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MG Midget and Sprite Technical - Why does my Weber hate me!

Hi guys,
I am so frustrated, my Sprite goes really well with a 1 3/4 SU, but I got a brand new 40 dcoe a while ago, on a nice maniflow manifold with all new Mishabs etc, which when put on the car ran really badly, which I quickly deduced was the jetting. I have replaced all the jets in line with Moss recommendations, and the car now revs freely and pulls well, now here comes the but! About 2000 revs on a feathered throttle it hunts and there is an obvious misfire feeling, which was absent with the SU with no other changes, so I am guessing that the float level might be out, that the Weber needs more advance on the timing (but why the misfire?) or the idle mixture is way off. The other thing I noticed is when I got back from a run and blipped the throttle, the revs took a while to come back down, and I know it's not the carb not closing as the springs are quite strong and working well. I've got a malpassi pressure regulator and have set the fuel pressure to about 3-4 psi, compressions are ok at 12-13 bar and I set the tappets today as well. I would mess about with it myself and do my usual bury head in the sand trick were it not for the fact that I have the car's first competitive sprint at Curborough next week, and I would hate to make a prat of myself as I've done it too often in the past with various cars! Any advice you guys can give will be most welcome.

Cheers
Neil
Neil Williams

How is the carb set up and what is the spec of the engine?
Dave O'Neill 2

was just about to say much the same.... A 40 is a tad small if you are 1330 or larger, but in any case, don't mess about, get yourself off to a good rolling road and have it set up properly.
David Smith

Hi Dave,
The jets etc are as follows;

32mm Choke
145 Main Jet
45 Aux Vent
F16 Emulsion Tube
180 Air Jet
40/45 Pump
50F8 Idler Jet

The rest of the spec is a bit of a mystery, as it is a Marina block, MG Metro head, points and std 45d distribuor, apparent fast road cam, and a single box stainless exhaust. Sorry not much help

Cheers
Neil
Neil Williams

Thanks David, its a 1275, so a 40 should be fine. I may well end up getting it rolling roaded, but silly errors that can be sorted prior can save me huge dollops of cash! Oh the other thing I shoudl have mentioned is that with the kids of school, I'm bloomin broke for 6 weeks!

cheers
Neil
Neil Williams

While there is more than one explanation for the synptoms you describe (subtle variations of them can help refine the diagnosis) I'm inclined to consider lean mixture first. A few things to look for:

1. It is very common for the manifold gasket A series engines to be damaged where it is very thin above & below the inlet port, leaking air as a result. Sometimes the damage can be very slight, and the leak will be very small. When this occurs, it usually doesn't affect full throttle conditions, but is most evident at light cruise situations at lower speed - say at a steady 30-40 mph.
Two other symproms when this happens: slow to drop engine speed (you have this one) and rough idle (or in the case of lumpy cams rougher than it should be.

2. It could be the progression system not working correctly, either through blockage or wrong jetting. Check that the progression holes are not blocked. They are accessed via the large blanking screw just behind the idle mixture screw.

Camshaft choice can affect progression requirements, so it is enirely possible you have a mismatch even though your main circuit is good.The progression system draws fuel through the idle jets but the mixture for it is not controlled by the idle metering screw which adjusts only the idle mixture itself.

To change the progression mixture, you need to change the idle jets, so I'd suggest that if you can gain easy access to a range of jets it would be worth trying to see if it makes a difference. After each jet change, you will nedd to adjust the idle mixture screw to optimise the idle setting.

Note that there is a complex mix of jet orifice and air bleed (F number) so don't assume that the next size up (or down) in jet size automatically makes it richer (or leaner). If the air bleed changes in the opposite direction the effect might be different from what you anticipate! The F number has no proportionality to the amount of air bled - a list of the sequence is at
http://members.aol.com/dvandrews/webers.htm (but not the jetting examples alsewhere on this page are not for siamesed inlet engines)
Paul Walbran

Thanks Paul,
I will have a look at the inlet manifold gasket, as I've got one of those, and the SU has the extra ring bits which join the carb to the head, so perhaps this is why it didn't happen on the SU?

Cheers guys
Neil
Neil Williams

The locating rings certainly do make the SU's less sensitive to gasket failure
Paul Walbran

Dave a common misconseption with webers is the difference between a 40 and a 45. The airflow through a weber is determined by the choke size, in this case a 32mm so therefore whether that is fitted into a 4o or a 45 will make no difference at all. A 40 DCOE is capable of having 36mm chokes fitted and anything above that would need to be fitted into a 45. Thus achoke size of 38mm would need a 45. A 1380 running at 8 thousand RPM would just about need a 36mm choke so in reality a 40DCOE would be suitable for the vast majority of Spidgets

Neil assuming it is not an air leak which would certainly cause these symptoms then you need to look at the idle jets.

The idle jet is very important on a weber and actually does most of the fueling in normal running. If the idle jet is too small then as you "slowly" progress the throttle the car will misfire badly and stutter. On the other hand a jet is too big then the engine will hunt around alot at slower speeds/ small throttle openings. You need to establish which symptom fits your car.

From my point of view your idle jet sounds too big probably a 40 or a 45 idle would be better, or at least worth trying, although with a high overlap cam that could change.
Bob (robert) Midget Turbo

Neil Also check the thickness of the mounting flanges of your exhaust and inlet manifolds where they mount up to the head. If the exhaust is thicker than the inlet it might be preventing the inlet manifold from being sealed up on the gasket properly causing an air leak the result of which is the same as what you've got. Hope this is of some help Gary
Gary

I am getting ahead of myself here as the car is some way of being ready for this but, can anyone recommend anyone who could set up a pair of dellorto carbs on a 1500? Sorry to hijack the thread but it just seemed the right type of people might be following this one.

Cheers Carl
C Bintcliffe

Hi Gary thanks for the idea, I've made some plates that go behind the washers to make the thicknesses equal, as like you suggested the inlet flange is 2mm thinner than the exhaust. I've now changed the gasket, having filed the flanges flatter thsn they were, as the finish on the inlet was poor, and I though this might help. I've put 45f9 idler jets in, which were the only other ones I had, so this might help too. I've checked for leaks now with the car running, using anti freeze spray and the revs stayed constant, so that's one less to think about. Now all I have to do is road test it and hope. Thanks for all your help guys it is really appreciated.

Cheers
Neil
Neil Williams

Took it out for a test run, and soo much better initially, although a misfire when hot, but I guess that is because I was a) tanking it and b)it's got a grp front and gets bl**dy warm, but that is another subject I am guessing, anyway so far so good, I'll now take a gander at the ignition side

Cheers
Neil
Neil Williams

Glad to hear it is improving. Naturally I think the 45 f9 pilot jets sorted it but it may have been also a leak.. :-))
I have found in very hot weather with high underbonnet temperatures the weber does suffer from fuel vapourisation and after struggling a little in traffic needs to have a bit of a blast to cool it down. I would try insulating it and the fuel lines or reduce the temperature.
Bob (robert) Midget Turbo

Good news! Given the direction of your jetting change, which indicates it was a bit rich before, your remaining flat spot when hot could also be residual richness. If you can get some slightly weaker idle jets they might be worth a try.
Paul Walbran

Cheers Bob, I've got some Metro bonnet vents which I am thinking of putting in to cool it down, just got to work out how to fit them without weakening the bonnet too much, and I might wrap the exhaust too.
Thanks
Neil
Neil Williams

I think they were air inlet vents on the Metro, weren't they, because of the high pressure area in front of the windscreen?

To get hot air out from under a GRP bonnet, one of the best spots is a vent in the low pressure area behind the front wheel, as on the Dick Jacobs midgets (developed with wind tunnel assistance).

Tom




Tom Coulthard

Thanks Tom,
You are far more knowledgable than me obviously. I think I'll stick them in there once I've made a template for the hole. It would explain why the edge of the bonnet on my old hillclimb car was cut away, as in the accompanying photo (courtesy of Peter May's website), which did not suffer from overheating. The Metro ones probably aren't ideal, but they were cheap.

Cheers
Neil


Neil Williams

This thread was discussed between 26/07/2008 and 28/07/2008

MG Midget and Sprite Technical index

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