BMC 1.5 - Idling speed

[Bloomsberry]

A couple of questions

1) What is the correct idling speed for a BMC 1.5 ? The manual has this as between 500 - 600 rpm but wasn't sure if there is a different range for when it's installed in a narrowboat.

2) How can you tell the RPM of an engine from the exhaust sound.

The reason I ask is because my BMC 1.5 cut out when put into neutral yesterday before I set off, first time it has done this in 5 years. It started ok after it ran a while but I have always thought that the idling speed was too low anyway.

 

From the exhaust sound when idling , it does 150 'phuts' a minute, does this equate to 150 rpm ? Would you recommend increasing this ?

[Tony Brooks]

Its a four cylinder four stroke engine so you will get two power strokes per revolution. If you are sure you are accurate the 150 phuts a minute equates to 300 rpm and I doubt a BMC would idle at 300 rpm, let alone when in gear - even with the bulk ring added to the flywheel.

 

Best Google the forum for how to use a cheap cycle computer as a rev counter or just set it by ear so it sounds OK but does not hurtle along on idle.

Edited May 13, 2016 by Tony Brooks

[Bloomsberry]

Its a four cylinder four stroke engine so you will get two power strokes per revolution. If you are sure you are accurate the 150 phuts a minute equates to 300 rpm and I doubt a BMC would idle at 300 rpm, let alone when in gear - even with the bulk ring added to the flywheel.

 

Best Google the forum for how to use a cheap cycle computer as a rev counter or just set it by ear so it sounds OK but does not hurtle along on idle.

 

Thanks for the reply , Tony.

 

I've got some old video footage taken 4 years ago and the idle speed then with the engine warmed up was 285 phuts/min which will equates to 570 rpm which sounds ok. Nothing has been altered to the engine since so don't think anything would have changed to effect this.

 

I assume the engine idles slowly when cold so maybe I need to let the engine run a bit longer and warm up a bit before setting off.

[Tony Brooks]

Most 1.5s have a hydraulic speed governor and it uses very light springs so is very sensitive to dirt etc. In fact even globs of water can upset it. More more modern pump with a small bleed screw in the centre of a large hexagon between the throttle and stop lever and the whole thing is an anti-stall device These can and are upset by boater when they try to bleed the pump. However I do not think either is particularly likely from the stated symptoms.

 

I suspect that the throttle lever on the pump used to be held slightly open by the Morse cable rather than bearing on the idle speed stop screw so now the cable has wear in it and a bit of slack the throttle is closing more. It is important that when you adjust the idle speed you make sure the cable is not trying to force the lever tight against the stop. This can cause leaks from the throttle spindle. Anyway I can see no reason not to adjust the idle speed.

 

ON the older pumps the adjuster looks like a carburettor mixture screw and spring set at an angle between the throttle and stop lever while the more modern one its one of the the vertical hexagon setscrews beside the throttle lever. Its the one that does NOT have a small aluminium cylinder wired over it. If neither have the cylinder inspect the ends to see which one stops the throttle lever moving as you reduce speed.