DCOEs and Timing

[hoffstock]

OK, I've searched and read but have a question. I keep seeing people talk about adjustment of timing after DCOE install and replacement of vacuum advance dizzy with mechanical advance. Should I fully expect to have to play with my timing after installing my 40's? My timing was totally dialed in with a 38/38, 292, oversize valves and P&P. Will it be OK to leave as is with the 40s? What do I do with the vacuum port on the stock dizzy?

I'm admittedly a total novice when it comes to timing setting and adjustment. Can anyone recommend a solid book on this topic - general theory, adjustment and setup?

thanks in advance

[02for2]

Weber DCOE's were bred as Race Carbs. As such, there's always a compromise when using them on a street car.

You must first decide your goal. Do you want POWER, TORQUE, or MILEAGE? Properly set up, the DCOEs will give you one, maybe two of these, but not all 3 of those chracteristics.

The reason for swapping a dizzy with mechanical advance is because the DCOE's do not have a vac take-off to power the vac module on the dizzy.

You could drill and tap a take-off from the manifold, but you'll get into lots of other things, so not recommended.

You'll need to recurve the dizzy (or get another) so your advance comes in earlier (like the vac advance did). This is a calibrated exchange of weights and springs and must be done by someone with the proper equipment such as a SunTune distributor machine. Advanced Distributors is a company which can convert your old vac advance dizzy to a recurved mechanical one specifically setup for the M10 motor.

An ignition with no vacuum advance will require more initial mechanical advance than a vac advanced one for part-throttle tractability. 5 degrees at least and you may end up with 10 degrees or more.

But, there is much more to getting a set of DCOE's operating properly. You need to have the correct jetting, incl. altitude compensation and such. Static Timing is important. There are baselines available here using Search, but to truly dial in your motor, you'll likely need to go off these for variances in altitude, individual volumetric efficiency of your motor, avg. local temps, local fuel quality, etc. This can really only be accomplished on a dyno.

You may get the car to run by just plugging and playing, but I doubt you'll get max benefit from the carbs until you specifically dial them in.

Cheers!

[c.d.iesel]

...."My timing was totally dialed in

with a 38/38, 292, oversize valves

and P&P. Will it be OK to leave as is

with the 40s? "

= yes leave the timing alone - IF

IT IS INDEED 'CORRECT' and work on

optimizing your air/fuel mixture jet settings

[Preyupy]

Depending on what distributor you are running there is 4-6 deg of vacuum advance, and the possibility of 5-7 deg of vacuum retard if you have the 0 231 180 008 distributor.

DO NOT hook the vacuum advance up to a port seeing manifold vacuum, even worse if you do it to a single runner on a Ti manifold. The vacuum needed for the advance to work correctly is generated by the airflow depresion in the venturi of the carburetor so as you open the throttle more and there is more air flow throught the carb more vacuum is generated and ignition timing is advanced. If you hook it to the intake manifold you will see higher vacuum at closed throttle and lower as the throttle is opened causing timing to advance at idle and retard as the throttle is opened. If you hook it to a single cyl intake runner you will get a pulsed signal and you would be able to watch the timing jump around.

Depending on what compression ratio you are running, what plugs and fuel you are using I would suspect you need somewhere around 33-35 deg BTDC at full throttle above 3500 rpm. The "curve" in the distributor will need to be set so that it runs well (no detonation) at all loads below that speed. A all centrifugal advance distributor (ti- Tii) has about 12-14 degrees of advance in the distributor (double that to 24-28 deg at the crankshaft) with the advance starting at about 1000 rpm (crankshaft) and ending at all in by 3200-3500 rpm. You can change the total amount of advance and the rate the curve comes in by changing the weights and springs but it does take someone with the knoledge, equipment and parts to do it.

If you try and use the distributor you have and just leave the vacuum hose off you may wind up with too much advance at lower engine speeds if you set your timing at 35 deg BTDC at 3500 rpm or not enough advance at full throttle if you set it so it runs best at lower engine speeds. But then again it might work just fine. You are going to spend a lot more time getting the drivability set on the DCOE's than playing with ignition timing.