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mcman56

4t Carb - Vacuum Chamber plug and jetting

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I have a 2016 300 4t and it runs OK but sometimes spits and dies when opening the throttle.  Yesterday, i even got a very serious spit back through the carb, (80 degrees F).  I have had bikes spit the carb right off the manifold with spit backs like that.  I pulled the carb today to check jetting and noticed that the big vent for the vacuum slide was not capped off like it should be.  There was a little dust up there like it had been off for a while.  Could missing the cap create issues I have seen?

Otherwise the carb looked clean, 27.5 pilot, 120 main, idle mix 2 1/2 turns out, needle 3 positions from top, standard quiet muffler,  I don't know how to tell the difference between slide springs but assume it is the original.  I have no other 4t to compare it to but the fan does seem to run a lot and sometimes it is finicky to start when hot so I thought it may be lean.  The bike does have extremely low hours so maybe that is why the fan runs a lot.  I'll install an adjustable idle mix screw.  It seems like the only other option is to raise the needle.  Are there any recommendations? 

 

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Not sure your elevation.  I'm at sea level and changed my 16 200 4T jetting to 132.5 main, 25 idle jet, and 4th slot from the top on the needle.  It was the same as yours (27.5, 120, 3rd clip position, but only 1-1/4 turns out - I'm now at 2 turns).  I also changed from the blue spring (>5,000 ft) to the silver spring (<5,000 ft).  

PO rode at 12,000 ft, so my bike was extremely lean at sea level.  Lots of decel pop and flameouts.  I also did blow the rubber intake boot off the carb.  Then it would only start with the choke on, and would have a major backfire and die if I pushed the choke off.

With my changes, the bike runs fantastic, idles very well, no decal pop and no flameouts.  Fans on these things do run quite a bit from my experience.

Make sure both boots are very well mounted when you put it back together.

 

Edited by markbxr400

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1 hour ago, mcman56 said:

What exactly is a flame out

When your bike simply dies on you (typically when letting off the throttle then getting back on it).  Usually happens at the worst moment possible, like just as you are hitting the second blip on a double blip.  

Edited by markbxr400

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Flame out, when you left the barbecue well alight, well you thought it was and when you came back to throw the meat on it was out! 

When one jet fighter flies too close to the other and catches the jet wash ( serious lack of oxygen already burned by othe jet) and the effect is ‘flame out’ too low and boom you crash, too high and it’s a hell ride to earth unless you are a very good pilot.

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13 hours ago, mcman56 said:

I have a 2016 300 4t and it runs OK but sometimes spits and dies when opening the throttle.  Yesterday, i even got a very serious spit back through the carb, (80 degrees F).  I have had bikes spit the carb right off the manifold with spit backs like that.  I pulled the carb today to check jetting and noticed that the big vent for the vacuum slide was not capped off like it should be.  There was a little dust up there like it had been off for a while.  Could missing the cap create issues I have seen?

Otherwise the carb looked clean, 27.5 pilot, 120 main, idle mix 2 1/2 turns out, needle 3 positions from top, standard quiet muffler,  I don't know how to tell the difference between slide springs but assume it is the original.  I have no other 4t to compare it to but the fan does seem to run a lot and sometimes it is finicky to start when hot so I thought it may be lean.  The bike does have extremely low hours so maybe that is why the fan runs a lot.  I'll install an adjustable idle mix screw.  It seems like the only other option is to raise the needle.  Are there any recommendations? 

 

Block off the vacuum thingy and run the engine, see how it behaves.

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Move the needle clip to the bottom slot (or the next one up if it "blubbs" in the midrange).

These bikes are shipped from the factory very lean in the mid range to get them past emissions. Moving the clip to this position really opens the bike up to a much stronger midrange.

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