I have a 1999 grizzly 600 that backfires. When I am hard on the gas or running steady it runs fine, then I let off the gas and the rpms get almost down to idle speeds i get 2-4 backfires. If I adjust the idle speed up to just at the point that the clutch starts to grab it will almost never backfire. I have made sure of no exhaust leaks and it still backfires, and I have also tried putting a new exhaust on because the old one was full of pin holes and it still backfires.

Any Ideas?

Also does anyone know how to tell it the atv has stock shocks on it, the ride is rock hard and I think the previous owner put heavy duty shocks and springs on it for hunting.

Thanks

Mike

 

The idle circuit is lean, and thats causeing the backfire. You can verify this by pulling on the choke just before letoff, and the backfire will stop.
If the carb has never been cleaned out, its posible the pilot jet is getting plugged, and thats responsible for the lean. If youre sure the exhaust leaks are fixed, and theres no other issues, try going 1/4 turn richer on the mixture screw, that should richen it up enough to stop the popping.

 

Thanks for the speedy reply I will try that tomorrow morning and let you know if that corrected it.

Thanks again

Mike

 

Anyone have an exploded view of the Mikuni BST 40mm?

 

I hope this link works

 

Thanks

I think I found the same one on a parts ordering page.

Thanks again for your help

Mike

 

I have had a very similar problem with mine after rebuilding the carburator. I am thinking I have something out of adjustment but can't figure out what it is. This may be a stupid question but where is the mixture screw? An item number in the diagram would do if someone knows what it is. Thanks

 

Well that is where I am stuck.

Here is a parts list for that picture above

Tag Description
1 SKU: 4WV-14901-00-00
CARBURETOR ASSEMBLY 1
1 SKU: 4WV-14901-10-00
CARBURETOR ASSEMBLY 1 CALIFORNIA CALIFORNIA
2 SKU: 4WV-24411-00-00
DIAPHRAGM
3 SKU: 4WV-14933-00-00
SPRING, DIAPHRAGM
4 SKU: 4WV-14272-00-00
VALVE
5 SKU: 4WV-14147-20-00
O-RING
6 SKU: 4WV-14918-00-00
RING
7 SKU: 620-14231-29-00
.JET, MAIN (#145)
7 SKU: 620-1423E-S7-00
JET, MAIN #143.8 CALIFORNIA CALIFORNIA
8 SKU: 4WV-1490J-00-00
NEEDLE SET
8 SKU: 4WV-1490J-10-00
NEEDLE SET CALIFORNIA CALIFORNIA
9 SKU: 4WV-14941-90-00
NOZZLE, MAIN
10 SKU: 4WV-14985-00-00
FLOAT
11 SKU: 4WV-14190-28-00
NEEDLE VALVE ASSY
12 SKU: 98502-05008-00
Loading... .SCREW, PAN HEAD
13 SKU: 239-14172-07-00
PILOT JET, AIR (0.7)
14 SKU: 4WV-14996-00-00
PIPE
15 SKU: 4WV-14193-00-00
PIPE
16 SKU: 98502-04006-00
Loading... SCREW, PAN HEAD
17 SKU: 4WV-14147-10-00
.O-RING
18 SKU: 4WV-14105-01-00
PILOT SCREW SET
19 SKU: 4HR-14142-42-00
JET, PILOT ( 42.5)
20 SKU: 4WV-1410A-00-00
STARTER SET
21 SKU: 12R-14191-00-00
.PLUG, DRAIN
22 SKU: 4WV-14197-00-00
PIPE
23 SKU: 87M-14137-00-00
.CLIP
24 SKU: 98502-05014-00
Loading... .SCREW, PAN HEAD
25 SKU: 97602-04112-00
Loading... SCREW, PAN HEAD WITH WASHER
26 SKU: 4WV-14940-00-00
DIAPHRAGM ASSY
27 SKU: 522-14161-00-00
.NUT, CABLE ADJUSTING
28 SKU: 1UY-14155-00-00
.GUIDE, CABLE
29 SKU: 4WV-14103-00-00
Loading... THROTTLE SCREW SET
30 SKU: 98502-05010-00
Loading... .SCREW, PAN HEAD
here it is
<<-- New Search

 

I am thinking it is assy 18 but wanted to verify with someone before I start adjusting stuff. When I put it together the kit in the carb it did not come with any instructions so I ran that in until it bottomed out. I was going to back it out 3 turns but would appreciate any feedback.

 

The mixture screw is #18. And about 2.5 turns out is a good place to begin.