timing

[tide2085]

i think my timing is off?? what would it be doing if the timing is off?? it will never start and when it does start it backfires and bogs down and has very little power and dies after letting the rpms down.

[honda400ex2003]

http://www.atvriders.com/articles/valveadjustment400ex.html

that is a good way to check it out. it will run pretty bad if they are way out of adjustment. I would say that could be your first thing to do on the list if it was running good at one time with nothing changed. implying that no jetting changes or performance mods or anything were added and it started acting up after this install or change. another thing to check would be the coil, spark plug, and spark plug wire to make sure all are in good working order. steve

[NacsMXer]

tide2085, those are all symptoms of retarded timing.

I just went through this after my winter rebuild. I took my HRC cam to a local machine shop to have the sprocket flange pressed off/on, so I could remove the decompression mechanism from the cam. Well I installed it and triple checked to make sure the timing was perfect before reassembling.

I go to start it and it "hits" for a second like it wants to start but no go. I try to start again with my thumb lightly on the gas and it hits again but lets out a sharp backfire out the carb and dies. It kept doing this over and over every time I tried to start it.

I racked my brain for weeks going crazy, taking the carb apart a million times, rejetting, re-checking the timing, checking for intake leaks etc, but no improvement.

Then one time I was getting really frustrated and sort of forced it alive by thumbing the crap out of the gas while holding the starter. The second I would let off the throttle at any given time it would die. It sounded/revved weird and was extremely down on power. It would also shoot black peppery looking deposits out the exhaust. It also had a nasty hanging idle where the rpm's sort of surged strangely back down after you revved it. Also would backfire sharply out the intake and die at random. I was totally dumbfounded why it was running like this at the time.

Well I took that cam back to a different shop this time. Turns out the idiot at the first shop pressed the cam sprocket one spline off (retarded) when he put it together. It only goes on one way since there is a single wide spline on the cam, what a fool! I made a point to show him this before. He actually forced the thing on so hard that it cut its own spline in the wide spline of the cam as it went back on.
:eek2:

So the new shop pressed the flange on/off correctly this time. I installed the cam again, and boom, fired right up like nothing. All along my timing marks were perfect, but the actual cam at the sprocket flange was retarded. Totally bogus situation.

What changes did you make since your quad started running badly?

[honda400ex2003]

very good info nacs! thank you, learned something new from that info! steve

[tide2085]

I havent done any changes recently.. but about a month ago i rebuilt my top end and put a hotcams stage 2 in and it ran fine until the other day. im wondering if maybe the cam chain jumped or maybe even the cam jumped? It is doing exactly what yours was doing by the sounds of it. so tomorrow im going to take the valve cover off and put it to top dead center and see if the lines on the cam sprocket are lined up right and just check everything out.