TDC / Dizzy Advice Please

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jimbob
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TDC / Dizzy Advice Please

Post by jimbob »

HI,

I had to replace my vacuum advance today as the diaphragm had split so the timing was out a little.

I marked the side of the dizzy with a scribe.

Turned it anti clockwise, to replace the advance no issues.

Removed the dizzy cap, rotor arm had signs of pitting, so i pulled it off, then all my trouble started, i pulled the bob weights off. Stripped the dizzy down, cleaned the weights & fitted back, now when i turn the rotor arm a few degrees it returns back by itself, no issues.

Moved the pulley to TDC, then i put the dizzy back in, for firing on cylinder 1 / lead one, all good so far, but will it start not a chance i have to rotate it about 145deg anticlockwise before it will fire, and then really rough idle.

Before i did this it was a smooth idle and performance was ok, just my vac advance was U/S.

What have i done wrong Gents?

Any help would be great.

Cheers

Jim



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Post by DEVONMAN »

You need to make sure No 1 is at TDC on the firing stroke and not the exhaust stroke.
The valves on no 1 should both be closed at TDC for no 1 to be on the firing stroke.
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Post by DaveEFI »

You've got the dizzy 180 degrees out. Set the engine to your timing mark again with it pointing at number 1. Now turn the engine one more turn to the same timing mark - the dizzy will now point at No6, IIRC. Remove the dizzy and replace so it points at number 1.
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Post by jimbob »

HI,

Thanks for all the replies & advice.

Ive had to go back to basics, as it just wouldn't turn over and run steady, only when the advance was touching the alternator, i knew this wasn't the correct position.

To check the valves i would have to drain the system remove all the cables and plenum chamber to remove the rocker cover, so I made a piston stop gauge.

Rotated one way until it hit the stop marked, then carried out the same again in reverse, than halved the difference, for TDC, as a check i used a dowel rod and it was at the top, out of chance i wound it back to the timing mark on the pulley it was out by at least 30 deg. Set the dizzy to plug lead on with no advance to start with. Confused me on how it ran but it did. Was this correct on how i did it? I know i turned the engine with the dizzy out, but doing this I'm sure I'm starting at first base so to speak.

Just to add insult to injury my starter motor is toast, so can't even check it, the plastic gears on the drive end are all crushed, cant complain its given me 18 years service. Now I'm on the hunt for a Bosch starter motor, so many cheap brands out there i just want one to last.

Cheers

Jim

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Post by DEVONMAN »

Using a stop to find TDC on no 1 is good for finding TDC but the piston could be either on the compression stroke or the exhaust stroke. So you should make sure the piston is at TDC on the compression stoke before you fit the dizzy and put the rotor to no1 in the cap.

If you can't easily get to the valves to see if they are closed, a finger over spark plug hole will detect some pressure on the compression stroke. You would need someone to turn the crank for you.

As a last resort just fit the dizzy at a TDC (No 1)position and if it fails to fire then remove the dizzy and turn the engine 1 rev and refit the dizzy.
1950 A40 Devon Hotrod with 5.0 twin turbo RV8.
EDIS8 wasted spark, Holley Injection.
Been as far as the Moon and back in 57 years of driving. Same Car, 5 engine upgrades !!!


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Post by jimbob »

Hi,

Thanks, for the tip i thought the same.

At least I know its either there or 180 deg out. Heaps better than where i was before.

Started to doubt my sanity, plenty of fine Anglo Saxon words were bing used when it wouldn't fire.

Cheers

Jim

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Post by jimbob »

Hi,

Engine is back up and working new starter motor is a Bosch rebuild, must say good quality first turn and spins up and cranks over, well pleased.

Just my timing is a bit lack lustre, its set at 10deg, in all honesty i can't for the life me recall what I set it too.

Hopefully my final question, may be a newbie one, my apologies, if i set to 8deg, would that help with the sharpness or would 12deg be best @ idle of course with the advance disconnected.

Cheers

Jim

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Post by DaveEFI »

Wonder if when you pulled it apart by mistake, a spring got stretched? If so, the advance curve could be well out. Might be worth checking the full advance setting with a strobe.
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Post by sidecar »

jimbob wrote:Hi,

Engine is back up and working new starter motor is a Bosch rebuild, must say good quality first turn and spins up and cranks over, well pleased.

Just my timing is a bit lack lustre, its set at 10deg, in all honesty i can't for the life me recall what I set it too.

Hopefully my final question, may be a newbie one, my apologies, if i set to 8deg, would that help with the sharpness or would 12deg be best @ idle of course with the advance disconnected.

Cheers

Jim
Advancing the ignition is more likely to 'pep' up the engine rather than retarding it but you need to be looking at what the 'all-in' figure is. There are well known all-in figures for these engines and unless yours is radically different you would be wise not to stray too far away from these figures. For a 3.5 lump you want to be looking at around 36 degrees all in.

The other thing to look at is what carb you are running, if you are running an Edelbrock carb it does not work too well with the vac system on a Lucas dizzy. If you are running SU carbs then that is a different kettle of fish. I know from experience that most 3.5 lumps want to run about 12-14 degrees on idle, regardless of how that figure is obtained. (This does depend a lot on the cam, my 4.6 runs 20 degrees at idle) In other words if your vac system functions at idle and therefore adds advance at idle you need to take this into account. If it adds say 12 degrees and it is doing this at idle then you don't want to go setting the dizzy up with say 10 degrees with the vac disconnected. Clearly what will happen here is that when you connect the vac system up you would be running 22 degrees at idle. In this case you would need to set the timing up at 0-2 degrees with the vac system disconnected. (The engine won't run well whilst you are doing this!)

I'm not familiar with SU carbs so I can't tell you if the vacuum connection is 'ported' or not.

Personally if it was my engine and the vac system does add advance at idle I would carry out some tests and write down the results so that I know what I'm working with. Now you can obtain some from information stamped on the vac canister and from the dizzy but in my humble it is better to test for this stuff.

1. Disconnect the vac system, make a note of the timing figure at idle then rev the engine so that all of the mechanical advance is 'in'. Note down the second figure and the RPM. You can then work out the 'sweep' that the bob weights add. Lets say the idle was 8 degrees and the all in was 36 at 3000 RPM. This would mean that the dizzy has a sweep of 28 degrees.

2. With engine ticking over with say 8 degrees whilst the vac system is disconnected then connect the vac system and note the timing figure, lets say it goes up to 20 degrees, that means that the vac system adds 12 degrees.

After doing all of that you can connect the vac system and just twist the dizzy round listening to how the engine responds, you should be able to find a dizzy setting where the engine runs best (Highest idle speed/highest vac level, you can then the pull the idle speed down using the idle screw). Check the timing figure but leave the vac system connected, lets say its turns out to be 14 degrees. That figure is fine, we now know that the mechanical advance must be 2 degrees, we know that the all-in at 3000 RPM will be 2 + 28 = 30 degrees AS LONG AS the throttle is wide open. At 3000 RPM with the throttle just cracked open the vac system will add another 12 degrees to the 30 which makes 42 degrees, this is fine, if you then boot the throttle the timing will back off which is what is needed.

Even from the above you can see that it is possible to end up getting figures that are not ideal, in the example above the all-in at WOT is only 30 degrees, really you could do with another 6 degrees. You are then into modifying the dizzy which is possible if you are handy with files!

AJMHO!

(The above a quite a lot of waffle, I'm at work and bored!) :D

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