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  1. #1
    Join Date
    Dec 2002
    Location
    Gloucestershire
    Posts
    5,384

    Default Engine won't rev

    Finally getting to know the quirks of my BMC 1.5.

    However, maximum speed through the water is a staggering 2.43 knots.

    Engine starts easily and ticks over around 700 rpm. When the gear is engaged, revs drop to 500 and will not increase. Speed through the water is tranquil!

    First theory is weed round the prop. I deployed the underwater camera and SWMBO had a brief glance at the prop before the camera leaked. (Dismantled with great difficulty and dried out the interior and then gave it a dose of contact cleaner. Surprisingly it recovered).

    I am still on the Sharpness Canal and the weed, stink and the presence of the odd floating (animal) corpse are not enticing me in for another dive.

    Lifting the sole, I have access to the prop shaft. With the small leverage available, I cannot turn it.

    The possibilities seem to be that the prop is fouled, the cutlass bearing has swollen or the hull shape has changed after first launch.

    The shaft is connected to the gearbox via an Aquadrive so any hull movement should be taken care of.

    The prop was very free when on the hard so I can't imagine it has changed too much.

    The remaining (and hopefully easily fixed) possibility is a fouled prop.

    But why 500 rpm and no more? Could it be to do with the power/revs relationship in the engine? I.e. at 500 rpm there is insufficient power to speed up to a greater torque figure.

    I will move (tranquilly!) to a clearer part of the canal at the weekend and don the dive kit.

    P.s. The engine normally starts pretty easily and I think my previous problems were down to insufficient battery ump. But the other day, it would not start. The reason was that the stop valve had not closed completely. A useful tip for others perhaps.
    I'd rather be naked
    www.mastaclimba.com

  2. #2
    Join Date
    Jan 2011
    Posts
    5,753

    Default Re: Engine won't rev

    Does it rev in neutral? If not, then nothing to do with the prop.

    Have you checked things like the air filter and fuel filter. Usually all a diesel needs to run is fuel and air, and denying the engine of either one will kill it.

  3. #3
    Join Date
    May 2007
    Posts
    38,249

    Default Re: Engine won't rev

    Quote Originally Posted by Graham_Wright View Post
    Finally getting to know the quirks of my BMC 1.5.

    However, maximum speed through the water is a staggering 2.43 knots.

    Engine starts easily and ticks over around 700 rpm. When the gear is engaged, revs drop to 500 and will not increase. Speed through the water is tranquil!....
    Does it not increase in speed at all as the boat gathers pace from 0 to 2 knots?
    No black smoke or signs of unburned fuel in the exhuast?
    I suspect the governor/pump control is stuck or broken.

  4. #4
    Join Date
    Dec 2002
    Location
    Gloucestershire
    Posts
    5,384

    Default Re: Engine won't rev

    Quote Originally Posted by Bobc View Post
    Does it rev in neutral? If not, then nothing to do with the prop.

    Have you checked things like the air filter and fuel filter. Usually all a diesel needs to run is fuel and air, and denying the engine of either one will kill it.
    Yes - freely up to 2000 +.
    I'd rather be naked
    www.mastaclimba.com

  5. #5
    Join Date
    Dec 2002
    Location
    Gloucestershire
    Posts
    5,384

    Default Re: Engine won't rev

    Quote Originally Posted by lw395 View Post
    Does it not increase in speed at all as the boat gathers pace from 0 to 2 knots?
    No black smoke or signs of unburned fuel in the exhuast?
    I suspect the governor/pump control is stuck or broken.
    "Quote;- Lifting the sole, I have access to the prop shaft. With the small leverage available, I cannot turn it.

    Please explain the function of the governor. Governors on other engines, seem to govern the rotation speed according to some mechanical feedback
    I'd rather be naked
    www.mastaclimba.com

  6. #6
    Join Date
    May 2007
    Posts
    38,249

    Default Re: Engine won't rev

    The governor adjusts the fuelling to obtain the rpm set by the 'throttle' lever.
    If it is not capable of setting the fuel above a low level, it will only allow the engine to rev at zero load.

    I'm not familiar with the BMC motor.
    If it was a Yanmar, with the engine stopped, setting full throttle would move the fuel rack to max fuelling. When the engine reaches the desired RPM, the centrifugal bit of the governor backs off the rack so fuelling is reduced to the minimum needed for the set RPM at whatever the load is.
    As you said, feedback.
    But, it's a complex system of weights, springs, damping, pivots and levers.
    Easily made worse by amateurs!
    RTFM. And then read it again before touching anything!

    Your inability to turn the prop shaft could be the gearbox rather than the prop?
    If the prop were stiff, I'd expect the engine to be using lots of fuel fighting it, balck smoke and (diesel) oil in the exhaust.

  7. #7
    Join Date
    Dec 2002
    Location
    Gloucestershire
    Posts
    5,384

    Default Re: Engine won't rev

    Quote Originally Posted by lw395 View Post

    Your inability to turn the prop shaft could be the gearbox rather than the prop?
    If the prop were stiff, I'd expect the engine to be using lots of fuel fighting it, balck smoke and (diesel) oil in the exhaust.
    Thanks for the explanation. For reasons I can't justify, (something I read somewhere) this governor "doesn't do much". I had the injector pump serviced professionally so and haven't fiddled since.

    I hadn't considered the gearbox. Oh er!

    From the manual;-








    I can't see anything that looks like a conventional governor.

    Ah the links didn't work. ASAP used to have an on line version which I have a copy of but which will not allow reproduction.

    https://www.asap-supplies.com/media/...1.5-manual.pdf.
    section 51D
    Last edited by Graham_Wright; 06-08-19 at 15:24.
    I'd rather be naked
    www.mastaclimba.com

  8. #8
    Join Date
    Dec 2008
    Posts
    147

    Default Re: Engine won't rev

    The standard DPA pump on a BMC 1.5 has a hydraulic governor.I believe the lift pump fuel pressure is used in some way.
    Jim

  9. #9
    Join Date
    Feb 2009
    Location
    Greece (Aegean)
    Posts
    2,760

    Default Re: Engine won't rev

    I remember my old BUKK DV20 exhibiting problems with revs when in gear. It was many years ago but as far as I remember the symptoms were similar. The engine would start easily and could be revved to max. in neutral. The throttle ceased to have much effect as soon as gear was selected and only a very slow speed was possible.

    The spiral wound rubber exhaust hose had de-laminated slightly inside. It seems that the volume of exhaust gas in neutral was too low to cause problems in neutral. It did seem to be a bit weird that revving in neutral was OK but I assume the increased gas flow was still too low. I assume that the loose flap of rubber was blocking the exhaust when the engine was under load and throttle opened.

    Luckily, the hose was only a few feet long and I was able to see the internal damage. I could poke the flap open with a screwdriver but it always flipped back into place as soon as it was released. I swapped the hose around so that the flap was now being blown open and the engine ran perfectly. The quick fix allowed me to get back home under engine.

    Pretty certain that others have reported similar failures in other threads over the years. It is one possible answer in addition to other things already mentioned. Only worth checking if you do eventually discover that the prop is not actually seized as that is a very obvious reason for low revs.
    Last edited by Mistroma; 07-08-19 at 19:55.

  10. #10
    Join Date
    May 2010
    Location
    Dublin
    Posts
    2,279

    Default Re: Engine won't rev

    had something similar earlier in the summer but I could rev. It was off a lee shore and against a 3.5 knot tide all we could manage was 1kt sog. When we got into harbour I had an arse like two walnuts in a sock from clenching and discovered that some polyprope was stopping the prop from unfeathering properly

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