Timing a new SVDA dizzy...

[rev-al]

I'm confused.........
I was under the impression that using a strobe timing light, one timed dynamically with vac hose off and plugged, at about 3000 rpm.

I've today read around the forums and seen it written that you are supposed to leave the vac hose connected in place, otherwise full advance won't be achieved and therefore simulate high rev driving condition.

So, what's the truth ?? :?

Cheers
(Also - how about with a Type 4 dizzy which has both advance AND retard vac hoses ?)

 

[sparkywig]

Do you have a workshop manual?
What engine have you got fitted?

All at 800-900 rpm
0 deg for a B series engine (left notch of 3 on crank pulley) with vac hose disconnected and plugged
5 deg ATDC for an AD engine up to 290 640 with vac hose connected
7.5 deg ATDC for an AD engine after 290 641 with vac hose disconnected and plugged
5 deg ATDC for an AE series engine vac hose connected

Set max advance at no more than 30 deg at 3000 rpm.

 

[Trikky2]

Please note - the 5 degrees after top dead centre timings with vac hoses on are only for engines fitted with double vac cans. (these are getting rare now since most have been replaced by single vac units which tend to run better).

The 5 atdc must not be used on single (svda) type dissys - regardless of engine numbers.

Also theres a typo in the list in the previous post - where it reads 7.5 atdc it should be 7.5 btdc.

Dont get these things wrong - it will likely cause engine failure if its set that far out.

So, rev-al, assuming you have a normal 1600 twin port engine then set your new dissy at 7.5 advance static.

If you want to check with a strobe then disconnect the vac hose and the timing should advance, as you rev the engine, to a max of around 28 at 3500rpm. You can then repeat with the hose connected and this should give you around 4 to 6 degrees more advance on top of the previous 28.

 

[rev-al]

Many thanks guys.

The 1600 sp in the crew cab is a B code.

The Type 4 has a dual vac can dizzy at the mo- but the new svda is for this one until I get the original carbs on.

So Trikky2 - I'm assuming then that I'll make a final timing setting of 28 (-30) deg advance TOTAL - this being vac hoses ON ? Otherwise I'll end up with potentially 32-34 advance, which presumably would be too much ?

Cheers
Al

 

[Trikky2]

Many thanks guys.

The 1600 sp in the crew cab is a B code.

The Type 4 has a dual vac can dizzy at the mo- but the new svda is for this one until I get the original carbs on.

So Trikky2 - I'm assuming then that I'll make a final timing setting of 28 (-30) deg advance TOTAL - this being vac hoses ON ? Otherwise I'll end up with potentially 32-34 advance, which presumably would be too much ?

Cheers
Al

No the 28 advance without vac hose connected is correct as I posted earlier.

The whole point of the vac advance is that it gives extra advance over and above the mechanical advance of around 28.

It only does this on light throttle, which is why its vac supply comes from a drilling right where the throttle butterfly rests in the carb.

When the throttle is shut (like at tickover) and when the throttle is well open, there is no vac signal and hence no vac advance. That way when the engine is under load it has 28 degrees but, when cruising on light throttle, it gives the extra advance, which is where the better fuel consumption comes from.

 

[rev-al]

Thanks for your help Trikky2 - much appreciated. Timing light ordered and I'll try it this weekend if I get some time.
Al

 

[rev-al]

As a final re-cap then (as I have more than one aircooled that needs to be checked);
It's the dizzy which dictates the required timing, not the engine. ?

28-30 BTDC for a SVDA or SVA ( @ 3500 rpm)

5 ATDC for a dual vac type dizzy. ( @ 3500 rpm)

???

Cheers
Al

 

[Trikky2]

As a final re-cap then (as I have more than one aircooled that needs to be checked);
It's the dizzy which dictates the required timing, not the engine. ?

28-30 BTDC for a SVDA or SVA ( @ 3500 rpm)

5 ATDC for a dual vac type dizzy. ( @ 3500 rpm)

???

Cheers
Al

Sorry to say this but I am concerned that you are going to do the timing on your engine whilst your understanding of how an engine works could lead you to think that it would be ok to run any engine at 5 degrees ATDC at 3500 rpm.

The only times I use a timing light is on non stock engines/dissys or if the dissy is suspect and needs checking.

I would suggest just sticking to the basic settings and forget about using a timing light.

Dissy with advance system using weights and a single vac is set to 7.5 BTDC static or at tick over.

Dissy with advance system using weights and a double vac is set to 5 ATDC with the hoses connected only at tickover.

As posted previously, dont get these settings wrong because incorrect timing will destroy the engine.