12 replies to this topic

[darkcat]

Hi guys, Im just trying to get my car back on the road after an SPI to carb conversion.
I am having a bit of trouble with emissions (suffering high HC's) so need to be sure my ignition timing is spot on.

So i need to know, what should the static timing be at idle (1000 rpm) both with and without the vacuum advance attached?

It is a 1275 A+ with a minisport CA4 cam and a dellorto dhla 45 carb


Thanks!!

[jaydee]

Look for around 12 degs at 1500 rpm with vacuum disconnected.

Optimal setup can only be done using a gas tester with engine under load.

Unless you have a megajolt, you wont tune it with vacuum connected. Theres no point doing that because its run by the dizzy vacuum advance unit which you wont tune in a conventional dizzy. You're likely to get pinking under load or too retarded timing under load. Its a waste of time.

[darkcat]

12 deg ok - i'll see what its at once ive got the petrol tank back in!
Surely the vacuum advance should be somewhere specific at idle with vacuum connected? 

[jaydee]

Yes but you need to go megajolt route to tune it properly.

[darkcat]

yes but im not going to (yet) 

 

my point is if im getting too much or too little vacuum at idle then the advance could be way out with the vacuum attached.

[jaydee]

Actually its the wrong way round, advance is likely to be way out if you tune it with vacuum connected, and will surely be out if tuned basing on idling, you need to put it under load. 

Vacuum advance will mostly effect idle (which you wont care about as the CA04 is a 544 cam profile), and it will affect cruising and pick-up. 

Max load usually falls into hte mech advance curve of the dizzy.

Tune the timing on mechanical advance first, then tune mixture on your ingition timing.

Vacuum tuning will improve cruising mpg and pick up, resulting in a better overall performance and fuel consumption, but its much more important that you have the correct advance curve with no vacuum.

[darkcat]

Yes understood, but im looking for SOME IDEA of where it should sit at idle when connected!

[KernowCooper]

You should see a 3 digit code stamped on the side of the vacuum unit, the three-digit code indicates starting advance, full advance and the total degrees of advance. 

 

Between the many different unit codes, units can have a starting range of mercury vacuum from 2 to 10, full advance mercury vacuum from 4 to 18 and total ignition advance as little as 3 to as much as 12 degrees.  “At the crank", this is 6 to 24 degrees ignition advance.  

 

A unit with a code of 8-18-7 will start the advance at 8 mercury vacuum.  Fully advance at 18 mercury vacuum and has a total ignition advance of 7 distributor degrees, that is 14 degrees at the crank.  Installing a 3-13-12 would advance too soon and be 10 degrees over advanced if your vehicle called for an 8-18-7.

 

So the first code 8 is where the vac advance will start to operate at 8 mercury vacuum

The second code 18 is the maximum mercury vacuum advance

The third code is how much advance in degrees the vacuum advance will add 7degrees

 

Remember the distributor runs at half crank speed so 7degs at the distributor is 14degs vacuum advance maximum.

Edited by KernowCooper, 02 September 2013 - 04:30 PM.

[darkcat]

Thanks Dave - good info if not QUITE what im after - So working on those figures, if it was a 8-18-7 vac unit, and i had the static idle set at 10deg btdc, i should expect to see an idle at 24deg btdc on full vacuum...

[KernowCooper]

10degs static would be a bit much, 10degs at 1000rpm with the vac pipe off, then vac pipe back on and 24Degs mate, and at idle thats where max vacuum would be

[darkcat]

well yes, my idle is at 1000 rpm (not happy at all on that cam at less)

[KernowCooper]

So your timing is 10degs BTDC at 1000rpm with vac off,which is whereabouts you'd expect with the cam

[jaydee]

Yes understood, but im looking for SOME IDEA of where it should sit at idle when connected!

 

You cant extablish whats better without a gas tester showing the O2 (oxygen) values.

Actually any value between 20 to 28 degrees will work with a rich mixture. You need a rich idle with such cam (obvuiosely NOT while running in the engine!!).