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Discussion Starter · #1 ·
Got some questions about my 400 here. First off, it’s bored to a 440, has stage 2 hotcam, jetted, air filter mods, and white bros e series. A few other small things but that’s just about the majority of the serious things. I had to do a one way bearing for the starter, or as some call it a starter clutch, so she sat for a while as I waited for parts (about 3 weeks). Now, I put the one way bearing in, sealed everything up, and she won’t fire. So, I take her apart. Everything seems fine valve wise, none are stuck or anything, hasn’t jumped timing, but I get to the piston and it’s covered in oil and grime like dirt. Physical chunks of really oily grimey substance. So, I clean everything off and throw her back together, timing seems good, but I notice that I can’t get it exactly perfect. With the tensioner hand tight, flywheel perfectly at top dead, the can gear shows a bit ahead, so, I take the chain and bring it one ahead, but now it’s too far the opposite way at top dead. It’s like I’m half a tooth out. Now, I don’t think it would cause it not to start, but it’s a concern because it’s never been like that, and this spring I did valves, chain, and top sprocket, so it should be good. She ran great before, and won’t even fire off of ether down the cylinder. I took it apart and rebuilt it 3x, still to no prevail. She’s getting spark, I’m feeding her fuel and air, so my main concern is the timing just being a tad out. I readjusted the valves 2x, messed with the chain countless times back and forth, removed flywheel again and retorqued and redid the key, and she seems like she has compression. What are some thoughts? sorry for the stupid long question, but I just want to give as much detail so I’m not getting 9 year olds telling me to “clean my carb” or “try 2 taps of the throttle when you crank works perfect on my 1976 go cart”. Thanks guys!
 

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Well let's see; being 1/2 tooth off on the timing is semi normal - as the timing chain wears (it does not stretch, but wear will effectively make it longer) so the can comes to TDC a little after the piston does.ened

"With the tensioner hand tight......." The tensioner is spring loaded, how did you make it hand tight? If you have installed a manual tensioner, it is easily over tightened and causes the cam chain and slipper to wear prematurely.

So back to the basics: if it has adequate compression, spark, spark at the right time, gets sufficient air with the correct amount of fuel it will run. The law of physics governs it, so........ what is missing or not right? Numbers count here; how much compression does it have at normal cranking speed with the throttle held wide open? How many degrees before TDC does the spark occur? Important information; what grade and brand of fuel are you using? Is it non-ethanol or alcohol contaminated. NOTE: alcohol blended fuel will absorb water from the atmosphere and the water/alcohol mixture will settle to the bottom of the fuel tank and carburetor. The engine will not fire on water and alcohol, but it would still fire on starting fluid, so the problem is probably compression or spark related.
 

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Discussion Starter · #3 ·
As for chain tensioner, yes it’s aftermarket. I tighten it until i barely feel the chain, then set the lock nut. When it comes to the compression, I don’t have a trustworthy compression tester on hand, I got a cheap harbor freight Pittsburg compression tester, but it’s proven to be wrong pretty consistently. I’ll put my hand on the spark plug hole or exhaust and it feels very strong when I turn it over, like to the point where I can’t keep my hand flat over either surface, so I don’t think compression is the issue, it’s got enough to at least fire off. As for spark, how would I go about checking if it’s perfectly on time? It’s getting spark, but maybe a bit late? It’s backfiring a lot, through the carb too, which makes me think timing of some sort is off, so my main guess right now is electrical and spark. What do you suggest?
 

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Backfiring through the carb is classic for a lean condition in the carb - backfiring out the exhaust (when trying to start is classic for a late spark - 30 psi of compression will blow your finger off the spark plug hole, but it takes 100 psi minimum to make the engine start and run fairly well. Average TRX400EX with a std. bore and compression is between 120 and 170.
 

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Discussion Starter · #5 ·
Ok, I’ll get a compression tester and do a test as soon as I can. In the meantime, what can I do to retard the spark if that’s what it needs? New coil? Cdi box? And it’s sorta sputtering through both, not really backfiring through just the carb. And I know the carb isn’t too lean because it ran great with the same jetting and settings for the past year I’ve had it at this setting.
 
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