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[ChrisD]

Regarding fuel consumption rates, and remembering the discussion a few months back, I recollect driving a rental VW Caddy with fuel flow rate readout. Cruising at 60mph gave an indicated and actual ~60mpg at ~half throttle. Accelerating WOT to pass a slower bus, the fuel flow rate increased to 10-15mpg. Surely the same could be true of our Bullets? So with that in mind, I did the sums – they are mostly in kms and litres ‘cos that’s what we use in Cape Town.

I have stripped and cleaned my petrol tap (a lever type on my 1996 Classic 500cc, not available from Mr H). On Main, the tap flows 250ml/min (on reserve it is only 170ml/min). If I remove the long plastic filter inside the fuel tank (‘cos I’ve fitted Mr H’s glass filter, p/n90007) those rates increase to ~310ml/min and 210ml/min respectively.

At 60mph, the bike is thrashing along at ~4000rpm and I am travelling 1 mile in 1 minute so that’s 1.6kms/minute. So assuming I am doing, at very worst, (at WOT) 10mpg, what is the fuel flow rate per minute? That 10miles/gallon equates to 16kms/4.55litres or 0.28litres/kilometre or 0.45litres for the 1.6 kilometres needed in that overtaking minute. So my tap does not supply enough for WOT overtaking lasting a minute. At 15mpg and ~0.3litres/minute, I’d just make it. Does that sound about right?
Cheers, Chris D

[Alan R]

Hi CHRIS D ------ WOW !! that's v.impressive.... The nearest I can get to that is I generally estimate my partially-tuned Classic 500 to have an OVERALL consumption figure of 80mpg, which with the 3 gallon tank fitted gives me an estimated range of 200 miles riding before thinking about re-fuelling...or until the reserve is needed...BUT, given the general rise in the cost of everything these days then maybe a CASH-FLOW METER would be of more use ?? (LoL)

[Norm]

Chris, your clutching at straws there, 250mm per minute means 1 litre every 4 mins which means that you would empty your tank of 14 litres in 56 mins

[ChrisD]

Norm
Yes, 250ml/min dpoes indeed equate to a tank ful every hour or so. There is nothing wrong with the facts I listed but note that I did guess the part about 10-15mpg when at high revs and WOT. It cannot be the same 60-80mpg that is the average. So what do you think the fuel consumption is for that ~1 minute of WOT at 60+mph? And if my assumption is so wrong, then why is it that at WOT, when the bike starts to misfire at ~70mph (GPS true) the glass fuel filter appears empty at the lower of those quoted flow rates? I think the possibility of fuel starvation could be more true than we want to admit. Note that my Mikcarb float is set at 22mm height. Anyone who has looked at a fuel flow meter will know that WOT increases the average rate several times. In that same vehicle mentioned, at slow cruising (~50mph) the reported rate was 80-90mpg, so there is a wide rate variation with small throttle variations.
Chris

[Norm]

Chris,
Try it withot the fuel filter they can airlock and cause fuel flow problems

[Nico]

Chris,
as you have shown, there is obviously a difference between an average fuel consumption (in l/100km or mpg) and the instantaneous fuel consumption depending on the current rpm and load (i.e. airflow) of the engine.
It is actually fairly simple but involves a little bit of maths.
I assume roughly that a Bullet running at a power of 20PS draws about 70kg of air per hour (rough number, but good enough). Now, if you ran this at a fuel air mixture of lambda=1, it would mean that this air is mixed with 4.76kg/h of fuel. take the density of fuel and this equates to a fuel consumption of 106cc/min.
Usually, our engines run rich, more likely something like lambda=0.85. With the same assumptions, this would equate to ca. 124cc/min if you are asking for these 20 ponies to run.
With a tuned Bullet producing 40PS (at the same friction), this would be 246cc/min at lambda=0.85.

If this is not clear and you are interested, I can send you a little Excel sheet that might explain it better.

I.e. a std fuel tap can be too small for "big" power Bullets. I remember running a lot of 624cc Egli-Bullets on the dyno in the early 90s. When fitting a transparent hose parallel to the float chamber you could see the fuel being sucked out of the float bowl at high rpm/load until the engine started to stutter. We usually fitted high fuel flow taps to resolve this. Beware, the float needle valve can be an obstruction on the Mikcarbs, too.

[seen some changes]

I vaguely remember reading somewhere in an article about the Egli Enfield that to overcome the extra demand for fuel created by the improvements made to the engine he actually fitted an extra tap on the other side of the tank.This prompted me into getting into the habit of opening both taps on my constellation in case I felt like "playing" with car drivers in traffic. I was a bit younger in those days(in my early fifties ) I admit but after dismounting at around 80 mph one day I find the urge to be heavy on the twist grip easier to ignore. My friend who was following me at the time did say the sight of me doing a handstand on the bars just prior to, as he put it, the bike spitting me off and then cart wheeling down the road after me was rather impressive.

[ChrisD]

Norm, that's a distinct possibility. I'll take off the glass external filter and replace the tank one and test it this weekend.

Nico, that sounds fascinating and I would appreciate a copy of the calculation sheet. It certainly sounds like the fuel flow rate is important but maybe not as important is the external filter. [email protected]

SeenSomeChanges - ouch. I've done that off a full-on downhill MTB in a national race, and that hurt when I met the rocks.

Be funny if the fuel filter has been the problem all along, woudln't it?
Cheers, ChrisD

[simon]

The difference between the VW and the Bullet is obviously the work being done. It takes a lot more fuel to displace the air anf move the weight for a van than it does for a motorcycle. The worst average economy I have come across for a 350 is a CZ twin of a friend of mine. He gets about 40 mpg if he drives fast and 50 if he uses a feather welly. If your fuel flow rates were too slow it would certainly result in an under filled float chamber and extreme leanness which would either bring you to a stuttering halt or blue up your pipes and cause overheating. Fitting a vacuum gauge would probably be the best way to establish the economy of your driving style and also allow you to establish the ecomomy using different throttle settings over the same bit of road with a measured amount of fuel.

[Beezabryan]

All this pseudomathematicalscientificcalculatory stuff is way over my head except the overtaking bit, so WTF is WOT please?