Waking my bike out of its winter sleep and I'm back to troubleshooting an issuethat i had last year. When acceling with maybe 1/2 throttle from a stop, my engine will stumble hard around 4k, enough to be scary sometimes. I think I might be lean on the mid, but I'm looking for some advice first.


My bike is a 2001
Current setup, 22.5/65/150
VH exhaust and KN lunchbox
Ignition advanced
Brisk plugs

I've tried fresh fuel to no avail. Thanks in advance!

I don't have one of the newer carbs, but that sounds like needle jet range. Do you have your needle shimmed at all?

I didn't have them shimmed at all. I just got back from a ride with 2 washers on each needle and didn't see a difference.




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How do the floats look?

Do you have top end power still?

Top end power is fine. I'll check the floats tomorrow. They were fine last time I checked (about 1000 miles ago), but I'll check again.



Edit: Floats are both high according to the Carb drain hose method (1/4" and 1/8" above the gasket line) .  Will get in there to fix as soon as I get a chance. Would this likely be causing my issues?


Edit 2: did some more riding and I'm noticing that the stumble almost always happens when turning from a stop. I haven't experienced it while acceling straight.




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Quote from: nmh5028 on April 08, 2013, 06:57:30 PMEdit: Floats are both high according to the Carb drain hose method (1/4" and 1/8" above the gasket line) .  Will get in there to fix as soon as I get a chance. Would this likely be causing my issues?

Possibly. My first instinct is to say no, but regardless of whether or not that's the problem it should be corrected so you may as well try. While you're in there check your slides for stickyness and their diaphragms for tears. Outside of that, I probably can't help you, hopefully someone with a lot of experience on the newer carbs will chime in.

I'll be sure to report back after adjusting the floats (hopefully tonight). I paid a lot more attention to the behavior of my engine on the way to work today, and I noticed a few things that weren't happening at the end of last season.

-The big stumble on 1/2 throttle accel in a turn at the 4-5k mark
-Cruising at around 25mph (5-5.5k), the engine isn't smooth. It sounds almost like it is "sneezing" through the air filter.
-Occasional backfire at the 25mph (5-5.5k) condition. Only happened twice in about 10 minutes at this speed, ususally when slowing
-Popping on decel (I think this is common with the V&H enxhaust)

I know my carbs are different on the 450 but your symptoms sound like mine before fettling.

I ended up with my needle in the richest possible position and am currently 8 sizes up in main jets (137.5) but need to go bigger still.

My pilot jets are stock but my mixture screws are actually set too rich.

I would definitely raise your needle some more.

My original symptoms were stumbling just off a closed throttle to about half throttle, and if I held it around an eighth to quarter throttle I would have constant hesitation which felt more like surging (but wasn't).

My issue now is I have great throttle up to 3/4 and then it seems to have nothing more to give, and I still have an occasional pop through the carbs at idle because I think my mixture screws are still a little on the rich side.

Oh, I have K&N pods and a custom free breathing 2 into 1 on my 450, engine completely stock.

Finished adjusting the float levels last night, and I didn't notice the stumble this morning. I didn't have a long ride today, but it looks promising.

I still have the roughness at about 1/4 throttle 4-5k in 1st and 2nd gear, with what sounds like backfiring in the carbs. (Almost like a sneeze from under the tank). Any suggestions on that one? I've tried with both 0 and 2 washers and it didn't make a difference.

Are the carbs synced?

Quote from: RossLH on April 10, 2013, 10:17:25 AM
Are the carbs synced?

Carbs were synced towards the end of last season after I put the new exhaust on. Should still be ok unless changing the float height affects the sync.

I noticed something odd about my carbs when I was doing the float heights... Apparently the PO had the fuel starvation vaccuum issue. Instead of replacing the petcock, there is a second vaccuum line coming from the left carb (so one from each carb) to a T, then to the petcock. I checked it for leaks and it looks good. Not sure if this would throw off the performance at all.

The extra vacuum line shouldn't affect performance so long as there's no leaks. All the vacuum does is open the petcock, fuel is gravity fed.

I'm not 100% sure on this, but something in my head is telling me that when you mess with the fueling, it can mess with the carb synchronization. It's worth a shot, only takes a few minutes if you have the right equipment. Setting the floats and synchronizing the carbs can fix a majority of carb issues.

Definitely check the sync. again as any adjustment can potentially throw it out, especially things like mixture screws and float heights where there's a greater chance that the effect is even a little different on one side to the other.

sounds to me like you have an air leak in one of the rubber boots between the engine and the carburettor .

Carb sync is good and no air leaks from what I can tell (removed/reseated the carbs and checked with WD-40 while running). It seems to be running better, no stumbles today either. I'm still getting the jerkiness at 1/4 throttle 4-5k. Seems to happen in any gear, but is much more noticable at lower speeds.  0 washer/2 washers didn't seem to make a difference. Is it still possibly a jetting issue?

You could be looking at mixture screw adjustments there... if they're CV carbs like mine then they can affect the whole throttle range so they've got to be right, but being more noticeable at low speed would make sense for that.

I'm not 100% on the 500 carbs... actually probably not even 50% 

Quote from: peteGS on April 11, 2013, 12:54:23 PMYou could be looking at mixture screw adjustments there

That's a good point, especially if they're rejetted. I completely overlooked the mixture screws.