300 ex ignition or fuel issue

[dualincome]

I have just purchased a race modified 2003 honda ex and have some engine issues. When you rev it up past 1/4 throttle it missfires. Sounds like a rev limiter cutting in. I am new to this and need some help. the quad has a 12/1 piston and updated cam, plus air box top is off. Is this a jetting issue or a rev limiter problem

[aDviSol2y]

Are you running race fuel, 110 octane? with that compression you need to. Its not a rev limiter issue if it starts at 1/4 throttle. Do you have a k&n with outerwears?

[dualincome]

no when I purchased it I was told it only needed 97 octane

[aDviSol2y]

You should try running 110. I also updated my first post, you were to quick for me. :)

[dualincome]

it has a just a standard foam air filter with the air box top off I am going to check jetting size tonight I live in sault canada and the bike is from southern wisconsin there is a bit of a temp change the bike has not been run since last summer

[aDviSol2y]

If its been siiting for that long, you should clean the carb. Jetting condtions probably wont effect it to drastically. I've gone from sea level at 80 deg, to 4000 ft at 35 deg, and didn't re-jet. Most likely, your carb is dirty, and you need 110 octane. I know with a 400ex, anything 12:1 and up you need race fuel. Pretty sure it would be the same with a 300ex. The person you bought it from might have been filling you full of crap because to the average rider, requiring 110 is a turn off.

[dualincome]

I did do a quick clean of the carb and it improved however it is still not right. running the 110 is not an issue I just want to get it right I purchased a 140 hex head main jet just in case do they use hex head jets or the slotted head jets was reading on another site that the issue could be with the wires coming out of the engine on the right side other sitre said to remove ,clean and reinstall them

[aDviSol2y]

Honestly, I'm not sure what the jetting is on the 300ex. And I'm not sure about the wires either. Sorry.

[dualincome]

I have changed over to running 110 fuel and it did improve slightly however still having issues in 1st through third gear bike begins to missfire right away in 4th or 5th you can get to about 3/4 throttle till it does it , the carb and fule tank has been clean, checked electrical connection, could it be a coil issue:

[aDviSol2y]

Not sure about the coil. Electrical is not my strong suit. It may be a timing issue. But I think you are on the right track. You have eliminated the fuel as the problem. Does the bike have an after market pipe? If so, try taking off the air filter and running it. Don't run for very long at all, and don't run it in the dust. Just want to see if breathing is the issue.
Your bike needs 3 things to run. Fuel, air, and spark. I always eliminate fuel and air first.

[dualincome]

bike as hmf exhaust system tried to run it with air filter off same issue

[aDviSol2y]

Do you have a service manual for the bike? If not, get one. You should check the timing. Also change your spark plug if you haven't already. As far as electrical, in my experience, when I had an electrical issue, the bike did not run at all. If you check the timing and everything looks good. Then I would go ahead and try re-jetting and adjusting the needle and A/F screw.

[fmftrx300ex]

start ur quad let it idel and warm up next kill it take ur air filter off and then cover the hole (airintake tube half way with ur hand rev it see wht happens and try more and less of the hole covered if it runs best 3/4 closed off by ur hand ur reallly lean, also replace ur spark plug and ajust ur valves easy stuff when u have a manual good luck lett me know as i too have a 300ex and have figured out a fair share of issues

[honda250xrider]

what size jet are you running? along with how far down on is the clip on the needle?

[honda250xrider]

to give you a round about on my 250x i used a 155 main, with the clip 4th down. This was with exhaust, cam, lid off, 12:1, port work. I also used a 38 or 40 pilot jet with 2 and a half turns out.