chasing heavy fuel consumption

For as yet unknown reasons I've had in the last year three instances of heavy consumption - I run out or almost at 13 mpg (usually I get around 22.5 mpg). It seems to happen when the bus has been standing for several weeks, so last week I filled up first time since September. You'd suspect a leak, but there's nothing, no vapour smells, no fuel in the oil, and the chokes both appear to work and stay open.

It runs OK, no hesitation or down on power, no air leaks, correct timing, brakes not binding. The only other fault is poor oil consumption but that's another story.

In summer when it gets plenty of use the mileage is normal.

Ideas? It's a 2 litre t4 with stock PDSIT 34 2/3 carbs.

 

Short runs, cold weather might have an affect.

 

there's someone on here who gets thirty something miles per gallon probably best getting some advice off them, it might have been @Dub and Dubber . my type 4 1700 used to do a similarly low mpg

 

Old fuel gives lower performance so less mpg with old fuel.

Petrol will generally last for around 3 weeks at summer temperature in a vented fueltank, after which time the performance will suffer, and it will be best to add fresh fuel to restore the performance. Petrol in a sealed container will last for more than 6 months before the performance suffers too much

 

Our T2a with 1600 tp type 1 was running rich and a local mechanic spent about thirty seconds with a screwdriver and a "leaning ear" to tweak the two adjustments on offer.
(Single carb ... Solex copy?)
I knew there was a danger of running it too lean, but he made such a slight adjustment .... Mid 30's we had from then on.
Almost entirely summer driving, and "touring" mostly at 50 to 55, and quite hilly in places.
Not sure what I can offer really, given a different vehicle, different unit and different driver ... though in your case, it was the same driver before at 25mpg, so 13mpg nothing to do with "a change in habits" I'm guessing?

 

the chokes appear to work you say but have you actually checked them?

 

Sticking float valve? Making it run rich?

 

I reckon thats about what I am getting, 1 day I will look into why, 1 day. Today was not that day, tomorrow isn't looking good either

 

All good ideas for me to follow up on. So far it doesn’t seem to to be getting over rich, and the plugs are no different. This looks like something that happens due to lack of use, but it’s only recently happened - twice last winter, then a trouble free summer, then back to previous. I’ve had the bus 16 years.

 

Your not getting dodgy fuel from anywhere ? With water in tanks ? Worth checking . Winter choking ? Chokes not going off ? Or taking longer to warm up ?

 

Is it really an issue if your not actually using it? If it aint broke ,,,

 

I call BS as well .. unless it’s been part of the emissions scandal... The only way to tune is with an AFR and lamda sensor . I’d be happy with 20+ ...

 

I noticed that mine had gone down from around 24 mpg a year or so ago to around 15 mpg recently - I have now discovered that the odometer on the speedo only works intermittently.

 

Ethanol in the fuel may be partly to blame. Petrol and ethanol don't really mix and leaving old fuel in the tank for a while lets the heavier ethanol sink to the bottom of the tank and worse is ethanol absorbs water easily and then forms a layer below the petrol which doesn't burn well compared to pure petrol.

In summer the van is used more and fuel topped up with fresh and the engine would also come up to temperature much more quickly than in winter.

 

That’s another good place to look. I usually set the trip meter each fill up and could confidently drive 350 km even though the fuel gauge is dodgy. In actuality, I reckoned it would run out nearer 400. Now it’s almost out at 250.

 

I'm interested to find out if this actually happens. The last tankful went from September to January, not that long really so it would be quite a dramatic change.
As per your previous suggestion I'm looking more closely at the chokes. I did have an intermittent problem on my old bus. One of the choke units used to turn itself off when it got hot, and there was a strong smell of fuel. I don't get that, but it could be a more subtle effect.

 

Take the plastic tops off both carbs. press throttle once to activate chokes. both choke flaps should be in the 3/4 closed position. turn ignition on. don't start it. wait say 5 mins. look at the choke flaps, both should be in the vertical position. Sometimes a carb backfire can blow the choke flaps shut, if this happens you have to physically push them back to the open position. Also you need to oil the choke flap spindles were they fits into the carb top.
Also there is a tiny mixture screw on the side of each carb. These have to be set right. Set these when its fully warmed up.
The 3 and 4 carb has an air adjusting screw and a mixture screw on the top of the carb.

 

Google 'ethanol in petrol' for loads of interesting info.

 

I was thinking along those lines but leaving it for a longer period. I know they open OK but from previous experience, the heater element can break, with the two ends of the wire still making contact. Eventual expansion disconnects them and the choke flap shuts. You need to make sure other circuits on the van are off if doing this parked up - in particular electronic ignition.