Bruin 350 Crappy Idle

LBZ

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Buddy just picked up an '04 Bruin 350 with 700 km on it and it doesn't idle very well. Backfires and stalls once in awhile but off Idle it runs good.

We did the obvious, changed plugs, cleaned the carb, but to no avail. For some reason it's idling very lean no matter how we adjust the air/fuel screw. Starting at two turns out and went up to 4 but wouldn't make a difference. Still backfiring through the carb. Checked timing and it's good.

Ended up changing the pilot jet from the stock 17.5 to 18 and that seemed to improve things-at least it would Idle for longer with fewer backfires but it's still lean as per the plug.

Was thinking of trying a 20 pilot and see what it does but I find it odd that a stock atv would need such drastic carb changes with no other obvious issues like torn boots or leaks.

Thoughts or ideas anyone??
 

LBZ

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Tank was dumped when he got it home and only had a couple litres in it anyways. It's got fresh premium and fuel cleaner/water remover. Fuel quality isn't the issue.

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LBZ

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It does. Trying to get it to idle without needing the choke though because slight choke speeds it up.

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Lund

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Hey Dale, generally a 4 stroke that will back fire is because it is running lean, i doubt it came originally with lean jetting, i would reinstall the original jets. My self i would be checking for a vacuum leak, air intake issue. Your vacuum will be the highest at idle and a low vacuum will indicate an air intake leak some where.
Make sure all your connection and hoses are good and carb mounting, intake manifold ect. Being your issue is at idle i would double check your idle circuit, it must be pulling air from some where, some how. It almost sounds like your pilot passage is plugged. Actually partially plug because if it was completely plug it wouldn't idle at all.
 
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LBZ

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Went around with the butane torch around all the lines, boots intake etc and the idle never changed so I'm sure we don't have a leak. The more I think about it I kinda believe it still has a slight blockage in the pilot port. We blew air and cleaner throughout all ports but it doesn't take much to cause a problem. Virtually nothing for vacuum lines so I don't believe that's the issue. It's definitely running lean. Ground electrode is white on a new plug.

As I said anything off Idle and it runs good. Under load with throttle etc so it's just the idle circuit causing the grief.
 
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Lund

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Being you have checked the basic's, that is the most common idle issue with atv's the idle circuit, pilot passage. The passage is at the bottom of the bowl and water will tend to collect inside if they are left sitting for a while. This will cause corrosion and partially plug or completely plug the circuit. You cannot visually see that and sometimes air will flow but the restriction is still there. Changing jet size to little effect to the idle is a good indicator of a partially plugged passage as it is acting like a restrictor in the circuit. You might have to replace the carb if you can't open up the passage

I have unplugged some using a propane torch to the bottom of the bowl. Heat the bottom where the passage is, then while still hot put water in it, then blow air through it. You need to take care doing this though not to damage the bowl.

BTW, you need to take the bowl off the carb...LOL
 
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Bigblack

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A buddy's carbed Griz had these issues....ended up that the main jet was partially plugged because the machine sat so long...fuel dried up, left deposits in the carb. We changed the main and pilot, then ran Seafoam in the gas...fixed 'er up.
 

LBZ

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Well a pipe cleaner through the pilot hole which didn't yield any gunk didn't fix it. 20 pilot tuned it right up though. Runs like a top now. Weird it needed jetting to fix it but it's not the first time I've heard of this needing to be done.

FWIW it never did have any gunk or dirt in it. With only 700km cruising around an acreage it is pretty clean.
 
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