Carb'd bike getting really poor mpg's

Discussion in 'Road Warriors' started by elementalg20, Nov 20, 2012.

  1. elementalg20

    elementalg20 Been here awhile

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    I've got a 98' trophy 1200 with I believe mikuni bst36 carbs(o.e. fitted carbs) that have a "stage 1" jet dyno jet kit installed, slides are drilled as well. I didn't do the work, just bought it this way. I don't know the jet sizes or needle positions or any of that useful information at this point.

    What I do know is this, it starts easy, idles fine, only the slightest of sub 2k rpm "issue" when riding(it likes to run over 2000 rpm under load....like most any bike imo).

    No smoke or anything crazy, but stock 36mpg should be easy(2up loaded riding hard one gent claims 36mpg). Riding with nothing but me (200lbs) and empy bags I'm returning 22-23mpg....you can practically watch the gas gauge fall.

    I'm not a carb guru, I do most all my own auto repair but carb's are a dark art to me, especially a bank of them. It may well go to a shop, but I'd like to have some insight into the situation of at all possible. I don't trust any shop nor have a clue where I'll take it if I do but I need to get to the bottom of this. Range sucks this way and frankly it makes traveling just that more more expensive as well.

    Thanks for any insights, and yes I'll probably see about having it jetted back to stock but I'm wondering if that can really drop my mileage this far alone?

    Also installing new rubber, and new "insterstate friendly" gearing(19/41 replacing stock 18/42). New chain too of course.
    #1
  2. sailah

    sailah Lampin' it

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    Does the bike smell rich at idle? How easy is it to get at the carbs?

    Id find out a good jetting recipe from a triumph forum and get into the carbs to see where you are. Check floats and their seats too could be dumping fuel
    #2
  3. BikePilot

    BikePilot Long timer

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    People rarely tune carbs well when they go about modifying and most off-the-shelf "stage-whatever" kits are pretty bad and tend to make things way rich. Probably the case with your setup. I'd go about tuning the carbs to get the AF right. I bet your mpgs go way up.
    #3
  4. larryboy

    larryboy Chopper Rider

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    Return it to stock, drilled slides you can't fix, but that just improves response.
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  5. usgser

    usgser Long timer

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    No idea what mpgs you should be getting but if it's running good with the tweaked carbs it may just be it is what it is. If it were me I'd pull the carbs and rejet set them up back to stock specs then sync them and see where you're at. If it has a hot rod free flow exhaust you'll have to fatten up the jetting a bit to keep from running lean but just go up one size at a time and test it till it runs good through the entire throttle range. Most advertised jet kits are for power at WOT but you'll lose some driveability/smoothness at lower speed. Takes some trial and error to get the right set up that works good at all throttle openings. Most MC carbs are pretty simple but it means diving in more often than most folks like to do to get them right. I don't and never will own any bikes that have 4 carbs. The thought of syncing 4 carbs to all run the same seems more grief/frustration than I'm willing to deal with. Again you say the bike is running good for you it may just be that much fuel is what the motor needs for the way you ride it.???? Tou might try riding less aggressively. I've run tweaked sportsters that got 45-50 mpgs ridden sanely but dropped to low 30's flogged. Ya wanna go fast, ya pay the price.
    Go back to stock carb specs and work your way up till you're happy. At least you'll have a known base line to work from. Keep good notes regarding any deviations from stock. Since it's running good now it's probably not an issue but make sure your airfilter(s) are perfectly clean.
    Good luck but you're in for some leaning/experimenting or pay a tuner to do it for you.
    #5
  6. Triumphrider

    Triumphrider Live to Ride

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    I am a Triumph Mechanic and also have a Trophy 1200 that came with Dynojet kits installed. It also got terrible mileage even though it seemed to run OK. If you have or can get the original jets, emulsion tubes and needles, you can bring it up to 38 to 40 MPG.

    Steve
    #6
  7. elementalg20

    elementalg20 Been here awhile

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    Lucky for me the p.o. saved the stock parts....
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  8. willis 2000

    willis 2000 neo-quixote

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    This is correct, return to stock. But if you want to play around, (1, drop the needles one notch, (2, synch the carbs.
    #8
  9. MrBob

    MrBob Out there

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    Before further entering the hell of carb tuning, you might want to find a local shop equipped with an oxygen sensor or gas analyzer and start from those readings. This could tell you immediately if your engine is running fat.
    #9
  10. concours

    concours WFO for 41 years

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    Very good! Put them in, along with a new OEM air filter and enjoy the three cylinder goodness!
    #10
  11. Triumphrider

    Triumphrider Live to Ride

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    This is good advice, I had forgotten to mention that I also dropped the needles 1 notch and synced the carbs to get to 40 MPG. Thank you for the reminder.

    This is a four cylinder engine so it does consume more fuel than the three cylinder models but has a good bit more power.

    Steve
    #11
  12. warewolf

    warewolf Tyre critic

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    It's a known issue with Mikuni BSTs. They suffer chronic needle jet wear which causes rich running. It's not so much that the carbs are not stock, but the inherent wear on the moving parts; and in that regard stock is as bad as aftermarket. Having said that, I'm not a fan of Dynorich kits. Do yourself a favour, throw away the dynojet kit and get a factory pro kit and factory pro high-dispersal needle jets. Get it dyno-tuned, report back here! :D

    Have a read of this: The BST-40 Bible
    and this: Mikuni Needle jets (Factory Pro web site)

    Parts that wear are the slide guide, the slide (not so much but replacement is the only way to return to standard bleed holes), the needle and the needle jet (aka emulsion tube). My Trophy 900 was needing new emulsion tubes every 20,000km when the slide guides were badly worn.

    In several states of tune, the dynojet kit never really felt like it was dialed in, despite lots of dyno testing. However the Factory Pro kit and needle jets were dynamite! :clap
    #12
  13. Beezer

    Beezer Long timer

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    also be aware that Mikuni, Dynajet, and Kehin all have a different size hole in jets of the same number. needles have different profiles too.... best not to mix them together
    #13
  14. elementalg20

    elementalg20 Been here awhile

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    I know this is a long shot question as it can obviously vary a great deal.

    But....

    If I took it into a shop and had them restore it back to stock and synch the carbs back up what sort of bill should I be expecting?

    I've seen guys claiming that the slide guides were still looking good in there triumphs at 80k plus miles, mine is at 33k so I hope not to need a total rebuild of these things just yet. I may keep my eyes open for another rack to completely return back to new down the road.
    #14
  15. larryboy

    larryboy Chopper Rider

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    I don't know that bike all that well, but just looking at a picture of one I'd estimate 4-5 hours labor...$100 per hour around here, might save some bucks if you stripped the body work before you took it in.
    #15
  16. bwalsh

    bwalsh UUU, UUU!!!

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    Been there...

    Who re-jetted it? Were the correct jets installed(too big a main jet)? Pull the carbs and verify!!!

    Reason I say this I just bought a XR650L that had already been re-jetted. First full tank I got 37mpg. WTF? My older XRL gets 44 average. I also pulled that carb on that bike to check and make sure it had indeed been done correctly.
    I took the new bikes carb apart to check to see what had actually been done. It had a way to small slow jet(50) and a too big main jet(165)(Stock is 50/152 ) and the slide had not been drilled. Idiot!
    I replaced them with what should have been used along with the Q4 pipe that's on the bike(58/162). I haven't gone thru a tank with the new jets yet but I can already tell by the amount of gas still in the tank I will get WAY better fuel mileage.

    Good luck getting your bike sorted out!
    #16
  17. elementalg20

    elementalg20 Been here awhile

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    I'm going to need it. I have the stock parts, and I tore into a single bst40 earlier this year. I didn't pull the jets or emulsion tube out put none of it looked like rocket science to me. I'm sure I can get it apart and back together again with some patience. I may need to take it to be synched when I'm finished but hopefully that won't cost nearly as much as a full tear down. I also know a guy who used to wrench for a big shop and now works in a diesel semi shop that'll do the rack on the bench if I want him to for a reasonable fee. I'm just not up for paying somebody $400-500 to get this thing were it needs to be if I can at all avoid it. With the current mpg though, it''ll definately pay for itself quite quickly......
    #17
  18. ibafran

    ibafran villagidiot

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    Once the carbs are back to stock and tuned, if the poor mpg persists, I would expect that one or more of the enrichening valves is leaking fuel. Also, if the fuel valve at the tank is one of those vacuum things, the valve is probably not reliably shutting off like it should. It does not take much loss of fuel to skew mpg a whole lot.

    I had a 95 Trident for 10yrs and the thing was a gas hog no matter what I tried. I felt like a econo rider whenever I got better than 40mpg on that thing. I even geared it tall in an effort to improve the mpg. Others with the same bike got high 40's and road like maniacs, wtf? Keep us informed as you make progress.
    #18
  19. plodalong

    plodalong Now if we just.....

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    As has been mentioned the needle jet wear is a known problem. Replace the air filter while the carbs are out as you have to remove the carbs to get to it. They're a nice bike but there's a lot of bodywork, covers, coils ect to remove to get to some basic service items. Wouldn't hurt to check the valve clearance as well. loved my 94 model.
    #19
  20. warewolf

    warewolf Tyre critic

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    It's a bitch of a job to get the carbs In and out, but the carb work itself is easy. You could do it yourself. Mine really benefited from a carb sync every 7000 km. Get yourself a vacuum gauge and use it. When Triumph stretched the service interval from 5000 km to 10,000, my bike didn't like it. The oil was too dirty even using full synth, and the carbs were long out of balance. So I did oil, filter and carb balance at 7 thou, 14 thou and then at 20,000 km also did valve clearances and sometimes emulsion tubes.

    33k miles is what, 50 thou kays? Chances are good they'll be poked if the slides have been drilled. The BST40 on my 640 Adventure lasted about 20-30,000 km, utterly bog stock standard.

    The main thing to watch is the needle jet wear: that's what causes the rich running. Worn slides just wear the needle jet (and needle) faster.

    Triumph changed from Mikuni to Keihin carbs in '97-'98 sometime, partly because of this issue with the BST's?? The Keihin-equipped bikes return better fuel economy for longer because of it. And the later bikes therefore won't have such dramatic slide guide wear 'cos they're not BST's.
    #20