Lotus Elan

low compression

PostPost by: daverubberduck » Sun Apr 16, 2017 1:05 pm

I bought my +2 as a part complete project. It is now on the road and seems to be running well having finally sorted out the carb jetting etc. I decided to do a compression test so bought a kit. Did the test with engine warm, airbox cover off, throttle wide open. Here are the results:
Cyl 1 130
Cyl 2 135
Cyl 3 132
Cyl 4 135

So fairly consistent but low. I?m not sure what this means. Engine has been apart, head was converted to lead free. The history of the bottom end is uncertain but the pistons and bearings all looked like new so I think it has been rebuilt. I took the cam cover off and the marks on the cam sprockets aligned perfectly. Seems to run smoothly, and idles nicely. Not using significant oil and not smoking. Has only done about 800 miles since rebuild. Oil is Halfords Classic which is a 20W/50 mineral oil. Difficult to tell if it is down on power because I have not driven one of these for 30+ years.

Does it just need more time for the rings to bed in? Should I try the test with oil in the cylinders, and what would this tell me? Any other possible causes? Would appreciate any advice given.
Dave
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PostPost by: vincereynard » Sun Apr 16, 2017 1:27 pm

As far as I know, if trying the test again with oil in the bores gives a significantly higher reading, it would indicate bore / ring wear. (or glazed bores?)

Either way if it runs well, doesn't smoke and sounds fit I would leave well alone.
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PostPost by: ericbushby » Sun Apr 16, 2017 1:56 pm

Dave,
Before you blame the engine, I suggest you check your pressure gauge against another.
Just because it is new does not mean it is good.
Perhaps someone on here is not too far away.
I would have thought that the differences between cylinders would be more than you have measured, if it was worn or valve faults.
Best of luck,
Eric in Burnley
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PostPost by: daverubberduck » Sun Apr 16, 2017 5:26 pm

Thanks Vince and Eric. A bit more background, a couple of months back I was having real problems getting the car to run properly and I took it to a chap down the road who I thought might be able to help. In fact he didn?t sort the problems, I managed to do that myself in the end, but while he was working on the car he did a compression test. He got ~140 on all cylinders. Having sorted the other problems, I decided to re-visit the compression and bought a kit with the results mentioned above.

So the readings are definitely low, but I have a theory. The starter is not the original, it is a modern high torque starter, can?t tell which model until I get under the car and have a proper look, but I?ve always had the impression that it turns slower than the original starter. I?m sure I?ve read somewhere that compression should be measured at a certain revs, 200 springs to mind. Could that be the reason?

I think I will leave it for now and maybe try to measure again in a couple thousand miles.
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PostPost by: nmauduit » Sun Apr 16, 2017 5:46 pm

a quick thing pops in mind : did you take out all spark plugs before running the test, and did you run it with the throttles fully opened (using a block of some sort if non weber) ? I understand you are not running a wild engine with camshaft diagram having a lot of overlap...

then if you are definitely suspect low compression, a leak down test could provide you with further info
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PostPost by: daverubberduck » Mon Apr 17, 2017 8:03 am

Yes all spark plugs removed and throttle pedal to the floor. Carbs are webers. I can't be certain that the cams are not standard i.e. long duration, but I doubt it. A leak down test is a good idea, if I can find someone who can do it.

The +2 manual on page E9 says engine should be turning at 200rpm. I will measure what speed my starter is turning at, to see if it runs slower.
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PostPost by: nmauduit » Mon Apr 17, 2017 10:45 am

daverubberduck wrote:Yes all spark plugs removed and throttle pedal to the floor. Carbs are webers. I can't be certain that the cams are not standard i.e. long duration, but I doubt it. A leak down test is a good idea, if I can find someone who can do it.

The +2 manual on page E9 says engine should be turning at 200rpm. I will measure what speed my starter is turning at, to see if it runs slower.


ok, so no issue of under reading from the procedure. Unless your engine is cranking really slow I'm sure you've seen the needle going up and stabilizing at the top for each cylinder.
If you had a wild race engine (long duration etc) you'd know, would it only be by the minimum idle higher and more erratic...
One thing you can try is to poor a spoon or two of thick oil (even gearbox) in the cylinder under test so see if it goes higher then (in which case a worn bore is likely, the thick oil restoring the bore air tightness for the duration of the test.

a leak down tester is not very expensive but you need an adequate air supply to use it : that would help to confirm if the bores are indeed worn out (which is compatible with the 4 homogeneous readings), you basically push air trough the spark plug hole when both valves are closed and try to see where is is going and at what rate.

An other thing : did this happen all of a sudden, or did you see performances degrading gradually over time (how well did it run best last, and when was it)? a tooth jumping at the distribution chain (by accident or mishandling) would alter all cylinder equally... I don't see the modern starter being the source of such a problem, it should be an improvement if anything (has been on my car).
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PostPost by: daverubberduck » Mon Apr 17, 2017 12:00 pm

Hi Nmauduit, taking your last point first, you are right about it not being the starter. I just measured the crank speed at 200rpm so that?s perfect.

Engine idles nicely so it?s not a wild race engine. The car has not been on the road long and I bought it as a project, so I can't answer your question about performance. The car runs well but it is difficult to say if its performance is down because I have not driven an Elan or +2 for over 30 years, so it's difficult to make a comparison.

I will try the oil trick next, and then I will look for someone who can do a leak down test.

Many thanks for your input, Dave
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PostPost by: Craven » Mon Apr 17, 2017 12:24 pm

Hi,
Definitely checkout you camshafts, I have had readings similar to yours with a L2 cam 46/70/70/46.
296 deg duration, 272 for sprint, but a long duration cam can give low reading, It?s the overlap that causes the low reading, that is both inlet & exhaust open at the same time L2 overlap 92 deg, sprint cam 52 deg. and worth a check if you can.
Ron.
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PostPost by: daverubberduck » Mon Apr 17, 2017 12:58 pm

Hi Ron, I understand what you are saying but I have no idea how I would check that. If I remove the cam cover and turn the engine by hand, is it something I would be able to check visually? Dave
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PostPost by: Craven » Mon Apr 17, 2017 1:35 pm

Hi,
Only a rough guide but if you look to see when there is zero tappet clearance (inlet valve) on No 4 cylinder coming up on the inlet stroke, check the ignition timing mark on the front cover, this will show if the inlet opening is before the 30 deg first mark. If it falls somewhere within the marking then probably not the problem.
Ron.
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PostPost by: Chancer » Mon Apr 17, 2017 4:06 pm

Your test readings have a variation of +/- 2% of each other and are within 5% of the readings from a completely different guage on a different day under different conditions.

You must live a charmed life if all you can find to worry yourself with is this (IMO) non problem.

Cranking speed will affect the guage reading as will the throttle opening and dont forget atmospheric pressure varies, a compression tester should be used as a comparator for readings between cylinders and yours appear fine, no point at all comparing it with readings from another guage or even from itself in the past unless you calibrate it each time.

As has been mentioned hotter cams with more duration and overlap will reduce the static compression ratio compared to a standard engine.
Last edited by Chancer on Mon Apr 17, 2017 4:07 pm, edited 1 time in total.
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PostPost by: daverubberduck » Mon Apr 17, 2017 4:06 pm

Thanks for that Ron. I will give it a go.
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PostPost by: daverubberduck » Mon Apr 17, 2017 4:14 pm

Hi Chancer, I am not a mechanic so I don't know if this is something to worry about or not. Your message is reassuring because I'm not living a charmed life and really don't need another worry in connection with this car. I will check the cams anyway, just out of interest, and will stop worrying :)
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PostPost by: daverubberduck » Mon May 15, 2017 5:28 pm

Hi Ron
I finally got round to seeing if I have long duration cams. I measured the tappet clearances on cylinder #4 which were perfect (inlet 6, exhaust 10). I then used the smallest feeler gauge I have which is 0.0015? and I turned the engine until the inlet tappet just closed. Result was 20 deg BTDC, so seems to be a standard cam.

I then decided to see what happened with the exhaust tappet so I continued to turn the engine. I was expecting the tappet to open at about 20 deg after TDC but it actually opened at about 0 deg TDC. This doesn?t seem right to me? is it? And such a short overlap doesn?t explain the low compression.
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