DG ignition fault

[1360rallye]

I have DG engined van that runs quite roughly when pottering along 10-15 mph. It doesn't misfire it's just not it's usual smooth self - it kind of shunts - it also is quite lumpy on idle. At the weekend when we had to set off from cold up a v steep hill it bunny hopped away from the start and when it settled down up this hill in 1st it would not take full throttle. It also sometimes hesitates for a second or two when changing down.

We generally run on LPG. However the problem is more pronounced when we run on petrol - on the motorway it occassionally lost power, backing off on the throttle would see it eventually pick up again

All of which leads me to think it is an ignition fault. I've checked the plugs - they look ok. I've replaced the plug leads, distributor cap and rotor arm (the contact points on these were black). All the little vacuum hoses look ok.

Any pointers on where to look would be useful.

Thanks

[Mocki]

generally, if it were an ignition fault it would be more pronounced on LPG than petrol.

it may be a mixture fault, choke or air intake flaps?

[RedSquirrel]

Fuel starvation?
I'd check the vacuum hoses and change the fuel filter for good measure.

[1360rallye]

Just taken it out to double check - rough running seems pretty much the same on petrol or lpg when pottering and doesn't improve when the engine is at working temp.

Mocki - how do I track a mixture fault? (I haven't altered the lpg mixture in at least a year) I didn't think that the carb did anything other than open/close when running on lpg?

[Mocki]

Doubt it is LPG mixture, although they do generally need a tweek after a few thousand miles, just to mae sure it hasnt "wandered" since install.....see your installer!

I meant that you should check your choke is working, and the flap in the carb isnt staying half shut or maybe the flaps in the air intake depending on installation and model, the LPg is "injected" above the choke flap, with the air, so it maybe restricted if the choke has gone all "awkward" on you. which will also make it run like pooh on petrol.....

however it could well be a vacume fault also, check the big vac pipe on the end of the inlet manifold with the one way valve in it ( Vac to brake servo) as these often leak causing crap running and that would cause trouble on both fuels.

I am not ruling out an ignition fault, just pointing out that LPG shows ignition problems to a bigger degree than Petrol , unless its a major fault, where both will show it.

Process of illimination is the way to go, check / replace the simple stuff first, then when you are sure all is well, move onto the next thing to check.

[Pootle]

You might also want to check the operation of the vacuum advance. Check that it actually advances the timing.

--
Pootle