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Prairie 650(etc)  ATV carburator synch
By Chet Walters
Return to Chet's Prairie Pages

Click any picture to see enlargement

Tools: 

  •  Very long #2 phillips (can be "fudged" with a hand driver w/ extension and 1/4" socket with #2 phillips bit stuck in it with some gum ;-) 

  • 10mm socket or wrench

  • very long needle nose pliers

  • suitable vacuum gauge (I use CarbTune which I bought for my six banger Valkyrie. Here's a link to a six banger home brew carb synch tool... but for a twin, you'd just need a simpler "T" fitting and a gauge.)
WHY
There are two cylinders and each needs to breathe equally to yield the best performance.  If one cylinder is working harder than the other, you risk vibrations, hot running, plug fouling and in the worst case, fuel washdown on the cylinder walls causing the rings to die a quick death - not to mention loss of power.   For normal running, twice a year is ok to check the synch.  But if you do any modifications to the carbs, top end, intake, or exhaust, then you need to synch as a final step.  The most noticeable symptom of out of synch carbs is engine vibration.

PREPARATION
Remove the air box cover body panels (two phillips screws and the battery holder). Set the battery on the frame near the fuses etc.  Leave your filter and air box top in place (set 'er up like yer gonna be ridin'). Keep the engine cool at this point unless you want to wear gloves.  Set the pilot screws to even steven (book says 2¼ turns out). With the long pliers, remove the caps from each intake (see pic).  Squirt a little WD-40 into the CAPS (not the nipples) to make them easier to remove/replace this time and next. Put your vacuum gauge hoses onto the nipples and clamp them off if need be.  Start the engine and let it warm up some. Set the idle a little faster than normal.  

ADJUSTMENT
Adjustment is easy, but a little tricky.  Just putting the long screwdriver on the screw will affect the synch.  You need to adjust the screw and watch the gauges.  When you get it close, let off the screw with your long phillips and let the motor settle.  Blip the throttle very little then let it settle down again.  Check the gauges.  If they match fairly well, your done.  If not, readjust.  Note that if the fan comes on at any time, you'll have to shut off the engine and let it cool some before continuing.  If you can't get it within range, some pilot screw adjustment may help.

If you're using CarbTune, it must be held perfectly vertical!

Put the caps back on the nipples and re-assemble.  After your next days ride - DO IT ONE MORE TIME! (second time's a charm). 

 

 

 WHAT IT LOOKS LIKE

If you click the pic at the left, you will see what the CarbTune looks like when the synch is correct.  You will likely not get an exact match, but if you are within the same full across red lines, that's good enough.

Click here for a 2meg QT movie of Carbtune running....
You'll need QuickTime player to see the above and can get it here http://www.apple.com/quicktime/download/
Click here for a 400K AVI movie of Carbtune...

Questions? Contact Chet at Chet says 'Be sure to write!'Rattlebars.com

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