740 - B230k idle woes

[islipaway]

Hi, can anyone point me in the right direction with regards to me idling problems?

I have a 2B7 Carb.

The car will start from cold with no problems. As it heats up the idle gets rougher and rougher and the idle speed drops.

When I drive under acceleration the car is fine, as soon as I come off the pedal the revs drop dramatically and normally stall the car, sometimes the car will recover slightly and the idle will jump up and down between 400/800 until it stalls again.

I have recently removed and cleaned the air box to check the operation of the thermostat in the airbox. It seemed to be working fine (opened right up under hot air from hair dryer).

I also cleaned the idle jet, and the other jet above it, it seemed to get worse after this but I triple checked the jets have been put back in the correct places.

I have read one report that there is a relay which controls the fuel pump and some other things which may be the problem. I looked at my relays and the one which should be in this location is missing and just a small blue fuse is in it's location, however it seems to be factory as it as some alignment tabs.

The car is also fitted with an LPG system but I am using it on fuel only at the moment until I can get the idle under control.

Thank you for any help you can give!

[aardvarkash10]

sounds like the main fuel circuit and the idle circuit in the carb are fubar.

Its highly unlikely to be the fuel pump etc since the car performs ok under acceleration.

Is the car new to you? Some LPG conversions were pretty rough and commonly led to performance issues - often on both fuel types.

[islipaway]

Yeah, had it about 4 months. I'll try and clean the idle circuit again, would it be okay to squirt carb cleaner down the holes the jets go in? I don't have easy access to pressurized air near the car. Is there a solenoid or something that could be stuck and not allowing the carb to properly access the idle circuit?

[islipaway]

So I did a quick bypass of the relay (or where the relay should go?!) and no change. However just for interests sake here is an image of the relay area.

The blue line is a sort of built in bypass fuse thing, the yellow line is the bypass wire I used to the terminals my research told me should be the bypass (now I think I should have done the wire on the built in bypass too just to check, oh well tomorrow). Should there not be a relay here?

[Laird Scooby]

Sounds like an air leak on the inlet manifold to me. Does it behave the same way on gas or have you not tried?
Also when were the plugs last changed? If they are just standard plugs and been in a while, LPG is much harder on them than petrol. Thing is, that means they will fail before you'd expect them to so will run rough on either fuel (petrol or gas) but.............

given the cold-start enrichment, a rich mixture is easier to fire than a lean one. That means if the plugs are on their way out, you'll notice this first with a hot engine.

Before you start getting into the "complicated stuff", i'd do a basic service/tune-up - oil and filter change, new air filter, new plugs etc and also check the dizzy cap, rotor arm and HT leads. Also check your breathers/flame trap and other parts of the PCV system are clean internally, connected properly, not split, perished or otherwise leaking.

As you're running LPG as well as petrol, i'd set the plugs at 0.65mm rather than 0.7mm, both are within spec but the tighter gap gives it a better chance when hot and/or running on gas.

[islipaway]

It used to only do this on gas, but was fine (though dropping, it was driveable) on petrol, which is why I stopped using the lpg actually. I put it down to the lpg system being old and crappy and possibly not serviced properly.

Plugs, dizzy cap, rotor, air filter have all been changed in the last 4 months, to genuine volvo parts. I also replaced the flame trap and cleaned the black oil separator box thing.

Inlet leak has been suggested by someone else too. Do you mean the gasket where it connects to the main block? it's frustrating to check as I can't spray around it if it won't idle at all.

I will try and check that tomorrow when it's a cold start thank you.

Hmm! Just had a thought related to your lpg/plug gap comment.
Could this be the HT leads? I had similar behavior on lpg before but fine on petrol, now it happens on petrol also. Could weak sparks caused by failing HT leads cause rough idle/stalling?

[Laird Scooby]

All that suggests to me it might be a heat-related fault on the ignition side. Your ignition amplifier module should be on the inner wing behind the left hand headlamp, you may have to remove the air filter box for access.

If you have some heatsink compound, remove the ignition amplifier, clean the back of it with something non-abrasive and a solvent such as isopropyl alcohol, meths or similar (not white spirit, that's oily) and clean where it mounts on the body work.
Once dry, apply some heatsink compound to the mating surface of the ignition amplifier and refit. Also check all the coil terminals for security and undo and clean the coil mountings and refit.

The air leak might be the inlet manifold gasket but it's more likely to be a vacuum hose off/split or similar. Could also be where the diffuser plate for the gas fits to the carb and/or where the carb fits to the manifold - could simply be loose.

[aardvarkash10]

Unlike the Laird (apologies yer honour!) I lean toward the problem being fuel side. Vacuum leaks are one of Dave's favourite go to's and with good reason. The Volvo intake is plastered with vacuum take-off trees each with a leaky, thirty year old rubber hose attached to it.

They are the automotive equivalent of a urology ward near a brewery.

The carb itself is a nest of vipers. It was a horrid piece of kit when new regardless of what it was fitted to, and age hasn't improved it. Parts are almost unobtainium as I understand it, so regardless of the cause, its well worth considering a swap to something more common, simpler, easier to get bits for and reliable. Like a Weber ( the carburettor, not the barbeque).

[aardvarkash10]

Originally Posted by Laird Scooby

... and/or where the carb fits to the manifold - could simply be loose.

Ha! Reminded of my old Datsun 1600 - the entire top plate of the carb would slowly work loose and fall off! Fortunately, it was clamped to the air cleaner, so never dropped onto the road. It all just hung there like a whipped dog, apologising for the inconvenience and asking to be replaced - again, and again, and again...

[Laird Scooby]

Here's what i would do :

1) Remove, clean and re-paste the ignition amplifier module as described above. This is good preventive maintenance regardless of whether it fixes the problem or not.

2) Check your plug gaps, not sure who makes plugs for Volvo but consider a set of NGKs (BCPR6ES, very cheap in most places), set the gaps to 0.65mm

3) Check all the vac lines, just to be on the safe side. It did this on both petrol and LPG so points away from the carb BUT (i'm with you on this one Ash!) the Pierburg is known to be a troublesome unit and has a few faults that could upset the running even when on LPG such as the vacuum controlled secondary throttle, vacuum controlled part-load enrichment valve etc Any leaks in those could not only allow air in where it shouldn't be but also squirt unwanted fuel in as well.

4) If all else fails, fit one of these :

http://www.webcon.co.uk/shopexd.asp?id=839&bc=no

It also gives you a manual choke which leads me to another possible fault - the auto-choke unit could have failed resulting in the engine choking itself when hot, regardless of fuel.

On petrol, it would cause excessively rich running but as the vacuum dropped due to engine speed dropping, the flaps would open a little to let more air in, vacuum would build again and the flaps would close again causing rich running - this results in hunting.

On LPG, because the mixer sits above the throttle butterflies (usually on top of the carb) it has the opposite effect. As the engine speed drops, the drop in vacuum causes the choke flaps to open slightly allowing more vacuum to present to the mixer pulling gas in. As the revs then rise, the choke flaps close reducing the flow of gas - again this self-repeats and causes the hunting you described.

Gut-feeling is it's something simple but probably vac line and/or carb related.

Good luck and let us know how you get on!

Ash - i've only owned three carb-fed Datsuns, a 120A F-II with a 3-speed semi-auto (great fun!), a 1.2 Cherry and a 1.3 Sunny, no carb problems on any of those. All other Datsuns/Nissans i've had have been injection, three C210 240KGT Skyline Coupés (L24E) and a Y30 300C with the VG30E - nearest i got to a fuel problem was a pin-hole in the fuel feed hose on the 300C squirting a thin jet of fuel onto the underside of the bonnet!