running problems on part throttle

[ethelred the ready]

Hello everyone, i wonder if anyone has any ideas what could be causing running issues on my 1989 1.9dg waterboxer with weber carb and lpg conversion with romano reducer and twin coke mixer plate on the carb. The van idles fine on petrol and lpg and pulls at full throttle past 70mph. The problem is at intermediate throttle, the van hesitates and stumbles, and the fuel air mixture goes lean, same on petrol or lpg. It only does this sometimes, at other time the mixture is fine across the range, and the van drives smoothly.

So i think it cant be fuelling as problem happens with both fuels,all ignition components were all replaced a year ago, and you can feel the extra pedal resistance when the second throttle butterfly opens and when it opens the engine runs great at full power. Perhaps something in the carb is getting stuck intermittently?

I intend the recheck all vacuum lines and the carb for air leaks and dissasemble the carb looking for anything that sticks, does anyone have any weber carb experience or any other ideas what to look oit for and check? Oh yes timing is fine too, checked withbstrobe recently.

Sorry for long post, all ideas welcome, thanks.

[ethelred the ready]

Any ideas anyone? Even crazy suggestions welcome as I don't really have time to check absolutely everything. Thanks.

[CovKid]

Not too many of us with weber carbs thats why replies thin on the ground. New leads (well a year old) can still break down or short to earth via the plug caps. I wouldn't suspect fuel either if the effects are same on both. More likely to be ignition, or part of.

Substitution may be the only way to nail it, but don't just buy new parts in the hope you'll sort it, better to try one thing at a time and ideally borrow or use known good used parts.

[kevtherev]

personally I would be off to the nearest rolling road to have the advance curve and fueling checked and if needed rectified.

[ethelred the ready]

Thanks for that both of you. Not sure it advance as timing is the same as when it was running great and no broken springs in the distributor. I would like one of those dizzys with the selectable advance curve but not much cash at moment. My 2cv is has replaced van as daily driver until i get this sorted, will have a fettle at weekend and hopefully get to bottom of it. Thanks again.

[kevtherev]

The advance curve is controlled by vacuum.

[Cheeky V]

could be that the main jet needs switching out for a bigger one, if you take one of the plugs out what color is the insulation around the electrode? pic would be lovely if possible

(as its running fine on idle it might be difficult to tell with an older plug because deposit build up, best thing to do is stick a new one in take it for a short thrash and then see what color it is)

also which weber is it?

[T25Convert]

Hi,

Had a weber (without the LPG) - they have a 'convertor block' at the base which is a lump of rubber with metal blocks embedded in.

These are pretty weak points and can spilt in a way that you can't easily see - I'd start with this, check its all in one piece. May be split at the front (i.e. facing you) which would only mess with the first throat not the second which opens at full throttle....

Let us know how it goes!

Cheers

[ethelred the ready]

Thanks again interesting about the conversion block will have a look at that. I seem to have solved 90% of the problem hopefully (although as it was intermittent it is hard to be sure until i have done more miles) i took the lpg mkxer plate off and filed the adaptor on top of the weber so it was perfectly level and the mixer plate sealed on top of the choke barrels properly. I also unscrewed the choke butterfly plate and removed it (not needed on lpg) as i thought it might have been moving around despite being disconnected from the metal coil spiral thing. After driving and adjusting mixture (lambda probe and gauge) last night it drive much more smoothly

Engine still slightly shudders at light throttle, think engine mounts might be weak, will try manually rocking engine. Thanks for all suggestions, it helped the thought processes.

The dizzys i meant kev were the ones where you can manually switch between different advance curve profiles to suit lpg or petrol, seen them in practical classics, dont know if you can get them for t25's

Thanks again, will get back if it needs more fettling.