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MG MGB Technical - Ported vacuum for DCOE? Am I mad?

I've switched my dizzy to a 25D4 (41288) that would normally take ported vacuum. Using manifold (as I've been doing up until now) doesn't seem to be an option as I can't get the idle low enough with all that advance. I was about to disconnect the line and plug it when I thought of tapping off some vacuum near the throttle plates, as many have DIYed on SUs. I've just had a look at my spare DCOE and the casting seems clear around that area and I'm thinking why not? Someone elnlighten me before I wreck a good carb! If I were to try, how close to the butterflys would the takeoff need to be?

Steve Postins

Don't bother. If I correctly understand how the DCOE works, vacuum advance won't help anyway, unless this is on a daily driver. Then you need to install a crossover tube with a "T" and tap into that for the vacuum. This was just recently discussed.
Jeff Schlemmer

Hi Jeff,
Yes, this is a daily driver and it was the earlier thread that got me thinking. I have tapped the manifold for vacuum already with good results, but need ported vacuum for the new distibutor I'm trying. Pros like Mr Burgess et al only discuss adding manifold vacuum with a DCOE (presumably for very good reason?), but also comment that the advance chracteristics are all wrong for a tuned engine as a result. Whatever, the idle is uncontrollable with an 5-13-10 can. I've drilled a pair of HIFs to reintroduce ported vacuum in the past, and can't see any obvious limitation on doing similar with the DCOE. But then there are many things I can't see!
Steve Postins

Doesn't matter what MGB dizzie you have, they all can use ported vacuum with no other changes. The change to manifold was made purely for emissions reasons, about 1971 for North America which still had 25Ds, not until Sept 76 for UK cars which by that time had been on 45Ds since 1974. Manifold vacuum gives maximum advance at idle, which increases the idle speed, so you reduce the idle screws to compensate, so you end up with a smaller throughput of mixture and hence lower emissions for the same engine rpms. All a frig really, because as soon as you start opening the throttle ported vacuum rises rapidly and manifold falls slowly, and very soon they meet, thereafter both fall gradually as the throttle is opened further, hence misture throughput and emissions are the same.

Ported is better as it gives less advance during cranking, which makes starting easier, and 'works' the vacuum capsule less so possibly lasting longer.
Paul Hunt

Hi Paul,
I was hoping to use the 25D with manifold vacuum as you describe. The trouble is the idle adjustment backed full out and the idle speed is still about 1500rpm thanks to all of that advance.
Steve Postins

Steve, is the cab tuned for your engine, or is this something that you just started working with? Is it new or used? Are the idle mixture screws1/2 - 1 turn out or are they all the way closed? There's no reason you can't get the carb to work with vac advance, but I think most people who install the DCOE don't idle much!
I can't say for certain how far forward from the throttle plates to go with the ports, but you will need to port each side of the carb and "T" them together. Maybe copy the vac port location on a DGV?
Jeff.

Jeff Schlemmer

The carb is tuned for the engine and I've run it with and without distributor vacuum. The previous can I was using was a 5-8-5 so it didn't push the advance/idle as high. The mix screws are about a 1/2 turn out but it's the idle speed screw that I've run out of adjustment on. I've adjusted the linkage and have got the idle to hum at about 1200rpm now, and can drop it to 1000rpm if I stand on the brake and clutch, so I will try living with that whilst I decide if the new dizzy is a better option than the old one. If it is, I may get brave with a power drill. The DGV sounds like a good starting point as a template.
Steve Postins

Steve, if you can't get the car to stall using the idle speed screws, something else is wrong - I'd solve that before I drilled into the carb body. The throttle plates should seal the throats. Is it possible that the vacuum canister is bad and is giving you a vacuum leak? You can test it by sucking on the tube, see if it's air tight. If not, it's probably the linkage or cable binding, or the return springs are too light, or perhaps misaligned throttle plates - was the carb ever rebuilt? Unhook the linkage first, see if you can set the idle speed properly, that should give an idea on where to look.

If you do decide to go ahead, you need to drill off to the side or bottom to avoid the idle and progression circuits, like you can see here: http://www.webcon.co.uk/weber/perf_operation.htm. As you can see, the weber throttle plates move in the opposite direction as the SU's so you may want to put any holes in the bottom to emulate it. I've never tried it, I'm not sure how likely it is you'll hit any passageways - but that will ruin the carb.

The big question to ask is why? Once you solve your idle speed problem, I doubt you'll see much of a difference either way. I've tried manifold vacuum with a 45DCOE and a dizzy canister built for ported vacuum, it works okay - it tended to make the a little less stable (too much advance). Starting wasn't noticably more difficult. It didn't make any measurable difference in mileage, but did give slightly better acceleration at part throttle (pretty subjective). I don't spend much time in my 'B cruising and I use WOT on most starts, so YMMV - literally :).

Fiddle with it, but I bet if you don't think the mainifold vacuum helps, I doubt you'll find ported vacuum much better. I found it was best just to set the static timing as far as I could get without any pinging or drop off of top end power.

HTH, good luck!



Mike Polan

Steve - if you can't stall the engine with the idle screws then something is wrong. ON SUs this could be maladjusted linkages resulting in one butterfly being fully shut but still holding the other open; butterfly not seating in the throat properly; fast idle screw holding the throttle open; insufficient slack in the throttle cable; faulty poppet valves (if fitted); or sometimes wear in the throttle spindle and bushes meaning mixture is being sucked past the butterflies even though they are fully shut. Some of these are applicable to your carb and some not, and your carb may have other possible causes.

It is not caused by a simple vacuum leak, as that only lets in air, and if our engines would run solely on that we would be considerable richer and the Chancellor considerably poorer :o).
Paul Hunt

Thanks for the pointers Mike. I'll have a good check of them all before deciding about making any permanent alterations. The ported option just hit me as I trying to set the timing/idle with the new dizzy. I've always accepted it's just not an option with a DCOE, but then thought "why not?". I've drilled the manifold for vacuum already and found a big improvement part throttle. I use the car for commuter crawling as well as more entertaining driving, so whilst I want the WOT performance, I'm happy to play around with modifications that improve mid-range and part throttle driveability at the same time.
Steve Postins

Steve, the standard 25D4 take the vacuum advance before the throttle and not in the manifold. I have put a plastic tube throw the air filter and it ends 1/2 inch in front of the trottle. I have run it for 8-9 years with fine idle. Also I put the vacuum advance first in the manifold then a mechnical told me that it was wrong when he tuned my weber on a rolling road.
Tjellvar

Thanks Tjellvar, that sounds a much less destructive approach. Did you notice much difference in running compared with the manifold vacuum?
Steve Postins

Steve, its the nature of the vac advance can to leak - that's how it regulates itself. If it didn't leak, it would always pull full advance from manifold vacuum. Its no surprise to me that your idle is high. I doubt if you would get it to idle under 900rpm anyway, even with all this fixed. If you did, you would have occasional off-throttle stalling problems. If you can get it to idle at 100rpms, live with it!!!

Try spraying carb cleaner or propane into the throttle plates at idle. The plates should be sealed closed with the idle screw all the way out. A rise in rpms should tell you otherwise - fix the throttle plate problem.

Keep us posted!!!
Jeff.
Jeff Schlemmer

When switching from manifold to carb vacuum made a hole lot of change. The manifold vacuum is the inverce of carb vacuum. When running at idle the manifold vacuum is at its maximum and the carb at its minimum. At high rev. the vacuum manifold is at minumum and carb at its max. Its better to dont connect the vacuum then connect it to the manifold. Maybe ther are dizzy that works the oposit compare to the D25, I dont know. At least now I have a nice idle at 900 rpm.
Tjellvar Harbom

Thankks everyone. I sprayed some ethylene or similar around and only got the revs to rise when it was directed at the air filter. I then pulled and plugged the vacuum line, disconnected the linkage and made sure the throttle plates were fully closed and instead of the engine stumbling to a stop it continued to burble away at just a marginally slower 1100rpm. I'll take it off this weekend and will have a look at the seating of the throttle plates. I'll try to keep my hands off the drill until that's sorted.
Steve Postins

Sounds like you're getting a handle on it Steve.

One other thing it occurred to me you could check is the bypass screws (mine, a 152, are under white caps on either side of carb). One should be closed, and the other open just enough to balance the air flow between the 2 throats at idle. It's possible one or both are open far enough to allow the car to idle with the throttle plate closed, that shouldn't be the case. If you know they're correctly balanced you can count the turns to close them to double check that's not the problem.

I'm pretty sure you don't want them open too far, as the throttle plate needs to be slightly open to unshroud the idle holes. It's the air flow over edge of the plates that gives the pressure difference to pull fuel from the idle and progression circuits.
Mike Polan

The vacuum advance can should *not* leak, if it does it is faulty. It contains a sealed diaphragm pushing against a spring, and within defined upper and lower limits it is the amount of vacuum (from either carb port or inlet manifold) in conjunction with the back pressure from the spring that determines how much the 'output' shaft of the can pulls the points plate round. You should be able to suck on the can input with the MkI mouth and move the output shaft to some extent, seal the port with your tongue, and the shaft should stay where it is, until you remove your tongue from the port. If you can continually suck air through the can it is faulty.

Manifold vacuum is *not* the inverse of carb vacuum - for an SU at any rate, don't know about a DCOE. Whilst they *are* at opposite ends of the scale at idle (manifold on max and carb on min) as the throttle opens the carb vacuum rapidly rises while manifold vacuum slowly falls. Very soon they are both at the same level, thereafter they *both* fall gradually as the throttle is opened further. I don't know how *any* carb would be able to deliver maximum (or anything above marginal) vacuum with the throttle fully open, as it is only the throttle being closed that allows the vacuum to be developed in the first place.
Paul Hunt

I agree with Paul on the vacuum can, I had one that was good, it held the vacuum, one that was bad, it didn't. It wasn't a particularly appetizing test :).

The DCOE would work the same way as the SUs (assuming Steve was planning to put the hole in a similar place). The vacuum port on the SU is just upstream of the throttle plates, but downstream of the venturi (the piston). Drilling through the DCOE's venturi is probably a bad idea, it's floating and removable, held in place by the aux venturi.

What Paul says is correct - as usual :) - ported SU vac is not the inverse of manifold vac. Manifold vac IS roughly inverse to *venturi* vacuum as Tjellvar suggests. But I didn't think the venturi vacuum is near as strong as manifold vacuum, I suspect there'd be very little vacuum advance if any happening if taken from there for an MGB can - even at WOT / high RPM.

It was recommended to me to have my distributor recurved - w/o vacuum advance the springs can be weaker to bring centrifugal advance in sooner for part throttle (max advance should stay 32-34). I decided to pass, without a lot of dyno time it's pretty much working in the dark, and I suspect the results would be only be slightly better than using manifold vacuum.
Mike Polan

Update: I pulled the carb today and found the stop on the throttle shaft wasn't quite allowing the throttle plates to close. It was only a matter of a couple of thousandths gap, but enough. I used my precision angle grinding skills (bit of a rush job) to adjust, and now it stalls as it should with the idle screw out. I'm glad that was the problem because I have no idea how you get the throttle plate screws out without resorting to more power tools?
I have left the ported vacuum for now, but I'm still curious to try it. I completely follow what you are saying about the similar vacuum above idle Paul. I just wonder if Tjellvar's experience means there are still subtle differences, or maybe it's that I can't believe that any BL mod to what the MG designers penned can't be for the best! Otherwise, why not use manifold in the first place?
Steve Postins

Good stuff on finding the throttle problem Steve.

Manifold vacuum on my DCOE made the idle hunt, and I'd get slight surging at low speed cruising (30mph). With no vacuum advance idle is rock steady with no surging. I've heard vac adv at idle is supposed to improve emissions, perhaps that's why they changed.

Part throttle acceleration, something you need to do with DCOEs at low RPMs to get the most from them, should benefit from vacuum advance - ported or manifold. It's hard to be sure, estimating acceleration improvements under part throttle is pretty subjective... I *think* it's better with the vacuum attached. At high speed, WOT, high RPMs, there's no vacuum to be had and you wouldn't want it anyway.

The DCOE is a performance carb - I'd rather live with manifold advance and hunting at idle than disrupt the air flow at speed with a tube or a badly placed port. Truth was I couldn't be bothered with either, and ran without any vac adv. In real life it's all pretty subjective, and with these relatively low powered engines probably doesn't make all that much difference - I'd just try it and see which I preferred.
Mike Polan

This thread was discussed between 22/01/2005 and 28/01/2005

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