JOG asked about mpg. The worst my (stock) S40 ever got was about 39 mpg, running somewhere around 70-75 into the wind on I-80 in Nebraska. I was westbound, so was going slightly uphill (averages about 9 feet per mile) as well. That comes to roughly 2 gallons per hour, which also translates to about 22 horsepower.

Charon wrote on 09/11/12 at 06:05:59:

So how much horsepower were you making when you ran down hill.

Charon wrote on 09/11/12 at 06:05:59:


When I was doing the research on the stock and Raptor petcock, I was surprised to find that the flow rate on PRIME is less than in the ON or RES positions.  The flow rates in minutes and seconds to drain a gallon were:

Stock Petcock:  ON 2:34, RES 2:24, PR 3:25
Raptor Petcock: ON 2.15, RES 2:15

This was done with the tank nearly full, and I put a vacuum pump on the diaprhagm when checking the original petcock in the ON and RES positions - but not when checking the PRIME position.  At the time I was not sure why the PRIME was slower - but I now know it is because in the PRIME position the petcock moves a little plunger over and it opens the diaphragm mechanically - but it doesn't open it as far as the diaphragm does when it is operated with a vacuum pump.....I am not sure how far the diaphragm is open during actual engine operation.

In the PRIME postion - the flow rate is 1 gallon in 3 minutes 25 seconds......or about 17 gallons per hour.

I found the following for fuel consumption per HP on the internet:

BSFC

The brake specific fuel consumption of an engine depends on many factors including thermal efficiency, mechanical efficiency and air to fuel ratios. Most piston engines have a BSFC of between 0.5 and 0.55 lbs. of fuel per hp per hour at maximum power, set fairly rich with AFRs between 12 and 13 to 1.

IF our engine is 30 HP and I do this:
     30 x 0.5 = 15 gallons per hour.
     30 x 0.55 = 16.5 gallons per hour.

Now I have not done any tests to see what the flow rate through the carb is for the needle and seat....but it sounds like the prime position may not provide enough fuel flow for an engine making full HP when you add in some flow restriction for the needle and seat and the little filter on top of it?  (This is assuming the vacuum line is unhooked while in the prime position and the diaphragm is not opening more than the little plunger allows).

Dave, I think there might be an error in your calculations. You multiplied 30 bhp-hr times .5 lb/bhp/hr for a result of 15, but that should have been lbs instead of gallons. Gasoline weighs approximately 6 lbs/gallon. 15 lbs of gasoline is about 2.5 gallons.

I have a book about the Nebraska Tractor Test lab, including hundreds of reports of fuel consumption of tractors. The approximate median of those results (for gasoline tractors) is around 11 hp-hrs/gallon, so I used that in coming up with 22 hp at 2 gph.

As information, those tests were run with the tractors exerting maximum pull - in short, at wide-open throttle. Gasoline engines lose efficiency rapidly at less than full throttle, because they have to do a lot of work pulling mixture in past the restriction of the throttle butterfly.

I really like where this just went, but, this is Halvors tuning thread.
Charon, I think what ytou were talking about is interesting enough to expound a bit, & the Hp/Hours thing,, Ive never seen that, another cool topic,

Would anyone want to fire up a new thread so we can follow these rabbit trai8ls a while?

I did not know Prime had a lower flow rate. But, 50 MPG at 75 MPH or 40 MPG at 60 MPH, 1.5 gallons/hour = 6 quarts in 60 minutes= 1 quart every 10 minutes.THats a cup every 2.5 minutes. Thats a trickle, not flow, so, if the bike is going lean, surely its not because the petcock isnt delivering.

IF its the cap vent, then it would show up quickly with a full tank, less so with a near empty one. I would ride to where I could run at speed, shut it off, pop the cap, put it back on & GO, hit top speed. If it goes lean, its not a delivery problem, unless the needle/seat/float stuff just arent playing fair.
Shut off bike & fuel & coast to a stop, measure what comes outta the bowl. Tho, I would expect it to coff & sputter like its outta gas more than just go lean if theres just not enough in the bowl.


How is it that a carb is jetted & provides a nice Air/Fuel mix at 40 & just cant keep the mix rich enough for 70 MPH? Air flow increases thru the venturi as the slide climbs outta the way, the needle lifts out of the way,,




& Dave said

30 x 0.5 = 15 gallons per hour.


& I said,, Wow,, Ill just hafta circle the gas pump..Thats 1.5 gallons every 6 minutes,,

Get to doin math & get yer head all full of numbers & mess up on the Units,, easy to do,,

JOG....Well I do believe the petcock flow was still related to Halvors tuning thread, as I am trying to figure out why it is going lean at speed.  Obviously there is a fuel restriction issue somewhere.....or a source of air that does not show up at lower throttle settings.  Can air bleed in through a faulty slide diaphragm or through the TEV (Throttle Enrichement Valve). 

Or perhaps the O2 sensor is just not accurate......and it is doing something weird when the bike is actually starting to work and make some heat in the exhaust. Does the bike start to act lean.....or is it just the O2 sensor and meter showing that it is going lean?

Something else to consider. You have direct control over the throttle butterfly. You have no control over the slide throttle itself, which is controlled by airflow through the venturi. When you open the butterfly airflow through the venturi increases, but the slide probably will not open completely until engine speed increases. Perhaps it isn't opening completely until you approach top speed and maximum venturi airflow, and that might be the reason for the "delay" in going lean. Just a thought - I don't know whether it is correct.

I think it would be interesting to put a T in the vac line to the petcock  a gauge in it with a hose long enough to get the gauge up on the bars so vacuum at high speeds can be seen, to know what vacuum the petcock is getting at 70.