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MG Midget and Sprite Technical - AUD502 Change Manifold to Port vacuum

I still have a thread running (AUD502 adjust om MKIII 1275) about setup/tuning my MG midget RWA MKIII 1275 from 1971.
The car is export build for US market. The engine is a 12CJ type (not the original acording to Herritage Certificate)and probably low compression type. Originaly the car was equiped with emmision regulating stuff, but it has been disabled/dismantled. I got the car with a 25D4 Dissy (with vacuum advance) Now i run a 23D4 (with vacuum adv) wich came of from my donor car (also a RWA MKIII 1275 with AUD502 carbs). Both cars/carb's dont have a vacuum port on the front carburettor but use vacuum from the manifold.
Yeasterday i talked with Eelco Hofman from Hofman Classic Cars in Holland, and he told me that the manifold vacuum was/is used for emmision regulation only. For good use/running the vacuum should be ported (from the carb) and not from the manifold, because you want more advance when you hit the trottle (ported) and not from manifold wich gives vacuum advance when trottle is closed (for emmision reasons)

Is it possible to change my vacuum from manifold to ported and can i drill a smal hole in the front carb as shown on the picture (copied from and plug the manifold vacuum.

Eelco also told me to setup ignition timing at 15 degr BTDC instead of the 8-10 wich is mentioned in the haynes manual. Can anyone subscribe this?

Niek Lammerts

Bad advice. With the correct vac advance unit, the cars will run exactly the same wherever the vac comes from, but it takes two different vac units and possibly other (mech) curve changes.
Vac capsules have part numbers and function descriptions to tell you the difference between units.
23D is the designation for no vac advance distributors, ala Cooper S and some early 1275. Don't know what you are looking at.
No vac advance is not a good idea, predominately for fuel economy reasons.
Go to the MGB Tech board and find Paul Hunt, who has on his website very good detailed explanations re the different vac options.
The position of the drilling for advance is fairly critical, so not recommended unless you have a drilled carb for a sample.

FR Millmore
Nigel Atkins

Super wicked awesome post, Fletcher! And Nigel, for the link!

Finally the penny drops (in my thick head) and now I understand.

"The important thing is that both carb and manifold vacuum give the same results in most normal driving conditions. The only reason for the change is that manifold vacuum results in a higher idle speed than carb vacuum. This allows the idle screws to be backed off slightly to achieve to same idle speed, which reduces fuel consumption and hence pollutants."

thanks guys!

Norm Kerr

just to confuse us all:

Niek Lammerts

unless I've missed something the second link seems to confirm PaulH

seems to me correct dissy advance curve is required to particular engine as it actually is which may not be as original

can I also throw in the fact that dissys were apparantly not that accurate after a couple of years when new so probably wont be after many years of wear plus how often do you hear of the dissys being oiled at services or ever

I swapped to a fully new and electronic 123 dissy and found noticeable improvements plus if you need it you can get programable versions for those non-standard set ups -
Nigel Atkins

Why not go the simple easy direction ...drill and tap the front carb spacer block and add a vacume port for the dissy

Just buy the 1/2 mm spacer block that has the vacume port already installed

Minimania and there UK sister site has them...about $15 IIRC

Prop and the Blackhole Midget

Correction... That would be

" OR " get the 1/2 mm spacer block with the built in vac port

Sorry for the confusion
Prop and the Blackhole Midget

I couldn't get the second link to work. The first one has two reliable contributors, Paul H and the ex GM guy, and various levels of folks who sound more informed than they are.

Prop - That is the same as manifold vac. Unless and until you can make your car work properly, how about spending more time reading and thinking, and less writing?
Love ya-

FR Millmore

The spacer block with vacuum connection seem the same to me as manifold vacuum, because you are already in the manifold system. Ported vacuum has to be on same hight as the throttle valve.........

It all started with my interest in the 123 ignition. I already bought one here in Holland and it is the one with a vacuum port on it. Then started the confusion/discussion wich vacuum, the manifold or the ported one. Also got the advice to tune valve's, carb before putting in the 123 ignition, and also this started new "problems" :-), At this moment i run a 23D4 dissy with manifold vacuum, but when i did a test run last week the car has holding back/hessitating when accelerating, no problem when cruising. (you can also read this in my other post, tuning AUD502). So i changed back to the red spring and shock oil in damper, checked gasket between engine and manifold but still she is not running like i want it to. Tonight i setup the carbs again, but when ideling, short rev up and let loose, it seems she goes to stall but then pickes up again at idle (1000rpm). So i have to solve this first before throwing in the 123 ignition.
Niek Lammerts

If you search the net for "MG vacuum manifold versus ported" you get a huge amount of hits, when you start reading you get all kinds of sugestions pro and/or contra ported/manifold, or that it doesnt matter what kind of vacuum, only at idle there is a significant difference...... so for every pro you get contra messages

Niek Lammerts

Sorry FRM,

I obviously was mistaken or had forgotten that the vac port for the dissy was on the front side of the butter fly and not on the backside... So your correct the spacer with the vac port wouldnt work as the dissy vac port appeartly needs to be In front of the butter fly... NOT the back side

Thanks for the correction

Prop ...MY bad !!!
Prop and the Blackhole Midget

Yes, there is much confusion, The fact is that the whole deal is a SYSTEM, and must be set up as such. People saying they did this or that and it "fixed" it, are talking about few or single cases, which may not apply to yours, and are certainly not definitive. When you have a "bitsa", with unknown parts and unknown mods, you sometimes have issues which require deep understanding of basic function. When you start changing multiple factors, you may have the "same" problem = symptom, with completely different causes.

Stumble on accel is usually taken to be lean mixture, and your "hunting" on return to idle tends to confirm that. Low float levels will cause it - old VW & Mercedes with Solex carbs were famous for stalling when you stopped for traffic lights, and raising the float almost always corrected it; I used a hard stop as a basic troubleshooting check on those cars.

Lack of advance at begining of accel will aggravate the "lean stumble", and excessive advance may mask it, but introduces other, possibly engine killing, side effects.

I have had "bitsa" cars which I found I could only correct by using a distributor with more low vac advance. On one MGB, I found that simply putting a restriction in the vac pipe to manifold cured the stumble, because it held the vac advance until the engine speed increased enough for the mech advance to come in. The restriction I used was simply another 8 feet of the small diameter plastic vac pipe, coiled up between the manifold and distributor. On another worn out bitsa MGB with manifold vac, it would actually stall when you snapped the throttle open at idle. Fitting an Austin 1300 ported vac distributor hooked to manifold vac cured that.

FR Millmore

I know that it is a bits and pieces car and that it also affected by the emmisson regulating stuf, wich has been taken of some 15 years ago when the car was imported to the dutch mountains :-). So overall the the whole system is not complete anymore and compromisses have to be taken. As a technician and perfectionist i always am looking for the perfect setup (wich is impossible i know now) but it keeps nagging. As i discribed in my other threat the change took place after i renewed the needles and Jet's as i thought that the old ones (40 years old) would have been affected by wear. Now it seems that i have trouble to find the basic setup. The "problem" with the almost stall after rev up is new, i didnt have this with my old needles and jet's. The one thing that is the same with old and new needle/jet, is the not smooth running at idle and so the search for perfect setup keeps on going
Niek Lammerts

This looks like a useful reference...


James B

check with 123 forum but if you fit the 123 dissy I think (don't know) that the electronics will deal with the vacuum regardless of source, so you might be trying to takle a redundant issue as regards the vacumm sorce -

I had to cut and paste the second link into the adrees(?) box to get to it but no trouble that way
Nigel Atkins

TBH I'd forgotten you'd already got the 123

I ran my 123 without the vacuum conected for a while because of another problem and I couldn't tell the difference

that when I discovered that such things as vacuum plugs and caps exist in different sizes, in the photo below you can see a cap fitted to the carb vacuum oulet, the plug in the 123 dissy is obviously out of shot

Nigel Atkins

although I could not notice any difference with the vacuum not fitted as I'd paid extra for a 123 dissy with vacuum I thought I ought to have it connected and tried fitting a larger internal diameter (and OD) pipe which seemed to cure the previous problem so I kept it on, as shown it goes from front carb to dissy, secured by one p-clip on alternator bracket, one tie-wrap on breather hose and a second on fuel pipe to front carb (oil filler vent cap strap was just left that way from last oil change)

note also the HT leads which I've found to be very good, better than the (2 year old) 7mm silicone ones I had fitted before

the new blue HT leads are from a company that makes leads and other stuff for other well know names, so I recommend 'perfomanceleads' from Fast lane to you -

Nigel Atkins

Maybe this can help identify the original curve of the dizzy you've got.

If you want I also got a huge listing of lucas type numbers with their designation.

Alex G Matla

I made 2 videos with my iPhone and placed them on youtube. The sound is slightly different from real because of mic placement of iphone, but it gives an impression. The video was made after a drive af 30min. When engine is cold and running on choke and slightly more RPM it is running smoother, also is the engine less shaking then when warm, wich is not good noticable on the video.

Suggestions are welcome
Niek Lammerts

second vid is show as private are you sure that's supposed to be shown :)

if you've taken the emissions stuf off what's that vent pipe on the rocker box cover for as toy appear to have a vented oil filler cap and breather chain cover to carbs, try blocking that off rocker cover pipe off and see what happens

also to eleminate dissy-to-cap you could fit the 123 if the problem remains the 123 can remain fitted or go back to old dissy
Nigel Atkins

oeps........ ;-). I changed the setting of the second video, nothing to hide :-).
The vent pipe of the rocker cover is going to the charkcole canister, this is blocked.
As i have a breathed filler cap i dont think the vent pipe makes any difference....
Next step is to fit the 123 dissy..... let you know what the results are

PS: i use these tools for adjustement (see image)

Niek Lammerts

please post a larger size version of that file!
I tried to print it but it is just too blurry to read, at whatever the pixel size that it is posted at.

Or, can you post a link to where I can download the original?

The table looks like it would be great to have as a reference!


Norm Kerr

the vent pipe going to the charcoal canister is it blocked before or after the canister, as you have the y-pipe breather and vented oil filler cap you donít need that rocker cover pipe so it can be blocked very close to the metal outlet pipe

you need someone who knows what theyíre talking about to comment on the videos and thatís not me- but to me it just sounds like you need to go over the basics, block that rocker vent pipe tight, then again carefully check your plugs and leads, fit the 123, get it timed up, adjust the mixture as required and see where you are then

those instruments look like they may be to do with that private video, Iíll not look further :)

it was Alex that posted that, all blurry my end too
Nigel Atkins

I just thought I'd add -

you might be going into this too deeply with the timimg numbers, vacuum curves and those lovely instruments

subject to all the parts and components on the car being fitted and working correctly

rather that consulting figures in book and on gauges that relate to different set up cars to yours

simply block off that pipe, set up the 123 dissy to what sounds right by ear and adjust the carbs to what sounds right by ear then adjust from there if required

back to basics - that sounds like a good political slogan I might offer it to our government :)
Nigel Atkins

Maybe this is better....

Alex G Matla

Hi Alex,
yes, that is much better, still somewhat blurry, but now I can read it


Norm Kerr

well according to that chart 12CJ engine is high compression with 25D4 dissy so there's two variations from Niek original post

putting the 123 dissy on will eliminate the changing from 25D4 to 23D4 variances

Iíd suggest you put that Haynes away and get a copy of the DVD with factory Workshop Manuals, Parts Catalogues and the invaluable Driverís Handbooks -
Nigel Atkins

Sorry, made a little typo. The Current dissy is a 25D4 wich came of my donor car. The dissy fitted on this car, when i bought it 13 years ago, was a 45D4.

@Nigel: The private video was the last one i posted on youtube about exhaust, and i checked the box "private", so i changed it to "hidden".
Niek Lammerts

well the fitting 123 might show if the 25D4 is faulty, running badly/worn

I was pulling your leg about the second video, the thing I noticed about it was the nice birdies :)

to me the exhaust didn't sound bad just a bit out of setting perhaps but as I put I don't really know it needs some one who does but I'm not sure you could pin much down from it

the combined information on that DVD will help you more than the Haynes with its wrong info in places and variable instructions quality
Nigel Atkins

This evening i fitted the 123. Overall there is little change. When warm the exhaust is still "splashing" and also its still running irregular, however, less the with the 25D4 fitted. I pulled off the line to charkcole cannister an plugged the pipe. With the 123 she is running a little smoother but when i set the stroboscope light on, the mark is "dancing" all over the timing mark. The 123 is trying to regulate the timing to a steady value i think. I did i run this evening and she runs ok, still a little jerky at speeding, but steady on cruise with no misfire. It seems that there is some play between cam and dissy thats the only way i can explain the "distortion" with the timing light. I will try to make a video of it and put it on youtube for you guys to inspect/comment
Personaly i had expected something more with the 123, but i is not getting worse
Niek Lammerts

keep the pipe plugged

when it was jerky at speed did you try pulling the choke out to see what difference that made?

did you match up the carbs -

have you got a spare coil?

are all your LT and HT electrical connections clean, secure and protected?

once you get it set up and running right the 123 will give you more
Nigel Atkins

You should be able to get back to where you started by reversing everything you know you did, If not, you did summat else, or it is progressive.
You said it all was the result of carb fiddles, so this thread is a confusion. But, It is true that fooling with ignition can alter fueling issues.
The two threads is crazy making!
The vac capsules have numbers that tell you what they are supposed to do. The numbers you gave are in the ballpark for emission distributors, good for lean idle mixtures. The mech advance weights have numbers that tell you the advance limit, but not the rate which is controlled by the unmarked/unknown/possibly worn/maybe unavailable springs. This can only be checked accurately on a distributor machine. You can, with a timing light, measure and plot the mech advance curve you have now. That will not tell you what you need.

You say you want it right, as a technician.
So, do it right. Check and measure:
Compression, hot, throttle wide open.
Vacuum at manifold
Cam timing
Lift on each valve

These engines were notorious for chain wear, and cam wear if oil and filter changes were neglected. Even with decent maintenance, cam wear is not unusual at the stayed mileage - which may not be true.

Subtle poor running issues at low speed are frequently the result of this sort of wear. It only takes one or two valves opening less than they should to do it. Late cam timing from chain wear causes oil consumption, power loss, poor economy and somewhat low idle vacuum. A common first notice of really bad timing chain is a cloud of blue smoke catching you at a stop sign.

FR Millmore

Splashing exhaust is overrich mixture isn't it?
Alex G Matla

Possibly is fuel related but timing had to be set up and 123 fitted so I thought might as well started there

check lists below with a couple of things added

keep the rocker cover pipe plugged

Electric side of things:
have you got a spare coil?

∑ are all your LT and HT electrical connections clean, secure and protected
∑ set tappets (0.012 cold?)
∑ checked spark plugs gaps and sparks plugs for cracks and fractures
∑ checked HT leads for wear and connections are secure
∑ checked all LT wires and connections are clean, secure and protected
∑ timing 13(?) BTDC @ 1,000 rpm vacuum pipe disconnected and plugged

Fuel side of things:
when it was jerky at speed did you try pulling the choke out to see what difference that made?

∑ have you got a vented petrol cap and is the vent fully clear
∑ checked for full steady flow of fuel to carbs
∑ checked float chambers levels and for needles sticking
∑ sprayed WD40 on and around manifold and gasket for leaks
∑ lubricated carbs and linkages
∑ check suction chambers match -
Nigel Atkins

"Splashy" is lean.

Niek - Meant to say, I cannot get video on my horrid internet connection, so video helps me not!

FR Millmore

When fitting the 123 ignition i purchaced new HT leads, also put i an new coil. Spark plugs are fine and gap is per manual. Set timing on 10 BTDC for start, measured with stroboscope and vacuum on manifold pluged @1000RPM
The petrol cap is vented and the fuel system is ok. Sprayed WD40 all arround but no effect. Al linkage and cables are lubed. Also matched the suction chamber a month ago (same video)

I did not put the choke when accelerating and "jerky",althoug it is less now the 123 is fitted

The exhaust sounds splashy, meaning that mixture is lean. But when i turn jet 1 flat to richer the RPM start to drop, so i turn back again to 1 flat to rich (from starting position of 2 turns)

Strange thing with the 123 is that engine seems to run a little smoother but the timing mark on the pully is jumping 3-5 degrees up and down the marking point. The 123 is trying to compansate the rough running on idle with compensating timing i guess.
Niek Lammerts

Timing moving is another sign of chain wear and cam jumping around. How far can you turn the crank to and fro without the distributor rotor moving?

FR Millmore

I replaced the timing chain about 10 years ago. since then it has not run more then 5000 miles. From 2005 till 2011 it drove only 10mile a year because MOT was suspended.
Niek Lammerts

It is largely immaterial what you did, or think you remember you did, ten years ago - been there, did or did not remember correctly - ten years is a long time in a real life! What matters is the state of things NOW. So measure.

If it needed a chain then, the sprockets may have been bad enough to eat a new chain, especially a crappy repro one - regardless if it came in a Reynolds or other decent box. Double or single row? Ever heard of nuts coming loose or keys sheared or left/fallen out?

Did you measure the cam timing and valve lift, now or ten years since? While you are doing that, you can give me the chain slop number.

You have what appears to be a puzzlement. Such require precise and adequate description, for anybody to figure out, especially on long distance verbal comm,

FR Millmore

I changed the chain because the engine was out and i wanted to change it to new because i had the engine out. I will check the movement soon. What is a chain slob nr?
Niek Lammerts

Chain slop = How many degrees can the crank be moved back and forth without moving the cam?

That, plus wear in the distributor drive gear, plus endfloat in the cam, add up to how much the timing can vary as things bounce around. It usually will not vary that much, but some part of that, possibly wildly different at different engine speeds.

Worn cam lobes can contribute to timing bounce because they cause unusual changes in the load on the chain. These can become resonant at various speeds.
Example: timing might bounce at 800-900, 1600-1800 & etc for multiples, but be stable in between.
Or it might bounce at low speeds but not high.
When they bounce at high speeds but not low, it gets really difficult to find, but people have, as they try to figure out why race engines blow themselves to bits.

FR Millmore

I lost my post to you last night when the computer went terminal - trying now to use a laptop with silly pad that I can't use properly

what I lost was basically:

you're not putting the engine as standard so don't expect to set up as standard

12CJ has IIRC(?) air pumped to the head so what have you done about that (unless you've changed heads)

your 12CJ is high compression as it has a 'h' in its number, 8.8:1

it would have been 10 BTDC at 1,000

it wouldn't have had a vented oil filler cap (one with a hole in the top of it) but you need a vented one now with the way you have it

that's all I can remember
Nigel Atkins

This thread was discussed between 01/07/2012 and 07/07/2012

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