Topic: fuel economy

[bitsa750]

I wonder what accounts for the marked improvement in fuel economy for the later K series 750's over the earlier ones.
I have a K2 with an 810 kit running 105 main jets, it gets about 210km from 13 litres before it hits reserve.Other K2's in my club get about the same.
A friend with a K6 [with accelerator pump carbs] goes over 300km before reserve. I believe the k6's also had higher compression pistons, is this so? I wonder which of these changes accounts for the improved mileage, or if it's a synergy of them that makes the difference?
Pete

[Schnell]

Is the tank capacity the same?

[TwoTired]

I don't know what was available in Oz.
But in the US, accel pump carbs (PD type) were introduced in 1977 (K7/F2).  The change was instigated by the EPA's desire to reduce hydrocarbon emissions and drive engines to be more frugal with the fuel they consumed.  The laws went into effect in 1978, which was the last year of the production CB750 SOHC4.  Following models from Honda saw the switch over to CV type carbs, and then fuel injection as the emission standards became more restrictive.

For the most part, it worked.

But, for a more precise determination, fuel consumption is tallied by distance per quantity of fuel used.  This rules out factors of tank size, and/or the height of the stand pipe in the tank where reserve must be selected.

[CB750F2]

Pete. There are many variables that have an impact on fuel economy - driving style, gearing, state of tune, motor condition etc. I am pretty sure that the accelerator pump type carbs were not fitted to the 750s until 1977 so it seems that your friend with the K6 has fitted a set of carbs from a later model.
I suggest that you carry out the 3000.00 km tune up and then go from there. I have not checked my fuel consumption recently but when I did it was approx. 5L/100km - highway use. The bike at that stage was a 1978 F2 - standard bore. I have since bored it out to 836 and I do not know the fuel consumption. Pat

[Deltarider]

This 'EPA' thing, is that a federal thing? Could it be that prior to 1978 a state like California already had legislation in place to reduce pollution by cars and motorcycles? Does anyone remember?

[Retro Rocket]

Pete. There are many variables that have an impact on fuel economy - driving style, gearing, state of tune, motor condition etc. I am pretty sure that the accelerator pump type carbs were not fitted to the 750s until 1977 so it seems that your friend with the K6 has fitted a set of carbs from a later model.
I suggest that you carry out the 3000.00 km tune up and then go from there. I have not checked my fuel consumption recently but when I did it was approx. 5L/100km - highway use. The bike at that stage was a 1978 F2 - standard bore. I have since bored it out to 836 and I do not know the fuel consumption. Pat

You are correct Pat, the K6 didn't have accelerator pumps on the carbs....

Welcome Bitsa.. 

[Bailgang]

This 'EPA' thing, is that a federal thing? Could it be that prior to 1978 a state like California already had legislation in place to reduce pollution by cars and motorcycles? Does anyone remember?

The Environmental Protection Agency is a federal agency and was founded in late 1970 so they no doubt played a factor in California pollution legislation prior to 78.

[TwoTired]

I lived in California through the CARB and EPA mess during the 70s, 80s, 90s, etc.  (We are unable to dissolve agencies once they have been established.  We can only add more of them and increase their size and purview of these bureaucracies.)
Anyway, California kinda started the whole thing with instantiating "smog laws" on cars and trucks.  Attacking the biggest contributors to smog first.  Fed politician's simply adopted a new "cause" to rally popular support and ride the voter swell of "righteousness" into office, and resulting in the creation of the EPA.

The California version of history is here.  Some of it is interesting.  Start at the bottom of the timeline.   http://www.arb.ca.gov/html/brochure/history.htm

Motorcycles were ignored in the earlier years, but the EPA included them in their 1977 law, which became enforceable for the 1978 models.  Honda tried to meet the upcoming standards early and show cooperation with fed agencies.  So, many mechanical changes were introduced on earlier model, like crankcase gas scavenging and later, lean burn idle operation for 77.

California did drive earlier "smog control" efforts.  Then the big dog Feds came in to show who was the "boss" of the nation, allowing California a "waiver" to enact more restrictive laws.

[RJ CB450]

Sneaking into this thread, see some nice mileage numbers.  Makes me wonder if my 82 650 fuel economy seems meh.  I seem to be about 7.7L/100km/30.5mpg  mix seems good.  Guess need to recheck and tuneup. 140km safe range city riding is kinda  I guess?

[Lostboy Steve]

My MPG's (which is aggravatingly THE buzz word of the last decade) are good enough for me to not care, or worry.
On another note, if you want OPTIMAL Miles per Gallon, Google "Vapor Carburetor".

[KRONUS0100]

1976 cb750f........................fitted with k8 tank...........5 gallons
just ran her bone dry to the tune of 42mpg.   didnt get 15 miles out of the reserve tho..............thought reserve was at least 20 to 25 miles.

[lrutt]

seem all my bikes get about the same, solidly mid to upper 40's. Except the 305 dream which gets almost 60. but all the others, 750's, 650 triumphs, etc. etc. etc. all are solidly in the 40's. Just seems a sweet spot for most bikes.