviously in the water! I have a 18ft speedy that at 5k revs does 25knots. at 6k it does 35knts. all on gps so this should give you an idea. Nik
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Old 25 June 2015, 17:33   #6
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OK,

So, both absolutely correct, in 1999 when the engine was built it was indeed rated at 5-6K. I have just dug out the manual and refreshed my addled memory.

On that basis and without news on how what we did could be the cause I am going to withdraw for the evening (sleep) and discuss with locals tomorrow. Then maybe I can give you a solution or enough information to answer the questions I might ask!

Thanks so far!
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Old 25 June 2015, 17:41   #7
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if you do only one thing tomorrow get a compression test done

i assume it will be okay and can look at common issues like bad fuel, tank breather etc

also, confirm that you have lost speed would be good or it could be your rev counter is on the blink.

cheers
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Old 25 June 2015, 17:43   #8
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Originally Posted by dubrus View Post
if you do only one thing tomorrow get a compression test done

cheers
Done, no issues there...
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Old 25 June 2015, 17:47   #9
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what were the numbers?

also, have you lost speed?

cheers
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Old 26 June 2015, 02:28   #10
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if it's a carb model, then that's what I would be looking at. Drain fuel at carb bowls first to see if there's any discolouration or sediment. If there is, then full strip down and re-adjust according to the service manual. Could be the timing is out too.
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