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Drop in MPG after valve adj and TBS???


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Last month a buddy of mine called me up and said he just got done adjusting the valves on his '04 RT and was wondering if I could synch his TB's for him. I say, no problem come on over. Anyway, he got here and his bike is idling at like 800rpms. BBS are only about 3/4 turns out on both sides. I furged I just start from scratch and set them both at 1 1/2 turns out. Fired upo bike and it idled right at +- 1050rpm. Cool! A little fiddling got it centered on the twinmax at idle and a small twist on the one cable centered the needle at 1500 and 4000rpm. I test drove the bike and it was awesome. Well, I just saw him tonight. After running 3 tanks of gas through it, he is at a consistant 5mpg less than he was before. He aslo said that he was looking around on the bike and found that one of the lines was off his charcoal canister. He said he adjusted the valves toward the tighter end of the spectrum meaning there was a bit more of drag on the feeler guages when he was done. What would cause the drop in MPG? Having the BBS out farther than before, hose of canister, valves adjusted a bit tighter? I told him I would post this on here and would get an answer for him. Thanks, Terry

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What is the current MPG?


My GS (01) routinely gets 42 in town (commuting) and close to 50MPG highway.


Might try new plugs....I use Autolite 3922s summer, 3923s winter.


Snugging valves does increase cam duration slightly. I prefer loose valves for slightly better mid-range torque. I rarely rev more than 6000RPM...just no point...my GS pulls very hard from 3500 to 5800 RPM with loose valve lash settings.


Was there any other adjustment made? Anything?


What about weather? Did climactic conditions shange? Denser air requires more fuel.


Check tire pressure...it has a major influence on mileage.

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Eckhard Grohe

Maybe he is just riding it faster now that it is properly tuned!


Tighter valve lash means more valve overlap which means more mixture going into the engine at higher rpm. Ergo lower gas mileage.


Perhaps they are too tight which means mixture and combustion product leakage. Lower mileage.

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Dr. Grohe is correct. But, the boxer valves can be adjusted about 0.0015" (both int and exh) tighter without concern...but, you will pay the MPG penalty. And performance likely drop off a bit in the mid-range. Go stock and you'll be fine.


Running them tight means they run a bit hotter because they are not on the seat as long.

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I'm thinking a couple of things. First clue, idle speed way down after valve adjust. I'm thinking he screwed up the valve adjust and has them way too tight. Tight to the extent that he is no longer getting good valve seating. Yes the engine will run under those conditions but power will be down all across the band and the resulting larger throttle openings to maintain speed will cause the drop in fuel milage. If this is the case, there is a real need to fix it, post haste as if the exhausts are left too long they will burn.


I doubt valve timing will have much bearing on this as that mostly just moves the peak torque around a bit. A couple of thousandths is not all that critical. By all means, reset the valves and use a go/no-go method to double check. You should be able to insert the 0.006" gauge easily but not a 0.008 or even a 0.007 if your anal.


Do go and pick up a couple of sealing plugs for the vacuum ports at the throttle bodies. When you have finished the valve adjustment and TB synch, use the sealing plugs rather than the lines to the charcol cannister and run the bike like that for a bit. In deference to DEF, put them back afterward. Leaks in those lines, or a missing line, will introduce lots of air into the system leading to very lean mixture and consequent loss of torque. The O2 sensor will attempt to richen the mixture to compensate for the extra air and that could cause the drastic drop in fuel economy as well.


If you have such available, a vacuum gauge, or better yet a mercury manometer is a useful tool. Measure the absolute vacuum at both cylinders with the motor idling. Measure both before and after the new valve adjustment. If the vacuum is higher after the adjustment, you have found your culprit.


One last thought. If, during the throttle synch, you somehow managed to get the left throttle body hanging from its cable, you might have messed up the TPS setting causing the engine management system to no longer cut fuel on traiing throttle. This is a long shot but certainly worth the 5 minutes it takes to measure the TPS voltage at idle. (should be less than 0.400 VDC). Do the TPS relearning dance anyway. Just pull the Motronic fuse for a bit and then replace. Key on, motor off, wind throttle from full closed to full open twice, key off. That procedure will also remove all long term data from the CPU though and could result in a slight drop in fuel economy, depending on your buddies riding habits, until the Motronic "learns" his pattern again.

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Ok,here's the deal. He usually gets around 46mpg. After the agjustments he was got 36mpg with the first tank of gas. Then he found the disconnected line from the canister and reattached that. Now, 2 tankfulls later, he is getting an average of 41mpg. He said that the idle was that low even before he did the valve adjustments. Said it was always like that from new. I thoght that to be a bit wierd because he bought it new from a dealership, but with customer service the way it is nowadays, I don't doubt anything anymore. I feel bad about this and hope that it was nothing I did during the TBS, but I got it dialed back in to where it shoul be and it runs great. He just can't figure why the drop in MPG. I myself wouldn't be worried about it as long as the bike is running tops. I don't even know what I get. I just run it till I need to fill it up again and that's that. Thanks for the input so far. I have passed the info onto him already. We all met for a ride this AM to a big bike gather that takes place the 1st Sunday of every month. Lots of Beemers there amongst almost anything else you can think of. Even saw a newer Kawi ZZR1200 that had a snow mobile engine mounted in the frame. Talk about a smoker!!! Thanks, Terry

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