High speed stumble...? Scrambler. - Triumph Forum: Triumph Rat Motorcycle Forums
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post #1 of 13 (permalink) Old 12-08-2012, 03:59 PM Thread Starter
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High speed stumble...? Scrambler.

My 2007 Scram has had the air box removed, TOR pipes, Dynojet kit, etc. done before I came into ownership of the bike.

The bike starts very quickly and idles fine after a minimum of warm-up. At anything below 4000 rpms it runs beautifully, no misses or hesitation of any kind. No stalling on deceleration.

Above 4000, it starts to stumble, perhaps miss or hesitates; to a consistent degree. It will keep pulling at WOT as far as you want to go- I've had it at 100 mph, but that miss and stumble remains until you get back below 4000 or so.

Is there a "most obvious place to look" first? Dirty main jet in the carb? Coils?

I'm out of my depth, but the fact that it runs so well in all other ranges makes me think it's a specific part of the puzzle.

Any observations or experience much appreciated.
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post #2 of 13 (permalink) Old 12-08-2012, 04:08 PM
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Look for an air leak ? I had a stumble that was intermittent around 5000 rpm which I believe was caused by cracked in the rubber holding the air box to carbs.

Does your have the AI delete are the rubber caps new ?


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post #3 of 13 (permalink) Old 12-08-2012, 07:50 PM
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Sounds like either of these:

-Vacuum leak

-Improperly sized main-jet

-Improperly shimmed needle

-Damaged rubber diaphragm
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post #4 of 13 (permalink) Old 12-09-2012, 04:18 PM Thread Starter
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Quote:
Originally Posted by luckyduck View Post
Sounds like either of these:

-Vacuum leak

-Improperly sized main-jet

-Improperly shimmed needle

-Damaged rubber diaphragm
Dang. Well, I guess I can start with vacuum leaks.
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post #5 of 13 (permalink) Old 12-09-2012, 04:19 PM Thread Starter
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Quote:
Originally Posted by Mekagojira View Post
Look for an air leak ? I had a stumble that was intermittent around 5000 rpm which I believe was caused by cracked in the rubber holding the air box to carbs.

Does your have the AI delete are the rubber caps new ?


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All the plumbing looks OK. It does have the air injection removed. Looking at the vacuum lines now. Thanks!
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post #6 of 13 (permalink) Old 12-09-2012, 04:36 PM
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Check the fuel tank breather pipe isn't trapped or blocked at the exit under the bike.
If it isn't that make sure the filter / strainers in the petcock and left hand carb aren't blocked.
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post #7 of 13 (permalink) Old 12-09-2012, 04:42 PM
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I am with the not enough fuel issue. I have had the fuel tank vent clogged and a partially clogged fuel line.


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post #8 of 13 (permalink) Old 12-11-2012, 01:59 PM Thread Starter
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I pulled the plugs today after a long high speed run, about 140 miles at 80-85 mph. They're pretty new (<1000 miles) Iridium plugs, and both sides looked great. I understand a lean condition can be tougher to spot than rich, but these plugs really looked good.

Does this indicate anything about looking elsewhere?
Is a fuel starvation problem at high load going to result in a lean condition- like jetting problems, or does the mixture stay the same and the engine just stumbles.

Plainly I have a lot to learn about carburetors.
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post #9 of 13 (permalink) Old 12-11-2012, 03:43 PM
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Has it been like that since you bought it or is it a problem that's developed recently ?

Pods ? Main jet size ?
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post #10 of 13 (permalink) Old 12-12-2012, 05:55 PM
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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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