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MG Midget and Sprite Technical - Unable to drive at constant speed

hi,

My midget (1973, RWA, MK3, with a replacement 1275CC engine type 12CC) has a very very very strange problem that is beginning to drive me crazy:
The engine is idling perfectly at 1000RPM, accelerates fine at every speed, gear and RPM. But it does not perform at all when cruising (constant speed, same gear, accelerator pedal hold at a fixed position) speed fluctuates constantly or stutters (I don't know what the right term is). It can be felt as if the car constantly accelerates and decelerates very subtle. When (trying) to drive above 55Mph the engine occasionally has no power and a bang can be heard from the exhaust.
This only happens when driving and trying to maintain constant speed. Choke does not seem to have any effect on this. When tuning the engine I don't have any problems: engine revs nicely and idles fine.

I'm suspecting it has something to do with ignition timing.

When I bought the car a 45D4 lucas distributor was fitted with the vacuum advance appearing to be connected. Closer inspection showed that the vacuum line was blocked on purpose.
With the vacuum connected (to the inlet manifold) the engine does not idle at all. So it is clear to me that this type of engine (12cc) does not require vacuum advance, or at least it should not be connected to the inlet manifold. I've tried to connect the vacuum to the carburetor in which case the engine idles normally but the problem remains.
The replacement engine fitted by one of the previous owners is of an earlier type (12cc) than should have been installed in this midget. From this website: http://www.teglerizer.com/mgstuff/advance_curves.htm I've found that a 23D4 without vacuum should be used on this engine. A local MG workshop told me that with the 45D4 the midget engine can never run properly. So I replaced the 45D4 with a 123-ignition just to get rid of all other possible problems (contact breakerpoint, condenser etc).
But even with this fully electronic 123 ignition the problem is not solved. I've tried the 123-ignition on the midget from my parents (also a MK3, 1973 but with a different a-series engine type) and it runs perfectly, almost like a modern car.

The carbs, coil, air filters, spark plugs, ignition leads, fuel filter are new. Engine compression is perfect and equal for all cylinders.

Am I overlooking something? When I bought the car it ran fine without this problem (as far as I can remember) even with the 45D4 distributor. I had the engine stored for quite some time well preserved in oil and in a dry storage.


Hopefully someone has experience with this and an idea to solve this problem?

Thanks,

Rens
H.L. Voogd

you may get a better answer from someone else in a moment, but it sounds to me like your fuel mixture is too lean. Lean running can cause a surging sensation where the engine seems to speed up and then stumble, recover, then stumble, etc. A lean mixture can also cause a backfiring noise, which could be what you are hearing. If you haven't changed the mixture on the carbs, you might check for a clogged fuel filter or anything that could cause a fuel restriction.

just something to check,

Chris
Chris Edwards

Check for air leaks at the inlet manifold to cylinder head joint.
Dave O'Neill 2

If you've got the timing correct (for your car) with the 1-2-3 dissy then the carbs must be out (or wrong needles for your set up)

I've got the 1-2-3 dissy and with the vacuum connected I was getting petrol inti the dissy from the carbs which no one could explain

It was suggested that I try disconnecting and blocking off at both ends which I did and the car runs fine without the advance

So I'd suggest you disconnect and block off the vacuum advance

Also check the 1-2-3 dissy is set to the right number for your car (at bottom of dissy under grub screw
Nigel Atkins

This effect can happen when carb needles get worn. When this is the cause, it occurs thus:

The needles wear more at the thick end of the needle (idle) than anywhere else, which effectively changes the profile a bit. When tuning, the mixture is set at idle, and if the needle has worn a bit then we screw the jet up to compensate and the engine idles beautifully. However, because the needle has not worn as much (if at all) elsewhere, the engine runs lean everywhere else.

The effect is minor at full power because the change in needle thickness at the jet exit is small compared with the gap between needle and jet. However, at light throttle openings such as steady speed conditions (where the needle runs at about no 3 position) then the difference in needle thickness is much larger in comparison with the gap, so the leanness is significant.

This effect is even more likely if the throttle spindles have been replaced but the needles not renewed as worn spindles leak air, most notably at idle, and so compensate significantly (albeit roughly) for the wear on the needle at idle. When they get fixed, the compensating leak is stopped and the problem becomes more evident.

It is easy to check if this is what the cause is. Pull the choke out a little bit when the symptoms occur and see if it makes any improvement.
(Note: The first 1/2" or so of the choke cable's movement operates only the fast idle adjustment. It is a good idea to note/mark the position of knob at the point where the linkage just starts to move the jet.)

The cure is simple: fit a new needle.
Paul Walbran

Wow thanks for all suggestions. I'm probably focussing too much on the ignition timing...

I've checked for fuel line blockage by disconnecting the fuel line and timing how much fuel is delivered at the carbs: 60 l/hr which is probably okay. I can't measure any pressure though. A Facet fuel pump is installed instead of the su pump.

The 123 ignition is set to the recommended setting 4:
10,0 degr. @500-1000RPM,
16,0 degr. @2000RPM,
30,0 degr. @4500RPM

I don't really know how to check for leakage at the inlet manifold. When installing the carbs I replaced all gaskets between in- and outlet manifold and carbs but maybe I didn't install correctly. With google I've found a couple of suggestions to spay WD-40 on the gaskets and listen for changes in engine noise. Maybe I can give that a try.

Today I will also check the effect of using the choke to see if worn carb needles are the cause. I've tried it before and it didn't seem to have effect but I want to make sure that I pulled out the choke knob enough to move the jets and not just the fast idle.

H.L. Voogd

short update on progress (it really is an interesting problem):

I've checked for leaks at the inlet manifold but didn't find any. I noticed the inlet manifold and outlet manifolds are not of the same thickness so just to be sure the inlet manifold is tightend evenly I cut the rings so it now presses equally on both manifolds.

Pulling out the choke does not seem to have effect the surging effect is still there. I'm going to try other carbs tomorrow just to rule that one out.

With the stroboscope light i noticed that with the 123 ignition - vacuum disconnected - the timing fluctuates heavily. So I installed the old 45d4 again just to see if it also fluctuates.... but it didn't it was spot on! Maybe this is caused by the spark balacing done by the 123 ignition and could this be an indication of some other problem?



H.L. Voogd

Hi Rens

Where in the Netherlands are you?
And do you mean THE MG workshop in Monster?

Check the overflow's (pipes hoses)from the SU's they can be blocked and give verry strange symtoms.

I have a spare set of su's and several distributors (and a 123) if you want we can try to mix and match to get it runing properly
Onno Könemann

Hi Onno,

Thanks for your reply I really appreciate the offer to help out! Amazing btw that you use your midget as your main car, must drive really good.
The info that the 45D4 is not suited for for the a-series midget engine came from another workshop but I've actually been to The MG Workshop in Monster on Thursday last week. On the phone I was advised to check the jets while they could clog up with the small rubber seal fitted on the tube to the float chamber. This rubber somehow can't withstand fuel very well(sounds really strange). After a short test drive the backfiring was gone but the stumble was still there. At the MG workshop they checked the ignition timing and set the mixture a little richer – and I was thinking it was already very rich. Also the throttle linkage between the carbs were not synced very accurately... and I thought that I worked very precise!
After a short test drive it appeared to be a little better. They also gave me a good advice just to use the car more often (and to get used to all noises in the process :-) ). There still is a mechanical unbalance somewhere in the powertrain. I never had the prop shaft balanced so I ordered a recondition shaft to see if that helps.
Cheers,
Rens
H.L. Voogd

If the ignition is wandering (which it should not do) it can be something in the drive train, (timing chain, pulley, distributor drive) I live in Badhoevedorp, it is not far from Monster. This weekend I am not home (Spridget weekend, there are 2 places left..) but next week I can help you.
Flip
Flip Brühl 948 frog 59

Flip, the spridgetweeken is NOT this weekend but the next: 4 + 5 september.

Rens check out the eventpage or check www.spridgets.nl
A realy recommomandable weekned with loads of spridgets, original and many modified and a great group of people of truely al ages.
Arie de Best

Hi Rens

Where did you hear i drive my midget as my main car? Where Thijs, Bart or Cathelijne talking again?

It is combined with a GT (my girl drives that daily) and a perfect train commute from Leiden to Haarlem.

Some carb parts can indeed have some trouble with fuel (strangely).

If you want we can meet up once and you can have a drive in my fairly sorted midget (though it is slightly sporty) just to compare with your's
Onno Könemann

Onno: No I looked at your 'vehicle profile' which mentions usage as main car. The guys at MG workshop are very discrete they normally don't talk about customers who are not present - no worries :-)


Here is a link to some pictures of my midget (and re-build process):
http://picasaweb.google.com/117861495662395460607/MGMidget?authkey=Gv1sRgCOGtn8yqouOeNA#

Sounds interesting this Spridget weekend. Is it possible to join on Saturday or Sunday? I’m home very late on Friday (3rd of September) from work. That would be a good opportunity to test and use the midget.
H.L. Voogd

Rens - they all told me that it is possible - but then went on to persuade me that I have nothing better to do with my Friday but to go and get drunk ... ;)

Best thing would be to contact directly I would think and not believe anything that Arie/Alex/Onno say about it - as they all told me they were going to go - and now only Arie has kept his word!

Really lovely car by the way :) Hopefully get to see it on Saturday week
rachmacb

Hi Rens, thats a beatifull looking midget!
I like it that you too left all the american stuff on(sidelights and triple wipers).

Ofcourse you dont have to go all weekend, if only saturday suits you thats fine too.
Better call Sipke, he is a great guy and he can arrange what ever works for you.

"not believe anything that Arie/Alex/Onno say about it"
Excuse me young lady????

If my car doesnt break down ill be there
...Altough I did got VIP tickets for the AmnestyInternational-lesbian-mudwrestling fundrasing event.
Diffecult decision... :)
Arie de Best

LOL - you want to borrow a pressure hose?!

If you don't come - I'll know it's coz you're scared ... :)
rachmacb

As Arie said great car!
I like the ashely hardtop (got one waiting for me to fix it up)

As for the workshop crew not chating about customers that is true but they do make exemptions when talking about cousins ;)
They are the ones that got me hooked on MG's and midget's.

Be aware of Rach she has the power to lure you in with friendlynes and when you make one little mistake she starts telling every about that one little mistake.
Yes you might have got it she is a woman! ;P
Onno Könemann

LOL Onno - is that possibly an admittance of guilt for certain drunken comments made at Silverstone???? And to think, I'd believed you ;) That's your bottle of Pimms I was thinking of bringing over gone!
rachmacb

You know the saying: drunken people and kids always tell the truth.

What was it that Onno mentioned when being under influence of alcohol? :)
Arie de Best

LOL - how much are you going to make me drink to tell you?!
rachmacb

Im not, you can pay for your own drinks.
Im a cheap/tight Dutchmen remeber? :)
Arie de Best

no i still stay with my sttement that Bob was drunk ;)

i was just admitting i first promised to come to the weekend and canceled due to other commitments
Onno Könemann

LOL - I've always believed you on that one - not least coz he confused Alex and Arie ... ;)

No worries - will still be nice to catch you and Alex on the Monday - sorry we probably won't make the sprint event you're going to, it looks great fun and look forward to hearing about it.
rachmacb

This thread was discussed between 14/08/2010 and 28/08/2010

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