Two questions please.
What if there is no number stamped on the VA?
Can I assume that there are hard stops for the spring adjustment both fully compressed and relaxed?
Thanks
Yes and no.
on the arm is a stopper. You can see it in post 34 from HalifaxHops. As vacuum pulls the arm in, eventually the stopper will hit the back of the V-can and that stops it from advancing any more.
Those stoppers vary in size for allowing different amounts of maximum vacuum advance. You can cut the stopper off partially, to increase the max. The most I have gotten is 22*, but Trailbeast got 24* out of his.
On the other-side, as already mentioned, if you drive the spring to maximum compression, it will eventually limit the amount of vacuum advance possible to whatever vacuum your particular engine can generate.
The adjusting screw is torque-limited. If you back it out thru the stiffness, it will fall out.
Here's how I use the Vcan;
Part I
I map my PowerTiming.
I map my Vacuum advance unit.
I observe my cruise rpm.
I look on my map to see how much mechanical advance I have available at cruise rpm.
I look at the Max that my can is able to generate with the vacuum my engine can generate at cruise rpm.
I add those two up.
If the number is too high, the engine may detonate.
If the number is too low, the engine may suck gas.
The cruise timing should total more than 44* and less than 60*, with a good compromise being 50>56 degrees.
Here's how to determine the best cruise timing for your Combo;
Loosen the V-can so that it can pull-in all that it is able to.
Advance your idle timing until the rpm stops increasing.
Rev the engine up to cruise-rpm, then keeping it at that rpm, Without regard to the numbers on the balancer, just add more advance again,until additional advance produces no additional rpm. If the rpm goes down at any time, you have gone too far, back it up.
NOW
install your timing light, rev it back up to cruise rpm, read the TOTAL all sources timing. The number you read is what the engine wants, at that rpm and load setting.. Subtract THREE degrees for load compensation. This will be your target.
Suppose your engine wants 56*@2800 with something like 3.55s.
Suppose your total mechanical timing available at 2800 is 28* Then; 56 less 28=28 more needed in the Vcan. Suppose your particular can, is only able to provide 14*. Thus your engine is 28 less 14= 14* short. it will suck gas big time.
So then, you take the Vcan out of the D and cut the stops, and say you get 22* out of it. Now you are 56* less (28+22)= just 6* short. That is as good as it gets, unless you want to try running Power-Timing of; 28+6 =34* at 2800. It will cruise just fine, but what will happen when you floor it in Top gear at 2800 with 34* of Power-Timing? My guess is that the engine will rattle itself to pieces..... over time. Don't do that.
Part II
I use the v-can to augment a slow mechanical curve. That is to say if your engine will not tolerate more than 24*/WOT at 2400stall, then 24* is all you can run. But suppose that at that same 2400 rpm at Part-throttle light load, your engine likes 46*. Well , just modify your Vcan to make 22* and adjust so it all comes in at whatever vacuum your engine can generate at 2400rpm/light load.
So now your engine is safe at WOT(24*), and fuel-efficient at 46*cruise. AND
Since the V-can is operating at max of 22*, then by varying the throttle opening, you can vary the amount of all-source timing anywhere in between that 24* and that 46*, thus making the throttle very responsive.
If your particular combo, detonates anywhere in that range, readjust the V-can to kick itself out quicker.
If you still cannot get away from detonation, you most likely have poor chamber characteristics, and will need better gas.
If better gas won't solve it either, then check your cylinder pressure; it may be that that your combo will never work properly at the theorized numbers I have provided.
Finally, you may have to reduce the load on the crank with a higher stall or racier gears.
If you are able to get this right for your combo, you will be rewarded with a very fun engine, that also gets great fuel economy.
But be warned; not every combo will accept the kinds of mods to the extent that mine does, so you may have a summer of dinking around ahead of you. Take it slow and sneak up on it. Start by disconnecting the Vcan, and by mapping out your PowerTiming.
If it was me, I would try to use a two-stage curve, using the one-long loop spring. This will get you a fast stage-one, then a delayed and slower stage -two, and you can fill in the blanks by trading initial-timing for all-in timing, using the slots in the advance-plate.
I would start with 20 degrees in the slots and 12* at Idle, for a total of 32* .. This will give you 2>4 degrees on the final approach, and allows for fooling around, during the preliminary adjustments.
If you run too much Idle-timing you will upset the Transfer-slot fuel to mixture screw fuel, and cause problems like tip-in sags, hesitations, maybe a bog, and an all-the-time wrong AFR. Not to forget the possibility of a too-high idle-speed and/or clanging the power-train on transmission engagement. Once you get the T-slots sync'd up, your engine does not care about idle-timing. No matter where you set it, it will NEVER be optimum, anyway, because no carburator that I know of will actually operate at the extreme idle-timing that most engines actually want or will tolerate.
If you want to actually find out what yours wants do this;
Sync up your transfer slots to show about square underneath the primary throttles. Close the secondaries up tight but not sticking. Open your mixture screws to in the center of their working range, which on a Carter-type is 2.5 turns. Make sure the float-level is where the designer specified. Make sure the PCV system is functioning correctly and if you have a brake-booster, clamp the charge line shut.
Now, increase the timing without regard to the numbers. Keep on increasing until additional timing does NOT produce additional rpm. Now put the timing light on it. Whatever you read there, is what the engine wants at whatever the rpm got up to, with no load on the crank.
Ask yourself, how that will run in gear and accelerating......... There is no way to slow the idle back down, on any carb I know of, that will not destroy the T-Slot sync, which has to be set that way, for tip-in and drive-away considerations.
Now
if you have a factory EFI car, go put a scanner on it, that has a graphing capability, and watch the timing. It's a thing of beauty to watch.
