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Compression - problems?


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Hi everyone,

After running problems with my 1976 504 Elite jumping from one place to the next: fuel supply, vacuum, carbs, ignition, etc I decided to test the compression.

All tests done with butterflies fully open, all plugs out and a good battery pretty much at sea level. Hot tests with engine at temperature, cold test at ambient - about 25 deg. Cold wet test with 5ml of ATF in each cylinder.

Please have a look at the attached image and see if you can help me interpret the results.

Firstly I can see that my tester is probably quite bad - it seems to measure high as the compression readings are a lot higher than they should be for this engine (150-170 for Lotus 907 LC hot as per a thread I found). That being said, I can see carbon buildup on the pistons (I imagine cylinder head too), so perhaps compression is just high due to the reduced volume.

Then, it seems like the inter cylinder readings are okay, but not great. It seems cylinder 3 is a bit over the 10 psi between cylinders spec.

Then with the "leak down" wet test, this seems to indicate a rings problem, particularly on cylinders 1 and 4.

Question is, how bad is this really and what am I looking at. Will I ever get the engine to run okay with these measurements, or is a rebuild in order?

Screenshot_20211025-125445_Office.jpg

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The other test you can carry out is a proper leak down test whereby compressed air is introduced into the chamber via the spark plug hole when the piston has just reached TDC on the compression stroke. Most such test instruments have two gauges which show applied pressure and "retained" pressure. If the valves are defective you can hear a hiss of air at either the exhaust outlet of carb inlet. If the rings are defective you can hear a hiss of air into the sump (listen at a breather hose). Around 10% leak down is where one starts to ask questions.

Edited by dmottram
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You started the thread because you are experiencing running problems, perhaps it would be useful if you gave us a bit more detail of the actual problem.

I am inclined to agree that your compression readings while not perfect, are unlikely to  be the problem.

assuming that the ignition timing and spark is/ok, I would be looking at the carbs, is there flooding, are the sealing rings ok ,is the inlet manifold gasket ok, is there a leak on the vacuum system for the lights. Just my 2p worth

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Thanks for the thoughts! Glad to hear that the feeling is the compression is okay.

I made some headway this weekend. Having the plugs out to test the compression led me to clean them up a bit, and somehow the gaps were ridiculously out. I don't remember setting them that way, but the gap was closer to 1mm than 0.6. Setting back to normal fixed my running issues for now.

 

Something else I discovered was that the accelerator pump linkage was loose on the shaft on both carbs. Tightened that and set the pumps up correctly also to get the right volume and rate. Something is still not 100% right with the carb setup, but everything is running more or less okay for now.

Next step is suspension and exhausts and then should be ready for roadworthy.

Thanks again.

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