400EX - Hard Starting? Carb or Timing?

[Chris S.]

Here is my situation. I bought a motor that was running perfect. The person shipped it to me and I put in my quad and it wouldn't start. I put a smaller pilot jet in and it idled and reved perfect. I then moved the quad from my basement to outside the next day and it started hard, ran rough, backfiring and hising through the intake. However it started and ran and I rode for 10-15 minutes with out much trouble. Now it won't start again and when you crank it, if you put your hand over the exhaust you can feel it suck and then push air. That doesn't seem right to me. Is it possiable the timing slipped during shipping or when we lifted the quad on its side to take it out of the basement or maybe the chain tensioner broke. What sounds like the problem? I can't imagine the exhaust tip is suppose to suck air at any point of the engines revolutions. Is there something obvious that I am overlooking. I just don't want to take everything apart to check the timing if its uneccessary. The first time I had it running it ran so perfect, that is what really has me confused. I am using the same carb as the original owner. Its a 500 stroker motor with a 450r carb with HMF full exhaust and open lid air box with a uni filter. Please help, I'm in desperate need. My friends with banshees are laughing at me!

[2004exrider]

Try putting the bigger pilot jet back in, it is probably starving for fuel and cant idle or rev very easily. It probably ran good in your basement because it was warm, but as soon as you took it outside my guess is that its cold and is messing with your jetting causing it to be lean.

Jimmy

[Chris S.]

The air temp outside has been between 40 and 55 degrees and inside it was 65 to 70 so I was thinking that was not the problem after my last question that I posted. What about the exhaust sucking air?

[Chris S.]

Well, I tried a larger pilot jet, had a 38 in there and tried a 42. Still no luck starting. Do you think I should try going even bigger? Next time someone gets a chance could you crank your quad motor over with you hand over the exhaust and tell me if you feel it suck air on the down stroke. To me that does not seem like it would be normal but I never checked that before. I am thinking that timing has got to be off.

[DUNEZRUNNER]

Have you tried adjusting the valves. If you think the timing might be off the best way to check would be try adjusting the valves. One of your valves might be adjusted too tight and that could be causing the air to be sucking in. I cant see how the timing could just by just moving the quad out of the house, unless the tensioner broke also at the same time. If it was me and I thought that the timing was off I would check the adjustment on the valves, that should tell you right away. Hopefully it is just a tight exhaust valve and nothing else though. But it shouldn't suck any air in the exhaust only in through the intake.

Hope this helps

Thomas

[WheelerBob]

Sounds like valve overlap, I'm not sure of the volume of air your talking about, but on a four stroke motor there is a brief period of time at the end of the exhaust stroke/ begining of the intake stroke that both valves are open, this causes a slight suck back effect , its called valve overlap due to the fact that the cycles of the intake and exhaust valves actually overlap. That is why if you ride in mud you will find mud up in the packing in your muffler. hope this helps.:D :D