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440
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11/28/2010
Location
Frisco, TX, USA
bish153
Edited Date/Time
12/13/2014 9:29am
I have a question for the people on here that know, can diagnose and rebuild a 4 stroke motor. For the better part of my life I have raced and worked on 2 stroke motors, and I take pride in knowing the motor in and out. I felt like once I had a good understanding of the motor, it made me a much better rider. Anyhow I own 4 strokes now also and I just thought about this, but other than standard maintenance and preventive maintenance, I don't know much else. As far as tear one down and rebuilding, I couldn't do it. So my next project is to find a blown 4 stroke motor and rebuild it, for know other reason than just learning how. My question to the techs on here, what are some books, on line sites or places I can find some very in depth info to help. Thanks in advance for any advice.
The head, valves, cams, and cam timing are some of the most critical parts of a 4 stroke. Piston & Rings don't wear out as fast as a 2 stroke, so most of your attention should go into keeping the valve train working properly.
The Shop
Luxon 4-Post Bar Mounts
$189.95 - $239.95
DeCal Works Huge Plastic Inventory of UFO and Polisport kits.
30 years experience hear with too many completed to mention. This may sound crazy but I learned on an engine that worked fine. Took it completely apart and put it back together. This way it only cost a new gasket kit. 79 TT 500 was my first 4 banger tear down.
If you have experience rebuilding a 2 stroke, you are in front of where I was. I bought a 2006 KX250F last Feb, with a major bottom end knock, with the same intention as yourself. As a project.
After watching a 4 part series of videos by a fellow rebuilding a CRF250 motor, and obtaining the manual for the KX, I gave it a go.
I took my time, bought quality parts and the bike now runs great.
Best of luck. Go for it I say.
Below is the link to the videos I got a lot of info from.
Cheers
Steve
https://www.youtube.com/watch?v=RLnmTRyqMIA&list=UUy4-4AFgc21Z9QKdGSOD9…
Swapper, no offense i was just curious.
first lesson
motors are electric
engines are what we have in our scooters.
take lots of photo's and make some notes, you will do fine.
I've done several tear downs on CRF's. The only time I need a manual now is to look up torque specs. Good luck and have fun with it.
You probably know someone that services/tunes/rebuilds heads & engines. Ask them about brands and type of replacement parts...
Sometimes you use a head, valve, spring, guide or bucket from another motor or a cam from a different year or brand...I know that sounds weird; but, I've learned these things from some of the best builders out there.
How & Who machines things REALLY MATTERS...
Seats
Guides
Ports
Valve Surface, Size, Material, Coating, Surface Finish, Brand, Year
Cylinder Parallelism & Surface Quality
Head Sealing Surface Flatness
Gasket Material/Type, SIZE, Brand, Year
The Little Things can add up to a lot of performance and longevity on a four stroke.
Good luck & make it fun.
Pit Row
Between youtube and resources like this (and it's not like it's the motor out of your weekly racer), you have nothing to lose!
What's the suggested piston replacement schedule on a modern 4 stroke?
The skirt is so short that I'm thinking it can't be much longer than a 2 stroke piston.
Here's mine at 120 hours (I ride my bike like a grandpa, so I probably got away with keeping it that lon)
Manny hit it dead on. It is all about the details in getting good performance. The reality is that not too many people have the proper tools to get the valve seats back if they are worn or even check to see if they are off a tiny bit. Anyone can lap a new valve into a poor seat and get it to seal but very few could get them 100% correct.
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