Valve Timing

Pertinent ramblings though jon d

Kinda makes you see why they did away with the old distributors . . .

jon_d wrote:

Then I loosened the locking clamp and backed the dissy way back then slowly rotated it in the right direction until the light flickered then went out.
This is where it is suppose to be, 8' BTDC.
Going by an uneducated guess the way it was certainly was not right as it was firing after TDC.

Just some ramblings...

This method is known as "static" ie, setting it with out the engine running. It's a good start. But to do it properly, the engine must be running and with a timing light.

The spark timing is set when the light goes out. The light going out reflects when the current in the primary coil (12V winding) stops flowing. This triggers a 'breakdown in flux" which then generates the HV pulse in the secondary HV coil.

The HV voltage will increase to a point where the air/fuel mix across the plug breaks down (ionizes) and sparks.

If the gap is to large, or the air fuel is to 'dry', then the spark voltage has to be higher to ionize. If the coil voltage cannot reach the required ionization voltage, the engine will miss. If something else (eg HV wire) breaks down, the engine will miss.

If the rotor rotates off the brass probe n the cap before ionization, the engine will miss. (aka play in the shaft)
If the rotor moves off the cap, during ionisation, the spark will be short and a full burn/ignition may not happen. (aka miss)

If the HV lead or something else breaks down at a lower ionization voltage, the engine will miss.

If the dwell is set wrong, the LV coil will not saturate and therefor the HV ionization voltage will be low and possibly not trigger a spark. (or a weak spark)

If the plates holding the points, or the rubbing block opening the points is moving about, the dwell will change. (see above for impact.)

Vacuum timing advance impacts dwell. If it's faulty or not set right it can change the dwell at different RPM's.

You want the dwell set such that maximum HV is generated at high revs. Which would mean a longer dwell at idle... but not to long becuase then the mechanical timing components come into play.

Thank you for that jon, that's more than I know about and very informative
If we had some were with "The Golden Rules of Getting Our Buckets of Bolts Going" on here this type of information should be at the top of the list

Very helpful stuff

Paul

Thanks everyone for the replies once again, it does help a lot.
Put it all together again then started from scratch with the spark side of things.
Cleaned and gapped the new plugs, checked the point gap (not dwell) and all was good here. The dissy is still spotless inside from when I last stripped it down to inspect. There is no advance in this old Lucas dissy so thats not a problem. Although there are bob weights below the staor plate the stator plate has no way of moving it is screwed in place,
Double checked the HT leads from inside the cap to the brass connectors and get all 000 on the mutimeter. I set the flywheel to position "M" and then rotated the dissy around until the light went out and this is when the point gap has just broken as the manual wants it to be.
Time to start up and it jumped to life quite well and I ran it for a short time checking the gauges are right then started lifting the revs from idle to full. Got a flat spot in the carby. After a short time I had the air bleed screw out 1 3/4 turns and the flat spot was gone. Then the engine died. Out of bloody fuel. Fuelled it up again and started again and the flat spot was back. How does that happen?
Went inside for lunch and ponder on the why the flat spot vame back and got a phone call to go and save another stationary engine from going to China so that stuffed the rest of the day. So back at it tomorrow to see what happens.

cobbadog wrote: Fuelled it up again and started again and the flat spot was back. How does that happen?

There is a variable you haven't thought of

If you only keep on the checking and adjusting the same things you will always get the same result

As to what this is I dont know as Im not there

How ever I feel it isnt spark, or at least it isnt totally spark

I reckon I would be checking mating surfaces of manifolds and the like

Something is changing when it gets warm

Also I would look at mounting the coil a long way off the motor to prove to myself it isnt getting affected by heat

Also make sure it has a good fuel supply as it had just run out of fuel and when you put some in it may have sloshed something over the fuel outlet from the tank
And yes I recall this tank was lined so maybe a bit of the lining has come off
If your filling out of jerry cans thats another source of stuff to get in

Not saying any of these are the problem but I would be checking everything if it was mine

Paul

Got a flat spot in the carby. After a short time I had the air bleed screw out 1 3/4 turns and the flat spot was gone. Then the engine died. Out of bloody fuel. Fuelled it up again and started again and the flat spot was back. How does that happen?

Just a long shot.... How long (time) after you adjusted the flat spot out did it run out of fuel?

If the bowl was emptying when the flat spot was gone, then, there may be a float level (or fuel height level) issue.

Just something like to high fuel level may change mix as the jets cross over from the idle to main. The emulsion tubes and their little holes come into effect.

Even things like adjusting with the air cleaner off, vs driving with it on can have subtle effects on old designed engines.

When I rebuilt this engine I knew there was an issue with the surface of the manifold to head so I had that machined way back then and only the other day while the engine was running at idle I sprayed either around the carby joints and shaft as well as along the manifold top and bottom using a suirt tube from the nozzle of the can to get to those hard to get to places. Made no difference to engine revs so I feel that there are no vacuum leaks.
Now that idea has merit about it running out of fuel before the engine stopped. Problem is I cannot remember how long before but just might be what happened.
To adjust the float level on these upside down carbies you undo 4 screws, drop the bowl off and turn it 90' then turn on the fuel. It is to rise up to a certain level on the main jet which last time I checked this is where it was exactly. Only way to adjust it is to then remove the needle and seat from under the top of the carby and add a flat washer. When the new needle and seat came it came with 2 different washers, thin n thick and you are to use either or both to achieve the level. The thin washer was wrong as the fuel rose way too high. Fitted the thick one and got the right height. Many have to try lowering it more than the specs say.
The coil sits nicely against the rocker cover and gets a good supply of air from the fan and does not get warm never mind hot and the 2nd brand new coil made no difference to what is going on.
When I get back to it again I will start again on the "M" mark as I have moved the dissy since that time trying to improve it with no luck. I want to track down a new dissy cap so I have to ID the model Lucas dissy so I can get a cap to suit but I think it is the same cap as a Land Rover Series 1, yet to be confirmed.
Go back yet again to the fuel level in the carby and also double check the fuel supply from tank to carby. I still get a small amount of rust into the glass bowl at the tap and I think that is comming from the filler tube inside the front tank as this cannot be seen unles you cut the end off the tank so it is possibly not been coated by the tank sealer. The filler tube is what the cap screws onto and goes 3/4 of the way down into the tank.
Now the last time it was running and after I was opening and closing the throttle it was possibly getting a bit fuelled up and a couple of back fires up the chimnet happened too, again I had shut the fuel off to allow it to run dry like I always do.

cobbadog wrote: Thanks everyone for the replies once again, it does help a lot.
There is no advance in this old Lucas dissy so thats not a problem. Although there are bob weights below the staor plate the stator plate has no way of moving it is screwed in place,

Your distribtor has centrifugal advance. The bobweights move outwards as the revs increase, opposewd by springs. This is meant to advance the top part of the shaft in relation to the lower part driven by the engine. This part can seize. You are meant to lubricate this by removing the rotor and putting drops of oil on the felt in the middle of the shaft. This movement can be checked by turning the rotor in the normal direction of rotation, it should move a few degrees and spring back when you let it.

With old fashion magnetos the stator plate moves by manual control, whereas in the distributor as Paul says it all happens beneath the fixed stator plate. If your auto advance has been working as it should and you timed the static spark at TDC then as the revs increased it should have compensated for the late spark, but maybe not enough to stop a misfire. Keep at it.

So if you are using a timing light you should see the timing advance when the motor is reved up like on a normal car or truck motor ?

I believe this would be one way of checking its actually working as its meant to

Paul

OK, this is something else to explore. As for the timing light, I haven't menat to be ignoring you. I do have one but to connect it up to use I have to extand the power cable/s to allow it to reach from the power to the flywheel opening inspection hole or try and hook it up behind the dash and possibly short circuit something up there.
So when we get back from a short holiday down south I will hook up some extension power cables and have a better look. As it is now I am about 300mm short. Dissie on drivers side, timing marks on passenger side and a hot exhaust manifold in between as well.
There is some movement with the rotor button but thought this was possible back lash, again will check this out more when we get back.