rattly old BMC 1.8

[srod]

My old BMC rattles quite a bit in various ranges of rpm, seems noisier than other BMCs I've heard too. You can see the vibes down the side of the boat when the engine's running and we're tied up.

 

What can I do to calm things down a little?

 

Engine mounts haven't worked loose as far as I can tell, does the rubber wear out - would replacing them help?

 

There's also no flex in my exhaust system - the silencer is bracketed to the hull, the pipe from the engine to the silencer is rigid, would adding a flexy section here help much?

 

Do silencers become less effective with age and sooting up?

 

Cheers for any advice, great forum!

[Sir Nibble]

There's also no flex in my exhaust system - the silencer is bracketed to the hull, the pipe from the engine to the silencer is rigid, would adding a flexy section here help much?

hat could be it. Certainly an engine on flexible mounts rigidly bolted to a silencer clamped to the hull sounds like a recipe for a stress fracture.

[RobinJ]

My old BMC rattles quite a bit in various ranges of rpm, seems noisier than other BMCs I've heard too. You can see the vibes down the side of the boat when the engine's running and we're tied up.

 

What can I do to calm things down a little?

Engine mounts haven't worked loose as far as I can tell, does the rubber wear out - would replacing them help?

There's also no flex in my exhaust system - the silencer is bracketed to the hull, the pipe from the engine to the silencer is rigid, would adding a flexy section here help much?

 

Do silencers become less effective with age and sooting up?

If you clasp the engine firmly (when its not running!) can you feel any movement side to side which may indicate the engine mountings are going soft or perished?

There should be a flexible section from the engine to the silencer which will absorb some vibration (this may not be obvious if its wrapped).

Most of the silencers are absorbsion type and therefore should not provide any excessive back pressure when old and clogged (there is usually a date stamp on them somewhere and if older than 10 years it may need to be changed).

A rattly old engine can vibrate quite badly at different revs and this is caused by a number of things (wear and tear). Have the valve clearances been checked? How old is it, is the timing chain noisy? When were the injectors last serviced.

People say do not flush an old engine, but over time there can be build up of oil deposits on things like cam followers, valves etc., all of this can minutly affect the timing and cause irratic vibration.

Most boats have a panel that is susceptible to resonance (mines about six feet in front of the engine), but this will change with engine speed. You need to determine what vibration you can stop (at the engine or gearbox) and what you cannot and need to quosh at the point is appears (fitting braces, anti vibration membranes etc.). By far the most important things is whether there is too much noise when running at the steering position. Insulated deck boards, or anti vibration pads may help.

[Tony Brooks]

My old BMC rattles quite a bit in various ranges of rpm, seems noisier than other BMCs I've heard too. You can see the vibes down the side of the boat when the engine's running and we're tied up.

 

What can I do to calm things down a little?

 

Engine mounts haven't worked loose as far as I can tell, does the rubber wear out - would replacing them help?

 

There's also no flex in my exhaust system - the silencer is bracketed to the hull, the pipe from the engine to the silencer is rigid, would adding a flexy section here help much?

 

Do silencers become less effective with age and sooting up?

 

Cheers for any advice, great forum!

 

"Official" marinisations of the BMC (Newage/Tempest) used absolutely horrible rear flexible mounts. They are a sort of "top hat" metal tube fitted up through large holes in the lugs cast as part of the flywheel housing. If yours are like this you will find the rubber inside the top hat tube collapses so the three setscrew heads that fix the top hats into the lug drop onto the beds. This causes all sorts of vibrations

 

Once you have sorted the rigid exhaust pipe and if the thing still vibrates I would be wanting to check the timing chain for slack (The rubber tensioner slipper wears out) and also the the injector pump skew gear drive. If the lubricator and strainer (under the back of the exhaust manifold) become blocked the gears wear. Either of the above would allow the timing to jump all over the place and that may well cause vibrations.

 

Before you do too much check the valve clearances and shaft alignment.

[srod]

thanks for the advice.

 

i shall check the valve clearances, check the engine for movement when it's not runing (great tip, so simple!) and maybe re-do the exhaust section.

 

how tricky a job is checking the timing chain for slack, and tightening it if necessary?

 

 

 

Anyone feel like explaining or pointing me in the direction of a simple step-by-step guide to checking the valve clearances on a BMC?

I can see what to do in theory (and by looking in the BMC engine handbook), - but how does one know when the vavle is fully open/closed etc. could I check the timing while I was at it? Just a basic 'how not to make it worse than it was before' guide is fine.

 

My engine mechanical experience goes as far as replacing filters, fuel pumps and alernators, so I know which end of a screwdriver to hold. Never taken the rocker cover off before though (except on Tony's course, but that was 7 years ago and I have never dared do it on my own engine and have since forgotten)

Edited September 18, 2009 by srod

[Tony Brooks]

thanks for the advice.

 

i shall check the valve clearances, check the engine for movement when it's not runing (great tip, so simple!) and maybe re-do the exhaust section.

 

how tricky a job is checking the timing chain for slack, and tightening it if necessary?

 

 

 

Anyone feel like explaining or pointing me in the direction of a simple step-by-step guide to checking the valve clearances on a BMC?

I can see what to do in theory (and by looking in the BMC engine handbook), - but how does one know when the vavle is fully open/closed etc. could I check the timing while I was at it? Just a basic 'how not to make it worse than it was before' guide is fine.

 

My engine mechanical experience goes as far as replacing filters, fuel pumps and alernators, so I know which end of a screwdriver to hold. Never taken the rocker cover off before though (except on Tony's course, but that was 7 years ago and I have never dared do it on my own engine and have since forgotten)

 

 

Get a Hayns manual for any four cylinder car and follow the instructions.

 

3 methods, The one I cover on the course and is in the course notes so I will leave that.

 

Second method.

 

Count the valves from belt end of engine 1 to 8.

 

Turn crankshaft nut until one (any) valve is fully down.

 

Adjust the valve who's number when added to the valve that is down number makes 9 - that is valve 2 down adjust valve 7. Valve 5 down adjust 4.

 

Method 3. Divide the engine in half with 4 valves in each half.

 

Turn crankshaft until BOTH valves on one piston just starts to move TOGETHER (one will be about to close and one just about opens).

 

Adjust the mirror image pair so if piston 1 has both valves rocking adjust both valves on piston 4. If number 3 are rocking adjust number 2.

 

The cam profile will have make one of the last two methods more appropriate for a particular engine but either should do on a BMC>

 

BMCs are adjusted COLD

[RobinJ]

..

3 methods, The one I cover on the course and is in the course notes so I will leave that.

..

From Calcutt's 'Handbook':-

Unscrew the rocker cover retaining nuts and lift off the cover.

Check the clearance at the postion illustrated and in the order as follows;

Check No. 1 valve with No. 8 fully open. Check No. 8 valve with No. 1 fully open.

Check No. 3 valve with No. 6 fully open. Check No. 6 valve with No. 3 fully open.

Check No. 5 valve with No. 4 fully open. Check No. 4 valve with No. 5 fully open.

Check No. 2 valve with No. 7 fully open. Check No. 7 valve with No. 2 fully open.

Adjust if necessary by slackening the locknut and turning the adjusting screw until the clearance is correct. Hold the screw against rotation and tighten the locknut. Refit the rocker cover, checking that the cork gasket is servicable.

 

For a 1.5 the clearance is 0.015" (0.38mm), 1.8 (block without ribs) 0.017" (0.43mm), (block with ribs) 0.014" (0.36mm), all adjusted cold.

[nb Innisfree]

After all these things have been remedied Aquadrive or Python Drive makes a big improvement especially used with good softer mounts, bit pricey though.