Fuel consumption?

[Junkie]

I've been getting pretty horrible gas mileage on my '85 GS700 (very similar to the GSX750 everywhere but the US) recently. The past couple tanks have been right around 30mpg even. I have been filling it with Premium because I was planning on putting a V&H Ignition advancer on soon, so hopefully I'll see better mileage when I put that on.

I haven't pulled the plugs or anything, but it feels fine as far as mixture goes... I know, not the best way of telling but I'm not noticing any stumbling or anything.


Is it most likely a carb issue, or should I just expect it to be that bad? This is mostly freeway riding, probably averaging low 80s mph and includes a big hill - but the big hill isn't steep enough that I need to apply brakes, and am still slightly on the gas going down. It's 3 miles and 7%. It pulls hard up to plenty high speeds even up the grade. Tire pressures are at manual specs or maybe a pound or two higher.

Valves have been adjusted recently. It's got a 4->1 V&H header, and the can off an R6. No bodywork on it.


Ideas?

thanks

[BassCliff]

Lousy gas?

Mr. Junkie,

I don't know. I've noticed my gas mileage is a little off too. It's possible that it's just the lousy gas we get this time of year. There are different formulas for winter and summer. I'm going to dump a can of Sea Foam in my tank this weekend and ride for a couple hours straight, just for general maintenance. We'll see if that helps.

Thank you for your indulgence,

BassCliff

[earlfor]

30 mph sounds pretty low to me. My 85 1150 averaged 45 mpg. 03 Bandit 12 averages about the same. 01 CBR929 around 42. 79 Gs 750E around 48 mpg.
83 1100E about 40.

Earl

[trippivot]

any smell of fuel when the bike sits all night? on cold start up is there a white fuel cloud when it first fires up? leaking float needle into the engine symptom. they don't always leave a puddle.
does the engine oil smell like fuel?

air filter type? is it clean? ignition timing at proper setting?

I had vm26 that got the worst mileage 26 mpg and switched to rs 34 cv carbs and my mileage jumped to 45mpg. all problems went away with the old carbs..

yep after a 75 mile tank full of fuel and we ran out on an old country road I changed those carbs.

[Mark whiz]

I know that my 700 starts dropping mpg as soon as I cross 4000rpm - but not down to 30mpg. I usually get 45 to 50 while crusing to work at 55 and 60mph. When the weather is cooler it'll drop off about 5% due to the longer warm up time and thicker cold oil in the crankcase.

In your case I would definitely take the time to pull the carbs and give a thorough cleaning and change out at least the float needles, if not the entire needle/seat assembly in each carb. I had these go bad on mine about 8 years ago and MAN they flooded the crankcase with gas.

[Junkie]

No smell of fuel and no apparent smell of fuel in the oil (the petcock doesn't flow at all with no vacuum when it's on ON). There is a HUGE cloud on cold starts on cold days, but it doesn't really smell like gas. Basically, it only really blows a big cloud of anything if there's condensation on everything. A tank is 3 round trips if I'm VERY lucky, so that's 6 cold starts... I don't think even 6 sets of the carbs emptying themselves into the crankcase all the way would use enough gas to drop the mileage that much.

I got similar mileage on a long trip, but at lower speeds it wasn't too bad. 200 miles up (with 2 stops for gas in the middle), got maybe 32. 50 miles around town and 150 miles of fun riding, and then another 200 miles down - around town and fun riding I got considerably better mileage again.


Air filter is a drop-in UNI, and clean. Ignition isn't adjustable in any way.



Now, due to the fact that it gets decent mileage around town (not sure exactly how good - no odo, but I was very surprised at how little fuel it used on my fun ride, probably 40+mpg), it is possible that it's fine at lower speeds and that I'm simply riding fast enough that I'm getting horrible mileage... not sure that's what's happening, but it's certainly possible.

[Steve]

Is it most likely a carb issue, or should I just expect it to be that bad?

Personally, I think it's a steering issue. You need to adjust the nut that holds the handlebars. 8-[

First of all...why are you using 'premium'? :shock:
'Premium' gas, by itself, does NOT give you more power or mileage. In fact, it will probably give you less power and mileage.

'Premium' gas is designed for higher-compression engines which have so much compression that the heat from compression might tend to light the gas mixture before the spark. Therefore, 'premium' gas is harder to light and actually burns slower than 'regular' gas. This is why you usually have to advance the spark on higher-compression engines. It is not the advanced spark or the 'premium' gas that gives you the power, it is the higher compression.

In our 'normal' engines that are designed for 'regular' gas, if you run 'premium', it will finish the combustion process later than optimal, when the piston is alreay well on its way on the down stroke. This reduces peak combustion pressure and, therefore, power. Because you have less power available on each power stroke, you will have to add a little throttle to compensate, and your mileage will suffer, too.

Also, because 'premium' gas is not burning properly (in our 'normal' engines), it tends to leave deposits which will further decrease performance. Eventually, the deposits will be bad enough that they will retain heat and cause pre-ignition, in spite of the 'premium' gas.

Now you really have to ask yourself...based on the 20 cent or more difference between 'regular' and 'premium' gas, and a 4 to 5 gallon fill-up, is it really worth that dollar a fill-up for all that trouble?

Second, to increase the gas mileage, drop your speed a bit.
Last Labor Day, we went on a group ride to West Virginia with our GoldWing chapter. Our leader took us half-way there on the freeway, to get to a nice road that would take us to our destination. He was in a hurry. Not really mindful that half of us were towing trailers, and probably not noticing the approx. 10 mph headwind, he was cruising at about 75 mph. For the first 200 miles of the trip, my Wing got better mileage than my wife's 850. This was in spite of the fact that I had my dimensionally-enhanced son on the back and was towing a trailer. I got about 31 mpg, she got about 29.5. When we slowed down on the two-lane, her mileage came right up. For the rest of the trip (another 1000 miles) she averaged 47+ mpg, with a high of 59. My mileage ranged from 33 to 44. Just for the record, about half of that 1000 miles was riding up and down the West Virginia mountains, rowing through the gearbox to keep up with group, so we were not babying it to maximize mileage.

Mark whiz just mentioned reduced mileage when crossing 4000 rpm. I don't think engine speed has that much to do with it, because when we were in the mountains, engine speed was frequently above 4000 and mileage was still over 50 mpg. It is more the bike speed. Motorcycles are about the most unaerodynamic vehicles out there. Pushing through the wind is bad enough, but over about 65-70 mph really takes a toll. If you are truly concerned about fuel consumption, try riding for a tank or two holding your speed to the 65-70 range to see what it does. It just might surprise you.


.

[Junkie]

An ignition advancer most likely requires premium. I'm probably going to install it tomorrow afternoon, when I have some time to do so. Before I got it, I'd been running 87 with similar mileage. I know that 91 doesn't offer any more power if it's running fine on 87. And advancing spark can certainly give you more power - try retarding yours and you'll certainly get less.


The tank holds 3.6 gallons, and <$1/fillup isn't really a huge deal. I mean, cheap gas is nice but $3.60 vs $3.80 isn't a huge deal.


Thanks for the info about reduced mileage at high speeds, from the fact that I was getting better mileage having fun on it I'm guessing that's a large part of the issue for me too.

[Griffin]

Mileage on my GS700 has traditionally been about 42-45 mpg per tank, with a mixture of riding conditions. Interstate droning at 70 mph will get me about 48-49 mpg. Hard charging riding with rapid acceleration and constantly changing speeds drops mileage down to about 37-40 mpg.

I don't know if 30 mpg is typical while averaging over 80 mph on a tank. I've never done it. I do know that if you persist in riding that way, you won't have a license for long, if you don't wreck it and destroy it first.

In either case, your fuel consumption will then definitely go down.

[Dogma]

I don't know if 30 mpg is typical while averaging over 80 mph on a tank. I've never done it. I do know that if you persist in riding that way, you won't have a license for long, if you don't wreck it and destroy it first.

Maybe he commutes on the Atlanta beltway. No, that can't be right. 80 will get you run over.

[Junkie]

Mileage on my GS700 has traditionally been about 42-45 mpg per tank, with a mixture of riding conditions. Interstate droning at 70 mph will get me about 48-49 mpg. Hard charging riding with rapid acceleration and constantly changing speeds drops mileage down to about 37-40 mpg.

I don't know if 30 mpg is typical while averaging over 80 mph on a tank. I've never done it. I do know that if you persist in riding that way, you won't have a license for long, if you don't wreck it and destroy it first.

In either case, your fuel consumption will then definitely go down.

I move at probably the 85th percentile, at least that's what I'm shooting for. I'm paying a lot more attention than most of the people on the road, and really... 85 on the freeway is nothing. It was for a damn good reason, but one time in the cage I had the loud pedal down the entire way on the freeway... it was LATE, and I saw all of 1 other car the whole way, but I had no issues whatsoever with staying on the road and in my lane. That was doing about 50% faster than we're talking here. That should give you an idea of the difficulty of going fast on this freeway. If I had been caught doing that, it would've been a 2 point ticket, ~$800 fine, 7 day license suspension most likely.


If I do get pulled over at 85 and get a ticket for it, it's the exact same ticket as I would get if a cop decided to wire me for 66 in a 65. The fine would be a bit higher ($230ish the last time it happened, September 06), but it's a 1 point violation and you're eligible for traffic school if you haven't taken it in the past 18 months, which will take it off your record. It takes 4 points in a year/6 in 2/8 in 3 for your license to be suspended, I would stop riding fast long before I accumulated that many.


I've gotten insurance quotes for if I get a ticket, Progressive doesn't care what my record is like as long as no reckless, no DUI, no street racing. Even if I had a 100+ and a normal speeding ticket in the past month, it would be the same - currently $201/year. No clue why, but it's what they're saying.


If I do wreck it, it won't be due to excessive speed on the freeway. It's not hard to sit there with the throttle open a reasonable amount the whole time. There are plenty of freeways in CA where the average speed of traffic when there isn't much traffic is in excess of 80, some are more like 85-90. Different states are different. There are freeways where you're going to get run over if you're only doing 70, even if it's in the right lane. I'd rather get the occasional ticket than get hit on the freeway.

[t3rmin]

Did you already say how the carbs are jetted after the pipe and air filter?

[Steve]

And advancing spark can certainly give you more power - try retarding yours and you'll certainly get less.

Proper spark timing is all about getting the mixture to burn completely by about 10 degrees past TDC. If your spark is not quite where it should be, then yes, advancing it might help. However, if the spark is where it should be, advancing it more will hurt power and cause other problems. And, of course, retarding it from where it should be will definitely hurt performance.

If all you do is add 'premium' gas, you will need to advance the timing several degrees just to compensate for the slower burn time. Adding more timing will hurt performance because combustion will be peaking as the piston is still on the way up. Still no extra power from 'premium' gas and advanced timing.

Of course, all this assumes that the timing is set properly to start with. If your timing happens to be advanced on 'regular' gas to the point that combustion is completing too soon, switching to 'premium' gas might slow cumbustion enough to restore what was lost by the (too) advanced timing. It would be cheaper and better to just set the timing properly instead of applying the band-aid of 'premium' gas.


.

[Junkie]

Strange... so you're saying that the spark advancers that can be bought do nothing?

I'll install mine when I have the chance, and report on any percieved changes.

[bwringer]

On long interstate drones, I normally see about 33mpg with my GS850. I tend to clip along at 80-85mph. Nothing to see here in Indiana, folks, move along...

At lower speeds, even if I'm getting after it hard, I normally get 40-45mpg.


The point is that motorcycles are aerodynamically very inefficient, so speed has a much larger effect on drag than in a car. My Corolla gets about 33mpg on longer trips whether I'm going 60mph or 85mph.