03 400ex Not Starting - Page 2 - Honda TRX Forum
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post #11 of 22 Old 08-10-2018, 12:10 AM Thread Starter
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So i tore apart the top end on this 400ex and i am thinking the timing chain popped off. Should you be able to freely spin the timing chain?
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post #12 of 22 Old 08-10-2018, 01:19 PM
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No, the cam chain is driven by the crankshaft - there is a sprocket on the crankshaft that drives the chain - (working from memory here) the chain normally will not come off the crankshaft sprocket because the oil pump is too close to allow it to come off - if the chain is worn out, it may contact the top of the oil pump, wearing it enough to allow the chain to come off during service. You may just have to look down the cam chain well and manipulate the chain back onto the drive sprocket, or the chain could be worn to the point the lifter can no longer push the tensioner far enough to take up enough slack to keep it on the sprocket, or the lifter may not be functioning and taking up the slack, or there may be another undiscovered problem.

I worked on one that had a new cam chain installed by an out of state shop and had a lot of cam chain rattle. I found the "lifter" was fully extended (not normal for a new cam chain) - I removed the clutch cover, clutch, crank pinion gears and oil pump - found the anchor bolt for the tensioner was loose and about to fall out of the engine case. I tightened the bolt, retracted and installed the lifter, released the lifter and the chain again had proper tension.

The only way the chain will move without turning the crankshaft is if it's off the sprocket, the sprocket is broken or missing or the teeth are worn off the sprocket.
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post #13 of 22 Old 08-10-2018, 01:24 PM Thread Starter
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Im hoping the sprocket is good on the crank. I have the head off it already and i am going to pull the jug when i get home. I really dont want to pull the clutch but i am probably going to have to.
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post #14 of 22 Old 08-11-2018, 12:39 AM Thread Starter
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Now that i have the whole top end off how can i tell what it was bored out to? How can you tell if the piston was shaved?
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post #15 of 22 Old 08-11-2018, 01:24 PM
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Measure the diameter of the cylinder in mm - 85mm (397cc) is standard, 85.25mm (399.5cc) is first over bore and 85.5mm (402cc) is second over bore - anything over 85.5 requires the installation of aftermarket pistons like Wiseco, Namura, Pro-X or the like. Wiseco is available in 86,87,88 and 89mm.

86mm bore makes it a 407cc, 87 = 416, 88 = 426 and 89 will make it a 435cc (commonly referred to as the 440 kit) To bore to 89mm the cylinder has to be sleeved with a special heavy wall sleeve or the original sleeve will break in use.

If the piston has been shaved, there are indications of milling and there will be no markings on top of the piston. Before a piston is shaved it has numbers and sometimes an arrow indicating the exhaust side of the piston either stamped or engraved into it. You will have to remove the carbon to see the markings (if any) by bead blasting, buffing, or heating (burning the carbon off) - I usually bead blast or buff. Also, if there's anything left of the rings, oversize rings will have a number stamped on the top side of the ring near the end where the two ends of the ring meet when installed in the cylinder.
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post #16 of 22 Old 08-11-2018, 01:33 PM Thread Starter
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Piston and rings look good
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post #17 of 22 Old 08-11-2018, 02:10 PM
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Looks don't count - take the top ring off the piston and put it in the cylinder about 1/2" below the top surface. Use a caliper, tin can, pop bottle, old piston or whatever you can find that will fit the hole to push the ring to square it in the bore and measure the ring end gap. You can have a new, but incorrect size piston in an engine and it will not run. It has to be right.
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post #18 of 22 Old 08-11-2018, 02:13 PM Thread Starter
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I will check that and take pictures later
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post #19 of 22 Old 08-11-2018, 05:35 PM
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01 TRX400EX Not Starting

I just picked up an 01 400EX down the road and the starter spins along with the first two gears I can see behind the starter gear cover but haven't torn into it further yet. Hoping to get into it shortly here but was curious if there are any important "don't forget to do this" when I start on that side of the engine for a diagnosis? I have worked on/rebuilt snowmobiles my whole life but this is my first sport quad so looking for anything that might not be obvious pulling it apart for the first time.
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File Type: jpg TRX Starter Clutch.jpg (772.1 KB, 3 views)
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post #20 of 22 Old 08-11-2018, 05:40 PM Thread Starter
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My issue was first the starter clutch one way bearing and now the timing chain popped off. The first thing i would get is a gasket kit.
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