Engine and Transmission Tech

Actually, I have a question that nobody so far was able to answer in Greece. I own a LR 90 V8, with SU carbs, 1988 model. I have been trying to tune the advance and really got confused by the several instructions and advises I got from car mechanics in Greece. It seems that everybody is quite confused with the vacuum advance system:
1. The SUs with the orifice on top, do not produce any vacuum at idle speed. They produce a vacuum at high rpm. The vacuum pipe is connected to the inner vacuum unit side (facing the distributor) or the external?? Now it is connected to the external. Result: 6 degr. BTDC at idle with vacuum disconected (my setting) and 15 ATDC at high rpm (observed with the lighting strobo "gun". Is this compensation normal??
2. If not, what should be the expected behavior of the system?
I 've already replaced the vacuum delay unit and checked that the distr. vacuum unit works.


Thanks,
Alex

First of all, when revving, the distributor should advance. At higher rpm's it should be further BTDC, not less or into ATDC. This would be retarded, not advanced. Also, you didn't say if you have a points or electronic distributor. Production Rover distributors vary from +4 to -14 degrees static with various advance curves programmed by the "bobweight" springs.

Timing for stock 1980 3.5L Rover V8 with dual carbs

Crankshaft advance
rev/min Min Max
0 0 2
750 3 6
1500 6 9
2000 12 14
3200 17 21
4800 22 26
5500 22 26

Vacuum advance typically introduces an extra +6 degrees of advance but units vary considerably. Vacuum should be attached to outward side of diaphragm. Inward will cause a vacuum retard, not advance. Cap the pipe off on the inner side. To test the unit, pull off the distributor cap. Attach a hose onto the vacuum assembly and put the other end in your mouth. Looking down at the dizzy, suck on the hose. In normal operation, the distributor rotates clockwise. You should see the assembly pull in a counter-clockwise direction, advancing the spark. If it doesn't you've done something wrong.

General, non-ECU v8 timing wisdom appears to be to start with it set to 6 BTDC static, connect the vacuum, start it and see if it pinks when pulling hard. If so back off the timing 1 degree at a time until it stops pinking, otherwise advance it a little at a time and listen for pinking under load. When you detect it again, back off the timing a degree or so. Once you have set it like this it should be about the best you can do with the standard dizzy.

Regards,

Martin



Edited 1 time(s). Last edit at 10/06/2011 11:41PM by bsa_m21.

Thanks for your reply. it is very helpful. I'll try the test you describe. it makes perfect sense.
the distr. is the electronic.
Another test I did is the following:
I detached the vacuum unit off the distributor completely and the motor was reving up as a reaction to the bob weights centrifugal action alone.
when the the vacuum unit is installed connected to the vacuum, I observe a retard of 10-15 degr. at 2000rpm (the puley mark is moving to the AFTER zone, although at idle is set in the BEFORE zone).
This shows that the vacuum unit is over retarding the distributor instead of advancing it, correct:???
I do not know if you saw in my initial posting that the vacuum is increasing as the rpm increase, since the vacuum pipe is installed over the carb slides and not below (this is "ported" vacuum advance I think??) Therefor, the vacuum is increasing as the revs increase, instead of decreasing. Thus my question about WHERE to connect the vacuum pipe!
If I suck on the vacuum pipe, according to your test, this will simulate the high rpm situation in my car NOT the idle. Releasing the pipe will drive the vacuum advance to the idle rpm settings and it should be RETARDED vs. the high rpm situation and the distr. should be moving clockwise - correct??

Again, I thank you for your time and I'll try not to take advantage of it!

Are you sure you have the before and after right on the timing marks? Is it possible that you are misreading the initial timing as BTD and it is actually ATD? Then the normal course of advance would bring it to the other side of TDC. Had a buddy describe to me over the phone that same scenario on his car. I was confused until I went and saw what he was talking about.

Re: "If I suck on the vacuum pipe, according to your test, this will simulate the high rpm situation in my car NOT the idle. Releasing the pipe will drive the vacuum advance to the idle rpm settings and it should be RETARDED vs. the high rpm situation and the distr. should be moving clockwise - correct?? "

Correct.

You should have positive vacuum on the hose when throttle is applied. It should drop off when at idle. Check that the vacuum delay module is not blocked (if you have one). Check that the dizzy rotor will turn clockwise and spring back to its home position under good tension.

Pipe from top of carb goes to pipe on dizzy facing away from body. Pipe on manifold stub is for inlet air temp control valve flap on the air cleaner housing. One on dizzy, facing the body can be either left open or capped. Normally capped if not used, it's for vacuum retard with some stromberg carbs for reducing emissions.



Disconnect the vacuum pipe from the dizzy and plug the hose. At an idle timing should be about 6 BTDC. Rev the engine to 2500 rpm and you should see 12-15 BTDC. If this isn't what you get, something is wrong with the dizzy.

Hook up the vacuum pipe and repeat. Timing should be 6 BTDC at idle, but 20-30 BTDC at 2500rpm. If it is not reacting this way then either the vacuum unit is broken, the hose is leaking or connected to the wrong side of the unit, or it isn't connected correctly at the carbs.

M.

Guys using your advice above, I fixed it!!
The dizzy is producing the advance degrees as you describe in the table above (I tested up to 2500 rpm). NO pinking at any range so far but this could be due to losses of compression ration (??).
Solution was to remove the vacuum delay valve and maintain the vacuum unit, cause it did not move at all no matter how high the vacuum was!
I ordered a new vacuum unit to be on the safe side, anyway.

A big thank you to all who helped me.