View Full Version : Static Timing: 12*BTDC or 8*BTDC ?

06-14-2008, 02:54 PM
Before I try my new timing light, I thought I'd check the static timing. My engine is a '69 (not original to the car), and the Bentley manual shows static timing should be 12 degrees BTDC, and 4 ATDC at idle. (In fact those numbers are good regardless of the year.)

Sure enough, when I measure the static timing using a bulb and positioning the mark at 12 BTDC, it's bang on.

However, I have read in this forum several times that 8 degrees BTDC should be used.

Will setting to 8 BTDC reduce my fast idle?

06-14-2008, 03:51 PM
I dont think so, it will probably speed up, I think I set mine at 12 BTDC, 71 TR6, if you feel like experimenting try all at BTDC 12, drive, 10 drive 8 drive, get the pic. set it as what works the best. what works best for ones persons car doesnt work as well for another, I think age, dist wear, and advance spring tension wear affects timing. I need to send my dist to Jeff at advance dist now that I think about it


06-14-2008, 05:55 PM
Should knock it down some, but not a lot.

I've lost track ... did you check that the float bowl vent valves were not holding the throttle open ? Or do your carbs not have the valve ?

06-14-2008, 06:49 PM
Should knock it down some, but not a lot.

I've lost track ... did you check that the float bowl vent valves were not holding the throttle open ? Or do your carbs not have the valve ?

I haven't checked: I wanted to eliminate the timing before tackling the carbs.

06-14-2008, 08:33 PM
Should knock it down some, but not a lot.

I've lost track ... did you check that the float bowl vent valves were not holding the throttle open ? Or do your carbs not have the valve ?

That was the cause of my fast idle, disconnected the critters and life is good, they are there to make the anti- runon system work but mine stops without any run-on so I dont worry about them.

06-14-2008, 09:39 PM
So I set the static timing at 12 BTDC, and then I ran the engine up to temp, disconnected and plugged the vacuum advance tube, and looked at the timing marks with my new timing light.

The pointer was at about 16 BTDC!

So I twiddled the distributor and set it so that I had 4 ATDC. At that point the idle was about 900rpm: hoorah!

Oddly, reconnecting the vacuum advance to the distributor had no noticeable effect.

As I revved the engine, the timing went in the direction of BTDC.

The temp gauge was pegged to max when I looked at it, and when I turned the ignition off, the engine kept dieseling: it wouldn't stop!

I guess I've done something wrong. Perhaps I have rotation sense wrong ...

To be sure, if I'm looking at the pulley from the front of the car, the timing marks to the right of the pointer are BTDC, and those to the left are ATDC, right??!

06-14-2008, 11:44 PM
How fast was it idling when you saw the 16 BTDC ? If it was over about 1200 rpm, the centrifugal advance may have started already; accounting for the 4 degree difference you saw. Of course, wear in the dizzy (or even a slight error in setting the static timing) may be to blame as well.

Everything else (including the overheating with the ignition retarded and the running on) sounds pretty normal.

As I've tried to explain before, the 4 ATDC figure should only be used if you have vacuum RETARD connected and operating. With all the vacuum lines disconnected, you should see the "static" figure given in the book.

Speaking of which, Mr Haynes gives 10 BTDC for the TR250, and the Factory 1970 and 1973 emissions bulletins also give 10 BTDC for the TR6. But this isn't rocket science, so a few degrees either way isn't going to make much difference.

Retarding the spark will slow down the idle, as you have found. The factory set up the emissions carbs with wider throttle openings at idle to compensate for the retarded timing (thus burning more fuel at idle and increasing the tendency to overheat). You can choose to go either way : either keep the vacuum retard and the factory setup; or eliminate the vacuum retard and deal with having to modify (or remove) the linkage to the bowl vent valve in order to be able to adjust the idle rpm to spec.

My personal preference is for the latter approach; but if you go that way, you have to pretend you've never seen the 4 ATDC timing figure !

06-15-2008, 07:34 AM

As explained before in many threads this process is called tuning and one adjustment definitely affects another, so start with the basics.

1. Check point gap, make sure it is correct.

2. Insure that you have no vacuum leaks.

3. Start car and allow it to warm to operating temperature.

4. Disconnect and plug vacuum line to distributor.

5. Set timing at 12BTDC with light connected.

6. Check idle speed, if lower or higher than 850-900, adjust idle speed up or down accordingly.

7. Check timing again. It will not be at 12BTDC if you changed idle. Readjust to 12BTDC again, tighten distributor and check idle.

8. You may have to repeat this step 2-3 times if you are off the mark now, but that's why it's called tuning.

Then and only after the timing is locked in at 12BTDC, go for a ride. Your operating temperature should be normal and you should have no ping.

If you have no ping you should be fine. Now is the only time that you should ever mess with the carb settings. You can't adjust carbs if these steps are not followed and corrected.

06-15-2008, 11:51 AM
Thanks Paul and Randall.

Yes, I've read a lot about this :smile: I was unsure about the rotation sense, but now I'm sure I have it right.

My car has no emissions gear. The engine and carbs were completely rebuilt at Moss.

Today I think I will set the timing to 12 BTDC: I don't like the 4ATDC setting if it boils my coolant at idle :smile:

I am slightly concerned that the vacuum advance doesn't seem to have any effect, connected or not.

06-15-2008, 11:59 AM
I am slightly concerned that the vacuum advance doesn't seem to have any effect, connected or not.

Take the vacuum advance unit off, and "suck" on the vac pipe. if you have good strong lungs you should be able to make it move a bit. Of course using a vacuum pump is probably easier. In any case the diaphragms do break so it is possible that it needs rebuilding.

06-15-2008, 03:59 PM
I know Paul didn't want to confuse the issue, but I feel sure he'll agree that a vacuum gauge connected to the manifold nipple for the brake booster will give you a good indication of when you hit the optimum settings and carb adjustments.

06-15-2008, 04:08 PM
In General:
Vacume Advance:
There are three types of vacume advance setups.
1. Being straight manifold vacume { This type usualy incoprorates a V.R.V. {Vacume restrictor valve} {This valve or something like it can be incoporated into the vacume advance moduel itself Eg. a vacume bleed off port} to prevent the advance during idle conditions} Among many other various vacume operated valves on the emission system.
2. Being ported vacume, either connected directly to the carburator above the throttle plate or connected to a thermal valvebody attached to the intake manifold which in turn is connected via vacume hose driectly to manifold vacume, incoprorating many emission related vacume oprerated valves and gizmos. This thermal valvebody is only active at O.P. temp.
3. Bieng a dual advance/retard system. {two vacume lines from the vacume advance/retard pod on the distributor.}This system is dependant on the emissions system being left intact and working properly.
There should be NO active vacume advance while the engine is in an ideling condition. If there is, something is amiss, the V.R.V {If your system requires one} could be malfunctioning or removed ETC.
The primary function of the vacume advance system is
to increase engine timing on acceleration for improved power and to increase timing advance in a cruise condition to improve gas mileage, eg. economy.
Vacume advance malfunctions will have a great effect on engine timing at above idle conditions but should have little or no effect at idle conditions.
The centrifical advance will have an effect at ANY engine speed! It is best to ensure that the centrifical weights are intact and operating freely and the springs are correct for the advance curve you desire or your engine requires.
ANY advance in timing WILL increase engine speed at idle. Static timing is Just to get you in the ballpark.
As stated above, It all works in conjunction, idle speed, and timing. Once the initial timing is set { Vac Adv. dissconnected} reconnect the vac advance and recheck the full advance condition {Being a specified advance in degrees {example only EG. 34*} at a specific engine R.P.M. {example only eg. 2,800} set by the manufacturer}{usualy a cruise condition R.P.M.} to see if it meets the O.E.M. specs.
NOTE: Too much advanced timing is NOT a good thing for your engine!
<span style="font-weight: bold">This is ALL just GENERALY speaking</span>, Your car may have "other" gizmos, gadgets or equipment attached to the vacume circut that can {and usualy does} have a profound effect on vacume preformance items such as the vacume advance module.
I hope this info at least gives some insight to anyone that is going to attempt removing, rerouting, Vacume lines or related devices. The Vacume advance can be dependent on some or many of these devices especialy on newer vehicles. Or {Like me} trying to figure out what someone else that has no idea what they were doing and removed ALL traces of vacume hoses and related devices from an engine. Then wonder why it wouldn`t run very good!
P.S. YES ......... the Idiot left ALL of the vacume ports uncapped too! GO FIGURE!

06-15-2008, 05:49 PM
Just to clarify a bit, perhaps, the TR6 uses ported vacuum advance (on models with vacuum advance at all). That means no advance at idle, even with the line connected.

If you are worried your vacuum module might be bad, I'd suggest a tester, like this one :

06-15-2008, 05:51 PM
<div class="ubbcode-block"><div class="ubbcode-header">Quote:]I know Paul didn't want to confuse the issue, [/QUOTE]

Agreed. Let's get points, base timing and idle set, THEN look at other things and other ways of tweaking carbs, etc.

06-15-2008, 06:08 PM
You know, I am STILL confused about how to set the timing
on my car.

My 1969 dizzy has TWO ears and each carb has a tiny plastic
pipe that runs directly to a dizzy ear. My fan belt wheel
has one deep etch mark and one half across etch mark.

With both plastic tubes connected and my timing light set
at 12 - the metal arrow lines up with the deep etch on the
fan belt wheel; when the light flashes from No.1 spark plug.

Engine warm at 800 rpms.

Have I set my timing correctly?



06-15-2008, 06:18 PM

In most cases, with only two marks, the deep notch is 0 or TDC and the smaller is 4 ATDC. The smaller will be above the deep if looking at it from the drivers side. That is the direction that you DO NOT want to go.

Whether you have an advance and retard unit together, or just the retard, all settings are made on a warmed up engine, at curb idle (850-900) and with either one or both vacuum sources plugged to prevent a leak and lean condition.

Then follow my instructions above and don't think that it will all be right on the first shot. As I've said a hundred times, it's called "tuning" for a reason.

06-15-2008, 06:22 PM
If your distributor has been neglected it is likely to have the advance mechanism frozen. When you do a normal oil change you should remove the rotor button and put a drop of oil on that big screw head in the center of the shaft. Other wise, over time, the advance mechanism may lock up.

Perhaps some one with eloquent descriptive skills will tell you how to see if the advance mechanism is free to advance.

06-15-2008, 08:43 PM
<div class="ubbcode-block"><div class="ubbcode-header">Quote:]Perhaps some one with eloquent descriptive skills will tell you how to see if the advance mechanism is free to advance.[/QUOTE]

Well, pictures are better than words. Those will be on the link at the end.

If you rev the engine slightly above idle, with the retard and or advance disconnected, the centrifugal advance should begin to advance the timing, or cause the numbers on the damper that are further below BTDC to line up with the marker. This should continue until around 3,200-3,500 RPMS and it should move back down, just as smoothly. There should be no hesitation in the advance as the engine speed increases and vice versa on slowing down.

If it looks like this inside, it's probably not working at all, or even worse, sticking intermittently causing a very erratic idle or hesitating performance.

Send it to Jeff and have it done right.