Ignition advancer and its effect on fuel economy?

[mattd1205]

Hey guys


With gas prices knocking on the door of $5.50 in my neck of the woods, I’ve been looking to squeeze every bit of fuel efficiency out of my 78 GS750 that I can. While checking the function of the advancer, I noticed that it hit full advance at 2,080 rpm instead of 2500rpm (as per service manual). I bought some similarly sized, slightly stiffer springs at a local hardware store and was able to postpone full advance to 2,240rpm. The old springs started almost immediately off idle (1040 RPM idle) and the new springs don’t start advancing until about 1600 rpm. It seems like the idle might be more stable and the return to idle seems better with the new springs (placebo effect perhaps?). Most of my riding is in harsh city traffic so I’m wondering if getting slightly stiffer springs to delay full advance to 2500 rpm would be worth it as far as fuel economy goes.. any ideas?

Matt

[Brendan W]

Now there is something you don't often see people checking
At a guess the bike was optimised for tractability and power with economy a poor third.
Ignition timing is like getting the push right on the downstroke of a pedal cycle. Too early or late wastes effort. As the time per rev shortens you have to light the fire earlier to make sure you reach peak cylinder pressure at just the right time.
In theory getting closer to spec is closer to the intended map, whatever that was. At a guess what you have done will improve things.
Only way to find out is measure. Even if you did convince yourself you measured something a much bigger effect is had by gentle throttle and staying away from the brakes. A quantum leap in efficiency would be a bike of a quarter of that capacity. Air cooled, open fuel loop 750s, city traffic and economy don't belong on the same page

[mattd1205]

Now there is something you don't often see people checking
At a guess the bike was optimised for tractability and power with economy a poor third.
Ignition timing is like getting the push right on the downstroke of a pedal cycle. Too early or late wastes effort. As the time per rev shortens you have to light the fire earlier to make sure you reach peak cylinder pressure at just the right time.
In theory getting closer to spec is closer to the intended map, whatever that was. At a guess what you have done will improve things.
Only way to find out is measure. Even if you did convince yourself you measured something a much bigger effect is had by gentle throttle and staying away from the brakes. A quantum leap in efficiency would be a bike of a quarter of that capacity. Air cooled, open fuel loop 750s, city traffic and economy don't belong on the same page

I realize it’s kind of absurdist considering what the bike was designed for. Ha. I’m sure if economy was a concern Suzuki would’ve put CV carbs on them, electronic ignition with electronic advance, improved the combustion chamber design etc. etc. i guess I’ll have to see what the new springs net me as far as economy goes. I see that some of the Honda CB guys use the same Hilman 119 springs (540358) without complaint so that gives me some hope.

[Rich82GS750TZ]

I’m sure if economy was a concern Suzuki would’ve put CV carbs on them, electronic ignition with electronic advance, improved the combustion chamber design etc.

They did, starting with the 1980 16valve TSCC engines, with CV carbs and electronic ignition.

[rphillips]

Yes they did, But I don't remember ever a mention of them getting any better fuel mileage.

[mattd1205]

Well I decided to play around with the ignition today. I removed the martek ignition that I had on there and put the points ignition and original advancer springs back in. Lo and behold, the advance starts at 1,640 rpm and is fully advanced at 2,440 rpm. Much closer to spec. I definitely wasn’t expecting that going back to points would change things that dramatically. I’m guessing the weight difference between the points cam and martek rotor are a factor in addition to the drag created by the springs in the points being absent with the martek. I’ve got a Dyna S coming in the mail so I guess we’ll see if it does the same thing as the Martek. After that, I guess I’ll just enjoy the bike with points as it was designed.