I have 07 yfz 450 I have done everything possible to try and solve the problem of when it is running at an idle and I go to w.o.t. It falls on it's face and quits.I have no acceleration unless i work the throttle slow.Iv'e changed lean air jet,slow jet,low end jet,and even raised the needle also changed accelerator diaphram.Nothing seems to work.Can I please get some answers.

I suggest a second opinion would be in order as everything he did for the tune up was not altogether necessary. I consider that as the car was running ok before I would not touch the carby other than to fine tune the idle mixture to emission requirements. Any problems with the distributor cap would have showed up as a miss and the rotor button would have to be really bad but again as it was running ok. My policy is don't touch what ain't broke.Power valves in carbys are there to give extra fuel when the vacuum falls off under acceleration and do nothing else. To accommodate a bigger alternator more adjustment on the idle mixture screw would be necessary at idle but as alternator barely charge at idle the adjustment would be little. The low compressions may be from pumped up hydraulic followers and I would discuss that with the original engine builder as to what hydraulic units he fitted. but again if it was running ok before what the hell. Yes get a second opinion for sure from the engine builder.

You need to hone the accelerator pump well and adjust the throw of the pump rod as much as possible to get more fuel. Of course that is just a guess based on 100's of rebuilds i have done over the years. I have also found that Japanese carbs seldom turn out when rebuilt.

Have you checked the engine cylinders compression?

You have done everything except check the needle and seats. It is possible that there is debris stuck in the seat and starving the engine of fuel. How is your fuel filter? clean and serviceable?
Check your plugs, are they worn or dirty and sooty. If so give them a good clean and re check the gap.

THROTTLE BODY needs to be taken apart and cleaned. Hand clean the sensor and idle valve, they are electric do not soak them, check them also, Idle valve likes to stick and position sensor likes to go out. HUNT

Try testing for partially blocked exhaust

sounds like possible a sensor went bad or is loosly connected. O2 sensor??? Also check the air filter. Sounds like whatever it is, is making the truck run rich----that would be the black smoke.


Hope this helps

Sounds like your battery has one or more dead cells in it. If everythings work and then when you try to start it it won't start it is most likely a dead cell. Before changing batteries, clean the posts shere the cables attach to the battery and tighten grounds attached to motor and battery.

I FIXED IT BY NUMBER 1 : IGNORING YOUR CLAIMS OF" I CAN FIX IT"
NUMBER 2 : LOOKING FOR A VACUUM LEAK " THAT YOU NEVER SUGGESTED.

Sounds like the fuel supply is inadequate. Check for a plugged fuel filter, or possibly the fuel pump is losing pressure. I had a 93 Chevy V6 that acted the same way, and after checking everything else I replaced the fuel pump, which solved the problem.