Lotus Elan

low compression

PostPost by: oldchieft » Thu May 18, 2017 10:37 pm

This part of what I posted in the past and it might be worth a look.

"One thing that is missed is the type of Schrader valve fitted to the compression tester.

Has the valve been changed?

It should be a low differential type valve or you can get low readings.

I found this out when I begged some old tyre valves from a tyre shop to replace the leaking valve in my tester and found it was reading low even when compared with a tyre inflater.

I sent mails to Schrader to get the right type but got no answer, so I did my own fix.

If you get a long type valve with a spring on the outside then just pull off the coils and cut them till it is just a light pressure to keep it closed."

Hope it helps

Jon the Chief
oldchieft
Third Gear
Third Gear
 
Posts: 403
Joined: 17 Sep 2013
Location: Near Wolverhampton

PostPost by: daverubberduck » Fri May 19, 2017 10:48 am

Hi Jon
Thanks for your input but I don't think that is the cause of my compression readings. It was measured with 2 different gauges, and the second one is new, although it has been pointed out that new does not necessarily mean accurate. Anyway, this weekend I intend to try and correct the valve timing, then I will get back to compression. That's if I don't destroy the engine :(
Dave
daverubberduck
Second Gear
Second Gear
 
Posts: 172
Joined: 03 Aug 2009
Location: Essex, UK

PostPost by: alan.barker » Fri May 19, 2017 10:59 am

Good luck and think twice before you do anything :wink:
Alan
Alan.b Brittany 1972 elan sprint fhc Lagoon Blue 0460E
alan.barker
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 2446
Joined: 06 Dec 2008
Location: BRITTANY FRANCE

PostPost by: rgh0 » Fri May 19, 2017 11:06 am

You cant feel then piston hit the valve and bend its head as you do the cam timing and get it slightly wrong . So you need be careful to get it right before you turn the engine over past top dead centre. You only feel it when the timing is wrong in a big way and the valve actually stops the piston

Once the valve head or heads are bent on the stem you struggle to get them to seal and get a decent compression reading. Been there done that :oops:

cheers
Rohan
User avatar
rgh0
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 7572
Joined: 22 Sep 2003
Location: Melbourne, Australia

PostPost by: promotor » Fri May 19, 2017 11:07 am

Glad to hear you're going to tackle this yourself - the main thing to remember is that the T/C is a "clash engine". Even when the complete cylinder head is off the block (as an example) with the camshafts installed you cannot rotate one camshaft while the other stays still without the valves hitting. The valve/cam timing is such that the valves miss each other when running in the correct order. Change the relationship of the cams to each other and you can/will get issues.
Considering this is going off "unseen" with the head on the block can/will cause issues. Throw into the equation that if one of the inlet valves is at full lift and you try to rotate the crank/pistons/rods independently (ie with no timing chain fitted in the correct relationship to the cams) you will get piston to valve contact. This is less likely with exhaust valves at full lift but the possibility is not to be ignored.
Contrary to popular belief when the pistons are at TDC the inlet valves aren't at their maximum lift - the valve actually chases the piston and gets closer to the piston as the piston goes down the bore. The cutouts allow the piston room as the valve chases it down the bore - this is when the cutouts are effective NOT at TDC as it might appear.

Good luck with it. There's only one way to learn!
User avatar
promotor
Fourth Gear
Fourth Gear
 
Posts: 529
Joined: 16 Mar 2012
Location: Derbyshire, U.K.

PostPost by: daverubberduck » Fri May 19, 2017 3:00 pm

Thanks Rohan and Promotor. You have both scared me... and that's a good thing. I have gone through the procedure in my mind and this is my plan, please feel free to comment.

1. remove the half moon shaped plugs (what are they stuck with?)

2. loosen the bolt on both sprockets slightly by putting a bar through one of the gaps in the sprocket and turning anticlockwise until bar is jammed against top of head. This will fix the sprocket allowing me to crack the bolt. Is this OK? Is there a better way to loosen these bolts?

3. turn engine to TDC where it should remain for the rest of the process.

4. slacken the timing chain.

5. turn the exhaust cam and sprocket anticlockwise (facing front of engine) by just one tooth, making sure inlet cam does not move and engine still at TDC. This should create a bit of slack in the chain between the sprockets.

6. tie chain to both sprockets using cable ties as per Ron's suggestion. Remove a bearing cap from both cams and insert piece of card to stop cam turning, as per Bill's suggestion.

7. place rags into gap in front of sprockets to make sure nothing drops down, remove bolts and washers. Pull sprockets forward.
Question: what happens to the dowels? Will they stay in the sprocket or in the cam?

8. Drop the exhaust sprocket a bit, cut the tie, lift the chain and move it to the right by one tooth (facing the front of the engine). This will straighten the chain between the sprockets again. Tie another cable tie on to keep chain in place.

9. replace sprockets back onto cams and replace washers and bolts, doing them up finger tight.

10. tighten the chain, make sure engine is still at TDC, take a snap of the timing marks and compare to one taken before to check inlet cam is the same and exhaust cam has rotated anticlockwise by one tooth.

11. remove pieces of card and use the bar through the sprocket hole method to tighten the sprocket bolts.

12. check valve timing i.e. inlet valve opens ~20 deg BTDC and exhaust valve closes ~20 deg ATDC. Check ignition timing, although this shouldn't have changed.

How does that sound?
Dave
daverubberduck
Second Gear
Second Gear
 
Posts: 172
Joined: 03 Aug 2009
Location: Essex, UK

PostPost by: billwill » Fri May 19, 2017 4:46 pm

PS: The rectangular bits of card should be placed across the bearing with the ends sticking out so that you can clearly see them and won't forget to remove them before turning the engine. :shock:

And don't forget to check/redo the distributor timing, it may very well have changed as the jackshaft sprocket may have gone into a different link of the chain.
Bill Williams

36/6725 S3 Coupe OGU108E Yellow over Black.
billwill
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 4738
Joined: 19 Apr 2008
Location: London UK

PostPost by: JJDraper » Fri May 19, 2017 4:57 pm

And don't forget to check/redo the distributor timing, it may very well have changed as the jackshaft sprocket may have gone into a different link of the chain.


Been there, done that..

Jeremy
User avatar
JJDraper
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 1019
Joined: 17 Oct 2004
Location: Buckingham, UK

PostPost by: Craven » Fri May 19, 2017 7:10 pm

Point 5.
You cannot turn the exhaust cam anticlockwise, there is no spare chain between the crankshaft sprocket and the exhaust sprocket.
You will need to turn the cam independently after the sprocket is removed. Use Alan?s molegrip method. Remember it will be opening valves so some force will be needed then use Bill?s card trick to lock.
When refitting two things to watch, alignment of dowel pin but also the cam boss and the recess in the rear of the sprocket, it may not be in perfect alignment and you can spend an age figuring out why the cam won?t seat correctly with the sprocket, may need a tap but don?t try and pull it into place with the bolt.
Point 7. Be careful here as sometimes the dowel can come out of the cam and again can fall in the works.
Ron.
Craven
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 1113
Joined: 14 Sep 2013
Location: south coast uk

PostPost by: alan.barker » Sat May 20, 2017 6:20 am

Thanks,
I've always used the molegrip method and when you wiggle the Cam the Sprocket slips into place very easily
Alan
Alan.b Brittany 1972 elan sprint fhc Lagoon Blue 0460E
alan.barker
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 2446
Joined: 06 Dec 2008
Location: BRITTANY FRANCE

PostPost by: daverubberduck » Sat May 20, 2017 9:11 am

Well spotted Ron, for some reason I was thinking that the chain tensioner was on the other side. Turning the cam with a mole grip sounds tricky, turning it exactly the right amount sounds near impossible. What about if I remove the exhaust sprocket from the chain and re-mount it onto the cam, then use that to turn the cam by one tooth, measured against the top of the front cover?
daverubberduck
Second Gear
Second Gear
 
Posts: 172
Joined: 03 Aug 2009
Location: Essex, UK

PostPost by: Craven » Sat May 20, 2017 10:43 am

Use the new position of the sprocket, after one tooth move, as your aiming point, turn the exhaust cam to align with the sprocket.
There is little choice how you turn a fitted cam, you can use the detached sprocket but I would avoid this, you could undo all camcaps (EX) let the cam up get things all lined up then pull the cam back down. But think about valve clash if you do this, I think its ok closing valve rather than opening them?
All a bit tricky but again not much choice.
Ron
Craven
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 1113
Joined: 14 Sep 2013
Location: south coast uk

PostPost by: daverubberduck » Sat May 20, 2017 11:10 am

OK, thanks Ron. I will use the mole grip method to turn the cam and align it with new position of sprocket. I don't want to lift the cam if I can help it.
Dave
daverubberduck
Second Gear
Second Gear
 
Posts: 172
Joined: 03 Aug 2009
Location: Essex, UK

PostPost by: daverubberduck » Wed May 24, 2017 2:23 pm

Managed to change the exhaust valve timing. It was difficult fitting the sprockets back on but managed it eventually. Checked the timings of the inlet valve opening and exhaust valve closing on cylinders #1 and #4. Both now symmetrical about TDC. Had to re-do the ignition timing because it had gone way out. Car now runs very nicely. Also, there has always seemed to be a harshness in the engine when accelerating hard, which wasn't pleasant, but it now seems smoother. Could be my imagination but I don't think so.

Re-checked the compression:
#1 133
#2 142
#3 137
#4 141

So still low but at least they are consistent, so I'm not going to worry about this for the time being. I did try a teaspoon of engine oil in cylinder #4 to see what happened, the compression went up from 141 to 154. I don't know if this is significant. I can't believe there is much wrong with the rings because the engine doesn't seem to use much oil. Anyway, I will leave it for now, maybe try and measure again in the future with a different gauge.

Can I thank everybody who pitched on this. I am so grateful that people take time out to help those of us with less experience. I honestly don't think it would be possible to own an Elan or +2 without this forum, especially as the number of competent professionals who know about these cars seems to be dwindling.
cheers
Dave
daverubberduck
Second Gear
Second Gear
 
Posts: 172
Joined: 03 Aug 2009
Location: Essex, UK

PostPost by: nmauduit » Wed May 24, 2017 2:53 pm

well done ! as for the "wet" compression test (significant increase in reading with oil), it may be that the previous overhaul was a quick one, like manual honing and new rings but over specs regarding piston skirt clearance - or that the bottom has just seen a number of miles. Compressions are quite close to each other, so it can run for a while like this, just keep an eye on oil consumption.

You may want to do a leak-down test at some point if you want to keep track of the evolution of the bottom end, it's not a very expensive tooling (you need an air source though).
Last edited by nmauduit on Thu May 25, 2017 8:14 pm, edited 1 time in total.
S4SE 36/8198
User avatar
nmauduit
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 1530
Joined: 02 Sep 2013
Location: France
PreviousNext

Total Online:

Users browsing this forum: No registered users and 9 guests