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Help me understand carbs...

Old 06-23-2010, 10:07 AM
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Help me understand carbs...

When it comes to fuel injection I know it inside out, when it comes to carbs it's like black magic. All my cars have been FI, all bikes but 1 have been carbd, I have never touched the carbs. I've got a 98 Hawk with 2bros high mounts and who knows what else (most likely stock). I don't think it feels right, let me explain.

1500-2500rpm: Any more then light throttle results in serious vibration, low frequency, in tune with RPM, think jackhammer. It will power through this rpm range but it feels like you're seriously lugging it. Plenty of RPM hunting. Bike isn't drivable in this range.

2500-4000rpm: Plenty of torque, not a lot of vibration at light to medium throttle, if you go WOT it goes back to being a jackhammer. Bike hunts a lot in this RPM range, it's hard to be smooth with light throttle, it's pretty noticeable at 2500rpm but goes away by the time you hit 4000.

4000-5000: Very little part throttle vibration, no rpm hunting, this is where the bike feels smooth and I feel I have to keep it here for daily riding. Moderate jackhammer at WOT that starts to go away by the time you hit 5k.

5000-10000: POWAAAA!!!

The starts right away even when cold and idles on it's own after just 10 seconds of choke. It seems to run poorer in cold weather, will barely idle when it's below 50F. It runs best at 70-80 degrees, it's smoother and feels more powerful.

I think the carbs are out of sync and I need a jet kit. Or is this just the nature of the beast?
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Old 06-23-2010, 10:24 AM
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Not the nature - I'll let others provide more detailed carb suggestions as I'm a noob to carbs too, but when well sorted it should be driveable at part throttle (parking lot speed) at 1500 - 2500rpm and pretty much fine to open it up from there. You won't get a ton of power until your north of 5000 as you said.
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Old 06-23-2010, 10:55 AM
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Start with this stuff:

Check Throttle Position Sensor and adjust to 500 ohms
Check all hoses to air box are connected right.
Carb sync once the two above are done.
Check plugs

Questions to answer.

Has the bike sat for more than 2 or three months?
Has it always done this or is it a new problem?

If the bike sat the slow speed jets may be blocked some from fuel residue.

I would not start ripping into jetting just yet as stock settings should be a pretty good baseline with just aftermarket cans. Something else is not right or dirty.
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Old 06-23-2010, 12:02 PM
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If that's 1500 to 2500 rpm in 1st gear, you have a problem with your bike, could be a misfire...plugs, wires, etc. But if you're talking about any gear other than 1st, this ain't a Harley. You shouldn't be running at revs that low above 1st gear. Downshift. You are lugging it.

I usually downshift when it gets below 3k.

Other than that, it sounds like you may have some carb issues. It may not be jetted right for your exhaust. Is the air cleaner stock?

And +1 on cleaning the carbs. I had a plugged slow (pilot) jet which caused a misfire 3 months ago, and my bike has never sat for more than 1 week without being ridden. '05 with 40,000 miles, bought new.

Last edited by VTRsurfer; 06-23-2010 at 12:10 PM.
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Old 06-23-2010, 05:29 PM
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Originally Posted by cornandp View Post
Start with this stuff:

Check all hoses to air box are connected right.
One of the connections wasn't right, it's the hose that goes across the top of the airbox (right below the filter) and down to the front carb. Went through it with a flashlight and spotted it through the frame.

Haven't had a chance to ride it but so far it idled at 700-800 rpm without stalling, it had never done that before. Going to head out for a ride now.

EDIT: That was it!! I remember some time ago I rode a TLR, most noticeable was how smooth the engine was compared to my bike. Not anymore! Bike hauls *** from low revs, feels like I gained in the top end too.

So yeah, check your vacuum plumbing.

Last edited by FinalBoss; 06-23-2010 at 08:30 PM.
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Old 06-23-2010, 10:34 PM
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Originally Posted by FinalBoss View Post
One of the connections wasn't right, it's the hose that goes across the top of the airbox (right below the filter) and down to the front carb. Went through it with a flashlight and spotted it through the frame.

Haven't had a chance to ride it but so far it idled at 700-800 rpm without stalling, it had never done that before. Going to head out for a ride now.

EDIT: That was it!! I remember some time ago I rode a TLR, most noticeable was how smooth the engine was compared to my bike. Not anymore! Bike hauls *** from low revs, feels like I gained in the top end too.

So yeah, check your vacuum plumbing.
I am guessing that is the PAIR valve hose, except that hose is under that airbox - if you post a photo we can be sure. If you have not dePAIRED you may want to, and set the TPS while you are at it. You will be pleased by the results. See the KBs. JB
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