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Old 11-20-2012, 10:01 AM   #1
elementalg20 OP
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Carb'd bike getting really poor mpg's

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.
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Old 11-20-2012, 10:46 AM   #2
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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
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Old 11-20-2012, 11:04 AM   #3
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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.
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Old 11-20-2012, 11:25 AM   #4
larryboy
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Return it to stock, drilled slides you can't fix, but that just improves response.
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Old 11-20-2012, 11:45 AM   #5
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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.
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Old 11-20-2012, 07:21 PM   #6
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I have the solution to your poor mileage Issue

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
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Old 11-20-2012, 11:21 PM   #7
elementalg20 OP
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Lucky for me the p.o. saved the stock parts....
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Old 11-21-2012, 02:00 AM   #8
willis 2000
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Quote:
Originally Posted by Triumphrider View Post
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
This is correct, return to stock. But if you want to play around, (1, drop the needles one notch, (2, synch the carbs.
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Old 11-21-2012, 11:33 AM   #9
MrBob
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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.
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Old 11-21-2012, 05:13 PM   #10
concours
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Quote:
Originally Posted by elementalg20 View Post
Lucky for me the p.o. saved the stock parts....
Very good! Put them in, along with a new OEM air filter and enjoy the three cylinder goodness!
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Old 11-22-2012, 08:56 PM   #11
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Trophy 1200 Carb Tuning.

Quote:
Originally Posted by willis 2000 View Post
This is correct, return to stock. But if you want to play around, (1, drop the needles one notch, (2, synch the carbs.
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
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Old 11-23-2012, 04:00 AM   #12
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Quote:
Originally Posted by elementalg20 View Post
I've got a 98' trophy 1200 with I believe mikuni bst36 carbs
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!

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!
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Old 11-23-2012, 10:02 PM   #13
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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
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Old 11-25-2012, 10:40 AM   #14
elementalg20 OP
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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.
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2007 Victory Vegas Premium....finally I'm back on the road with another steed.

Gone...1998 Triumph Trophy 1200 2000 KTM Adventure 640, 1994 BMW R1100RS, 2002 BMW K1200RS,1999 VFR800, 1996 VFR750, 1985 Maxim X, 198x 250LTD
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Old 11-25-2012, 11:59 AM   #15
larryboy
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Quote:
Originally Posted by elementalg20 View Post
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 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.
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