Your questions answered: Carburettors and battery

Carb dashpot oil

In the May instalment of Torqueback, in answer to John Cameron’s question you advised using 20W/50 oil in carburettor dashpots. SU carburettors have always recommended straight 20-grade oil for the carburettor damper oil. Also applicable for use with the Zenith CD carburettor.

20W-50 is too heavy, especially when cold. On the downward recovery, the damper valve needs to recover immediately without any interference from the oil – 20W-50 will hold the piston up, resulting in weakness on overrun, and possible stalling. I think enthusiasts need to be cautious. Graham Nash

Ed Evans replies: I agree SAE 20 oil has been recommended for carburettor dashpots, and Land Rover did specifically stipulate SAE 20 monograde for the Zenith-Stromberg 175 CD-SE carburettor on the 2.6-litre straight-six engine fitted to Series vehicles.

However, for later engines in the UK, including the 3.5-litre V8 petrol engine which the reader was asking about, Land Rover recommended 20W/50 oil, 15W/50, 10W/40 and 15W/40 (depending on the oil brand) for the carburettor dashpots and the engine sump.

Land Rover’s (and other manufacturers’) recommendation of multigrade engine oil in the dashpots is confirmed by a Zenith Carburettor Co Ltd Service Bulletin on the Stromberg CD which states: ‘For all normal requirements engine oil SAE 20 is suitable unless the car manufacturer recommended some variation from this’. It continues: ‘As a general rule the same grade of oil may be used in the dashpot to that which is used in the crankcase of the engine’. This suggests to me that the grade isn’t too critical.

I have always found 20W/50 works well with Rover, Triumph, MG and Jaguar multi-carb engines, and have never experienced a lean mixture or stalling on overrun. But that’s not to say that other multigrades or 20 monograde aren’t as good or better, depending on how the vehicle is driven and how the person sets up the engine, and I think we all do that slightly differently.

Range Rover L322 loses power

My 2007 Range Rover L322 3.6-litre TDV8 is sometimes draining its battery very rapidly. The battery and alternator I am told , are both fine, so I think there is an intermitted current drain. When I took it to a garage for diagnosis, it was behaving normally, so how can I track down the fault?

I think it might be connected to the diesel-powered night heater, because both times I’ve used it, the battery has gone flat, but it has also gone flat when the heater is not activated. Occasionally I have heard a fan near the battery box coming on by itself and at unexpected times, but not very often, so I’m not sure if this is related. Last week I gave the car a 30-minute drive in the morning and it was fine, then an hour after I came back it was dead.

A garage, trying to be helpful, added a solenoid to the battery so that it would cut off before draining too low. This was fine for a few hundred miles, then it cut off while I was driving, repeatedly.

I’m now leaving the battery disconnected overnight. Any ideas? Theo Ford-Sagers

The magic eye battery indicator (not fitted to all batteries) provides a quick assessment: green colour means over 50 per cent, black is under 50 per cent and clear means the battery should not be used

Steve Jones replies: Your battery is four-years old and has repeatedly entered the ‘dead’ zone. Any battery that drops below 12.5 volts is suspect, and yours has gone to 12.35v, so the first thing to do is to check the magic eye battery indicator, if it has one. Green on the indicator means over 50 per cent, black is below 50 per cent, and if the indicator is clear do not charge it because it may explode.

Assuming it’s safe to charge, then charge the battery overnight preferably with a charger that has a regenerate setting. Then let it stand disconnected for 12 hours before checking the voltage. Usually they sit at about 12.8v.

The off-load battery voltage indicated here is 12.81 volts; that’s well over the minimum of 12.5 volts

The battery tester is a more useful tool. This one indicates a battery voltage of 12.76 and an internal resistance of 2.72 micro ohms

You cannot recharge a fully discharged battery by running the engine. And the only way to test it is with a proper modern tester which applies a series of current loads and monitors the recovery to give you a decision. Below 3 micro ohms is good, 5 is bad.

Once the engine has started, the whole car runs on the alternator with the battery at about 14v. If the car stops due to falling battery voltage, then this indicates the alternator is faulty. Having charged the battery as described, start the engine, then lift the revs to 2000rpm and measure the battery voltage. Turn everything off on the vehicle. The voltage should rise to 14v within about one minute. If below 14v, then alternator is not charging and you have your problem.

If alternator and battery are fine then you could have a parasitic drain. If this is suspected, remove the battery negative and put an ammeter between the lead of the battery terminal and the disconnected clamp. Makers say below 80ma (0.08A) but you may have to wait a few minutes for the vehicle computers to shut down, which typically draw 5 amps.

A clamp meter, which is positioned around the positive lead here, is a convenient way of checking current draw

If you have a load, remember that every amp represents 12 Watts, so try to think what could be drawing that power. A simpler way is to use a digital clamp-on meter on the main battery lead, with which you can also see if you have any charge current. A walk around the vehicle in the dark may reveal a light on somewhere.

If you are worried about the diesel heater, pull fuse F13 (59 on some vehicles) in the engine bay fuse box to disconnect it. The fan you hear is probably the one that cools the ECU. Personally, I think you have a weak battery due to repeated overdischarge,
and a failed alternator.

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