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Discussion Starter #1
Hey y’all, this my first post on here, I’ll try to keep it as short an to the point as i can. So my friend got a deal on a 04 grizzly660 I’ve been helping him work on it since i have an 2003 660 an know a little about them.
Originally it was hard to get started, once you kept it running an got it warmed up it would idle fine. However while sitting in neutral it would bog down at low throttle, less than 1/4 throttle, an if you tried to push it past the bog it would give a loud backfire that sounds like it’s coming from the front an then would stall out.
Gas tank is spotless, previous owner put on a new carb (looks like a very cheap aftermarket) but it is perfectly clean on the inside, and air filter is clean. Adjusted the mixture screw out to 2 turns. While the carb was off i did check the intake valves, an they were really tight less than .10mm, i double checked an it was on TDC of the compression stroke, however I had a brain-fart an forgot it had idled for a little bit prior to that an i adjusted the valves only 10mins or so after, so i know i need to recheck them cold an also do the exhaust, which I’ll be doing in a few hours. But was wondering if anyone else had some more ideas besides valves. A front end backfire isn’t something I’ve experienced, so it kinda surprised me. I’m still trying to learn how to identify something running lean or rich. Thanks in advance!
 

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Quick update, rechecked the intake valves and adjusted the exhaust valves. Intake needed a little tuning and the exhaust was really loose. Checked my grizzly and the mixture screw was set to about 2.5-2.75 turns, so set my friends about the same. Now it Idle’s but will die 3 times before it stays running steady an idles nice. Same thing, it will bog and sputter at less than 1/4 throttle but now once you get warmed all the way up, that goes away. Also, if you flick the throttle it’ll die sometimes. An i mean literally a tap an it dies. Sometimes it does it, sometimes not. Any ideas?
 

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Welcome from Colorado.
I bet the P.O. installed the new carb, and only needed the valve adjustment.
As for rich or lean, do a plug reading. Run it until hot, then turn off the engine, then as fast as possible look at the plug. I do this at speed, bump the trans into neutral while still moving (takes practice, be careful) and use the kill switch.
Tan is just right, white is hot and lean, black and possibly wet is rich.
Try a new plug now that the valves are right, that might do it.
When mine is cold and just started, I let it idle to warm or it will die, and it is in proper adjustment. Its the nature of the beast, after 15 seconds she is good to go.
 

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Discussion Starter #5
Sorry, things got busy at work an only just had a chance to test the spark plug. Turned out perfectly tan as you said it should, only burned myself 3 times getting to the plug lol, but issue is still the same. Still an awful backfire an wants to bog an die at very low throttle.
 

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Sounds like a timing problem then.
 

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Those cheap eBay carburetors are hit or miss at best. I would start by replacing it with an OEM Mikuni carburetor, or at least put the OEM jets in the carburetor you have.
 

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Update: Timing is perfect, replaced spark plug, ignition coil, and cdi box. Results are the same, which I didn’t expect those to be the issue, but had everything available to try it anyways. Did find that the starter plunger is kinda messed up where it connects to the carburetor, so going to replace the cable an get a different carburetor along with oem Jets like dezz suggested. A full Mikuni Carb is a little out of the price range at the moment.
 

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Discussion Starter #9
Once again, another Update: So I somehow managed to find my original Mikuni carb for my 03 grizzly an installed it on my friends to see if there was any change. Looks like I had cleaned it up prior to storing it, so didn’t take the time to go through it before installing, however it now Idle’s flawlessly an fires right up. Have to give it a little choke when it’s cold, but once it starts it runs perfectly without it. However, instead of a low end bog an backfire, it now backfires an dies between 1/4-1/2 throttle. Adding some choke doesn’t significantly change the amount of throttle you can give it, if at all. Seems to be backfiring through the carb still. It may be that i need to take it off an clean it throughly, but once again looking for other suggestions. Thanks again for all the advice so far.
 

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Forgot to add to my previous update, that I did adjust the mixture screw starting at 2.5turns out. Went an additional 1.5 turns out an went 1 turn in from where it started with little to no change.
 

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How are you checking the timing?
If she's timed and popping through the intake, check for a burnt valve.
 

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Discussion Starter #12 (Edited)
How are you checking the timing?
If she's timed and popping through the intake, check for a burnt valve.
Removed the pull start an used a timing light to check through the inspection hole. Sorry for the ignorance, but how do you confirm a burnt valve? Not something I’ve looked at before.
Also, one thing I forgot to mention in my previous posts, with the previous carburetor on it the machine didn’t lack power at all. It bogged at very low throttle (maybe 1/8 throttle) but after that It had scary torque. To the point it would do a wheelie if you weren’t prepared after it came out of the bog, but a lower top end speed. (At least compared to my 03)
 

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Removed the pull start an used a timing light to check through the inspection hole. Sorry for the ignorance, but how do you confirm a burnt valve? Not something I’ve looked at before.
:)
What if the timing chain is stretched?
You've said perfectly timed, I've been waiting to read how you confirmed this, so if you need parts we determine the right one's.
If you align the crank and the chain is stretched the cam is not timed, so the lash is probably off causing your problem.
That's why I use a straw to find t.d.c., then look at the crank and cam and rocker arm. And then rock the crank to make sure everything moves at the same time.
As for a burnt/open valve, do a leak down test.
Read about valve problems here;
.
 
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