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    xlt mod motor carb question

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    • Member since
      September, 2005
    • From: northern lower michigan
    xlt mod motor carb question
    Posted by olymon on Saturday, November 12, 2005 1:32 PM

    I have a xlt motor that was a heavily ported watercross motor at one time. It is now in an 93 chassis using tripple psi pipes and 38mm carbs. I'm trying to get the bottom end carb tuning figured out. All last year, and this year when I started it up there was a hesitation on the bottom end. It seems like it's in the transition from  idle (1800 rpm) to above 3000 rpm. It will idle fine but when I go to give it some gas it just bogs. it will do this no matter if you slowly open the carb up or you give it a quick flip of the gas. The only way to get it to "idle up" out of the low rpm is to slowly blip the throttle a few times until it slowly gets above 3000 rpm. Above that the sled comes alive and will tach up really quick.

    I'm going to go up one size on the main this year because last year when trail riding it would tahc up good but if you decelerated at all it seemed to be bogging. Then if you got back into the throttle it would feel boggy unless you pumped the gas a bit. I think it's starving for fuel on top but the plugs look pretty good.

    Here is roughly the carb set-up. 25pilots, stock needle and needle jet from the 38 mm carb ??, 2.0 slide cutaway, 280/290 main I can't remember from lsat year. Any possible  ideas on what it might be that's causing the transition problem?Sad [:(]

    Thanks, Todd

    so many little projects, so little time
    • Member since
      June, 2005
    Posted by Anonymous on Saturday, November 12, 2005 10:56 PM
    On many piston port engines, radical porting will give it a bog such as you describe, and it cannot be tuned out. I would try carb settings like the 97 XC with 38's, or the 96 XLT. 45 pilots, air screw 1 1/2 turns, 247 Q0 nozzle, 6DP10 needle #3. Or 35 pilot, air screw 1/2 turn, 247 Q2 nozzle, 6DH7 needle #3. Both use a 2.5 cutaway.
    • Member since
      June, 2005
    Posted by Anonymous on Sunday, November 13, 2005 2:11 AM

    try the needle clip position at 3 -4 or even # 5

    the pilot should be at least # 35

    the air screw at 1 1/2 to 2 turns out.

    you should confirm what needle jet is in it  QO  ?? Q2 ?   big diff here !

    Good luckwith it.  BTW   what RPM does this engine run at wot  ???????

    TMR.

    • Member since
      September, 2005
    • From: northern lower michigan
    Posted by olymon on Monday, November 14, 2005 7:45 AM

    This little 580 +.020  loves to run at 9400 rpm wot. I use the plastic shims from yamaha to get more of a fine tune on the needle position. I can make an adjustment of  about .015" using 1 shim versus going one whole clip size of.040"    I just didn't have much time to play with the carb tuning on this sled last year.

    The crank has been welded and I really don't see too much difficulty in running 9400 rpm with the crank welded. I ran it last year in a ski hill drag just kind of thrown together out of a bushel basket. Didn't do too bad I got third in the 600 improved class. I'll be digging into it soon and will see whats in the carbs for needles and needle jets. I'll have to borrow my friends avenger 3 again to take a look at what it's doing at each throttle position. What do you think I should set the idle at on this thing, 2500-2800rpm?    I've never had a motor tweaked quite this much before, just slightly upgraded trail sleds are my normal thing.

    so many little projects, so little time
    • Member since
      June, 2005
    Posted by Anonymous on Wednesday, November 16, 2005 6:08 AM

    now you got me curious about whats in my xtra lettuce and tomato.................

    gonna strip a carb today and check it out for you,

    my engine spins at 96-9800 RPM,   bored 38 mikes, very high comp on c-14, race ported by myself. I don`t go nuts with porting huge ports out......... there is such a thing as overporting,,,,,LOL                  I use storm ignition coils,    idles real nice at   2000 rpm,   pipes are old  school    PRO-5  triple  trail pipes and trail can.

    this combo is used stricly for racing............... and it ROCKS...........hehehe..................

    I will check out my carb calibration today and post it .

    John.

    TMR.

    • Member since
      September, 2005
    • From: northern lower michigan
    Posted by olymon on Wednesday, November 16, 2005 7:44 AM

    John, sounds good to me. Are your carbs 34's oval bored or are they 38's bored to about a 39.5 or so. Do they have thick flanges for mounting to the intake boots. I know mine look pretty thin walled already, I can't imagine boring them out any.... Tho, I do have a spare set..... :)

    Like I wrote, I'm just looking for some ideas on possible set-ups for this toy.  I had one time when I pinned (playing around on a road)it and it tach'd up to 10,000. Kind of made me jump, but it held together no problem. How long has your sled held together at that rpm?

    Thanks for the help

    so many little projects, so little time
    • Member since
      June, 2005
    Posted by Anonymous on Wednesday, November 16, 2005 9:57 PM
    Something that well help this motor alot is your belt clearence i have seen as little as 20tho the differance between falling flat on it's face and hangung the skees , we allways in the colder weather had to run the e clip one down for ritcher makes all the difference in the world,,
    • Member since
      June, 2005
    Posted by Anonymous on Thursday, November 17, 2005 5:39 AM

    my carbs are stock Polaris, round bored out to only 39.2 mm..   I have tried 38 mm carbs bored out to 39.5 mm  on many engines and they are N.F.G.  for tuning,,,,,,,,,, no velocity over the pilot jets.

    I have been xperimenting and racing this engine at 9800 rpm for about four years. and  the crank is not even welded, I use KLOTZ  oil ......... KLOTZ has saved my crankshaft.........

    Pockets is right with the specs. This is what I am using .........

    Pilot # 45

    slide #2.5

    needle jet #  247    # Q-2

    jet needle #6DH7

    mian jets  #  300 -320  depends on weather.      good luck with it  :)

    John.

    TMR.

    • Member since
      September, 2005
    • From: northern lower michigan
    Posted by olymon on Thursday, November 17, 2005 8:59 AM

    I have a major side clearance issue with this clutch. I'm going to borrow a spider tool from my friend and solve that problem. It will be my first time taking one apart to changew the side clearance so any suggestions on making it easier to get the belt clearance dialed in would be great.

    I may try oval boring the spare carbs at work.  I could keep the floor of the carb untouched by using one of our four jaw chucks in the lathe and offsetting the carb. I'd still be able to bore it out to say 39.2 -39.5mm and keep the bottom end velocity the same.

    Thanks for the help on the carb specs, I appreciate all the help.

    so many little projects, so little time
    • Member since
      September, 2005
    • From: northern lower michigan
    Posted by olymon on Thursday, December 15, 2005 10:16 AM

    I've torn down, cleaned like crazy, and put the carbs back on. 35 pilots, 247 q2 nozzles, 6dh7needle #3 position e-clip, 320 mains.

    The sled idles like *** unless I put my hand over the center carb and choke it off to draw some excess gas into the carb throat. Once it's running  it seems to be coughing quite a bit more fuel vapor out of the carb. It will tach up fine and run good unless I let the rpms drop down, then it doesn't want to run right on the bottom again.

    While increaseing the rpm, I looked at the inlet to each carb. On the mag and pto the carb had a thin trace of fuel getting sucked into the carb, but the center seemed to be stagnant. Then when I opened the throttle up a bit the fuel trail on the center seemed to almost come out a bit until the velocity became strong enough to pull the fuel into the carb.

    I have swapped the center carb with the mag, and even swapped the fuel lines  with no change in the centers response to low end idle issues....How could it be coughing more fuel back then the other two cylinders on this piston ported motor??????

    Any help out there??

    so many little projects, so little time
    • Member since
      June, 2005
    Posted by Anonymous on Friday, December 16, 2005 7:52 PM
    Put the 45 pilots in, and set the air screws to 1 1/2 turns out. Make sure the carbs are synced perfectly at wide open throttle, each carb reaching wide open at the same time. Also check that at 3/4, or just when the cutaway side of the slide clears the bore, they all do at the same time. Then set each idle screw exactly the same number of turns out from all the way in. I suspect a carb sync issue. If it is the engine, you should see a compression difference, or the intake skirts scarred up on the center one.
    • Member since
      August, 2015
    Posted by TripleT on Saturday, August 29, 2015 11:49 PM

    Hey! im new to the forum. Sorry to dig up a very old post but im building a sled right now and it has everything your xlt has. It has a very agressive port job, centurion 650 coils. High comp head. Bored 39.2 miks just wondering what elevation you ran 300 - 320 mains at. Im around 5000'. everything is fresh so I just want an educated starting point in terms of jetting...

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