GS750B fuel consumption

[GS750]

Im running my GS750B with the following (known) configurations:

4-1 transac? exhaust, open muffler
112.5 main jets,
4 x K&N pods,
15/40 sprokets
530 chain

Currently getting around 250Km for 17 litres

Does this sound average or what>?

I dont ride what I would call "hard" but I dont take my time either.

Most of my riding (85+ percent) is highway, 4200-4800Rpm, 5th gear

 

[earlfor]

Re: GS750B fuel consumption

My 79 750E with stock pipes and airbox/filter averaged about 12% better milage for the same rpm highway riding. My 750 was running on 102.5 mains and 15/42 sprockets. Considering you have an open muffler, bigger mains and pods, I'm actually surprised youre getting gas milage that good.

Earl

Im running my GS750B with the following (known) configurations:

4-1 transac? exhaust, open muffler
112.5 main jets,
4 x K&N pods,
15/40 sprokets
530 chain

Currently getting around 250Km for 17 litres

Does this sound average or what>?

I dont ride what I would call "hard" but I dont take my time either.

Most of my riding (85+ percent) is highway, 4200-4800Rpm, 5th gear

 

[ptm]

By my calculations you're getting 14.7 km/l which translates to about 35 mpg. That's about what I get on my 78 750 which is completely stock - airbox, jetting, exhaust and sprockets. I get in the low 30s if I'm really pushing it, and can get close to 40 mpg if I'm really taking it easy.

I agree with earl - that's good mileage considering the mods.

 

[KEITH KRAUSE]

With most of your riding being the hi-way and around 4,500 rpm's, you should get around 38-40 mpg if the bike is in good tune.
I'd be more concerned with how the plugs are burning. I suggest a solid 1/3 throttle test to see if the jet needles are set correctly. You spend the most time riding on the jet needles.
Also, my past experience says your main jets are too small. They're only increased by two full sizes (10). 102.5 is stock. For K&N filters and a quality pipe, two sizes up is just too lean. Something closer to 127.5's is my guess. Or more. Full throttle test will tell you how the mains are running.

 

[jonr]

With most of your riding being the hi-way and around 4,500 rpm's, you should get around 38-40 mpg if the bike is in good tune.

I'm with Keith on this one. The best I've got with my '78 750E is 38 or 39 and I usually get about 34-35 in town. I have a 4-1 exhaust with stock carbueration and a K&N factory replacement filter (not pods).

I've been told by several GSR members that I can run the stock mains as long as I keep the factory airbox. My plugs run just fine too. All those people also tell me that if I switch to pods, then I should jump up a few sizes on mains. However, you'll notice there is not a dynojet kit for 77-79 GS750's either.

J

 

[GS750]

With most of your riding being the hi-way and around 4,500 rpm's, you should get around 38-40 mpg if the bike is in good tune.
I'd be more concerned with how the plugs are burning. I suggest a solid 1/3 throttle test to see if the jet needles are set correctly. You spend the most time riding on the jet needles.
Also, my past experience says your main jets are too small. They're only increased by two full sizes (10). 102.5 is stock. For K&N filters and a quality pipe, two sizes up is just too lean. Something closer to 127.5's is my guess. Or more. Full throttle test will tell you how the mains are running.

Keith, the previous jets were 125's and with that config I averaged 150Km to the 17 litres.

I was of the belief that jet sizes were 2.5 each size?

When I get the chance I will do the plug readings and let you know the colour/s etc.

Thanks for the helpful indications.

 

[KEITH KRAUSE]

With most of your riding being the hi-way and around 4,500 rpm's, you should get around 38-40 mpg if the bike is in good tune.
I'd be more concerned with how the plugs are burning. I suggest a solid 1/3 throttle test to see if the jet needles are set correctly. You spend the most time riding on the jet needles.
Also, my past experience says your main jets are too small. They're only increased by two full sizes (10). 102.5 is stock. For K&N filters and a quality pipe, two sizes up is just too lean. Something closer to 127.5's is my guess. Or more. Full throttle test will tell you how the mains are running.

Keith, the previous jets were 125's and with that config I averaged 150Km to the 17 litres.

I was of the belief that jet sizes were 2.5 each size?

When I get the chance I will do the plug readings and let you know the colour/s etc.

Thanks for the helpful indications.

Mikuni jet sizes go up in STEPS (2.5). A full SIZE is (5).
I doubt your gas mileage is main jet related. The main regulates 3/4 to full throttle. Only if you spent a lot of time at these throttle positions can you say the mileage is main related. Typical cruising/steady riding will be the jet needle.

 

[KEITH KRAUSE]

With most of your riding being the hi-way and around 4,500 rpm's, you should get around 38-40 mpg if the bike is in good tune.

I'm with Keith on this one. The best I've got with my '78 750E is 38 or 39 and I usually get about 34-35 in town. I have a 4-1 exhaust with stock carbueration and a K&N factory replacement filter (not pods).

I've been told by several GSR members that I can run the stock mains as long as I keep the factory airbox. My plugs run just fine too. All those people also tell me that if I switch to pods, then I should jump up a few sizes on mains. However, you'll notice there is not a dynojet kit for 77-79 GS750's either.

J

If that pipe flows freer than the stocker and the K&N replacement WILL lean it out a tad, then your mixture is lean. How lean is the question.
Sure. You can run the stock mains, but performance will suffer even if the lean mixture "fools" you and you don't notice. This would be evident at 3/4 to open throttle.
But I'll bet your jet needles are still in the stock position too. So you'll be lean at just above idle to 3/4 throttle also. Really depends on that pipe.
Whenever you run pods, you have to richen all three jetting circuits, not just the main.

 

[Clone]

To the UK gallon, the true gallon, you get 41mpg with all your mods, I get around 45-47mpg with a stock config. So, the question is, does your bike run well?
If it does, then I guess it is ok.
If not, then start figuring out where to improve it in your opinion.
Otherwise be happy
AND DON'T F WITH IT, :lol:

 

[GS750]

If that pipe flows freer than the stocker and the K&N replacement WILL lean it out a tad, then your mixture is lean. How lean is the question.
Sure. You can run the stock mains, but performance will suffer even if the lean mixture "fools" you and you don't notice. This would be evident at 3/4 to open throttle.
But I'll bet your jet needles are still in the stock position too. So you'll be lean at just above idle to 3/4 throttle also. Really depends on that pipe.
Whenever you run pods, you have to richen all three jetting circuits, not just the main.

The jet needles are one notch HIGHER than the standard middle position, I found this is where the bike runs the best.

When I got it it had throwaway pods on it, had the main fuel jets done, and the 4-1 system. I dont know about the other jets, I havent bothered with them because as soon as I changed the mains down from 125 -112.5's the bike was completely different, rode nice, had as much power as it did with the 125's, without the eye burning exhaust gasses for anyone following me (even in a car)

My bike was obviously running extremely rich, the needles were in the middle clip, my pods were average (not filthy, not new), and my riding style used some throttle movement every now and then (highway)

Nothing has changed in my riding style, I go through a Metzler rear tyre every 7000-9000, and a front every 15K-20K. My fuel consumption is down, but my power feels the same. Ive ridden a couple of other GS's and my mates with GS's feel that mine definately goes better than theirs do. I can pull away from them quicker, and my mate with a GS850 shafty I infrequently ride with gets slightly better consumption, (17 litres, 280Km) his bike is totally stock.

I will do a plug read as soon as I have the chance, that will tell more.
What do I look for in the plug reads? colours? Fluffiness etc etc?


Im running on Aussie premium unleaded or Optimax 95-98ron.

 

[KEITH KRAUSE]

I'm not sure what you're saying about the jet needle position.
You say "the needles were in the middle clip", which caused it to run very rich. Now the needles are "one notch higher than the standard middle clip" as you describe it? This makes me think you've raised the jet needles to fix a richer mixture. Maybe I'm reading you wrong.
Just for correct terminology, you raise the jet needles and make them richer, by lowering the jet needle e-clip. If your needles were previously set at the factory 3rd position and it was very rich, you had other issues causing the rich problem such as poor float adjustment, etc. K&N pods and a decent pipe will generally require the stock jet needle to be raised as high as it will go (5th position), and even then the stock jet needle sometimes won't give good results, forcing you to use a jet kit or change the needles or needle jets.
I brought up the jetting questions because I just wondered what had been done. Your main and jet needle adjustments conflict with my past experiences. Two sizes up for K&N and a pipe isn't much, but now I'm just repeating myself. I have that habit when something unusual is happening.
Someone could have made changes to the carbs, I don't know.
As for your question about plug color, optimum color is a tan center electode insulator. A 1/3 throttle test will show what the jet needle is doing, full throttle for the main.

 

[GS750]

I'm not sure what you're saying about the jet needle position.
You say "the needles were in the middle clip", which caused it to run very rich. Now the needles are "one notch higher than the standard middle clip" as you describe it?

I brought up the jetting questions because I just wondered what had been done.

Sorry, it was meant to say that with the previous jetting (125 mains) my bike was obviously running extremely rich, the needles were in the middle clip, my pods were average (not filthy, not new), and my riding style used some throttle movement every now and then (highway)

I changed the needles DOWN 2 clip positions to try to alter fuel consumption and the burning feeling im my mates eyes when following me.

After changing to 112.5 mains, I noticed that the bike had sh*tloads more power, so I played with the needles in all 5 possible positions, I ended up back at tthe middle position, hence my feeling that the jeting is now spot on.

With teh current setings (now have REAL K&N pods) 112.5's, needles in the centre position, and running on premium or optimax 95-98ron, I get 250Km from 17 litres fuel, and the bike performs well with teh 15/42 530 chain and sprokets.

Hope this clarifys my answer.

Thanks for your help on the plug colours too Keith, but will they burn those colours with unleaded? my experience (and questions to other mechanics and fuel companys) shows that they only burn black or grey with unleaded.

 

[KEITH KRAUSE]

In my experience, there's a very fine line where the plugs will burn a tan color or a degree of tan. That's the optimum color. Certain shades of gray are acceptable but would need pic's to show the grays I mean.
You can achieve the tan colors with todays fuel. My bike and several I've jetted show light tan, sometimes a middle shade of gray is the "best" I can achieve. I've also been told by Vance and Hines and others that todays fuel won't allow these tan colors but that's not true. Black isn't acceptable, neither are lighter shades of gray, and of course white isn't acceptable.
If I pulled the plugs on my wifes Ford or my Blazer, you'd see a solid tan color right now. Same fuel, just lower octane for them.
Looked at the plugs in my friends 2004 Harley the other day, nice tan/coffee color.
Having your bike running well with your jetting set up is very unusual. But if you're happy with it that's what matters.