[Tracee Polcin]

I have a 02 CRF450R with a White Bros. R4 slip-on. I recently install the muffler and the bike has become harder to start. It would not fire kick starting, back fired a couple of times but refused to start. It would start by pushing it. Checked the plug to see if it was wet after it would not start and it was dry. Before I installed the pipe it was very easy starting. Too lean possibly?

[Asmith]

Wrong exhaust maybe???

The R4 is made for smaller bikes, not the 450. Who did the install??

[Tracee Polcin]

I did the install, I bought the pipe brand new in the box and factory packaging and it's for a 02 450R.

[Asmith]

I did the install, I bought the pipe brand new in the box and factory packaging and it's for a 02 450R.

And it definately says "R4" not "MX4"????

Strange...

[Tracee Polcin]

It's an R4 for sure, it's not made anymore from what I gather.

[Asmith]

You may need to open up the fuel screw a bit. try 1/8 of a turn counter clockwise.

What are your current jetting specs?

[Tracee Polcin]

Current setting are stock. I did turn the fuel screw in abit to see if that helped it to start, but it seemed to back fire more after that. So I guess I need a richer setting to get it to cold start?

[Asmith]

I would guess too lean. It might have more backpressure resulting in lower manifold vacuum. Push starting can over come this. Try opening the fuel screw and see if that allows more fuel to pull through.

[Tracee Polcin]

Thanks for all the help but I think my problem is no clearance on the intake valves.

[Asmith]

Thanks for all the help but I think my problem is no clearance on the intake valves.

That can do it as well.

What were the numbers when you checked them?

The left one is the one to go on the 450's.

[dirtbikermicah]

you might need to rejet the bike

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I did turn the fuel screw in abit to see if that helped it to start, but it seemed to back fire more after that.

That makes it leaner on a four stroke, but you've got it figured out anyway with the no clearance on the intakes discovery.
If a bike starts fine, then suddenly won't start the most common thing is a clogged pilot jet. A new pipe may require modified jetting, but not so much so that it's suddenly hard to start.
Besides a clogged pilot, if the bike in question is a modern, high performance,high compression, high rpm four stroke with some hours on it that's all of a sudden hard to start it can be just what you found, it needs a valve job. Doug

[Tracee Polcin]

Both intake valves faces were cupped very bad. Replaced them and bike fires on first kick everytime now. Thanks again for the input.

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What all did you replace, did you have the seats re-cut ? I hope. Doug