Hi guy and girls. You all seem pretty smart from what ive been readin. So hears my questian. My boss has a late 90s (i believe) 400 4x4 ac. I personally have a polaris so i dont no a whole lote about them. The problem he is haveing is that it starts fine and will idel all day but as soon as you give it even a little throttle and i mean a little it doesnt rev up at all but it back fires like crazy. The one time i actually have to check an make sure the muffler was still there. Now i do all my own work on my popo and my boss used to ride hair scrambles so we both no our way around the motor pretty darn well. Does anyone have any ideas on what it could be. He had the carb off cleaned it replaced some parts changed the spark plug and still no dice. Any help would be appreciated.

Oh yea one more thing, if I get on the gas when starting it it will rev fine for about 5 seconds or until i get off the gas. And just for reference of soud it sounds sorta like the rev limiter on my sportsman 500 when i back up and dont hold the over ride switch.

 

there is two issues here big guy.
most ac 4x4 400 class will bog down and back fire if the voltage regulator is going out. this generally will allow you to at least get to second gear before it starts backfiring. this is one possible problem.
second,if the carb needle is set wrong on the barrel it will back fire from to lean of fuel mixture generally when backing off the throttle.
good luck

 

Ok, I guess i didnt fully explain,it wont run above idel so I cant drive it at all. Could it still be the voltage regulater? Where is it generaly located and how can i test it. I guess that could explain why on certian days it would work so half and then some it wont. Lately it hasnt worked at all though. Thanks for any help though.

 

buddy i don't have clue how you would test the critter. i am a plumber not a electrican. second, a voltage regulator can go out gradually and it could very well be the bad guy here if it all the way out.
mind cost 219 bucks and would not fit. i had to buy another at 263 bucks before i could get it to run above second gear. i have seen more good ATV's that would barely run cause of a voltage regulator.
best idea is to put in the shop and let a good trained Mechanic look it over.

 

This could be Electrical are bad or comtaminated fuel if the quad has sit up for 3 to 4 weeks , are it could be that you didn't get the carb back together right ; but if it was put back together right you should check to see if there's an air leak between carb intake tube and carb to the air filter tube !

You can spray WD40 , Deep Creep or carb cleaner around the carb to engine intake tube ; and carb to air filter tube and if there's a leak the engine rpm's will increase !

If the Valve spec's are off to much they can cause the same syptom's you' are having , If it turn's out to be bad fuel after draining the tank before putting the fuel tank back on your quad check your valve clearance for proper gap ; make sure when you check them you're on the compression stroke !

The best way to find the compression stroke is to pull the spark plug and stick your finger in the plug hole , when you feel pressure again't your finger stop ; then turn the the crank shaft untill the timing mark's in the inspection hole's line up before setting the valve's !

These inspection hole's are accessable by 2 cap's in the crankcase , if your engine has the pull rope on the side of the engine you will need to remove the pull starter housing to get to the crankshaft ; and use the bolt holding the flywheel to turn the crankshaft !

The other inspection mark could be another srew off cap or a stationary mark on the crankcase up higher on the outside crankcase , on the same side as the flywheel and i think both the intake and exhaust valve's spec's are 0.006 inch ; but check with a Dealer's technition for the correct valve spec's most Dealership's will give out this info over the phone !

Good luck with the quad the AC 400 has a Suzuki Engine , so you can check with a Suzuki Dealership for the valve spec's ; and there are more than one mark on the crankcase make sure it's the " T " mark on the compression stroke !......

 

always the possibility that there is no compression, i had this problem with my dvx last year, turns out it had sucked itself full of mud and blown up, but those four strokes tend to run with little or no compression, pull the plug and put a compression guage in it, and compare the reading to the factory specs, if that is not the problem then its def. electrical