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MG Midget and Sprite Technical - 1275 tuning question thread

so, I am working now on the ignition system. it is actually running really smoothly, but when i try to time the system, the manual says to time it without the vacuum advance. But if I disconnect the vacuum tube... the engine stops. I cannot get it to keep running without the advance. I am inclined to think that there is something worn in the vacuum advance unit.

I have purchased, (but not installed) a brand new flamethrower distributor which i want to put in. I figure that will give me a new dependable distributor in the engine and a back up (original) distributor as a spare that I know will work.

anyway, what can i conclude about the vacuum line to the vac advance? the flamethrower recommends a ported vac line (off the carb?) rather than a manifold vac line which i can do, but i don't understand exactly the difference between these types of vacuum?

any thoughts or recommended procedures?
Chris Edwards

I'll ask the obvious... Are you capping the vacuum line after you unplug it?
Trevor Jessie

no, the engine dies so quickly, i haven't managed to get it plugged fast enough, maybe if i clamp it somehow to it is disconnected and plugged at the same time...
Chris Edwards

disconnect (and plug) it BEFORE you start the engine.


Norm
Norm Kerr

well now i just feel silly... LOL
Chris Edwards

Well, Chris, if you are feeling silly, I'll add that you need to plug the hose so that you block a leak from the manifold/carb. You don't need to plug the distributor.

Charley
C R Huff

You need not feel silly Chris because I am not convinced that capping the vac pipe is the issue. Normally introducing an air leak would tend to speed up the engine, not stop it! Plus the small amount of air that would pass through the vacuum pipe is very small to say the least.

However the problem maybe that the idle (speed) is set too low. Vacuum advance tends to speed the engine up so removing the vac advance will slow the engine. Thus if the engine is already slow diconnecting the advance will stop the engine. This is the reason why when you hear people say they set the timing by ear you should always cringe! The timing set in this manner will ALWAYS be set too far advanced and may cause detonation to tuned engines as a result.

If I am right (I could be wrong however) you will simply need to speed up the idle of the engine until you have got the iming to where you believe it should be then with vac attached reset the idle screws.
Robert (Bob) Midget Turbo

Good point Robert, i knew i wasn't completely daft.

Since i have the experts on the line, can someone explain why a ported vacuum connection vs. manifold would make difference to a distributor?

the only thing i can think of is that vacuum would increase more with the throttle operation, which might help make the advance curve more specific to the engine (since the flamethrower dist. has a 'generic' curve) am i close at all?
Chris Edwards

AFAIK, the vacuum part of the distributor is there to help with fuel economy by advancing the ignition under periods of high vacuum in the manifold, i.e. under deceleration.

Rob
Rob Armstrong

Thanks Rob.

So really, why is a vacuum line from the manifold good for the stock distributor, but the new distributor recommends a ported vacuum line instead? Anyone know?
Chris Edwards

ok here's the plan then:

1. start engine and adjust idle up from 1000rpm to ~1500rpm.
2. turn off engine and disconnect vac advance.
3. restart engine and time dist. (9 degrees btdc at 1500rpm)
4. connect vac line
5. adjust idle back down to 1000

sound right?
Chris Edwards

Chris -
Bob is 100% correct for reasonably set up cars; the main cause of stalling with the vac adv disconnected is the timing retard from disconnecting the vac on manifold vac systems. Ported cars will generally speed up slightly from the increased lean mix, plus some more from the increased mech advance from the initial speed increase. The exception being US emissions arrangements which may be so lean at idle that a little more air will kill it, and certainly the two are additive, since lean mixture wants more advance to keep running.

Rob is also correct normally, but again the emissions setup changes things, and for the same reason. They are so lean at idle that they need a lot of advance to run at all. Ported systems typically have no significant vac advance until the throttle is well above idle, so you have the static setting, say 5deg, plus a bit of mech advance, say another 4 deg at 800 rpm, but zero vac adv, for a total of 9 at idle.
The manifold systems tend to have vac capsules calibrated to give a fair advance at idle, say 5 static plus 10 vac, and they hold enough advance as the throttle opens to let the engine get going fast enough that the majority of the mech adv comes in before the vac advance disappears at wide throttle openings. If they don't do this they bog horribly. At part throttle cruise, both ported and manifold tend to give similar vac advance, for good economy.
IOW, the port/manifold thing doesn't matter to the distributor per se, but it sure does to the engine, and the distributor is calibrated to account for this.

Then the question is, does your carb have a ported outlet? It will have no or very low vac at idle, and rise rapidly as you open the throttle off idle, then drop as you get to very wide throttle openings.

FRm
Fletcher R Millmore

Now, next question, what IS ported or manifold vacuum? Manifold is obvious, that would be a connection directly at the manifold. but ported? Does a SU have that?
Alex G Matla

Alex -
Ported is a little hole into the carb, just on the carb side of the throttle plate. At idle, the hole is pretty well sealed off from the manifold, so no vacuum. As the throttle opens, this hole becomes more connected to the manifold, so vacuum at the port increases. It actually increases above manifold vacuum, which is dropping since the throttle is opening more, but there is high velocity air passing the port, causing more vacuum. Once the throttle is more than about 1/3 to half open, the manifold and carb are both seeing low vacuum, but there is still high airflow past the hole, so it sees a bit more. In practice, the port vacuum will be a bit above manifold from above idle to about 1/3 throttle.

Different carbs may have the port more or less from the throttle plate idle position, so that can change transition characteristics. Some SU have the port, typically with an 1/8" hose barb on later carbs, many have the boss but it is undrilled. Early carbs have threaded ports that take 1/8 tube fittings.

Fixed jet carbs may have several similar internal ports used to do what the SU piston/needle/damper do so nicely, namely aid transition from one state to another.

FRM
Fletcher R Millmore

Well you learn something new every day!

Sneaksy extra emissions stuff... :)
Rob Armstrong

its so funny, the more i learn, the more i have questions about.

there is a ported vacuum connection on my carberators. I bought new SU's about 5 years ago, and I remember having to cap an unused vacuum outlet on the front carb.

but i am confused again... wouldn't adjusting the idle higher still leave the mixture lean (causing the engine to not run because the timing would be too retarded? would i be better to turn the jets a little richer?
Chris Edwards

"its so funny, the more i learn, the more i have questions about."
The more you know, the dumber you get! Such is life.
But, that gives you a wondrous world of new stuff to learn, better than Christmas!

One issue is that you gave us no reference as to what car or setup you have, so the info given is general.

Idle speed, mixture, timing are all interrelated, along with a lot of other stuff like valve timing, compression, weather, engine temp, etc.

Increasing idle speed puts the needles higher, changing the mixture; increases the mech advance, changing the timing; and gives some cushion above minimum running requirements, so that if the idle speed does drop the engine does still keep going.

Since you have non original SU set up by yourself, it is unlikely that you actually have the mixture set on the ragged edge of lean running. If you capped the port for vac on the carb, then you must be using manifold vac, and disconnecting it will retard the idle timing enough to drop idle speed below minimum required to overcome basic engine friction and drag, - so it stops. This is what Bob described.

You probably have one distributor that was set up for manifold vac, which you want to replace with one that is generically set for ported vac. So, you are in the realm of total darkness. At least you have one each ported and manifold tappings. Try what seems most like "somebody" meant it to be, then try every other combo you can think of, until you get it to work. Rolling road would certainly be your friend, if you could find one with a good operator!

FRM
Fletcher R Millmore

FRM you may be my new best friend. :)

I have had my midget for 12 years now and am finally starting to (i think) cross over from focusing my attention (and crude skill) on keeping it running/solving problems to improving performance/creating problems. (ha ha)

it is a 1275 1973 midget with 61000 miles from new. the engine has never been disassembled, (still on original rings, seals, head gasket, etc).

I replaced the carbs with new HS2s because i didn't trust rebuilt carbs with my limited tunign ability, but they are identical to the original HS2s except for the optional vac port being installed (and capped).

the distributor that is in now is the stock distributor. This was originally set up with manifold vacuum.

the distributor i would like to put in is a pertronix flamethrower (electronic) which is recommended to use ported vacuum.

according to my haynes manual:
static timing for my engine is TDC
dynamic timing is 9 degrees btdc without vac advance at 1500rpm
idle should be at 1000 rpms (probably because of the emission controls, it would get too lean with a lower idle.)
Chris Edwards

update: I have installed the new distributor and timed the ignition system correctly. Just as planned, i started the car, adjust the idle up to 1500, static timed the new distributor, re-started the car and dynamic timed. then i re-adjusted the idle.

I was going to hook up the vac line from the carburator to the vac advance, but realized my vacuum line was too short. So i tried it with the manifold outlet, and the sound of the engine changed slightly, i think it was a bit too advanced with the manifold vac, so i will be switching to the carb port. but this brought up another question:

How does the vac line cross over to the distributor side of the engine. for years I have had the tube arching up and over the valve cover, is that normal?

does anyone have a picture of an alternate/proper route?
Chris Edwards

Chris -
Not sure what you are trying to accomplish with the distributor swap. Other than wear problems, you probably won't gain anything; if the new one has a different curve, then other things need to be changed to match, and if it doesn't, it's the same. Notably, where did you get your new timing figures; the old ones are probably not correct anymore. OTOH, if the new carbs are generic replacements for the old ones, and are actually more like earlier carbs in their needles, the new distributor may also be closer to the older versions, and earlier car initial timing figures might be better (or worse!).

Aftermarket suppliers are very loose and vague about what they are actually selling you!

You should be able to set your idle down to 750-800rpm, adjust idle mixture to suit.

Vac pipes changed over the years, over the top is fine. I like to use a long enough tube to run around the front or rear of the rocker cover so it can stay in place when adjusting valves, keeps you from forgetting to put it back! Just don't let plastic tubes fall on hot things.

FRM
Fletcher R Millmore

according to my haynes manual, the normal idle for the 1275 engine for 72-74 is 1000rpm. probably because of the emission equip? I could set it lower, but it doesn't run as smoothly.

The carbs are not generic, they are the exact same model as the original (matched the model numbers) just without the throttle shaft wear that was making the old ones impossible to tune.

as far as why to change the distributor, half is just whim, and the other reason is i don't know how worn the old distributor is. Also the electronic distributor is lower maintenance than points (i hate adjusting points). by doing it while the old distributor is still functional (rather than waiting for it to fail) I now have a spare 'known good' dist. that i can keep in the trunk just in case.

Also, I am not an overly practical person. :)

thanks for your help.
C
Chris Edwards

This thread was discussed between 28/03/2010 and 31/03/2010

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