Lotus Elan

low compression

PostPost by: Craven » Tue May 16, 2017 6:55 pm

Hi Dave,
I would take a step back if I were you, the thread started with low compression now you are embarking on a complete cam timing exercise.
From albeit crude checks the inlet cam looked to be in about the right place, if the wrong sprocket were fitted then it would show up.
Stop!! think it through a little more. Exhaust sprockets I have seen are marked on the inside recess in the typical Lotus way using a vibrating engraving tool.
Ron.
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PostPost by: daverubberduck » Tue May 16, 2017 9:08 pm

But Ron, the sprocket on the inlet cam is clearly marked with "EX" which has been engraved as you say (see pic), so there is no doubt that the sprockets are the wrong way round. Also I have seen that the valve timing is strange with the exhaust valve closing at TDC. I am sure they are wrong and need to be swapped. I know the engine runs but I'm not convinced that it is pulling the way it should. As for the compression, maybe that is a symptom of the same thing.
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PostPost by: Craven » Tue May 16, 2017 9:47 pm

Hi Dave,
One tooth difference on the cam timing marks = a 20 deg difference in crankshaft position, exactly my original comment the exhaust timing could be one tooth out. What I?m not so happy with is if that is a true exhaust sprocket, then it will have moved the inlet timing by 20 crankshaft degs but you indicate inlet looks OK, which if we are dealing with a standard setup, can?t be.
Ron.
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PostPost by: daverubberduck » Wed May 17, 2017 10:43 am

Hi Ron
I've looked at the sprockets for sale on SJ Sportscars website and they sell only one sprocket, to be used on either inlet or exhaust (or jackshaft). It has 2 timing marks on it, one for inlet and one for exhaust. So what I didn't realise until now is that the sprockets are identical except for the timing marks. Please excuse my slowness but I am learning a lot from this. So the easiest solution for me is to ignore the timing marks and just move the exhaust sprocket back one tooth, as you say, which will correspond to ~20 degrees on the crankshaft. That will sort it.

So my next question - can I do that just by loosening the chain?
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PostPost by: Craven » Wed May 17, 2017 10:50 am

Short answer then NO you can?t
Last edited by Craven on Wed May 17, 2017 12:13 pm, edited 2 times in total.
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PostPost by: alan.barker » Wed May 17, 2017 11:07 am

I thought it had been understood the cam sprockets have been fitted the wrong way round. So what's the advantage of cable ties if the sprockets are to change sides.
When replacing the sprockets i use molegrips on a plain part of the cam to rotate to aligne cam dowel with hole in sprockets.
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PostPost by: daverubberduck » Wed May 17, 2017 11:28 am

Hi Alan
Inlet and exhaust sprockets are the same except for the timing marks. My inlet timing already looks fine, exhaust timing looks to be 20 deg out. So all I need to do is turn the sprocket on the exhaust cam round by one tooth. The timing marks may not align perfectly but the valve timing should be fine.

Hi Ron
So the answer to my question is no, I've got to remove the sprockets. That's disappointing. I was hoping I could just loosen the chain right off and slip it round by one tooth.
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PostPost by: rgh0 » Wed May 17, 2017 11:39 am

Typically Lotus marked the exhaust sprocket with "Ex" with a vibrating engraving tool and left the inlet sprocket blank. The difference in the sprocket is the location of the timing mark versus the dowel hole as others have said other wise both the cams and sprockets are normally identical. Different cams can and have been been used on the exhaust versus inlet both for tuning effect if you know what your doing as well as just through incompetence of so called professional engine builders.

I usually time using the specified cam timing and set it based on the maximum opening point and double check opening and closing timing which is not so accurate as it is affected by small changes in cam clearance. I ignore the timing marks on the sprockets and use offset dowels or vernier sprockets depending on the engine build if needed as so many things can change on an engine since it was originally built to make the original timing marks invalid (or the timing marks were wrong in the first place given the typical Lotus attention to detail - thats irony for those confused by that statement).

cheers
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PostPost by: promotor » Wed May 17, 2017 11:50 am

Photo as promised : this is an aftermarket sprocket but indicates the marks just as well as having two original ones together. The inlet position is noted by the dot and the exhaust position is noted by the line.

dsc01359.jpg and


EDIT : having read the above posts I now realise you have already seen these sprockets! Maybe the photo will be useful for other readers!
Last edited by promotor on Wed May 17, 2017 12:28 pm, edited 2 times in total.
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PostPost by: billwill » Wed May 17, 2017 12:13 pm

daverubberduck wrote:I think the answer to my last question is yes, I have to remove the half moon shaped plugs and undo the nuts. The tricky bit will be putting it back together. If I align the timing marks on the sprockets, then maybe double check by measuring the opening and closing angles of the valves as I did before. Am I on the right lines?


Take extreme care that you do not drop the sprocket-bolts or the tapered washers down the timing case to the sump, It will be pretty difficult getting them out again.

I usually first fix the position of the cams by taking off one of the bearing caps of each and inserting a rectangle of cereal-packet cardboard. Tightened down the cam cannot move,

Next loosen the sprocket bolts about a quarter turn.

Loosen the tensioner completely, tie a bit of wire or string to the top of the chain, so that you can lift it out of the timing case later. Lift the chain off the sprockets and lower it carefully into the timing case out of the way. Tie off the top of the retrieval wire/string.

Now you can carefully if you wish move the inlet cam by one tooth, by loosening the cardboard clamp just a bit and rotating the sprocket a little. You may need a tool made from a length of wood and two projections (nails or bolts) to lever the sprocket&cam around. Tighten the cardboard clamp.

Or take off both sprockets to check the marks are previously described, before refitting , possibly exchanging , them. {Note you might have TWO exhaust sprockets there and some other unlucky person has two inlets ones}.

When you have the cams &sprockets in approximately the right place, lift the chain back up and pull it straight up so that the bottom of the chain engages properly with the crankshaft sprocket deep below. Check that it is not kinked, peer down inside with a torch. keeping the chain taut, drape it over the exhaust sprocket so that it is taut all the way from crankshaft to exhaust sprocket.

Then drape it over the inlet sprocket with minimal slack between the sprockets.

Now gently screw in the chain tensioner, checking with a torch that it is seating ok into the chain. Take up most of the slack Tighten the sprocket bolts, then remove your cardboard from the cam cap and complete the adjustment of the tensioner to give the required half inch up & down movement of the chain between the sprockets. The sprockets& cams will move a bit during this final tensioning depending on tolerances and on wear of the timing chain. The marks on the sprockets are only approximations to within one tooth position.

Do a final tighten of the sprocket bolts, though the engine might start to turn, making it a bit difficult.


Now there is no way of being sure that the tensioner/jackshaft sprocket went on the same teeth as before, so it is essential to redo the distributor timing, from the beginning, including lifting out the distributor and reinserting it in the correct position. Remember that the centre shaft rotates a little as the spiral gear engages on the jackshaft.
Last edited by billwill on Wed May 17, 2017 1:11 pm, edited 2 times in total.
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PostPost by: daverubberduck » Wed May 17, 2017 12:39 pm

Many thanks to everyone who has contributed to this. I now understand the problem and what needs to be done. I'm just trying to decide whether to do it myself or get someone else to do it.
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PostPost by: billwill » Wed May 17, 2017 1:05 pm

daverubberduck wrote:Many thanks to everyone who has contributed to this. I now understand the problem and what needs to be done. I'm just trying to decide whether to do it myself or get someone else to do it.
Dave


OK, please re-read my extended description above as I edited it to add the full procedure and our messages overlapped.
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PostPost by: Chancer » Wed May 17, 2017 1:21 pm

daverubberduck wrote: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.
Dave


I got my knickers in a twist with the terminology but I think you are saying that the inlet valve closes at 20?ATDC and the exhaust valve opens at TDC giving 20? of overlap but assymetrical regarding TDC?

I would have to look up some text books but my gut feeling is that for standard road cams the overlap should be equal before and after TDC and 40? from distant memory is a more likely overlap than 20?

Perhaps someone more up to date could confirm my gut feel so as you dont tear down the engine for nothing.

I reckon the minute you restart the engine after correcting the cam timing, the first rev up will tell you that you were right (or wrong) to have done so.

I bought a Suzuki Jeep, it was cheap and my suspicions were raised that it had just had a new head gasket, it seemed a little flat compared to the one I had owned before but I could not really be sure, compressions and timing was fine but the cam timing marks were a little out, moving it one tooth made them out the other way by an equal amount so it was 50/50 for me and must have been for the guy who did the gasket, I started it with the cam moved one tooth and it revved up like a motorcycle and was much more perky on the road.
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PostPost by: daverubberduck » Wed May 17, 2017 2:35 pm

Hi Ron
I see you deleted your procedure on how to do it, but I understood it and it was very helpful. Didn't mean to offend with my reply. I'm going to try and do it myself. Fed up paying people to do jobs incorrectly.

Hi Bill
Thanks also for your procedure, very helpful. Jamming the cams tight with cardboard is a good idea.

Hi Chancer
No I meant that the inlet valve started to open (tappet closes) at 20 deg BTDC. I agree with the rest, the exhaust valve should have closed at 20 deg ATDC to make it symmetrical (overlap 40 deg), but it closed at TDC.
Encouraging that moving one tooth made such a difference to your Suzuki jeep.
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PostPost by: Chancer » Wed May 17, 2017 3:37 pm

Yes,my reply was incorrect just to add to the confusion, we are in sync now.

The Suzuki was a SOHC so both inlet and exhaust timings were Advanced or retarded, I think the latter but it was 30 years ago now.

I have seen lots of vehicles over the years with a little chain wear or belt stretch that could have been half a tooth out, often been tempted to try the 50/50 again!

A guy that I used to race against who had a real screamer of a X/flow engine would retard the cam timing in a wet race, it turned a tiger into a docile kitten, he had a small access plate to access the pulley.
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