Help(!): Missing in the mid range and tried everything!

[Terry W]

Looking for sympathy and good advice as I am well and truely stuck!!!

Have a Beta Spider with modified engine but original Weber type DAT Carb that was new when the engine was rebuilt and covered very little mileage, but has not missed a beat in the last couple of years.

Problem started half hour into a journey - symptom being stuttering under light throttle but iddling ok and full bore acceleration absolutely fine. Initial thoughts were blocked jet/ dirt in carb but interior of carb is spotless and have blown through airways/jets.

Furthermore car runs perfectly when cold and need engine to warm up before problem occurs. Engine stalls when taken slowly to 1500 rpm but can keep running by pumping accelerator (ie priming fuel injector in the carb).

Been the electrical route and change spark plugs, leads, coil and distributor cap with no impact so feel it must be carb related. But perhaps I should try new electronic ignition module? (comments?)

Have checked all hoses and looked for manifold leaks.

Guess it must be carburetor - but don't know what to try next.

Really bugging me now - so guess I have to try and find a professional outfit that understands Webers?

Any thoughts or suggestions much appreciated.

(Brassed off of Bristol!)

[Wallace]

Yes ! My Dad's got the same problem on his Manta - the vacuum advance is knackered ! Hopefully for you it's just a split pipe from the carb to the dizzie, if not, you might need a new capsule - and I wish you luck trying to get hold of one becasue I'm damned if I can !!

[Gregory Smith]

Sounds a lot like your carb's accelerator pump is failing. A worn pump cup can work fine on hard fast stabs but not so well with gentle pedal. Cheap fix, at least, if you can rebuild carbs yourself or are willing to try it.

[Shane Taylor]

I have had a similar problem, talked to my mechanic and we diagnosed that it was the condensor failing on the distributor.

regards,
Shane

[badboyboogzy]

i had the same problem turned out to be small paint flakes falling into the carb from the air filter box lid take it of to se if there is any flaking if so that may be it or any of the other things described above

[Terry W]

Thanks for the suggestions. Have decided to have another crack at this myself before going to the so called professionals (the problem here being who to trust with a problem like this). I have taken the carb completely off the car this time (rather than just taking the top off). Will clean out all internals and try again.

I don't think this can be the carb accelerator pump - as can keep engine running by repeatedly pumping (gently) the throttle. Vacuum advance is an interesting theory, but again would have thought this would just loose power rather than spluttering on gentle acceleration. Also with the electronic ignition there is no condensor as far as I am aware unless I have missed it?

Weather has been bad this weekend (what should I expect in the UK?) so carb still currently in bits - but will be back for more sympathy soon!

[Gregory Smith]

After giving it more thought, you could have a blocked idle jet. On some Webers the idle jet is also used at cruise and especially during transition. If yours was partially blocked it could exhibit the symptoms you have. It would make sense that it happens after warm-up and the choke is no longer richening up the mixture, since the blockage essentially causes a lean condition that the choke would mask.