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2007 Grizzly 450
4 Posts
Discussion Starter · #1 ·
Hi all! New member here. Excited to be a part of the community! Been having some recent issues I cannot for the life of me figure out, so I decided to try and pick the minds of some other grizzly owners.
Apologizing ahead of time for the long write up, but I just wanted to outline all the options I have exhausted and answer the common questions ahead of time lol. PS. Before posting this, I did research and read about every carb/idle post on every forum I have come across lol.

Problem Summary: No matter the idle fuel screw position, I cannot get atv to idle without exhaust popping and eventually dying after a minute or so of idling. I've checked and tried almost everything.

Background and things tried:
I purchased a Grizzly 450 new in ’07 off the lot. Since purchase, it would only run with the choke half open...thought that was normal at the time. The griz currently has 439 miles and 183 hours. Basic trail riding and casual cruising around some property. Always ridden a couple times a month. I recently decided to tackle the issue of only running with the choke on...the past couple of months I have been trying to get it to run/idle smoothly with the choke off…like it should actually do. I have spent A LOT of time and no matter how much I play with the pilot screw (idle fuel mixture screw); I cannot eliminate a slight popping through the exhaust at idle...it also will only idle for a minute or so before it dies. I have made zero aftermarket mods to the atv. Before troubleshooting and experimenting with the pilot screw, here is all I have checked so far as potential issues and verified are in spec:
  • Removed all diaphragms and rubber from carb, thoroughly cleaned carb and flushed all passages with carb cleaner and compressed air. Verified all jets are clear of obstruction. Verified all jets are the correct number and installed in correct location.
  • Removed rubber diaphragm and inspected, even holding to light…no holes or tears.
  • New pilot screw O-ring. O-ring and washer installed in correct order.
  • Air filter is freshly oiled. Let dry 48 hrs before installing. Oiled enough so oil is not running through foam and collecting at bottom of filter, but enough to leave residue on fingers/hand when you touch it.
  • Sprayed carb cleaner all around intake manifold and where boots connect to both sides of carb…no noticeably change in engine RPM.
  • Fresh gas (87) and seam foam added in at 1 oz./gallon.
  • Choke cable operates freely and doesn’t bind.
  • Throttle operates freely…snaps shut when released.
  • Intake Valve:
    • Spec 0.0024”-0.0039”.
    • Measured: 0.0035” feeler gauge fit, but 0.004” did not, so I know it’s between 0.0035" and 0.0039" (in spec, but near upper limit).
  • Exhaust Valve:
    • Spec 0.0063”-0.0079”.
    • Measured 0.0075” feeler gauge fit, but 0.008” did not, so I know it’s between 0.0075" and 0.0079" (in spec, but near upper limit).
  • Spark plug: new and properly gaped
    • Gap Spec: 0.6-0.7mm
    • Gap Measured: ~0.65mm (in spec)
  • Float height:
    • Spec: 13mm
    • Measured: 13mm (created gauge for checking) (in spec)
  • Fuel Level:
    • Spec: 4-5mm
    • Measured: 5mm (in spec)
  • Even taped around air box lid seal to ensure good seal.
I don’t have one of those 90deg pilot screw adjusters, b/c I measured the clearance under the carb, it’s about 1/8-1/4” shy of the clearance needed for the tool+tip (motion pro and pit posse both need about same clearance). So I did the tedious method of removing tank, loosening and twisting carb, making a 1/4 turn adjustment to pilot screw, twisting carb back and tightening clamps, reinstalling gas tank, and starting and checking adjustment.

Here are my results for each adjustment of the pilot screw from 1.5-4 turns out from seated:

Note: tests done with engine fully warmed, in neutral, with 1gal of gas in tank with petcock set to reserve (b/c only 1 gal gas). (1gal gas made taking the tank on and off quicker and easier). Also adjusted idle speed to recommended ~1450-1550rpm after each adjustment of pilot screw.
  • 1.5: noticeable popping. Deceleration (decel) popping.
  • 2: noticeable popping. Fluctuating idle (1470-1770). Occasional pop through intake. Popping on decel w/ light idle hang.
  • 2.25: Noticeable popping. Fluctuating idle. Hangs on decel.
  • 2.5: Light popping. Fluctuating idle (1490-1890). Occasional exhaust pop. Idle hangs on decel. Occasionally dies.
  • 2.75: Relatively light popping. More stable idle (1540-1650). Small hang on decel. Dies after short time idling.
  • 3: Light exhaust popping. More stable idle around 1490, but with surges up to 1770. Idle hangs on decel and idle floats around 1750 after decel. Dies after short time idling.
  • 3.25: Light popping. Fluctuating idle (1420-1800). Idle hangs on decel.
  • 3.5: Light popping. Fluctuating idle. Idle hangs on decel.
  • 3.75: More noticeable/louder popping. Slight decel hang.
  • 4: Noticeable/louder popping. Idle hangs on decel.
The best range of the idle fuel screw seems to be between 2.75 and 3.25 turns out from seated, but even still, the atv won’t idle without exhaust popping and eventually dying...seems to always be running lean. Short of starting to throw parts or a brand new carb at it (not thrilled at that option..$$$$) I’m really at a loss on things to try/do to get a smooth idle without exhaust popping and my atv dying after a minute or so of idling. Any thoughts or advice would be greatly appreciated! Would love to get his thing running properly and back on the trail! Thanks for taking the time to read!

Premium Member
12,259 Posts
I would do a compression test/check on it. The popping is a sign of a timing problem, like the valves are not closing when the plug fires.

2007 Grizzly 450
4 Posts
Discussion Starter · #3 ·
Ok I can check that. I guess I assumed compression was good since it's relatively low hours/miles, it doesn't seem to have any loss of performance while riding, can backup pull start in one or two pulls, and no smoke out the exhaust or anything. I will try to pick up a compression test kit here in the next couple weeks and report back the results. thanks!

85 Posts
This may be a dumb question but did you replace the pilot jet. If you get to the end of the range with the fuel screw if it has no effect you need to change the pilot jet to a bigger one. It sounds to me like it was jetted wrong from the factory.
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