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MG Midget and Sprite Technical - Weber carb hesitation on trailing throttle

Car is a 1293 fitted with a 45 dcoe weber. It pulls and goes really well but if you are pottering along behind traffic at around 50mph on a trailing throttle it stutters/hesitates.OK once you press the throttle again but hesitates when you ease off.Anyone had a similar experience, possible simple fix!?
Thanks,
John
J Sloan

What makes you think it is the carb?
Oggers

what cam are you using and what's it timed in at?
David Smith

Sounds like a manifold gasket leak. I had similar on a Midget, albeit with twin su carbs. Symptoms were it would idle OK, if you pressed the accelerator quickly it was OK. If you gently pressed it the engine would die. It could be a handful at lights in slow traffic.

When I came to change the gasket a couple of the nuts were only finger tight and on that inlet port the gasket had collapsed and was disappearing.

Rob
MG Moneypit

Agree with Rob. Or maybe vac advance? Just a stab in the dark - it's been a long day and I stink of diesel. Need a shower and sleep before I can think properly.
Greybeard

I think it is the carb as everything else is fine. It does not stop altogether but drops from around 3500 rpm to say 3200 then backup again and cycles like that whilst on the trailing throttle. It ticks over fine and accelerates well. It feels as if it is on some sort of transition point between too much and too little juice.
Engine was built around 15 yrs ago by Autosprint in Brum, Geoff Dodds (info from the files long before my ownership).It has one of their cams ,an AS125. Tickover is OK if a little lumpy....put down to the cam. It is top endy so really picks up over the 3500 to 4500+ rpm mark. Original spec was stage three,gas flowed,cooper s pistons and the lively cam. Don't know what the timing is set at currently.
It has lumenition electronic ignition (I think) and the vac advance is absent.
Recent new coil and rotor arm as it had been arcing out a bit. I know, I know....take another look!
I am competing in it at the weekend so will not be on a trailing throttle much but don't want it to let me down!
Thanks for the thoughts and ideas...
John
J Sloan

John
Is this a new problem or has it been like it for a while
If it's new, it could have a bit of a blockage in the idle jets as these feed transition as well or even the main jets could have a blockage----all easy to get at from the outside. Idle jets under the brass plugs and mains etc under the little round thumscrew cover
Also check the locator studs for the venturies, they are about half way along the body of the carb, down round towards the bottom at say 45deg ish--make sure they are both still there and secure- If one has fallen out or about to it could be a cause
Have you got your new coil round the right way
willy
William Revit

This is probably a red herring given that I'm a technical doofus but your engine, carb, etc sounds near-identical to mine. When I bought the car I also had a problem with severe hesitation off a trailing throttle, also it would sometime stall at lights/junctions. I fitted a Filter King (the glass bowl thing) which regulated the fuel pressure to the carb and problem solved. It never did it again.

Just a thought.
J Gibson

Sounds a bit over carbed for a 1293cc road engine, perhaps a bit of flooding, have you got a fuel pressure regulator in? I'd also wonder what cam you are running.

Plus all the above manifold leak, timing, ignition fault questions.

Weber gurus may want to know what carb emulsion tube etc sizes are you running?

Cheers
Mike
M Wood

Always difficult to diagnose without actually hearing/feeling it, but if it is a new thin and is happening at steady speed it sounds like progression issue. Progression is fed by idle jets, so check as Willy has noted above. I did once find a progression hole blocked, so worth checking that too.
Paul Walbran

Thanks for all of the ideas and advice.I opened the idle mixture screws a tad but then the tickover increased so adjusted that.Went to do the rally and it ran OK for a couple of tests but at the end of a run the revs were up at 2000 rpm which then slowly dropped to around 1000.At 1000 it was hunting up and down 200 rpm!? Carried on for a couple more runs on a forest stage and ended up with the gear lever stuck in 3rd but no drive.....end of rally. Jets are going to be checked and reset whilst stripped to see what the hell is going on inside the gear box.
The joys of Motorsport I guess. Four classics retired from the event (2 minis, mark 2 escort and us) none of the targa cars retired (mx5s etc) .....having a few bad thoughts at present about going over to the dark side so I can spend more time in a car rather than under it!!!!
I'm sure it will pass!?
J Sloan

Sorry to hear you are not having fun.

Maybe get it set up on a rolling road with someone who tunes twin-choke Webers?

Thanks
Mike
M Wood

This thread was discussed between 11/09/2017 and 19/09/2017

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