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Peter Thornton

BMC 1.8 idling speed

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We have an elderly BMC 1.8. One of our owners likes to have a slow idle, however our Marina engineers tell us that 900rpm is correct for these engines and warn that tappet damage can result from setting them slower..

i realise that this is probably a big can of worms, but any thoughts, based upon experience, will be welcomed!

Edited by Peter Thornton

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RLWP    1,038

We had our 1.8 idling too slowly. It was terrible

Richard

 

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alan_fincher    1,850

The original manual states 650 to 700 RPM for the 1800 engine.  Ours was in that range and sounded fine.

(If, on the other hand, it were the 1500 engine, the same manual says 550 to 600 RPM).

I would have thought setting idle speed as high as 900 RPM would potenially have meant too much progress when in gear, and you only want to be creeping along, (although obviously that depends on boat, gearbox, reduction and prop fitted).

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Mikexx    4
3 hours ago, Peter Thornton said:

We have an elderly BMC 1.8. One of our owners likes to have a slow idle, however our Marina engineers tell us that 900rpm is correct for these engines and warn that tappet damage can result from setting them slower..

i realise that this is probably a big can of worms, but any thoughts, based upon experience, will be welcomed!

900 rpm sounds high. The only reason I can think of is vibration being transmitted through the boat and that 900 rpm is a sweet spot where there are no 'rattles'! As another poster intimated, it will give a 'fast' crawling speed.

I have never heard of tappet damage at low revs. If there is low oil pressure then I guess it is possible where oil splash relies on oil failing to be being fed to the rocker shaft.

On a different engine I have known camshaft bearings to be so badly worn, no oil got to the rocker shaft, but in that case it was the rockers that suffered by wearing into the shaft. I don't know off-hand how the BMC rocker shaft is fed with oil?

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Tony Brooks    619

I agree with the tappet wear thing being very unlikely, I too have never heard of it.

Part of marinising these engines was to fit a bulk ring to the flywheel or a new heavier flywheel. This was to compensate for the missing clutch assembly and also to give more inertia so they were better able to idle in gear.

If it is a "Fred in a shed" marinisation and it still has an automotive flywheel then it probably will demand a high idle speed.

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Hi Tony, it's a Calcutt supplied unit. Actually I may have misdescribed the conversation with our engineer, he said that too slow an idle results in the tappets slapping down on the cams and wear in the cam components I.e. Cam and tappets I guess. I think we would like it at around 700 or so.

Edited by Peter Thornton

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Tony Brooks    619
1 minute ago, Peter Thornton said:

Hi Tony, it's a Calcutt supplied unit. Actually I may have misdescribed the conversation with our engineer, he said that too slow an idle results in the tappets slapping down on the cams and wear in the cam components I.e. Cam and tappets I guess. I think we would like it at around 700 or so.

So it will have a heavy flywheel then. Why the blazes should a marinised unit "slap down on the cams" more than the automotive one? It this were me I would be looking for a new engineer. Tempted to observe that maybe he cocked up the idle damper and does not know how to get the idle any lower but then I am rather cynical in such matters.

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cuthound Contributor    371

We had a shareboat with a Calcutt supplied, Turkish built BMC  1.8. Tickover speed was around 750-800 rpm according to the tacho. 

Camshaft wear is a fact of life with BMC engines, and is usually the first sign of an impending rebuild or a replacement engine. 

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X Alan W    61

Dependent on the type fitted I would guess that idling at 900RPM could have averse effects on the gearbox over a time I  guess if it's mechanical  it would engage with a "clunk"

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