Hello everyone- Hoping that perhaps some of my ATV rid?n partners can help me?.About a year ago I purchase a used 2001 400ex and I?ve been very satisfied with it. But just recently I took it to a local bike shop to have my clutch replaced and have it dyno tech?d. to get ready for my trip to the dunes (St. Anthony dunes). Anyway, at the time I took it in the engine was bores to a 416cc and told the guy to tune it for 3500 ft. elevation but once I got to the dunes my bike ran awful. It would bog down every time I gave it gas to quickly. Since then I?ve installed a 440cc kit and tried it out last week at Pismo beach (sea level) and the darn thing still ran sh$tty. A friend told me that perhaps it was running with too much air so I played with the choke to restrict some air to the crab and it made a significant improvement but still wasn?t enough to bet a stock 400.

Any ideas??

Sorry for the long post??

Dunes
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By giving it some choke...you richened it up. Sounds like the jetting is too lean, at least for sea level.

 

Thanks HB,
Any idea on what would be a good jet size to start on to get a better mix?

 

jetting

Here's a link to a good site on jetting it yourself, with a list of symptoms to help you figure what to do. Your quad is so far from stock, itd be hard to estimate what you need.
What other synptoms do you have? hard to start, breaking up at full throttle, popping in the intake?
Its posible the needle needs to move up(clip down), and maybe even a bigger pilot jet. If the trouble is at full throttle, then a main jet change would be the ticket.
Be aware that making it right at sea level is gonna be too rich at altitude.
Most of the time, the bog ( from rapid throttle change) is caused by the mixture screw being misadjusted, or vacumn leak somewhere in the intake area.

 

Thanks HB for that great link... My quad usualy starts on the fist or second crank so there is no problem there, the main problem is, what you would say, breaking up at full throttle and running hot. I also forgot to mention that I have a over size oil tank and the oil temp was running at approx. 190 degrees. Again, after using the choke to limit the air intake the quad ran better (wasn't bogging down at 3/4 to full throttle) and also was running around 140 -150 degrees range but I was still getting blowned away by stock 400's

Based on the link's discription I might be running to lean. Reading orther great post on this forum I think I have a good idea on where to start. Not sure what size jet I have right now , but I'll try going with a 170 jet and 42 pilot and see what that gives me. I'll post my results later and hopefuly I'll help somebody else with similar problems.

Again, thanks for your reply and hope you had a great turkey day!

 

With the K&N, open air box, and better pipe, you are flowing more air through you engine. You need to buy a few larger jets, ride with each one and pick the best one. Look at your spark plug coloring to help you decide which is best.

 

I agree. When you pull the main jet out, it will have a number stamped on it. Thats the size. Youll want at least 2 steps up from that.