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MG Midget and Sprite Technical - Running like crap mixture issue?! :-(

Damn... after yesterday and my first proper run and my nearly MOT, today has brought me back down to earth!

Here's the story:

Car is running a Weber DCOE.

Yesterday, had a five mile run out to the MOT station, bit of town driving and a bit of flat, fast country road. Car seemed to behave well in town and went well when you gave it some beans.

On the country road when running at ~45 ish mph (dunno exactly, speedo was broken) on small throttle opening in 4th there was an intermittent metallic popping sound coming from the engine. I *think* it was spitting back through the carb. It seemed to go away if you pushed the accelerator a bit harder.

MOT was OK, emissions of CO and HC were low (0.8% and 400 PPM). When I got back I pulled the plugs (see pic, plug 1 - 4). Based on this I thought I was maybe running a little lean esp. on no. 1 and this was causing the popping.

M Le Chevalier

This morning, I put in a slightly richer idle jet and messed a bit to get the thing idling smoothly again.

This afternoon I go to take it back to the garage to check the hand brake and collect my MOT.

Car warmed up and ready to go... except it won't. On small to medium throttle openings I just get a huge load of popping and spluttering and no go. I do a lap of the block, go back to my garage and put the leaner idle jets back in.

A little better but still pretty crap, worse than yesterday. Popping and misfiring.

Having lost all confidence in being able to make it five miles, I abort my run to the MOT station and go back to my garage and pull the plugs. Sooty as anything.

So... thoughts people? :-D

M Le Chevalier

remember 90% of fuel problems are electrical.??? you got electronic ignition or points and condenser.
bob taylor

Possibly incorrect float levels or float needle not closing properly, or maybe timing issue.

Trevor Mason

My vote is for timing being incorrect.
Chris Hasluck

Sounds like contaminated gas. Check for water in the tank. If you flush it out with new you can at least rule that out.

then check the electrical. Are you running points?
Steven Devine

I'm with Trevor.
My Elan has Dellortos (not the same but similar). Setting the float levels seems critical to smooth running
Simon Wood

Weber base line jet sizes.

Steven Devine


A mate had a similar issue with his Caterham which took a while to track down, turned out to be the main or auxiliary venturi had come loose and shifted.
David Billington

0.8% CO on a car without a cat and ECU is incredibly low. IIRC it should be about 3.5% on old cars like ours. It points to a weak mixture for whatever reason. Float level, as mentioned above, or incorrect jets.
Bernie Higginson

Is this an old weber or a new made in china weber

My understanding is the new.made in china webers are.loaded with issues and parts are NOT interchangeable no matter what the made in Vietnamese packaging says

Prop and the Blackhole Midget

Let me get this straight: A carburtor of Italian origin...with a German name is being produced by an Asian market specifically China thats shipped out of Vietnam. Eventually ends up in the U.K.

Now thats an international market!
Steven Devine

Get some S.U.s
Steven Devine


Sorry I just realised it wasnt a down draft. Are you running a 40 or 45?

Once you get it sorted out they do really well.

This might help.
Steven Devine

thanks for the thoughts guys.

pretty sure timing is not the issue, it was set and triple checked correctly, no reason to believe it has changed.

I am pretty sure it was lean, but putting richer jets in made it worse, unless it was another problem: the fouled plugs, that made it worse despite better jetting.

going to mess this afternoon, see what I can figure out.

M Le Chevalier

What plugs are you running?

Perhaps not relevant with A-series but in the world of Lotus twin-cams running on Webers/Dellortos, NGK BP6ES or BPR6ES are much favoured to combat plug fouling

Simon Wood

Was running BP5ES, do have a set of 6ES kicking about too. The 5 is a wee bit hotter, so should be good.

I am going 21st century on this 17th century engine, getting equipped with lambda probing...

Steady on Prop, lambda is not a type of alien ;-)


M Le Chevalier

Haha... ive got lambda In my set up also

Ive got 2 for each carb

Was the engine radically over hauled... if so it could be the dissy curve is no longer applicalble

Steven... id lay good odds that the printed paper work inside the box is probably printed in mexico or guam
Prop and the Blackhole Midget

Right, a successful afternoon I think.

- Exhaust back on, lambda hooked up, a set of cleaned up plugs in (BP6ES, although probably irrelevant, just the only spare full set I could find).
- Fired it up with the richer jets installed and let everything get warm.
- Turned it off and pulled the plugs, sooty as anything. Replaced with clean set of BP5ES.
- Adjusted mix to way lean according to the lambda, then brought it up until the lambda said that it was definitely rich.
- Tweaked each barrel until best balance/smoothest idle/highest and steadiest vacuum was achieved.
- Pulled plugs just out of curiosity: brown, not sooty, excellent.

Changed settings back to what I was running on Tuesday (leaner jet, but mix screws were fairly similar). Lambda reading lean and wouldn't idle well at the new idle throttle position (i.e. I have been able to close the throttle plates up a little using the richer jet which is good).

Pulled the choke out whilst running on these settings and it ran loads better. Should have thought to try this whilst driving to/from the MOT the other day. But forgot in the panic/excitement of driving my wee car for the first time!

Can only assume the richer jets didn't work yesterday because the plugs were fouled after waiting for the car to warm up on the choke. After then I was just fighting a losing battle and not thinking straight as I was racing to get to the garage (yeah, I never made it!)

Yet to be able to road test but a quick blast across the yard indicates it is running well. Fingers crossed! :-)

Now, do I get a lambda and vacuum gauge installed permanently in my dash... I am very tempted!

Thanks for listening and throwing in your ideas folks, you are always a much a appreciated sounding board.

It will be next week before I am able to give it a run, will let you know if I truly have been successful then.

When I finally get my MOT, I owe you all beer!

M Le Chevalier

I know your not legal at the moment.. but id still give a joy ride around town away from the donut shop and other popular cop hang outs

There is alot of differsnce between the open road and 200 feet of grass
Prop and the Blackhole Midget

This thread was discussed between 18/11/2015 and 19/11/2015

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