Stock carb issues, does it suck beyond repair?



7 replies to this topic
  • BigPopper

Posted February 16, 2002 - 06:07 PM

#1

I took my new-to-me BRP '00 out to Rowher Flats for my 3rd ride today. Been there a bunch before on my old CR250. About 1000 to 3000 elev.

Anyway, I've been struggling with tuning the off idle flat spot out of my stock carb.

When I got the bike it maybe seemed better then now, but I'm not sure. I've only messed around with the idle mixture, I'm now at 2.5 turns out and don't remember what it was when I got it. Does this adjustment really have much effect on the flat spot right off idle when you whack open the throttle? Unless I slowly roll on the throttle, it will stall the bike. This is unacceptable. Since I just bought the bike less than a month ago, immediately going to the QS is not in the cards. Is it possible to tune this thing close so It won't stall when I crack the throttle hard off idle?

Specs: Honda comp baffle, stock pipe, 172, 68S, B53F 4th groove ( I know it's supposed to be an E, but it looks like an F to me), Unifilter, stock plug.

I positively love the bike. All the hard to believe comments everyone has made about the "weight is an advantage over 5 mph" are almost believable at this point. The bike rips for hillclimbs, sandwashes, fireroads, chasing my daughter's Z50, and even does decent MX riding including jumps.

The flat spot sucks and I need to take this great bike from 90% to 100% without dropping 4 hun for a carb.

Amy suggestions appreciated,

thanks,

Steve Russ
Huntington Beach.

"Go big or don't go"

  • rhh4

Posted February 17, 2002 - 03:01 AM

#2

You can get it out to the point were the bike will not stoll but I have never heard of anyone eliminating it completely without getting a new carb.

  • terry_west

Posted February 18, 2002 - 04:57 PM

#3

Have you removed both baffles in the intake tract?If so you probably need a larger main jet. Other things to consider depending on how far you want to take it. I asked Scott Summers the same question about snatchoing the throttle open his response was " don't do that".

  • BigPopper

Posted February 18, 2002 - 06:55 PM

#4

Terry, thanks for the reply. Yes, both of the airbox baffles are removed. I had considered going to a 70S pilot jet to see if it would help with the stumble. I figured the main was fine, since the bike seems to be good once it gets beyond 1/4 throttle. One thing that seemed to help was bumping up the idle speed. I think I had it around 1000 RPM and the book says 1400.

I'm really not expecting to make this thing perfect with this carb. I just wanted to get it as good as possible and figured there was some more info out in BRP internetland about tuning this thing. I really would like more data from everyone, stuff like main jet, pilot, pilot screw settings, needle and settings, and pipe and aircleaner info. I know the stumble is objective and doesn't bother some people. I guess I need to stop riding it like a 2 stroke.

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  • MaXRSmart

Posted February 19, 2002 - 12:03 PM

#5

I had the same problem with my XR600. When the bike was in stock form, the hesitation was not that bad. When I started to open it up though, the hesitation got worse. I increased the jetting and that helped, but it did not get rid of the "kill switch" throttle. I learned to not ride it like a two-stroke bike, but every now and then you still need to blip the throttle.

Here is what I discovered was the cause of the problem. It is from the slide opening up and letting in air faster than the fuel can keep up. This will lean out the bike and cause it to hesitate or stall. In stock form, the intake and exhaust is very restricted, so the fuel can keep up with the air. As soon as you add a high flow pipe and air filter, the fuel can't keep up with the burst of air.

The only way I was able to eliminate the problem was to add a QwikSilver carb. My bike reacts like a two-stroke bike now and I get better gas mileage. I know this is not an option for you, but a pumper carb is the only way you will be able to keep the air/fuel mixture correct when you blip the throttle with a high flow engine. For now you can try and adjust your jetting, but I think you need to adjust your riding style for a four-stroke. Good Luck.

MaXRSmart :)

[ February 19, 2002: Message edited by: MaXRSmart ]

  • gtms34

Posted February 19, 2002 - 12:36 PM

#6

Steve,

You will drive yourself nuts trying to fix this. It CAN'T be fixed without a pumper carb. I ran that bike for 10 months with the current jetting. Don't change any jets unless altitude becomes an issue. You may end up with a hard starting, plug fouling, poor performing motorcycle. :) Jetting WILL NOT fix the flat spot. :D

Sorry, I know this is not what you want to hear.

  • BigPopper

Posted February 19, 2002 - 04:59 PM

#7

Well Kevin, I know you've been down this road before (with the very same bike!)so I'll stop trying to fix what can't be fixed. As you told me when I bought the bike from you, the jetting is about as good as it can get and I agree. I suppose if I wanted to fool around with slide cutaways and every needle Kehin makes for this carb, I might be able to improve it, but that's not going to happen. I'll just live with it until I suck it up and go with the Quick Silver. BTW, besides curing the off idle stumble, will the QS improve the top end as well? I've heard something about 3 HP increase with the QS.

Anyway,thanks everyone for the input.

  • gtms34

Posted February 20, 2002 - 05:04 AM

#8

Good deal Steve,

Just leave'er alone till the $$$ is available for a pumper.

I didn't notice any difference in top end with the Ebrock. It did get there quicker. It may make more HP but it's capable of 100mph now. :)

[ February 20, 2002: Message edited by: gtms34 ]





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