Lotus Elan

Top Dead Centre

PostPost by: LaikaTheDog » Wed Mar 03, 2004 10:32 pm

erm, apologies for asking something I really should know, I think I have this right, but just thought I would ask;

If I turn the crank pulley until the notch lines up with the timing mark, I figure this to be close enough to top dead centre, but how do you know if it is number 4 tdc or 1 tdc ? as indeed surely both are true.

I figured that the way to tell is to put a spark tester on number 1 cylinder turn the crank until number one sparks when th notch and the mark line up, I then know that I have TDC on one and setup my distributer as 1342 firing sequence.

I can then start the engine and use a lamp to set the timing to 12 degrees.

using the above I have got my engine to run and idle just fine, it even revs up and down the range just fine...but on the road the car feels like it has 2 bhp, have to rev it over 3500rpm to get out of 1st gear and i have to feather the clutch like a madman to stop stalling..I have got something wrong somewhere !

any advice or guidance appreciated
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PostPost by: Martin_StuartUK » Wed Mar 03, 2004 11:00 pm

Firstly, don't necessarily trust the TDC mark on the pulley. Best way to establish true TDC is by using a dial gauge down the spark plug hole, then mark the pulley accurately yourself (unless it is already spot on, of course!). Unfortunately the front pulley is not very accessible on the Elan, so you may need to remove the radiator to get a good view of it.

Secondly, are you sure the problem is ignition timing? If it idles OK the timing is probably not far out. What else have you changed since the car was running right, and why are you re-timing it in the first place? There is a long list of possibilities, from incorrect cam timing to not reconnecting the headlamp vacuum pipe, so it would help to know what recent work has been done and/or what faults were being attended to, to assist in diagnosis.

In any case, start by checking all the basics, like HT leads, points gaps, condenser, plugs etc.
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PostPost by: JACKJABBA » Thu Mar 04, 2004 9:03 am

Try using your strobe to see what the advance is doing when you rev the car, you will probably need another set of hands to do this.

If you dont have a dial test indicator, you can use a screwdriver through the number 1 plug hole to find TDC. Its by no means as accurate, but gives a good indication of if your notch is out. Make sure you have the ignition off when you do this, as accidents happen, and a srewdriver will make a mess of your engine if someone turns the engine over.

Like what Martin said, what else has been done?
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PostPost by: terryrds » Thu Mar 04, 2004 12:11 pm

Well, for what it's worth- Your symptoms sound exactly like what I experienced after putting my engine together- I had confused before top dead center and after top dead center. Amazing that it would run at all, but it did, exactly as you describe. Hope this helps. I know I was relieved when I figured it out.
-Terry
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PostPost by: type36lotus » Thu Mar 04, 2004 2:53 pm

Another trick to find top dead center.

1) Turn engine so cyl 1 is well before TDC
2) Inset a stick in cyl 1 spark plug, lay a straight edge across the cam cover, mark the stick even with the straight edge.
3) Now also mark the pully.
4) Rotate the crank, the stick will rise as it approchaes TDC. Keep turning till it goes over TDC and starts coming down. Stop when the mark on the stick aligns with the straight edge again. You are now the same number of degrees AFTER TDC as you were BEFORE TDC.
5) Mark your pully again.
6) Half way between these two marks is TDC.

The problem with measuring with a gauge or ruler is dealing with Dwell. The pistion will remain at TDC for a few degrees. The measurement trick eliminates the dwell issue. Mine crank pully was spot on after this process.

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PostPost by: LaikaTheDog » Thu Mar 04, 2004 11:36 pm

realised in the middle of the night that more infor may be required...

the engine is known to run just fine, as I used it daily for about 3 years before I broke the chassis.

what has changed since

engine is unchanged, cam coer hasn't even been taken off

other things....
lumenition fitted
both dellortos refurbished by aledged specialist
new HT leads
new plugs
new coil

as I said, it idles OK, but I am intigued by Before TDC and After TDC comment by terryrds, can you tell me exactly what you mean by this as I too would like to be relieved.

Strobe does appear to show the markings move about slightly even during idle, but only slightly

I actually suspect dodgy HT leads or a dodgy spark, can this cause such a poor performance under load ?

one other thing when I turn off ignition after engine is warm the engine cuts out and then gives one single sharp back fire, sounds like timing to me but the strobe says 12 degrees dead on. or is it too rich a mixture leaving fuel in the manifold ?

standard big valve engine..is that degree timing correct if measured using a strobe ?
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PostPost by: terryrds » Fri Mar 05, 2004 3:27 am

Hi- Didn't mean to be obscure. What I meant was it sounds like your spark is after top dead center, not before. That would allow the engine to run very smoothly, but absolutely no power at all. If it were too much before top dead center it would have plenty of power but would have a horrible, destructive knock. I see you replaced the HT leads. Is it possible they got replaced one position off? Was the distributer out? As I recall it's essential to allow for the rotation of the shaft (due to spiral gear) as it's replaced. It's possible it's a tooth off. Either of these would create a situation where the entire range of rotation of the distributer is all after top dead center. The solution is to either rotate the HT leads or reset the distributer. As I said, been there, done that. (Actually, either solution will correct either error.)
Using the strobe light the spark should be before top dead center at idle. As you rev it up it should move "more before", or away from top dead center. If it's moving towards TDC, you're starting out on the wrong side (in other words after TDC).
It's possible this isn't your problem at all, but your symptoms sound so similar to what I experienced I had to bring it up.
Good luck -Terry
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PostPost by: JACKJABBA » Fri Mar 05, 2004 9:05 am

If your timing is showing 12 deg BTDC on the strobe for idle, your distributor is set ok. Rev the engine above 2500rpm and let it come back down to 2500 and hold it there, you should have an advance of of 14.5, and 19 for 3500 rpm. This would show you if you have a distributor advance problem.

If this is ok I would fit your old leads, plugs and coil and try taking it for a run again. If its still the same then that leaves your carbs.

Have you balanced them correctly and set the idle mixtures.

Jack
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PostPost by: LaikaTheDog » Fri Mar 05, 2004 8:08 pm

JJ,
I have set the carbs up in synch and have set the idles to the factory recommended default of 2.5 turns anticlockwise from closed, I have actually set them to 2 turns as they were running verrrry rich.

Terryrds
I will check that timing, the distibuter has not been removed

but i recall from the dim distant past (about a year ago) that when I fitted the lumenition I was slightly disturbed to find that the 4 pronged 'light cutter' which replaced the rotor arm could be fitted wherever you wanted ( quater too, quater past etc) on the shaft. however if I set tdc on cyl 1 and have the spark on 1 set up accordingly this should not matter should it ?

one other thing, I am of course assuming, probably read it somewhere, that the notch moves clockwise by 12 degrees

L
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PostPost by: terryrds » Fri Mar 05, 2004 9:05 pm

Laika- I'm at a disadvantage here because I don't have a running engine or strobe available right now. Perhaps this can be confirmed by someone who does, but I think the notch should appear to move counterclockwise, when viewed from the front. If the distributer hasn't been removed I'd focus on the location of the HT leads. Not sure if the orientation of the "light cutter" could be the problem. Did it ever run properly after that was installed? -Terry
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PostPost by: LaikaTheDog » Fri Mar 05, 2004 9:52 pm

terry,

you are correct, it should indeed be counterclockwise and no it never has run right since i added lumenition, but then i did foolishly combine all the other changes as well, so i should start with the old stuff as much as i can and work from there...will let you know how it goes.

L
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PostPost by: JACKJABBA » Sat Mar 06, 2004 10:29 am

Your cutter will fit in one of 4 positions, but this does not matter as the blades are exactly the same. Mine fits the same as yours. The cutter fits beneath the rotor arm and does not replace it, you must still have it fitted otherwise the engine would not run. If you are getting a good spark then the Lumenition is working.

Remember you are setting static timing and should not have the engine at TDC (TDC is used to set the valve timing.) but at the static value for your engine. 12 deg BTDC for your Big valve with Dellorto's. You should then get a max advance of 26 deg BTDC at 5000 RPM. This is assuming that you have the correct 41189 distributor fitted.

If you have a 43D4 fitted then you can help me out with the code number from the side of the dizzy, and when it runs ok , a plot of advance against RPM. ;)

Regards Jack.
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PostPost by: LaikaTheDog » Sun Mar 07, 2004 11:18 am

spent a couple of hours in the garage.

plugs all tested OK
plug gaps are correct
HT leads tested ok
spark sequence is correct
top dead centre marking on the pulley is correct ( thanks every one for your advice)
12 degrees advance is perfect, the marks are steady in the stobe light
advance works perfectly
all earths are good
vacuum to the lights is good
very slight leak between the exhaust manifold and the head
compression is within manual spec
carbs;
as everything else appeared to be working, thought I would try some old technology..

when the engine is warm I removed the airbox and covered cylinder 1 carb with my palm..the engine slows slightly, you need a bit of throttle to get it back up to speed after you remove your palm
I then covered cyl 2 carb, engine slows significantly more than before but does not stall
Now cylinder 3 carb , engine stall almost immediately, frantic throttle pumping gets the engine back to speed after I remove my palm
Cylinder 4 carb covered makes no difference what so ever

I therefore believe that my carbs are unbalanced and way way out of tune, I am assuming threrefore that the terrible power delivery is due to poor combustion across all cylinders which I can rectify using a manometer for the balance and a rolling road for the rest...
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PostPost by: DrEntropy » Tue Mar 16, 2004 11:12 pm

Sounds like carburettors are at fault here: check to see if the throttle blade shafts are not "tweaked" (twisted). Unfortunately this would require you remove the carbs and physically examine the position of each of the plates in relation to its' counterpart... No light passing either one at "full shut." I have had the experience of a twisted set of shafts in throttle bodies... but it would usually only affect lower RPM running as you described by blocking one throat at a time... The other thought that comes to mind is the float levels. I know you had them gone through by someone else, but trust in these matters has never been my strong suite. Every thing else you have described seems to be in order.


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PostPost by: LaikaTheDog » Sat Mar 20, 2004 2:57 pm

wooohoooo, it works it works works !!!!

I stripped everthing back and found that everything was as it is meant to be. When I put it all back together I remembered something that a rolling road guy mentioned about 3 years ago to me. He noticed that the spring type washers betwen the carb bodies and the fixing nut were relatively loose, so this time I smeared some blue hylomar on the manifold face and on the carb face so that the rubber ring in the spacer had something to seal to , did up the nut as per torque settings.

Started first time, so no difference there, but the carbtune now says my vacumm is almost off the scale !

Suddenly I had this urge to go for a drive....

After 2 years I have finally driven my +2 they way it is supposed to be driven, the seal between the carbs and the manifold appears to have fixed this issue for good, off to the MOT station tomorrow.....at last I am almost there
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