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Whats need for 300ex Rebuild?
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Posted by: Sjorge300EX

Okay, so i picked up this 2001 300ex in april this year. The quad was never modified it was stock when i got it. So far I have elka fronts, raptor rear, Supertrapp exhaust, K&N, Dynojet jet kit, renthals, and other misc. things. Since it is a 2001 and was never touched im going to get it new piston ect. As of now it is blowing black smoke from the pipe when i rev so i know it needs the piston and rings soon. My main question is i have never rebuilt a 4 stroke motor before and I need to know what I need.

Should I get a honda or wiseco piston and rings and gasket

And do I need new valves.

Anything else that I need let me know.

Oh and i will be getting some cosmetic upgrades like PRM Summit bumper and Orange Maier plastic.

Posted by: Cody_300ex

Black smoke means its running rich. Blue Is oil which means it needs new rings and a hone, a new pistion and overbore, valve seals, or a 3 angle valve job. If you still want to rebuild it I would get a .080 11:1 weisco piston. Rebuilding a 4-stroke isnt two hard. Just make sure you have a service manual and follow all the directions. I just tore my top end down yesterday due to two broke intake valves, and it only tookl about 15-20 minutes max. The hardest part is getting the circlips in and out.

Posted by: Sjorge300EX

11:1 can i still run pump gas. So all you are saying is that I need to get a piston kit and that is it or do I need valves as well. Like I said never did a four stoke before. Just list all the parts that I will need.

Posted by: Cody_300ex

Piston kit comes with the piston, rings, wristpin, & circlips. You'll also need

Some hondabond for the valve cover
Top End gasket kit
what ever bolt you break(I have not torn mine down and not broke at least one bolt)
spark plug

Posted by: blasterfury7474

Pm Hondardr4life. He rebuilt his and it runs good.

Posted by: Sjorge300EX

so i dont need to get new valves? Just the piston kit.

And what is Hondabond?

Posted by: kd5sfq

If you buy a service manual ( I like Clymer myself,) 99% of your questions will be answered. It not only tells you how to do it but also what tools you need and other supplies...

I don't do my own valve work at home. Mainly because I don't have the equipment. I suggest taking your head to an ATV or motorcycle shop for that. They can also port it if your so inclined.

Call around and get prices before you buy any parts or services. Get at least 3 prices for each item and watch out for hidden shipping charges.

If you do it right, that 300 engine will be bullet proof... If you don't it won't be long before you tear it apart to repair it again...

If you beef up the engine, you should add heavy duty clutch springs... If you really hop it up be prepaired to replace the clutch soon... After that they seem fine.

Doing your homework before starting a rebuild project can save you time, money and lots of headaches... Take time and do things right... Haste will lay your engine to waiste...

Posted by: Sjorge300EX

i have a service manual coming for C&D and I was just wondering what parts I need.

Posted by: bwamos

I doubt you need a new piston, unless you just want one.

Black is definatly rich.

What size main jet are you running. I ran a 128 main with a "full" exhaust and K&N filter and NO LID.

Posted by: Sjorge300EX

the spark plug is black and oily. I have a dynojet 116 main jet. But that is dynojet, so that is probaly around a 125-130.

Posted by: ZeroLogic

mabey hes starting to burn oil and its passing oil up threw the rings but black smoke means its rich white is some kind of water problem and blue means it burns oil

Posted by: Sjorge300EX

its a bluish color i will probaly just buy the parts to have them for when i need to replace it. What is the compression supposed to be on a 300ex? That will tell me if I need new rings.

Posted by: bwamos

You dont do a compression test on a 4-stroke.. you want to do a leakdown test. Similar tester, but it tests how fast the air leaks past the cylider/valves. That's what you want. The tester will give you guidelines.

Does it smoke more on startup, or high RPM after the engine's warmed up?

At startup = need new valve guides/seals.
At high rpm = rings.

Posted by: 250rider1

i did my 250 ex and it wasnt too hard if u have a manual. i just used a honda piston and kept it stock bore. it runs good now.

Posted by: Sjorge300EX

the only time that I can see it is when it is in nuetral and I rev the engine. I dont know if I said it but this will be my first 4 stroke rebuild. How do you do this "leak test"? When I pulled my plug today, it was soaked in oil.

Posted by: REBSR76

Running rich means adjust your carb. Usually no repairs needed. If the compression tester your using is of reasonable quality it will also do leakdown tests. Both tests will give you an idea of your engine's internal condition. If it is black smoke in nuetral, it means there is no load on your enigine thus not running in the most efficient manner. If this happens while in gear you may have an adjustment to make on your carb. Again a blackish/ oilly looking plug means your running rich. You may want to replace that plug. A weak spark can also cause black smoke and plug fouling (most likley not your issue). I advise you to run it a while a few laps around your local track or field in a lower gear to get your engine and exhaust nice and hot to burn out any old residue, remove the plug and examine afterward on the scene. If it is grayish your know it is a carb adjustment. Also keep in mind if your change the JET and not the filter or exhaust you will run rich with stock components. Also clean you filter.

Posted by: Sjorge300EX

wow! This is an old thread. I since rebuilt my motor twice. The last time occured just last week. Motor is running strong. Oh and it did need a rebuild.