Valve Timing

[TimHaydnJones]

I have a 1979 Eclat 521 which stood in the garage for many years but which I'm trying to get back into use.
Last year, after some work, which included a new cambelt and complete re-build of carbs, I found the
performance to be poor, there was a lot of induction noise (partially caused by the fact that the bloke who did
the carbs couldn't get the mix right and ended up leaving the air box off).

On another forum, someone commented thus
"I had a lot induction noise, very poor performance, and mixture issues, which it transpired were because someone had set the cam timing incorrectly. Specifically, the exhaust timing was out"

I found this interesting, partly because the engine was non standard (Vegantune 200bhp in place of standard 160bhp)

I had difficulty cross-referencing because of difference in terminology
For Ignition Timing, the standard Lotus Workshop manual says
9° BTDC nominal static
25° BTDC @ 3500/4,000 rpm

but the paperwork from Vegantune says 21° strobe @ 1000 rpm
How do I compare these two figures? Are these the same or not?

Also, the Vegantune paperwork refers to Inlet and Exhaust valve timing of 104 full lift ATDC
The Lotus manual refers to 104° MOP
Are these the same or not?

It's not until I have ruled this out that I will revisit the carbs but can anyone recommend a top carbs bloke in the North East Wales/Chester area?

Thanks in anticipation

Tim

[AndyC]

If the engine has Lotus cams and lotus sprockets then valve timings is very simple (to check) and literally involves lining up dots.

With the engine at Top Dead Centre you will have the following points all in line (or no more than 1/2 tooth out_
A dimple in the outer edge of the exhaust sprocket, with it towards the inlet sprocket (else turn the crank one more revolution)
A dimple in the outer edge of the inlet sprocket, with the dimple towards the exhaust sprocket
The centre of the inlet cam
The centre of the exhaust cam

At this same engine position the belt pulley for the alternator and air con/ power steering (if fitted) will have a groove in its outer lip aligned to the TDC marker on the casting adjacent to it.

There's also a mark for the distributor/ oil pump sprocket but if the ignition timing is OK then you're OK there anyway.


As I stated, it's relatively simple to check, if it's out then you'll end up having to alter the tensioner, slip the belt partially off/ slide it over teeth etc so not too bad but more than 5 mins work.


Regarding ignition timing, go with the Lotus manual as there are so many different versions f tune of these engines that it's easy for Vengatune to state the incorrect one for a particular carburation, jetting and ignition system combination.

[barker_001]

It's not until I have ruled this out that I will revisit the carbs but can anyone recommend a top carbs bloke in the North East Wales/Chester area?

I think Skip Brown in Tarporley used to prepare Lotus Sunbeams. I haven't been there for years - a good friend of mine used to use them regularly with his 205 GTi - but it might be worth a call?

http://www.skipbrowncars.com/

Other than that, Pete Musgrove at PNM Engineering on the Wirral might be able to help:

http://www.pnmengineering.com

[Esprit2]

I had difficulty cross-referencing because of difference in terminology
For Ignition Timing, the standard Lotus Workshop manual says
9° BTDC nominal static
25° BTDC @ 3500/4,000 rpm
but the paperwork from Vegantune says 21° strobe @ 1000 rpm
How do I compare these two figures? Are these the same or not?

The stock distributor's centrifugal advance is nil up to 1000 rpm. Therefore 9° BTDC static also results in 9° BTDC at 1000 rpm. While the stock ignition timing calls for 9° BTDC at 1000 rpm, Vegantune specifies much more... 21° BTDC. What's not clear is how quickly the Vegantune distributor advances from that point onward.

For a mostly stock engine, I like to use 12-14 BTDC Static Advance. I've used more for motorsports, but I don't like it on the street.

Also, the Vegantune paperwork refers to Inlet and Exhaust valve timing of 104 full lift ATDC
The Lotus manual refers to 104° MOP
Are these the same or not?

They're the same. Lotus refers to MOP (Maximum Opening Point), while Vegantune says "full lift". Maximum opening/ full lift... same-same. Both call for 104°. That doesn't mean both engines use the same cams, just that the MOP is the same.

It would be best for you to ignore the Vegantune set-up unless you have all the details and all the same parts installed (cams, carb jetting, ignition advance curve, etc). Partial information is just confusing. Instead, focus on getting your engine set to Lotus' spec.

With the crankshaft set to TDC, the cam timing dots on the pulley rims should be adjacent to one another, and on the imaginary centerline between the cams. If the dots are adjacent, but above or below the centerline, the cams are not properly timed.

Check the ignition timing with a strobe light. The auxiliary pulley drives the distributor, and it that pulley wasn't properly timed when the new belt was installed, the ignition timing could be off by a considerable amount... one tooth equals 18 crankshaft degrees.

Good luck,
Tim Engel