Any ideas on this 'flat spot' fault?

[fairwynds]

Hi all
got a bit of an annoying 'flat spot' when driving my van.
Its a standard 1900DG running on a Gasure LPG conversion.
Recent work included dropping out petrol tank, which has not yet been replaced (but fault was there before this job).

Symptoms:
When engine under load (say 3rd gear and a flat road or slight incline) as I squeeze the throttle pedal to accomodate for the hill etc the engine just feels 'flat' and a bit lifeless. Have to contine squeezing the pedal down (maybe 2cm+?) before there is a sudden burst of life and all feels well again.
Come off the gas and decelleration is normal...put foot back on the gas and same flat spot is there, until I push pedal down again (beyond what is normally needed). In short, I'm having to drive really heavy right footed to get anywhere!
Though maybe it was the LPG 'power valve' needing a tweak, but that's made no difference.
Not had new Dizzy cap/rotor arm/HT leads for at least a couple of years, so will start by replacing those.

Any other ideas people? Ive read many posts here relating to the 'choke pull down unit' causing running issues....?

Am at a bit of a loss (of power!) right now...

[itchyfeet]

LPG needs to have a well sealed airfilter and trunking to the carb so make sure it's all sealed up properly, just a loose pipe clip in the air filter trunking can casue this sort of thing.

Also look for air leaks around the dizzy and air filter vac pipes, take the vac pipes off from the rear of the carb and suck, if you can suck air easily then there is a problem.

An air leak on the vac pipes can upset the second choke as that is vacuum operated once the throttle is sufficiently open.

You can check the second choke is working by removing the airfilter trunking to the top of the carb, take the vac pipe off that goes to ther rear disc shaped vac unit , open the throttle and suck on the pipe, you should see the second choke open and you should not be able to suck fresh air in.
Hard to suck on this pipe as it's short so extend it by borrowing the plastic pipe to the dizzy ( assuming it's origionl and still there)

check the choke pull down as well by removing and sucking on the pipe you should not be able to suck fresh air but if it's responsible i'ts becuase it's leaking air and upsetting the second throttle operation and not beacuse it's not doing it's function.

[fairwynds]

Thanks for the comprehensive reply Itchyfeet. I will be looking at all this later today, and will report back
....fingers crossed..

[California Dreamin]

Sounds like an ignition problem as the 'richer' mixture tends to extinguish the flame, so to speak. I would be inclined to remove those metal spark plug cap shields in the first instance, as this is where the spark tends to 'go to ground' under load.

Martin

[fairwynds]

A number of suspect air induction issues found. Have just typed long explaination together with pictures but laptop threw a wobbly and didnt save what Id typed! Grrrrr.
Will continue investigations tomorrow...

[ghost123uk]

Does it do it on both petrol & LPG ?