ignition timing

[JohnSchofield]

I have got my new Jupiter up on the ramp for the first time to give it a seeing to and to change the carburettors. Want to check the timing but there does not seem to be much info. in the manual/handbook. Some simple questions -
1. Where is the TDC mark to be found?
2. When checking the tick over timing with a strobe lamp, should you disconnect the vacuum?
3. What changes in timing do the vacuum and centrifugal advance provide?
4. Can you advance BTDC when using 97 octane petrol?
thanks in anticipation, John.

[Keith Andrews]

1. Where is the TDC mark to be found?

Im not familar with the jupiter

2. When checking the tick over timing with a strobe lamp, should you disconnect the vacuum?

Yes , that is to check the intial only....total intial + cent should alsp be checked , and it is this that the timing on an engine should be set to then check the intial is correct

3. What changes in timing do the vacuum and centrifugal advance provide?

The intial determins a good stable idle and being low endables light intial loads on the starter armiture current draws, easy starting....If the VA (vaccuum advance) is off manifold rather than ported vac this adds extra idle advance for stabilty and leaner idle mixtures.
When an engine is under load the vac is lower, ie under aceeleration, as the rpms increase the cent increases to give more time for the richer mixtures to reach max pressure of explosion...idealy this is between 12 and 18 degs ATDC, depending on bore to stroke ratios , fuel types , mixtures and ocatanes used.
As the car comes to a low load enviroment, ie cruise, the engine vaccuum increases, the mixtures lean out and one can then run far more economically by adding more advance via the VA..and even lean the cruise carb circuita even further.
To establish how far mixtures and time can go, requires tuning tuning with knock sensors and O2 sensors.

Can you advance BTDC when using 97 octane petrol?

97 octane is for hi compression engines, getting up in the 10:1 compression range and cams that devalope hi dynmanic compression ratios in thr mid to hi 9:1... which theseold cars dont unless very seriously borred out, heads planed and bores decked...far more can be gained by using a lower octane and tuning to suit.
It must also be noted that unless the cruise and power circuits have been tuned accuratly with Cox or preferbly O2 sensors, if the AFR are not correct and adjustment for higher octane is a total wate of time.

Also the more advance on puts in the engine cent (intial) the better the engine sounds, but the power/performance drops off ...its like an optical illusion what u hear is convincing, what is happening is very different...Also simply advancing intail and therfore the whole curve configeration also increases the cruise timing (intial +cent+ VA) and it is very easy to get into an inaudutable detonation at light load and/or cruise which slowly kills the engine, loads cranks bearings etc.

What the factory specs at the time where, will be very different now running on modern fuels with low compression engines that are most likely have been bored out.

[Forumadmin]

1. On flywheel. Three marks. 12deg BTDC cyl 1&2 and TDC cyl1&2 and TDC cyl 3'&4. But check using thin wire with plug out. Then mark pulley with Tippex.
2. Check static first with distributor cap off. Then idle which should be same place. Vacuum can be on or off and should not make a difference. If you give throttle then timing should temp advance 10 deg. When engine at 2000rpm centrif advance should be 25deg
sent from Paris hotel after celebrating rugby vict

[JohnSchofield]

Many thanks to both for the information. Just to be clear, should the timing marks on the flywheel be viewed through the circular hole at the top of the casing behind the oil filter?

[Keith Andrews]

Just another further note...Always set the pionts gap /dwell before the timing
Any change in the dwell changes timing
Therefore a quick check of timing with timing light is a basic check pionts dwell is ok.

[Forumadmin]

Yes from top. But be careful as there is a fan whizzing around! Hence reason for marking on crank pulley.
There is lots of advice on JowettTalk and in the technotes so please search.

[JohnSchofield]

Thanks again. Have an electric fan so that's one less hazard. I can go ahead now. John

[David Morris]

Hi John,

I do hope the timing marks on your flywheel are visible through the inspection hole? The flywheel on our Javelin has been lightened in the past, to the extent that it is just thick enough to hold the ring gear in place! A previous owner of our Javelin was Ken Lees, who was a past JCC Competitions Secretary, so I guess he was seeking to improve the response of the engine? I have always been surprised that we can still achieve a really slow tick-over with such a lightened flywheel?

If the marks are not visible, then it's then down to the 'thin wire down the plug hole' dodge, as I think Keith has mentioned. But, if doing this, take care not to get the wire stuck!

All the best,

David

[Keith Andrews]

If u cannot find the mark or have doubts if correct.....
Pull #1 sparkplug....put your thumb over the hole and turn the engine over till u feel pressure.
Now take a small screw driver or bit of wire and feel th top of the piston move, coming up.
There is a piont where the engine still turns but the piston doesnt move....
Mark the piont on the flywheel, crank pully, what ever....just where it stops coming up
Continue till u just feel the piston start to move down.....mark the piont where u just feel the piston move down
1/2 way bwteen these 2 pionts will be TDC with +/- error of 1 deg or less

Note the high lights..take those words literally...mark with the piston coming up, dont go past that point, back off so goes back down and mark the 'up' while going backwards, start coming up again amd then mark....and same the other side of the peak.
This relates to any engine....Jowett , Chev big block or Briggs and stratton.

[ian Howell]

David (et al): -

Your mention of Ken Lees reminds me of standing outside the venue of an evening meeting 'somewhere north of London' in the early 70's.

Standing next to another - unknown to me - Javelin owner I commented that the Javelin parked opposite was a bit 'grubby'.

The chap next to me said 'That's because I have been racing it today'.

Embarrassed exit stage left. Ken didn't seem to be all that upset though. Nice bloke.

[JohnSchofield]

thanks again all for further advice.
As a matter of interest, I just timed a 1927 Renault NN, and those clever engineers in Paris provided a screw in the cylinder head which you remove to put your rod down! Then you mark the flywheel when the rod is 'coming up 1/6" below its max height ', ie some degrees BTDC. You still need to get the right cycle, though, and do this by watching the valves. Then release the magneto strap....

John