Engine won't rev

[Graham_Wright]

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.

 

[Bobc]

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.

 

[lw395]

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.

 

[Graham_Wright]

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 +.

 

[Graham_Wright]

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

 

[lw395]

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.

 

[Graham_Wright]

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/faq/131000-bmc-1.5-manual.pdf.
section 51D

 

[bristoljim]

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

 

[Mistroma]

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.

 

[Euphonyx]

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

 

[Graham_Wright]

A bit more investigation.

I detached the prop shaft from the gearbox at the drive flange and, with forward gear engaged, revved the engine. It reached normal 2000 + as it does out of gear. At the same time. with better access and leverage than before, I turned the prop shaft. It was stiff and could not be described as free. There seemed to be an intermittent, once per rev, increase in the resistance. This afternoon I will dive and have a look, if I can find space amongst the weeds! I have a Fat Max saw (by Stanley) that is like a miniature sabre. Bought it on impulse but never found a use for it. This could be an ideal baptism.

However, pondering through the night (an unfortunate habit), it seems that the engine is not struggling to accelerate. It just seems happy with the speed it is at. That suggests, maybe, as someone has pointed out, it may be a governor problem.

Saturday morning, an expert friend has promised to have a look.

 

[vyv_cox]

It should be very easy to turn your prop shaft. The problem could be external as you suspect, prop fouled by something for example, or it just might be the Aquadrive bearings. I assume whatever stern gland you have has not been touched? It is certainly possible to over-tighten a packed gland.

 

[Bobc]

Sounds to me like you need to get the propshaft running freely and try it again, before doing anything with the engine.

 

[Graham_Wright]

It should be very easy to turn your prop shaft. The problem could be external as you suspect, prop fouled by something for example, or it just might be the Aquadrive bearings. I assume whatever stern gland you have has not been touched? It is certainly possible to over-tighten a packed gland.

I had a look under the boat yesterday afternoon (forgot the wetsuit but wasn't really cold!).

There were just a few strands of weed and the prop turned easily. The lack of leverage in the boat probably was the cause of the previous apparent stiffness.

The gland is a Deep Sea Seals one, i.e. ceramic disc to bronze, no tightening required.

Suspicion (don't really understand enough for more than that) now centres on the injector pump. This was recently professionally rebuilt. The engine doesn't emit clouds of smoke or struggle. It just seems happy with the revs it achieves. I don't understand how the governor works but that could be the cause (as someone has suggested). I have a spare pump and may swap them over. The access is abysmal.

Tomorrow expert friend has promised to attend and I will report back.

 

[Aardee]

Do you know if it used to rev OK in gear? - I'm wondering if it has a way oversized prop

 

[Graham_Wright]

Do you know if it used to rev OK in gear? - I'm wondering if it has a way oversized prop

The prop and the engine are as dictated by the hull designer. If the prop was too big, I would have thought the engine would have laboured and chucked out smoke. It doesn't. It is quite happy to "chug".

 

[black mercury]

Before condemning the injection pump and removing it, get yourself a small container of clean diesel and pipe it directly to the fuel intake of the injection pump, a priming bulb in the fuel line would make things easier. Then try the engine. Yes fuel will return to the main tank and not your temporary container but it will give you enough time to determine if the pump is at fault. You may have symptoms of fuel starvation, using the container will take the fuel supply and filters out of the equation. Also some pumps may have a small wire gauze in the intake pipe at the injection pump, take pipe off and have a look, you probably don't but just to make sure. If you can't get a priming bulb just pipe it without one, the dpa has its own supply pump.

 

[RIBW]

........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.

+1 for the air filter.
I can't see a reply that says you have tried taking off the air filter - I don't know your engine but a long-clogged air filter gave me similar symptoms once.
Cheers
Bob

 

[Aquaboy]

You said the prop was free on the hard so I assume you have just been lifted in.
Did they have a strop in the wrong place and its bent/ moved the shaft.
I would think the engine is fine as it revs freely off load and you seem to think the shaft is a little stiff so that would be my area of thought. No mention of what happens in astern?

 

[Graham_Wright]

Do you know if it used to rev OK in gear? - I'm wondering if it has a way oversized prop

The engine and the prop were as specified by the designer. Straight after launch, it seemed ok but then I'm an old git with failing memory!