TheSamba.com :: View topic - Mallory Unilite with Vacuum Advance

Tue Mar 08, 2016 8:43 pm

Just had my 74 bug (2020cc dual webers) out for first run of the season.

I have my carbs tuned to best lean and they seem to be idling very well (albeit, very fast idle.) Engine is not very responsive however, and lacks power (oscillates a bit between surging/dying when under load) There is no backfiring or popping, just lacks power. There appears to be a real dead spot right in the middle of the power band.

The engine seems to be idling way too fast (didn't have tach hooked up and old timing light broke, but estimate at least 1500RPM) and the idle speed screws were at default (barely touching the throttle) but I get NO power or throttle response if I back the timing off from there.

So I want to start from scratch and dial in my timing. I got a new light and will start tomorrow but I want to start taking advantage of the adjustments on the Mallory Unilite to dial in when my centrifugal and vacuum advance starts to come on. I think I still have the spring kit that came with it (I do not recall if Aircooled.net setup the kit or not, but I'll find out when I open it up.)

I'm reading from Aircooled.net about some basic setup workflow for my distributor (http://www.aircooled.net/vw-distributor-options/) and I wanted to just ask what you guys think. Can you clarify a few things for me and add other opinions?

Quote:
The easiest method to follow is very straightforward. Find the MAX advance point your engine tolerates (between 30-40 degrees BTDC) above 3000 RPM.

1) I'm assuming I should disconnect my vacuum line and lock out the centrifugal advance to find this point manually?

2) I'm not familiar enough with tuning to know what the warning signs are of detonation before it starts to happen - what am I actually looking for? The engine seems to respond better and better as I advance it (real qualitative analysis there) but I don't want to go too far and damage something.

Quote:
Now find the place where it responds best at idle. It may be 15-24 degrees BTDC.

3) Just to be sure: "Responds best" as in, quick response to a quick throttle change without coughing?

4) If the engine doesn't return back to idle quickly after letting off the throttle, is that a sign that timing is too advanced? Or is it likely to be an issue with fuel delivery? Or both?

Quote:
You may find that this much advance causes starting difficulties when the engine is hot, so be sure to check this out! Ignition before top dead center causes “negative torque” and that’s what suddenly STOPS your engine from cranking. Avoid this, or use a retard that is activated during cranking.

5) What devices are available to retard timing? (Like something to bolt on like a choke for timing?) Or is this something that a distributor would have to be equipped with already?

Quote:
Now, you know the two critical points in your advance curve. You only need to figure out how FAST you want the advance to come in. I recommend being conservative, and kick the advance in SLOWLY and see how the engine responds. Keep adjusting it (it requires distributor disassembly) so it comes in quicker and quicker, until you detect detonation (problem). Back it to the previous setting, and you are DONE with the centrifugal advance.

6) Should the vacuum advance be disabled still, up to the point that centrifugal is dialed in? (Assuming so.)

Quote:
Now, dial in 10 degrees of vacuum advance, and adjust the advance so the carb(s) will actually activate it and you are done! PHEW!

7) I'm guessing I'll need a vacuum gauge and some sort of pump to specifically adjust this. I haven't actually looked for any resources that detail this part of the adjustment but if you can recommend some, that'd be great.

8.) Where do you guys get a clean +12v/ground in the engine bay for doing timing? Alternator terminal? Last year I seem to have destroyed my timing light by using coil +12 (which was probably just a really bad idea?) (I was using batteries for it previously, just because the wires became a hazard)

Wed Mar 09, 2016 1:38 am

Wowzer. Ok, here we go:

1.Yes, start your Mallory tuning with the vacuum line disconnected, and plug the carb port(s). Do your carbs even have ported vacuum ports for vacuum advance? This gives you no vacuum at idle, and no vacuum at full throttle.

2. Detonation, or "pinging" sounds a little different in every car, but all the Volkswagens I've pinged in have sounded like someone throws a BB or pellet in the intake for each spark. It happens mostly under heavy throttle load, and goes away when you let off the throttle. Get the engine real hot, cruise at 60-65 and climb a grade. All of the sudden, floor it and listen. Engine should be smooth and the only noise you should hear is induction, no rattles or clickity-clackities. If you can floor it once the engine is HOT under load and not ping, your advance is safe. That doesn't mean it's ideal, but it is safe. Advance by two degrees and try again. Remember, HOT engine, meaning, the absolute hardest it has to work. For some, that may just mean a few miles to get ice cream on the weekends. For me, it means Southwest Desert hillclimbs on a 112* summer day.

3. "Reponds best" is going to be a huge opinion. Of course your carbs are jetted one way, and your induction (cam/venturi/valves/exhaust) is going to be a huge factor, but some engines like more advance at idle, others like less. This is just going to take time to figure out. If you have a drag bug, you' won't care as much as a weekend putterer. Smooth takeoffs are ideal, as well as power on demand when you goose it. I have never heard of a street car needing 20+ degrees of advance at idle, BUT John @ACN has tuned a hell of a lot more cars that I have. Here's a thing: quick throttle changes mask tuning issues. Quick jabs of the throttle look and sound impressive in YouTube videos, but they just compensate for issues with accelerator pump dumps. Try slowly opening the throttle. Then getting back to idle. Try a little faster. A little faster. All the way from glacially slow to quick jabs. The engine should respond well to all. Do this on the road too, since you don't drive it in neutral. :wink:

4. This could be a ton of things. Ignore it for now; it's burning off fuel and reducing hydrocarbon emissions! 8)

5. Uhh, nothing on a Mallory distributor. Volkswagen made DVDA distributors, but even they weren't designed for this. Again, I don't think the flagrant idle timing posted above will be necessary in anything other than extreme cases.

6. I personally think so. I had my best luck with my Mallory setup by getting my springs and advance stop tabs running as best as I could. There was the SLIGHTEST bog around 1,500 RPM under light throttle that the vacuum advance then cleared up once reinstated.

7. I don't know how to adjust your carbs for this. I don't even know if they have the right vacuum port. (see #1.) But I do know that if you set your mechanical timing to 28*-32* max advance, plugging your vacuum hose in while feathering the throttle around your max timing RPM should show up on a timing light as a much higher number now. 10* vacuum advance is a lot, but again, every engine is different and my bus engine wanted less. You don't need a gauge, just a tiny allen wrench to adjust the advance through the nipple, plus a timing light and a degree pulley.

8. This depends on the car and how good the paint is. :lol: On bugs, I use coil positive (#15) for the light power, and the coil bracket for ground. Others use #15 for positive and the carb throttle spring bracket for ground. Both of these keep the wires away from the belt! On Ghias or buses, you can use the battery directly, or any number of other sources. I sometimes use the decklid latch as ground for the timing light, tach/dwell meter, and test light.

Clear as mud?
Robbie

Wed Mar 09, 2016 10:22 am

Thanks for your feedback! I will try to digest it all :)

Quote: Do your carbs even have ported vacuum ports for vacuum advance? This gives you no vacuum at idle, and no vacuum at full throttle.

Yep - Weber 44 IDF

I wasn't aware of there being no vacuum at full throttle? That seems counter intuitive - I expected a somewhat constant progression of advance rather than a bubble?

Thanks for your info!

Wed Mar 09, 2016 10:52 am

bignic wrote: Thanks for your feedback! I will try to digest it all :)

Quote: Do your carbs even have ported vacuum ports for vacuum advance? This gives you no vacuum at idle, and no vacuum at full throttle.

Yep - Weber 44 IDF

I wasn't aware of there being no vacuum at full throttle? That seems counter intuitive - I expected a somewhat constant progression of advance rather than a bubble?

Thanks for your info!

See, the thing is, vacuum signal for an SVDA is different from a SVSA, which are both different from full manifold vacuum. We want "ported" vacuum for an SVDA, from a port near the throttle plate. This gives us a little extra help when taking off from a stop, since we're not floored here. (Or at least MOST of us aren't…) And it helps with fuel economy on the highway when we're cruising at partial throttle.

It works by connecting the vacuum hose to a drilling right near the throttle plate, which sees no airflow at idle, because the hole is blocked, then an influx of airflow as the throttle is cracked open or held partially open. The venturi effect creates negative pressure on the drilling, (and therefore the hose) when airflow past the ported hole is fastest. Under full throttle, the airflow is all going down the center of the carb, and the throttle plate isn't there to usher the air fastest past the hole. So even though total airflow is faster, the vacuum effect on the advance drilling is far, far less.

If you do have significant vacuum at full throttle, I might not recommend using too much vacuum advance, if any. The reason we set stock SVDA setups to 28-32 bfdc at max advance, is that the engine, under full throttle and full load, can't handle much more than that. The vacuum advance conditions are light load and light throttle conditions, so more advance is ok.

-

The constant progression of timing is taken care of by your mechanical advance curve. This is RPM dependent, as fuel takes time to burn, and with a faster RPM you have less time to burn it. The vacuum advance isn't needed on things like zambonis, combines, generators, or airplanes that run all day at the same RPM. But because automobile engine demands are unique, the vacuum advance option gives is better economy and flexibility of power and efficiency by adding to the mechanical advance baseline.

All in all, the vacuum and mechanical advance systems aren't really even connected. Related, yes, in result but not in design or function.

Robbie

Wed Mar 09, 2016 10:59 am

bignic wrote:

Yep - Weber 44 IDF

Most likely you're not going to have enough ported vacuum with the IDFs to get the Unilite's canister to work properly. Aircooled.net sells a brass "anti-pulse" valve that helps a lot but I'm still not sure T'in both ports with the valve will be enough.

On my person ride I run dual drla 40s with a single vacuum port into a unilite. To deal with the lack of available vacuum I retrofitted my unilite with an old-school 40hp beetle dizzy vacuum can. It was a little bit of effort but now I have immediate, off-idle vacuum advance which let me lean the carbs out for better MPG and performance.

Also, might want to add that the vacuum canister on the unilite is adjustable via an allen key (forget what size). Make sure you back the spring off to give yourself the best chance if going that route.

Wed Mar 09, 2016 12:02 pm

asiab3 wrote: Under full throttle, the airflow is all going down the center of the carb, and the throttle plate isn't there to usher the air fastest past the hole. So even though total airflow is faster, the vacuum effect on the advance drilling is far, far less.

...

All in all, the vacuum and mechanical advance systems aren't really even connected. Related, yes, in result but not in design or function.

Robbie

Ahh! Lightbulb! Thanks

sallittjob wrote:
Most likely you're not going to have enough ported vacuum with the IDFs to get the Unilite's canister to work properly. Aircooled.net sells a brass "anti-pulse" valve that helps a lot but I'm still not sure T'in both ports with the valve will be enough.

...

I retrofitted my unilite with an old-school 40hp beetle dizzy vacuum can.

Was the Unilite just not adjustable to the extent you needed it, or was this an experiment to compare the two?

I have the brass anti-pulse valve already, trying to nail down some of these issues. Didn't realize it would still be considered too weak of a signal.

Took a look through my documentation and dont seem to have any info from Mallory on adjusting the canister. Does anyone have a reference for what the adjustment does precisely - is it some sort of release valve / proportioning valve that lets air out of the diaphragm?

Wed Mar 09, 2016 1:04 pm

Fast idle with the speed screws almost out sounds like a vacuum leak. Get idle speed issues figured out before messing around with the distributor. :)

Wed Mar 09, 2016 1:36 pm

Good thread. I will be watching this one.

Wed Mar 09, 2016 3:18 pm

Slight Detour: I actually think I have some moderate-to-severe electrical demons at this point. My voltmeter was going crazy just being held in my hands NEAR the engine. Timing light would pick up an intermittent signal if one of the wires was just touching the rear deck lid. Thought my wires were just close/touching but it just seems to be a powerful electrical field in the entire engine bay.

Using an MSD blaster coil because I want a powerful spark but I'm guessing whatever plug wires I've got here are not well shielded and causing some RFI. The Unilite manual mentions needing "suppression type" / "spiral core" ignition wire but the Mallory wires they suggest appear to sold out / hard to find.

Perhaps I have some sort of grounding issue?

Recommendations?

Thanks for help so far, you guys are awesome.

Days like this I feel like a complete newb again. Been chasing weird issues like this since I got this build on the road like 5 years ago. Just wanna go for a cruise and make some noise. Never had a single weird issue with my stock builds... maybe time to give up the dream ;)

No.. must go faster.

Thu Mar 10, 2016 12:19 pm

A side question: can anyone provide some solid reasons for why we dont just couple timing directly to throttle position in a metered way? Like a gear reducers or something?

I suspect that giving direct timing control to the driver might inadvertently put the engine in the situation where the timing is completely inappropriate for the actual engine speed, and would thus be a problem, but I'm curious if that is true.

Thu Mar 10, 2016 1:16 pm

bignic wrote:
Was the Unilite just not adjustable to the extent you needed it, or was this an experiment to compare the two?

I have the brass anti-pulse valve already, trying to nail down some of these issues. Didn't realize it would still be considered too weak of a signal.

Took a look through my documentation and dont seem to have any info from Mallory on adjusting the canister. Does anyone have a reference for what the adjustment does precisely - is it some sort of release valve / proportioning valve that lets air out of the diaphragm?

Not enough adjustment. Basically I was pulling vacuum off one cylinder vs. the stock 4 cylinders because of the dual 2 barrel DRLA design (DRLAs only have 1 vacuum port vs. the two on IDFs). Since the vacuum appears at different times on each cylinder, I'm not so sure T'ing the signal will work all that great but the brass valve might smooth it out enough to do the trick.

To adjust the can on the mallory, i believe it's a 2.5mm hex key which needs to be inserted into the vacuum port opening and you turn a setscrew left or right to adjust the spring preload on the diaphragm. It'd be a good idea to get a vacuum gauge/pump see how much vacuum you get out of the T'd connection (at different driving conditions) and see where you stand. I'n my case I was making just enough vacuum at max signal to just advance the can when fully backed off. This doesn't work because you want the ignition to immediate advance when you blip the throttle and to stay advanced if your very light into the gas cruising. If you get it working right you can lean the mixture out at cruise (idle jets) since you have the extra advance to compensate. This improves MPG and helps the engine run cooler. Once you get into it more the vacuum signal drops out and your timing maxes out at whatever you set the mechanical at (all-in).

Thu Mar 10, 2016 6:06 pm

Quote: A side question: can anyone provide some solid reasons for why we dont just couple timing directly to throttle position in a metered way? Like a gear reducers or something?

I suspect that giving direct timing control to the driver might inadvertently put the engine in the situation where the timing is completely inappropriate for the actual engine speed, and would thus be a problem, but I'm curious if that is true.
Yes you have answered your own question. In a way the vacuum signal is tied to your throttle position in an indirect way. Less fuel takes longer to burn so the vacuum signal at partial throttle advances the timing to help squeeze out a few extra MPGs. At idle or full throttle it is not needed or wanted.

That said I have dual 40 IDFs on a 2.0L T4 motor. Ported vacuum provided from a source from each carb T'ed together. I also have a vacuum gauge in my cockpit which is normally on the manifold vacuum but one time I temporarily swapped it over to the ported vacuum to check it out. There are very few times that I have anything other than zero ported vacuum. Even driving very gently to try and max out the vacuum, I will only see at most 4 inches of HG for the very very briefest of times. So not only is the vacuum signal strength low; 99% of the time there is no sign at all

Also you might want to read up on the Unilite with the vacuum option. Years and years ago there was a post about the unit failing and baking a motor; I think it was Jake Raby who posted about it and I remember asked for clarification (might be on The Type4rum). Somehow when the vac advance unit fails it fails in a manner that advances the ignition beyond what it should be at causing damage to the engine.

I've just stuck with the factory bosch DVDA but it is set up as a SVDA (no retard) but is really only working as a SVSA (mechanical advance only) with some tweaking of the advance stops so that the unit only advances from 12 to 28 degrees.

Good luck

Wed Mar 16, 2016 10:21 am

Been there, done that... Still have the Mallory svda. I can tell you that the vacuum signal is not strong enough to do any good at all with the carbs your running. That's one reason why John is no longer selling this setup. Im using mine as basically straight centrifugal. The vacuum can on these at it's lightest setting is still too strong. I started researching canisters to try and adapt one to the Mallory to gain the benefit of more timing at cruise, but so far no go. :x

Wed Mar 16, 2016 10:22 am

And by the way, its best to have a key kit to adjust your advance, Mallory used to sell it as did john.

Mon May 09, 2016 12:45 am

So is it possible to remove the vacuum can if it is infact useless for drla's?

Fri Feb 24, 2017 3:44 pm

I have a 2017cc with 40 DRLA and unilite vac adv. distributor. I bought the anti-pulse valve. I love the distributor. I had to adj the vac. adv. on a rear wheel dyno machine with the engine at 3400 rpm. Was well worth it. I am very please with the Mallory but sad that they sold to MSD. I was told MSD sold to corporate so the quality goes bad quickly for profits.

vwdon

Sun Feb 26, 2017 9:07 am

bignic wrote: asiab3 wrote: Under full throttle, the airflow is all going down the center of the carb, and the throttle plate isn't there to usher the air fastest past the hole. So even though total airflow is faster, the vacuum effect on the advance drilling is far, far less.

Robbie

Ahh! Lightbulb! Thanks

This is true, but the venturi effect may not be the only law of physics at work here. As I'm sure you know, everything between the back if the intake valve and the bottom of the throttle plate is 'manifold' pressure, as opposed to 'ported' pressure. Obviously, when the throttle plate is fully closed, manifold pressure is at its lowest, i.e. manifold vacuum is at its highest (it's interesting to note that the term 'full throttle' is a bit of a misnomer, since 'to throttle' literally means to cut off the flow of air, so technically speaking, 'full throttle' would be the point where the least amount of air is entering the engine, not the most). As noted earlier, when the throttle plate is fully closed, the vacuum advance port is completely blocked off. As the throttle plate opens, not only does the port become exposed to a high velocity airflow, but it ALSO is exposed to the high manifold vacuum (one of the reasons engines with high duration camshafts have such a poor vacuum signal is that at low RPM, such as at idle or light cruise, the reversion from the cam overlap causes an increase in manifold pressure/decrease in vacuum). As the throttle plate continues to open, manifold pressure approaches atmospheric, and the vacuum from the manifold to the advance port is lost, which is another reason why there is no vacuum signal at or near WOT.

I also wouldn't sweat the high idle. The throttle plates in my Darla twins all came with a little hole drilled in them, which allows extra air into the manifold and raises the idle RPM (not to mention making it a bit tricky to adjust for LBI). I thought about soldering the holes shut to calm things down a bit, but on second thought, I decided that the extra 200-300 RPM at idle is also turning the cooling fan a little faster, which in turn should help reduce heat-sink in the cylinder heads when the engine suddenly goes from a high RPM, heavy load condition down to low RPM idle. 1500 RPM at idle might be a touch excessive, but I'd be perfectly happy with 1100-1200 RPM.

Good luck, man.

Mon Jun 11, 2018 3:23 pm

I recently bought on of these Mallory unilites with the vac advance. Just like the last one I tested the minimum amount of vacuum to get this vac can to start moving is 10 inches. Since my engine only makes 10 inches of vacuum with DRLAs this simply won't work. Others on here have mentioned using the vac can from a big cap 40hp distributor. I looked at that as well but ultimately I went for the vacuum can on my old favorite VJU4BR8. I just wanted to share this picture in case any others have one and not using it due to the vacuum advance issue.

Once I get it in the car and figure out where it will be clocked I will bend the vac line so it is out of the way, I am also not going to use the AMP style connector.

Mon May 27, 2019 10:03 am

Not used to the look, but love how steady the timing is.

Tue May 28, 2019 12:21 am

Aren't the Mallory vacuum cans adjustable?
Robbie