Gas mileage loss after major service?

[Kipp]

OK, I need a bit of help here, as I am perplexed to find that my gas mileage has decreased about 10% since a major service I just had done. I was getting 42-43 mpg religiously and now it dropped to 38-39 mpg. Spark plugs will original equipment and this was the first valve adjustment. My buddy did all the work and I totally trust his abilities as he is an engine builder for AMA Superbike teams and a great friend. I just dont see how this can happen? Any insight for me is appreciated. Here is what I had done for this service at 26,500 miles, why did my gas mileage decrease?

Valve Adjustment (six were out and tight, 4 exhaust and 2 intake)

Replace spark plugs with NGK Iridium plugs

new OE air filter

Coolant flush

oil / filter change (Purolator filter, Rotella full synthetic oil)

Shaft drive service lube / oil change

clutch and brake fluid flush

new tires (Bridgestone BT21's)

Not related to MPG probably, but boy does the bike handle nice now

Full rebuild of the front forks with new springs, and Race Tech gold valves using ultraslick fork oil

New Race Tech rear shock with weight matched spring, fully adjustable

 

[saread]

I dropped in Iridium plugs on the last go around got a drop in MPG as well. Idle was set a bit fast and I've since adjusted and I'll see if that's related to the issue. If I can't find it by the next service, I'll probably pull the iridiums and try a set of standard plugs for comparison.

 

[Kipp]

I dropped in Iridium plugs on the last go around got a drop in MPG as well. Idle was set a bit fast and I've since adjusted and I'll see if that's related to the issue. If I can't find it by the next service, I'll probably pull the iridiums and try a set of standard plugs for comparison.

Interesting, I did not have a high idle issue after install. I was wondering if the 02 sensors might have something to do with it as well. If these iridium plugs burn that much more efficiently than standard plugs, then the 02 sensors will tell the ECU to pump in more fuel which in turn would decrease gas mileage?

 

[Fred W]

Any chance this was a total coincidence? Like maybe they switched over fuel formulation for the up coming winter months?

One other thought... If the bike feels like it's running better in the corners, could you be twisting the right thing 10% more?

 

[Kipp]

Any chance this was a total coincidence? Like maybe they switched over fuel formulation for the up coming winter months?
One other thought... If the bike feels like it's running better in the corners, could you be twisting the right thing 10% more?

That is why I love this forum, people are on top of things!

To answer your VERY valid questions, the coincidence factor did cross my mind as far as the switch for "non summer" gas as California loves to have a reason to charge more. With that said, I thought that your gas mileage increased with "non summer" gas as their is less oxygen, water and all the other stuff that decreases the potency? You cant even light California gas on fire with a torch for petes sake!

And as far as being throttle happy, I am sure that is not the case. I ride a lot and am quite aware when I am riding out of my normal commuting tendencies. Plus I have mad a point the past 4 tanks to be consistent once I figured out my gas mileage was down on the first tank since the service.

 

[FJRBluesman]

Now that your bike is all hot rodded out you maybe on the throttle a bit more.

I usally try to gauge my mileage by my work commute. It's very consistiant and I can control the speed. On misc trips I'm all over the map, due to climing, speed, etc.

Good luck.

 

[petey]

I changed to iridium plugs on several bikes in the past, without the milage drop.

 

[Kipp]

Now that your bike is all hot rodded out you maybe on the throttle a bit more.
I usallu try to gauge my mileage by my work commute. It's very consitiant and I can control the speed. On misc trips I'm all over the map, due to climing, speed, etc.

Good luck.

Pretty much exactly what I said above: "And as far as being throttle happy, I am sure that is not the case. I ride a lot and am quite aware when I am riding out of my normal commuting tendencies. Plus I have mad a point the past 4 tanks to be consistent once I figured out my gas mileage was down on the first tank since the service."

 

[Kipp]

I changed to iridium plugs on several bikes in the past, without the milage drop.

score is one to one then.........the plot thickens?

 

[Knifemaker]

How are you figuring your MPG? If you changed (front) tires your speedo and odo may not read the same due to a slight increase or decrease in circumfrance.

KM

 

[GPRIDER]

I changed to iridium plugs on several bikes in the past, without the milage drop.

score is one to one then.........the plot thickens?

I had an 04 and put Iridiums in early on. I didn't notice any milage drops. I still got around 38-39 mpg in town and 42-45 on the highway. I was planning on putting them in my new 08 when it's due for them. Hopefully someone else on the forum with a little more technical knowledge can help.

GP

 

[Kipp]

How are you figuring your MPG? If you changed (front) tires your speedo and odo may not read the same due to a slight increase or decrease in circumfrance.

KM

I calculate MPG the old fashioned way. Total miles / gallons = MPG. I definitely don't think that the new tire would make a noticeable difference. The old tire was a Bridgestone 120/70 and the new tire is a Bridgestone 120/70. Now that I think of it, I dont think our bikes calculate off the front wheel?

 

[Knifemaker]

I calculate MPG the old fashioned way. Total miles / gallons = MPG. I definitely don't think that the new tire would make a noticeable difference. The old tire was a Bridgestone 120/70 and the new tire is a Bridgestone 120/70. Now that I think of it, I dont think our bikes calculate off the front wheel?

There is a sensor on the left side of front wheel that I believe relays tire rpm to somewhere. I know the ABS system needs sensors like this to work, but I always thought all motorcycles get speedometer reading from the front wheel.

Regardless...I constantly average 38-39 mpg so I'd say your bike is running properly now. These bikes are tuned for performance ..not gas milage, so a "tune up" can logicaly give you lower mpg figures.

Changing tires can effect speedometer readings. But if you did simply stay with the same tire , then maybe not here.

I'll join the "heavier wrist" club on this and say you are just using more gas on a better running bike and not aware of the change in your technique.

KM

 

[Afast03nNJ]

I'll join the "heavier wrist" club on this and say you are just using more gas on a better running bike and not aware of the change in your technique.

Got my vote on that one.

 

[El Toro]

No one has mentioned this yet (at least I didn't notice it if they did), but did your tire pressures change with the installation of the new tires? Race tires are meant to run at lower pressures than road tires. Is it possible that you're not close to the 39 front 42 rear that is often recommended? That would affect gas mileage.

I don't see any reason why iridium spark plugs would have any effect. The spark plug just initiates the combustion process. It doesn't control it.

I was surprised to see that nearly 40% of your valves were out of spec. Especially since you say they were tight. Fixing tight valves would tend to improve compression I would think, and this would tend to be positive as far as efficiency is concerned.

The seasonal change in fuel, and the right wrist syndrome are also possible issues.... but the one thing that I would want to check would be to see if this friend who is involved in AMA racing might have your tire pressure on the low side for your street bike.

 

[saread]

I dropped in Iridium plugs on the last go around got a drop in MPG as well. Idle was set a bit fast and I've since adjusted and I'll see if that's related to the issue. If I can't find it by the next service, I'll probably pull the iridiums and try a set of standard plugs for comparison.

Interesting, I did not have a high idle issue after install. I was wondering if the 02 sensors might have something to do with it as well. If these iridium plugs burn that much more efficiently than standard plugs, then the 02 sensors will tell the ECU to pump in more fuel which in turn would decrease gas mileage?

High idle was due to the TBS I did at the same time. I forgot to readjust the idle and it was running about 1250 RPMs. Too high for sure, but I wouldn't have thought it would have knocked 4-5 mpg off. But, you never know. Soon as I run some more gas through it, I can compare. The advantage of the iridiums isn't performance, but longevity. I find it difficult to lay the issue at the feet of spark plugs.

 

[Kipp]

I'll join the "heavier wrist" club on this and say you are just using more gas on a better running bike and not aware of the change in your technique.

Got my vote on that one.

Let me assure everyone here that "the heavy wrist" factor is not the culprit here. Although I am not old by any standards (38), I have logged well into 6 figure mileage (200,000+ miles) on a street bike in my life and lapped many a racetrack as well. I am completely aware of how I am riding at all times. Being consistent and alert are key to success on our dangerous roads today. Especially on my daily commute, I am aware of lane position every second, as well as speed and general relativity of how close I am to near vehicles. I am POSITIVE that the heavy wrist is not the culprit here.

Also, tire pressure is not the culprit here. I have run the same pressures front and rear (34 front, 36 rear) in both FJR's I have owned and check often.

I agree that iridium plugs don' make sense as the culprit, I am just really surprised that will all the service that was done, the gas mileage has such a sharp decrease. With that. I decided to ask the crew at FJRforum and I thank you for your opinions and comments.

 

[saread]

I did a search awhile back and someone had mentioned this effect with iridium plugs but was able to trace it back to not getting good contact with the wires after the install. This is an area I'll look into as well. I can't determine a difference in performance either way - pulls like a clydesdale when you twist the wrist. I also am mystified at Yamaha's spark plug change interval. While I don't claim guru status reading plugs, the ones I pulled out looked pretty good to me. Good color, gap was good, sharp edges. The drop in mileage with the iridiums was immediate and significant; that would indicate a decrease in performance. I need to double check my work and compare with the old plugs. Gives me something to do and keeps me out of bars.......

 

[El Toro]

Since your tire pressures are the same now as before, it may not account for the loss.... however, by all accounts, you would probably get some improvements if you were to bump them both up by about 5 psi. That's a lot in the grand scheme of tire pressures.

The other thing is that I've re-read the post, and I don't see anywhere in here where a throttle body synch was done.

I'm not usually a big fan of random throttle body synchs without specific reasons for doing them.... but when you make valve shim changes, you do give yourself a specific reason for checking and possibly rebalancing the throttle bodies.

A throttle body synch is supposed to match up the air flows into the four cylinders so that at a common throttle position all four cylinders are getting close to the same amount of air.

When you change your valve clearances, you make very small changes in your cam timing, but at the high speeds, these subtle changes could impact volumetric efficiency, and I wonder if you would find your gas mileage back again if this were carefully done.

The fact that you do not mention any engine vibration or roughness sort of discounts this theory though.

The other thing that can impact fuel mileage on a service is if the throttle position setting has been changed so that the engine control module is going into a different part of the fuel map as the physical throttle is moved through its space. On the Moto Guzzi Norge, they were notorious for coming from the factory so that closed put you into the table at 4.1% when closed was supposed to be about half a point higher. At WOT, the half percent at closed throttle was several percent and it had an impact on drivability, perceived smoothness, power, etc. If the TPS were changed, then for a given real throttle position the ecm/ecu is looking at a different part of the table than it was before, it could effect fuel economy, even as it made the bike seem to run "better."

These ideas may seem like a stretch, but you've excluded a lot of the other more obvious issues.