yfz450 hard start when warm

[darren250r]

I'm working on a friend's yfz450 that he recently got from a guy. I'm whiz when it comes to 250r's but these are new to me. He told me its been hard to start lately. The guy he bought it from didn't tell him/know what was done to the motor. It has a foam filter, no lid, full exhaust and esr stickers. I suspect something has been done to the motor whether its high compression piston, cam, porting or carb I'm not sure.

I changed the plug (its was pretty black), cleaned/oiled air filter and charged battery. It starts fine with the choke when its cold. I can turn the choke off right after it starts and it seems to run fine. Once it warms up a bit it sounds a little loaded up and if it dies it won't start back up with or with out the choke. I suspect the jetting is rich because of the spark plug color and the fact that when I rev it up, it backfires on deceleration of the motor.

I know its not much information to work with, but if anyone has a similar setup that has had a similar problem let me know. He said it never used to be hard to start and he hasn't changed anything. The plug I took out was a racing plug with the tiny electrode, but I replaced it with a stock ngk.

Sea level, 60-80 degrees.

[Sweendogie]

what year is it? you might want to check your timing, my 07 had the cam mod done to it and it was hard to start once it warmed up... i took it back to stock timing and the thing fired right up no problem everytime... while you have it open check out the valve clearance too....

[darren250r]

Thanks for the reply. I'm not sure what year it is. It's all powder coated and no stock stickers are on the frame. Are you talking about the cam timing? I don't think I'm gonna do anything more extensive than jetting it for him. I don't have the time to take the motor apart at all. Would valves out of adjustment cause the plug to be black?

I'm not sure it would be something like timing unless it can come out of adjustment. He hasn't changed anything on it to make this problem start.

[Toadz400]

I would check out the jetting and make sure the carb is clean. Start out with the easy things. Sounds like it's rich.

If your friends YFZ was hard to start after the cam mod then something was done wrong. Shouldn't be any harder starting after the cam mod as long as it's jetted correctly.

[Ride1Rob]

Your friend needs to call this guy or go by his house and ask him what's been done to the bike. He's in for a nightmare on trying to figure things out and replacing parts if he doesn't. Does seem like it's dumping too much fuel in the carb.