Bit of rough running at certain revs and road speed

Aerodynamica · Post by **Aerodynamica** » Sat Jan 03, 2009 5:44 pm

Hi all, the BX runs pretty well but ever since the very first motorway I used it on A30 in Cornwall to date up and around locally I have noticed that at around 60 - 65 and about 3K rpm, the engine is not smooth and there's mild jerking in the delivery - it almost always happens going up a long, mild incline at Mway speeds. When it does this the tacho fluctuates a little (say between 3K-3.2K)

Now I'd been aware of a little flat spot in the carburetor just at the start of the second choke that tends to cure itself after some time but now the car runs on LPG and the carb faults are all gone - it goes like a rocket! however It is still doing the same thing at 60 and 3Krpm always going slightly up hill and never does it down hill...

I'm thinking it's dropping a spark at these conditions but what could cause this?

I have still to look at the spark plugs for condition but would there be any common known causes of this? Timing has been checked and it was ok if a tiny bit advanced - not enough to cause issues.

Any ideas?

jonathan_dyane · Post by **jonathan_dyane** » Sat Jan 03, 2009 7:09 pm

Hi Graeme,

I assume that you have checked/changed HT leads, dizzy cap and rotor arm?

If so, assuming that LT leads are in good order, with good clean connectors, I'd be looking at coil (borrow another to test) and ignition module (I always like to have a spare in the glovebox in any case, as they do sometimes die without warning...)

On another tack, if the rev counter perhaps does not drop to an extent to make a definative diagnosis of ignition problems, I'd be checking carefully for signs of running lean (what colour are the plugs?); many 80s Cits were jetted rather on the lean side from the factory, with the result that a slight air leak or worn carb will lean the mixture to the point at which running is affected...

Kitch · Post by **Kitch** » Sat Jan 03, 2009 7:35 pm

Mine's doing something very similar at the moment. No idea what it is, so I'll watch with interest (although being injected it'll probably be something totally different!)

jonathan_dyane · Post by **jonathan_dyane** » Sat Jan 03, 2009 9:38 pm

Although if it acts up just the same on LPG you can rule out the carb

Jaba · Post by **Jaba** » Sat Jan 03, 2009 9:55 pm

If the rev counters dips while you are getting rough running then you are losing the low voltage ignition pulses. These are generated by the distributor and then amplified by the ignition module.
The rev counter pick up is from the HT coil.
Check all the connections and the condition of all the connector pins.
None of this is load or revs dependent particularly so you might have to investigate deeper by swapping out the components. Any of the wires could have an internal breakage that only shows up in your failing conditions.

If it was not for the rev counter I would say check the vacuum advance retard.

Hope this helps.

Aerodynamica · Post by **Aerodynamica** » Sat Jan 03, 2009 10:30 pm

Thanks for the replies guys!

I assume that you have checked/changed HT leads, dizzy cap and rotor arm?

- don't be silly, it's me here! I only 'thought' about them - not actually checked them!

I know what I'm looking for with misfire or no fire in a dizzy cap and rotor but what would I be seeing in a dizzy cap for this symptom?

Kitch, you're probably right but I'm sure this is ignition based so -might- be similar?

Jaba, yes, this is what I understand - I should say it's not a very large rev counter dip - just a 'ripple' . But I'll go through all the various spade connectors and look at the cap, rotor and (well nothing really to see on the module) - I know the vacuum capsule is alive as it was observed when the timing was checked - but not to be ruled out!
-will update

jonathan_dyane · Post by **jonathan_dyane** » Sat Jan 03, 2009 10:37 pm

Aerodynamica wrote:Thanks for the replies guys!

I assume that you have checked/changed HT leads, dizzy cap and rotor arm?
- don't be silly, it's me here! I only 'thought' about them - not actually checked them!

I know what I'm looking for with misfire or no fire in a dizzy cap and rotor but what would I be seeing in a dizzy cap for this symptom?

Hehe

I would remove the cap and give it a good clean with thinners or similar, carefully examine for hairline cracks, and clean up the copper bits, then ensure that the carbon brush seems as long as it should be and suitably springy and that the rotor arm isn't all green corrosion and had no obvious signs of wear/damage.

Citroen OE plug leads do last well, but nothing lasts for ever. Do splash out on some new plugs too!

jonathan_dyane · Post by **jonathan_dyane** » Sat Jan 03, 2009 10:42 pm

Although the rev counter ripple would certainly suggest that the problem is rather further upstream than the ht leads and cap/rotor arm, leaving principle suspects LT connections, Ignition module and it's connections, and the electrical jiggery pokery that lives in the dizzy. The coil may be indicated, but I'm not sure I've never been forced to get too involved with these systems before; usually if all the connections are fine as well as plugs/leads/dizzycap/rotor arm they just work