Lotus Elan

exhaust popping on changedown

PostPost by: MACCA.GLM » Thu Jun 27, 2024 7:03 pm

Hi all
I think this topic has been covered before.Although ive owned my sprint for 2 plus years I cannot stop the exhaust popping on changedown ive tried different fuels checked carb tuning all cylinders and plugs are sandy colour.Timing all good. Electronic ignition.There's no lack of power. could it have something to do with the float height. I am running 40 dellortos.Thoughts would be appreciated.
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PostPost by: Graham B » Thu Jun 27, 2024 9:14 pm

Hi,
Check for exhaust leaks.
My tubular manifold was leaking at the gaskets to the head. One of the 2 bolt flanges was not flat and was the cause of the leak. I had to carefully dress the flange flat to cure it. Use new gaskets and don't tighten too much.
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PostPost by: 2cams70 » Fri Jun 28, 2024 3:11 am

Some people of a certain age and demographic would give their right kidney to have their engine popping on downshifts.

Although you say the carbs. are ok you may have a lean mixture.
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PostPost by: h20hamelan » Fri Jun 28, 2024 5:13 am

How does an engine under power, load the timing chain & valve train, compared to under deceleration when there is not as much load on the pistons?
I would guess there is not slack on the chain, but there is less load. So what happens to the valves, do they open and close a bit slower than while under power.

How does one tell without a wideband.
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PostPost by: Craven » Fri Jun 28, 2024 11:10 am

+1
Graham B wrote:Hi,
Check for exhaust leaks.
My tubular manifold was leaking at the gaskets to the head. One of the 2 bolt flanges was not flat and was the cause of the leak. I had to carefully dress the flange flat to cure it. Use new gaskets and don't tighten too much.
Graham
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PostPost by: Andy8421 » Fri Jun 28, 2024 11:59 am

h20hamelan wrote:How does an engine under power, load the timing chain & valve train, compared to under deceleration when there is not as much load on the pistons?
I would guess there is not slack on the chain, but there is less load. So what happens to the valves, do they open and close a bit slower than while under power.

How does one tell without a wideband.

I may be wrong, but I don't think engine power output has anything to do with loading on timing chain and valve train. At constant speed, the load would be the same irrespective of throttle position.

I would have thought load would increase with increasing revs, and during acceleration there would be a rotational inertia component of additional load required to accelerate the cams.

edit: Thinking more about it, high combustion chamber pressure would increase the force required to open the exhaust valve. Whether that is significant or not, I don't know.

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Last edited by Andy8421 on Fri Jun 28, 2024 1:18 pm, edited 1 time in total.
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PostPost by: alan.barker » Fri Jun 28, 2024 12:02 pm

Last time i had the Engine out to fix "oil leaks" onmy Sprint lol. I refitted using VW Gaskets.
The other day during a car club rally she was popping very nicely lol.
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PostPost by: 2cams70 » Fri Jun 28, 2024 1:43 pm

Both Andy8421 and h20hamelan have been having a jolly great time on the turps it seems
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PostPost by: h20hamelan » Fri Jun 28, 2024 4:00 pm

An explanation might help as to why not if you’re bothered.
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PostPost by: elanner » Fri Jun 28, 2024 4:26 pm

I agree with earlier comments that exhaust leaks should be the number one suspect. I was convinced my exhaust was solid but eventually found a leak at the Y piece.

However, the real cure for me was when I discovered that the magnets in the sensor of my Pertronix ignition had come unglued and moved. So they were no longer separated by 90 degrees. The result being that timing for No 1 and No 4 didn't match, and the timing for No 2 and No 3 was unknown.

A new sensor ring with solidly fixed magnets not only improved the running but pretty much eliminated popping.

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