Can the advance on a DUI be adjusted for pressure, some other dist. have an allen adjustment inside.
Thanks.
Thanks.
DUI advance adjustment ? (1 Viewer)
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Nope.
I could use a weaker spring in mine but it's not too bad. The biggest problem is that it (the DUI spec'd advance unit) makes way, way too much vacuum advance. I modded mine to make around 12 degrees (IIRC). Less advance in addition to moving to port vacuum rather than ported vacuum helped driveability profoundly. Coolant temps is very hot weather dropped a tad too with less total advance.
I could use a weaker spring in mine but it's not too bad. The biggest problem is that it (the DUI spec'd advance unit) makes way, way too much vacuum advance. I modded mine to make around 12 degrees (IIRC). Less advance in addition to moving to port vacuum rather than ported vacuum helped driveability profoundly. Coolant temps is very hot weather dropped a tad too with less total advance.
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I did some searching and unported vacuum sounds like it would mess up the whole concept of vacuum advance, unported has high vacuum at idle. But.......it's hard to argue with " helped driveability profoundly".
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The vac advancer that fits HEI and has the allen adjuster inside is an aftermarket performance part. The DUI dissy is an aftermarket performance part. So maybe they use the adjustable advancer. Poke a small wrench in there, see what you can find.
Why would you need to adjust the DUI? Is your vacuum too low?
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The vac advance is adjusted based on the cam, compression, altitude, etc.
Yes, results>theory. There's plenty of theory to back up my results though. I won't bore you though.
Ported vacuum is for cams that make low vacuum at idle and to enable smog pump equipped vehicles to run cleaner at idle. If your engine doesn't have either of these, ported vacuum should get a second look.
I understand the basics regarding the dizzy, I'm just curious why he'd have to change the vacuum advance setting? (I'm assuming that he's running a basically stock 2F with similar components as I am.) Outside of running a non-stock cam, I can assume he may have a vacuum leak. Not trying to be dickish, just mentally masturbating the subject of low vacuum is all.
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Just curious about the adjustment, we all hope the DUI advance is set up correctly from the seller. When I had the Cap off the spring tension on the advance was really high, I moved it manually by hand just to check it out. I guess I was wondering how to test the adjustment with a vacuum pump, how much vacuum for how much advance.
I will try unported vacuum and give results.
I will try unported vacuum and give results.
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FWIW, a vacuum leak will have little affect on the production of ported vacuum. Ported vac is a result of the venturi effect right at the edge of the throttle blade in the throat of the carb. That is why it is different from manifold vac, it's a different source.
I suspect that the DUI would need adjustment because the builder of the dissy was not intimately familiar with the pre-war architecture of the 2F design. Perhaps they tuned the dissy curve for a 250 chevy or 258 AMC, engine that are of similar displacement, but 30 years newer.
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Did a google search and found this "TIMING AND VACUUM ADVANCE 101", nice article on vacuum advance. It really brings home the concept of lean/rich mixtures and when the advance needs to be applied. I can see that the ported vacuum should be used with emission stuff but if you are desmogged like me then unported(direct from manifold) is something to try ASAP. This could help engine temps, fuel economy, valve life.
This could end up being one of those best cheap mods. I think most folks just hook the vacuum line to the carb and don't pay alot of attention to what the port is and what is required...
This could end up being one of those best cheap mods. I think most folks just hook the vacuum line to the carb and don't pay alot of attention to what the port is and what is required...
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If the vac advance dissy is tuned correctly, then switching it from the correct ported vac source to manifold vac would be bad.
Yeah, that's pretty much right on.
FWIW, I'm pretty much satisfied with the mechanical advance that the DUI gives, but I've found that the mechanism would stick from time to time. I know you're not supposed to lube the weights, but some WD40 solved my problem of stuck weights.
The vacuum advance that came with mine allowed so much advance that it's plain to see they subscribe to the "advance it until it pings" mentality. The problem here is that the 2F, with it's super-low compression ratio, isn't going to make much noise when you've pushed avance over the line. It will, however, wear out much quicker.
I advise you to do a little googling over what other engines use for maximum total advance which is base timing+mechanical advance+vacuum advance. I think you'll find that the DUI goes over any recommended limits out there. I used an adjustable timing light which made it very easy to find out exactly how much mechanical advance and vacuum advance I had.
As mentioned, there are ways to fab up a travel limiter for the advance unit to reduce vacuum advance.
I just went through this with the HEI on my 40. I've never had it happen on a SBC w/HEI though. I used LPS1 to free things up.
This is basically what I was talking about above. When I was having problems I wasn't quite sure that I was hearing it ping at part-throttle. I finally took the vehicle to someone with an old-fashioned curve machine. I tried the full mechanical solution first. It was
okay, I guess. I eventually went with an adjustable advance mechanism and have been pretty pleased.It's fairly easy to limit the upper end of the vacuum advance curve but tougher to control when it comes in.
After the FJ40Jim post above I have a question ( I hope this isn't off topic) - Since everybody has always been trying to find a way to get ported vacuum from their older vacuum retard era carbs/distributors. Why couldn't an HEI or Non-USA OEM distributor be curved to work with manifold vacuum? I realize that manifold vacuum starts at idle and ported vacuum comes in at off-idle but don't both disappear pretty quickly as the throttle opens?
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The point of vacuum advance is not to have a bunch of advance at idle, then have it disapear @ WOT.
The point is to have no vacuum advance at idle, then bring in advance imediately on light throttle, then have it disappear at WOT.
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In my past life we always hooked the vacuum advance line to the port on the carb. That was the thing to do, now with more info and write-ups i'm not really sure what we were doing.
The first thing I see is if you go WOT then you want the engine to rev but your vacuum goes away temporally, you would think this would be a good time to advance the timing but according to the doc "TIMING AND VACUUM ADVANCE 101" it's OK to loose the advance because you just dumped in a bunch of fuel.
The logic seems OK but I don't know about the science.
The first thing I see is if you go WOT then you want the engine to rev but your vacuum goes away temporally, you would think this would be a good time to advance the timing but according to the doc "TIMING AND VACUUM ADVANCE 101" it's OK to loose the advance because you just dumped in a bunch of fuel.
The logic seems OK but I don't know about the science.
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It has little to do with adding fuel. The amount of advance needed is determined by two basic factors: RPM and cylinder pressure. The flame burns at a relatively constant rate, so as engine speed increase, the flame has to be lit sooner to allow time for the flame to burn across the chamber. Mechanical advance takes care of that.
At low cylinder pressure, the flame burns slowly because the molecules are farther apart. At high pressures, the molecules are squeezed tightly together, so the flame can jump from molecule to molecule faster. At WOT, cylinder pressure is highest, so the need for timing advance is least.
I disagree. The point isn't about vacuum at all. The point is a stable, efficient, clean idle as well as consistent power delivery at light throttle. Ported vacuum advance doesn't do any of that as well as port vacuum.
I would like to offer an apology here. I've been lurking for a while on this board and I've come to the conclusion that advocating port vacuum on an F or 2F is coming close to blasphemy. However, I feel the subject needs more scrutiny.
A few points:
Set your motor up at 5 degrees of base timing on ported vacuum then apply port vaccum where vacuum advance is limited to 12 degrees to net a total of 17 degrees of idle timing. Your idle speed will increase ~200 rpm, vacuum will rise, the exhaust will smell slightly less noxious (still nasty though). You'll have to reduce idle speed. Less fuel is being used to maintain a more stable idle. This is my experience and the results are tough to argue with.
I have no doubt that several of the port vacuum naysayers here have tons of experience with these motors and have tried this very trick themselves with less success. I suggest that the cause of these results is that the vacuum advance unit makes too much advance. Stacking 22 degrees of vacuum advance on top of 5 degrees of base timing isn't going to make a good idle. The amount of vacuum advance must be limited. Engines with low compression ratios should require less vacuum advance than the higher C.R. spec'd V8's that most of these advance cans were made for in the first place. You will most likely have poor results by switching to port vacuum if you are not mindful of the amount of vacuum advance you are feeding the motor.
Pick any motor with computer controlled vacuum advance. Strobe it or use a datalogger to verify timing at idle. You won't find a motor with timing anywhere near 10 degrees, much less 5.
I find light throttle cruising with ported vacuum irritating. Being a life-long tweaker of engines, I'm constantly tuned in to, and critiquing, the operating characteristics of whatever I'm driving. The change in power delivery that I feel when cruising in top gear at low speeds and light throttle due to the partial covering and uncovering of the vacuum port in the carb drives me nuts.
Vacuum advance allows the distributor to adjust the timing appropriately for varying engine loads. Vacuum is an excellent indicator of engine load. Less vacuum=more load=less timing. Throttle position is also a great measure of engine load. More throttle=more load which should =less timing. The problem with ported vacuum is that it gets the throttle position relationship to engine load backwards when you are cruising at light throttle with the throttle plate hovering near the vacuum port, covering and uncovering it as you adjust the throttle position. The result is less efficiency and wonky power delivery.
