Lotus Elan

Metallic rattle

PostPost by: andyhay » Sun Jan 06, 2019 9:52 am

Hi all

I was out for a drive recently and noticed a very brief metallic rattle at certain times while driving. At first I thought it was completely random but eventually narrowed it down to the following situation:
Around 2200 rpm
Only when gently lifting off accelerator
Doesn’t occur when freewheeling
Doesn’t occur under acceleration.
Occurs in any gear
Happens very briefly unless I very consciously keep it in that rev range.

I would describe the sound as like a bike chain on a metal surface

My engine has done 6000 miles since being rebuilt and is driving beautifully. Could it be that the timing chain is a little loose?

Thanks for any assistance

Andy
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PostPost by: JonB » Sun Jan 06, 2019 10:14 am

Check the exhaust clamps. If any are near the chassis, move / adjust them to increase clearance.

I'm guessing here - you haven't said where the noise appears to be coming from.
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PostPost by: andyhay » Sun Jan 06, 2019 10:27 am

Could be exhaust clamps. I can’t pin point where the sound is coming from to be honest. Without going for another drive I would have said forward rather than back.

Thanks

Andy.
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PostPost by: 69S4 » Sun Jan 06, 2019 11:28 am

Cam chain needing a tweek would be my first guess.
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PostPost by: RichC » Sun Jan 06, 2019 1:19 pm

slacken off timing chain tensioner locknut and turn tensioner bolt in 1/4 turn and see how that goes 8)
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PostPost by: richardcox_lotus » Sun Jan 06, 2019 1:45 pm

Does it happen at those revs when stationary, or just on the move ?
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PostPost by: andyhay » Sun Jan 06, 2019 2:05 pm

I couldn’t replicate the noise when stationary.

Thanks for the suggestions

Andy
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PostPost by: tvacc » Sun Jan 06, 2019 6:23 pm

I vote for the timing chain tension as well.
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PostPost by: nmauduit » Sun Jan 06, 2019 8:06 pm

andyhay wrote:I couldn’t replicate the noise when stationary.

Thanks for the suggestions

Andy


I vote against the chain tensioner then
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PostPost by: tvacc » Sun Jan 06, 2019 10:19 pm

Yeah, I thought if it could not be produced while stat, I said to myself, how could it be the timing chain? But I remember back in 86 with my Sprint, I seem to recall it would only do it under load and then releasing. And that issue was a chain that had stretched. Maybe Rohan can chime in.
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PostPost by: nmauduit » Mon Jan 07, 2019 8:56 am

In any case chain tension would be easy enough to check, one just need to remove the camshaft cover - could be the occasion to check valve clearances if one is game and that has not been done for a while...
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PostPost by: andyhay » Mon Jan 07, 2019 9:02 am

One more observation - rattle only happens when hot and is usually very brief.

I have never worked on the engine before so might seek professional help - unless it is a straight forward job?

Thanks again

Andy.
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PostPost by: rgh0 » Mon Jan 07, 2019 9:16 am

The timing chain gets tighter when the engine is hot so its unlikely to be a loose chain causing a rattle that only occurs when hot. If it only occurs when moving on transition during lift off it suggests its related to the drive train and not engine - i.e. clutch, gear box prop shaft diff etc and related to something moving with drive train load and coming loose or hitting something else.

Put a passenger in the car and get them to move around and try to identify the location more precisely is my normal approach with this sort of intermittent and hard to locate noise. Always check for anything loose in the boot or glove box or which should otherwise be bolted down tight that may resonate at the specific revs and load

cheers
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Last edited by rgh0 on Mon Jan 07, 2019 7:59 pm, edited 1 time in total.
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PostPost by: jono » Mon Jan 07, 2019 4:51 pm

I had similar symptoms which occurred on the over run at a certain rpm range. I became convinced it was the exhaust rattling on a heat shield or something as that is exactly what it sounded like - seemed to get worse when hot or after a long run

Turns out it was the gearbox - not stripped yet but the tail shaft bush is very worn (had a bad oil leaks here as well) or perhaps its a worn input shaft bearing? New gearbox fitted and the noise has stopped
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PostPost by: denicholls2 » Mon Jan 07, 2019 10:11 pm

While my first stop would be the forcing effect that deceleration has in changing the attitude of engine and drivetrain in their mounts, affecting anything attached to them that is in close proximity to anything else, "only when hot" might also suggest a mixture that goes rich for a moment when the throttle plates snap shut. On Italian cars and motorcycles, this often causes either burbling or backfiring as some of the unburned mixture sneaks out into the header pipe and ignites there. Contained entirely in the exhaust system as in minor cases, it might sound like a rattle instead.
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