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yamasaki

2003 yz450f valve/starting questions

4 posts in this topic

My bike has been getting progressively harder to start every weekend i've been riding it. When its cold, seems like it gets flooded, fouls the plug, and will refuse to start. This is with myself, and 3 or 4 other knowledgeable, experienced 4 stroke owners trying different starting techniques on the bike.

Once we get the bike fired in the morning (which recently has involved towing the bike behind the truck in 3rd gear for about 1/4 mile), it will run fine all day long, start on the 1st kick every moto, even after sitting 30-45 minutes. I even purchased a new plug for the bike and it worked fine the first day, but the next weekend (last weekend), we went through the same starting problems at the track.

as i just purchased the bike recently, and dont know much about its maintenance history other than it looks like it was VERY well cared for, cosmetically at least, i'd like to check the valve clearances before i delve into jetting and other minor "could-be's".

is it possible to check the valve clearances with the motor in the frame, or do i have to pull it?

anyone have any specifc recommendations for the yz450f i should keep in mind and check for?

p.s. the bike is in excellent condition, i got a GOOD deal on it from a fellow TT'er, and i'm not having buyer's remorse in any way, shape, or form. i just wanna solve this starting issue once and for all... i'm convinced there's something wrong somewhere.. not just my starting technique..

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yes you can check the valve clearances in the frame. Perform a leakdown test, or if you don't have access to a leakdown tester a compression tester will suffice. Sounds like compression is low, with the leakage test you can determine what is letting your compression out. Leaky valves, piston rings head gasket . It could just need a valve adjustment but if they pull enough to have zero clearance and hold the valves open making unit hard to start they are usually in need of a rebuild.

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but if they pull enough to have zero clearance and hold the valves open making unit hard to start they are usually in need of a rebuild.

rebuild, meaning new valves, piston, and rings?

also, what do you mean by "if they pull enough to have zero clearance and hold the valves open"

just trying to understand the 4 stroke better..

thanks.

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that's where the leakdown test is nice it will tell you what's leaking so you can target that in your rebuild. However everything else listed should be inspected while the top end is apart. When the valve face wears the valve "pulls" deeper into the seat reducing valve clearance.

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