So I purchased a 2006 300ex. It ran well for about 2 weeks then started giving me problems. The owner said he rode the quad every day for the past month without changing the oil. I let it sit for about 2 weeks, test drove it lightly every few days to make sure it ran well. Noticed the wiring was all messed up and rat-f*cked back together. The atv has an aftermarket exhaust (pretty sure there are no leaks), aftermarket CDI, aftermarket air filter (it wasn't even connected), aftermarket ignition switch, headlight, etc, and lots of non-essential components removed.

Changed oil, rode very hard for a few days, quad now has trouble starting when warm.

When I start cold, it starts 100% of the time, but there is a rattle in the engine. The rattle goes away as the quad warms up. Once the quad is warm it will start in 1st gear about 40-50% of the time. It will start in neutral 80% of the time. If it successfully starts in 1st gear, it will die about 50% of the time once the throttle is engaged, even with the clutch pulled all the way in. The quad seems like it pulls a little even with the clutch pulled all the way in. I linked a sound clip at the bottom.

I am not mechanically skilled, but did some research online and talked to a few people at the auto parts store. I did the following to the atv: Changed oil again with "better" oil, replaced the spark plug (plug was black), oil filter, k&n air filter, reconnected loose tubing to the carb, tightened a loose fuel line to carb, tightened loose wiring to ignition, ran a little seafoam in the tank and oil as directed, sprayed b12 in the carb and let run as directed, adjusted choke to see if it was fuel/air problem (it will not run with choke on), adjusted idle, ran without air filter lid, tried different gas, replaced battery, checked for loose wiring (though maybe not thoroughly enough?)

Here is a link to the startup noise: https://voca.ro/17BuLe2hQDUO
The starting pattern from the clip is as follows:
Start 1-2: 1st
Start 3: N
Start 4-5: 1st
Start 6: N

Here is a link to the sound of the rattle:https://voca.ro/198C8E7FZH56

I cannot pinpoint where the rattle is coming from, sounds like in the engine.

Can anyone tell me where to look next to fix these issues?

 

Rattle on startup is usually piston slap, but that sounds more like something loose on the bike, exhaust baffle? Always start your bike in neutral, most have lock-outs to stop the starter working if the N light isn't lit. That N light circuit also feeds into the CDI and gives a better spark, to aid starting, on most Hondas. If you have a sooty plug, the air fuel mix is probably too rich on the main jet. As everything seems to have been "fiddled with" may be best to try a smaller main jet and see how you go.

 

Thank you for taking the time to read this post and respond!! Since writing the post, I had the petcock bolt come loose at the gas tank causing a very bad gas leak. Took the tank off, cleaned it out with seafoam, , reconnected everything and ran straight seafoam for a few minutes. The bike is already sounding better and starting stronger - but this means I may have to check every bolt that may have been tampered with.

I checked the exhaust baffle - it seems very tight. All parts of the exhaust seem very tight, sturdy. The sound is coming from the engine area opposite the exhaust side. It doesn't sound internal as the sound isn't muffled at all, but cannot pinpoint where it could be externally.

Good info on starting in N - it's just that N is hard to find on my bike so I was just being lazy I guess. Plus there is no wiring going to the N, R lights so I don't have a visual indicator. I will only start in N from now on.

It's my understanding that the choke being on gives more gas, so with the choke always OFF, the new giant air filter on, the air-box lid off, etc. is there any way to increase air / decrease fuel without rejetting?

 

On most Hindas the main jet needle in carb slide or CD piston can be moved so it allows less or more fuel for a given opening. Usually, there are several grooves in the top of the needle and you move the circlip, which holds it in place, into a different groove, lowering needle gives less fuel.

If the wire to the neutral light is disconnected, it may not be connected to the CDI either, so not improving the spark for starting. In Hondaland this wire is usually pale green or grey.