High speed stumble...? Scrambler. - Page 2 - Triumph Forum: Triumph Rat Motorcycle Forums
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post #11 of 13 (permalink) Old 12-12-2012, 08:23 PM Thread Starter
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Originally Posted by Firstman View Post
I have been on a 450 mile road trip thinking about your issue. A constant rich/lean mix would show up on the plugs, but something killing the fuel would not. I worked on a carbureted bike that had a similar issue, at 65 mph the bike would stumble. I was able to reproduce the stumble reproduce the issue at certain RPMs, it turned out to be a piece of dirt that looked like a single flake of pepper that would lodge itself in the main jet only when X amount of fuel was being consumed.

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I really appreciate this. I'm coming to the conclusion that it's a fuel interruption problem too, either as you describe or (hopefully) in the petcock or that screen I understand is near the left carb. I'd really like to rule that out before getting into the carbs. I did take the top covers off the carbs and the - I guess those are what's called the diaphragms- the thin black rubber membranes through which the needles and springs go; were in fine shape.

I really want to get a good understanding of how the Keihins function and how to trouble shoot them, but I am not there yet, so I'm being cautious. Also because the bike really runs well in all other ways- starts fast, idles nicely, no backfiring, no stalling; so I don't want to screw that up. And I am capable of screwing it up.
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post #12 of 13 (permalink) Old 12-12-2012, 09:03 PM
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It may just be that the jets either need cleaning or replacement. I had chased a fuel delivery roast for months only to find myself right back at the same jetting I was before the problem started and everything seems to be at 98% better. Still have a slight hiccup, but over all, the bikes running much better.

Jets are cheap and easy to replace. While you have the pilot jets out, you may want to pull the air mixture screws, being careful not to lose the small spring, washer or rubber 'O' ring (but first, screw the mixture screws in and count how many turns it takes before they come to a dead stop[This will be the number of turns you set the screws to when you replace them]). With the air mixture screws out, blow carb cleaner up into the carb bodies until the carb cleaner comes out the other hole (i.e. shoot cleaner into the air mixture hole until the carb cleaner is coming out the pilot jet hole and vise versa), then before the carb cleaner has a chance to evaporate, hit it with compressed air trying to blow out any possible obstruction. Then make sure when everything goes back together, the air mixture screws are set at where they were before they came out.

Chris / RudeBoy BIR #309
2014 Scrambler: Arrow 2-1 high pipe. TTP Tune 4 with Breathe airbox cover no baffle.
Past rides:2008 T100, 1975 Honda CB400/4, 1989 Honda GB500, 1965 Honda CB450 Black Bomber, 1972 BSA A75 Rocket3
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post #13 of 13 (permalink) Old 12-13-2012, 01:01 AM
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And if you want to feel more comfortable with a carb before tearing into yours, find a shade tree mechanic in your area, give him $20 to teach you the basics on an old carburetor. While yours will be different, pulling the jets out and such on a workbench will give you the confidence to go forward on yours.

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