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Discussion Starter #1
I have a 2006 Hyosung gt650. It is a great bike but I am having an issue with the top end power. Power begins to go in and out when holding slightly over 80mph and around 5500 rpm. I can take it up to 110 / 120 mph but when trying to stay at any speed over 80-85 the power begins to come in and out. If you stay on the gas the bike will eventually die.

Some history:

It was a show room bike I got brand new. I (now) know the guy I bought it from is a terrible mechanic / dealer and did not do a lot of stuff to get it read ready he should have. It had this problem initially but after a carb clean it got better. The carb was cleaned and adjusted by another mechanic who has never worked on Hyosungs and it then backfired all the time on deceleration.

To fix that, and make it sound nice, I had a third mechanic (who does work on Hyosungs) put a 2 brothers exhaust, jetted, cleaned and synced the carb. It ran PERFECTLY! Then I took it out on a long ride. The top end problems started up again after the ride.

I am also am beginning to hear more of a rattle from the pipe when I really get on the gas. I highly doubt it is the brand new 2 brothers exhaust. I checked all the bolts, exhaust manifold bolts, its all tight.

My only guess is the fuel pump, and that I should replace that and maybe the tank. The tank is also an issue and not the best. It got a leak and I had it fixed but it makes me nervous. To make it worse the guy who fixed it took his sweet time (4 weeks) and it developed a bit of rust inside. Nothing too bad but need to replace it soon.

It has been dropped, there is a reason I chose a cheap first bike. The water pump cover had to be replaced after the drop but everythign workes fine now. The bike runs great except the top end stuff and the new rattle.

Some specs:

Hyosung GT 650 v-twin
twin carb- rejetted, cleaned and synced within the last month
2 brothers exhaust
~10000 km on it

Any advice is much appreciated. I am out of guesses how to fix this and I have no mechanic to take it to who actually fixes something without making something else worse. Thanks.
 

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I may be wrong, but a carburated bike shouldn't have a fuel pump, it should be gravity fed

It might have one if the tank were lower than the carbs, but that would really be the only reason
 

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Did you get the recalls done?

There are venting issues that could be causing random fuel starvation

HYOSUNG Recalls & Defects | HYOSUNG Problems & Service Bulletins


2005-2007 HYOSUNG GT650 FUEL SYSTEM, GASOLINE Recall
2005-2007 HYOSUNG GT650 FUEL SYSTEM, GASOLINE
Units affected: 3292 HYOSUNG MOTORS AMERICA INC.
Report Date: 2008-02-14
Summary: HYOSUNG IS RECALLING 3,292 MY 2005-2007 GT650 AND GV650 MOTORCYCLES. THESE MOTORCYCLES WERE BUILT WITH FUEL TANK CAP GASKETS THAT PREVENT PROPER TANK VENTILATION.
 

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Discussion Starter #4

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Let's see, you should have dual carbs, those could be out of synch too

Shop manuals are great things to own. Looking at the one for my bike, there is a
procedure for testing the fuel pump using a pressure gauge.

Given how cheap your pump is, probably cheaper to replace it than to have it tested or buy the equipment
 

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Hyosung top end power prolems

Hi there GT650sd

Just read your post now. I had a similar thing with the GT650R losing power as soon as you try to keep a top end run going. After checking everything I found the problem to be fuel starvation , but not due to faulty carbs ect, the problem was the ventilation hole on the top of the fuel tank was blocked, causing the tank to create a vacuum whenever the carbs start sucking down the fuel. I cleared the hole, and made it a tad b*****, the tank can now breathe and I can run the bike at any speed without any power loss or starvation issues.

I hope this helps, sure did the trick for mine.

Good luck
 

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DARKGTR
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Hi, have you by-passed/ eliminated the kickstand safety switch? As in highrevs the vibrations make it activate and it will "kill" the bike/turn it off and on while riding. Happened to me on my GT250R when I had it back on '07 now that I have a GT650R I've gone and eliminated/fixed the problems that have rised from '07 to now.. Hope that helps and that you have fixed your problem.
 

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[/QUOTE]
I have a gt250 comet 06 model I'm having a similar problem when I'm trying to ride at a flat 60kmh it sputers an pats as if going up a hill in too high of a gear but if I over rev it gaines speed but is too fast for legal speeds
 

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I have a gt250 comet 06 model I'm having a similar problem when I'm trying to ride at a flat 60kmh it sputers an pats as if going up a hill in too high of a gear but if I over rev it gaines speed but is too fast for legal speeds
That could be a dirty fuel filter or shorting spark plug wires. I've seen bad plug wires cause a bike to run fine until you're under load, like up a hill or you get on it in a higher gear, then it cuts out and struggles like it's losing power. Dirty fuel filter can cause all kinds of weirdness. start with the cheap and easy things to check M8, then work up to the more expensive and harder to check things.
 

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Are we not men?
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OP started this thread in June of '12...bike is probably scrapped by now.
 

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But not the rider three posts up that has a similar problem and is asking for help.
 
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