300ex

[RichM1983]

I have asked a ton of questions about this thing for the past 2 years almost. I cannot get it to run right and I'm really getting annoyed. Heres a list of what I have done.

--Its a 300 motor, on the last bore, 12:1 piston, something like 230 psi of compression after break in. I run it with premium gas because I was told that was good enough to run great.

--Complete White Brothers exhaust with the end opened

-- New filter with the lid still on because I ride a lot of mud

--Stock 250x carb with a 125 main and stock pilot, dont know the needle.

Heres the problem for one. The starter just doesnt seem to work good. The battery is brand new and charged. Also the starting solenoid is getting ready to go because all it does is tick when hitting the start button. Could this be because I had to wire it up and bypass that fuse?

THE IMPORTANT PROBLEM
The motor ran fine during break in. Plug would get pretty black though. One day my brother road it up the road and the screw came out that pulled the slide open. He limped it home and it took me while to figure out the problem. Once I fixed that the motor doesn't run right. It cuts out on top really bad. If you hold it wide open it will cut out once it revs and if you hold it wide open it will rev back up and cut out again. This cycle continues if you hold it wide open. It does this when its warm or cold. The bottom end is good as long as you dont rev it. Could I have an electrical problem? Jetted to rich? Take off the air box lid? Or is the compression so high that its blowing out the spark because of the low octane fuel. I have heard that this is possible (I think).

I need help and I'm sorry about the long post but this thing has been down forever and I'm gonna just sell it as is if I cant get it to run.

[fmftrx300ex]

to rich to lean i have a 300ex with no engine mods and im runing a 128...i have my lid off tho.. whats it boared to stock rev limiter....?

[RichM1983]

Originally posted by fmftrx300ex
to rich to lean i have a 300ex with no engine mods and im runing a 128...i have my lid off tho.. whats it boared to stock rev limiter....?

I was wrong the piston is a 76mm 12:1 250x piston. I don't think its to lean. the plug is black. I had to clean the carbon off of it. I am not good with 4-strokes but on a 2-stroke that means it to rich. And as far as the stock rev limiter yes it does have the stock one. BUT thats not the problem unless it can go bad and mess up the revs. It will cut out when your riding it and its nowhere near the top of the revs. When you ride it it goes bad after half throttle.

[RichM1983]

Does anyone else have any ideas on what I might try next. Thanks.

[honda300EXtreme]

ok, first, the clicking you hear when pressing the starter is one of two things,

1. its your solenoid
2. your contact brushes are going bad in your starter


as far as the cutting out on top end, that indicates a problem with your main jet, now if the core/ electrode is black then your too rich,, if its white your too lean, you want a dark tan color,
but personally a 125 main sounds way too lean to me, id at least go to a 128k main jet,

and then work your way from there,


also remember to adjust your valves, its critical during your break-in period


;)

[TRXRacer1]

Originally posted by RichM1983
I was wrong the piston is a 76mm 12:1 250x piston. I don't think its to lean. the plug is black. I had to clean the carbon off of it. I am not good with 4-strokes but on a 2-stroke that means it to rich. And as far as the stock rev limiter yes it does have the stock one. BUT thats not the problem unless it can go bad and mess up the revs. It will cut out when your riding it and its nowhere near the top of the revs. When you ride it it goes bad after half throttle. Did you put that same plug back in? If so get it out and get a new one. This isn't a lawn mower. Try gapping the new plug on the tight side of it's tolerances too (refer to service manual).

12:1 is a bit high for an air cooled engine, especially a 300ex. If you had a liquid cooled bike it would be a different story but you were told wrong, 93 octain in your case is not adequate. You'll want at least 105. Also anything over 11:1 can be too much for the stock starter. This is a big reason why 11:1 is common for the 300ex (electric start) and 12:1 for the 250x (kick start).

You don't really ever want to hold a throttle wide open when not riding. You might be bouncing off the rev limiter. I agree that having a stock CDI shouldn't be an issue here either.

My recommendation here is a rare one from me. I truly think you need to get a new plug, some proper fuel and shell out a couple bucks for some dyno tuning. Your problems sound fuel related. I'd also ditch the restrictive 250x carb and go with a 400ex carb before you get in to tuning.

Now if you don't want to do that you need to get back to us with a couple things.

1) Where is the needle clipped?

2) How many turns out is your air/fuel mix screw?

3) What pilot jet do you have?

4) Double check that Main.

[RichM1983]

Main jet is fine and is a 125. That was already double checked

The pilot is stock....cant remember what the number is though.


And as far as my information about the octane I was told by people who run 300ex's just like mine, same octane and compression.


Ill have to check the pilot and the air screw.

Thanks for the imput.

[TRXRacer1]

Originally posted by RichM1983
Main jet is fine and is a 125. That was already double checked

The pilot is stock....cant remember what the number is though.


And as far as my information about the octane I was told by people who run 300ex's just like mine, same octane and compression.


Ill have to check the pilot and the air screw.

Thanks for the imput. Rich if you still think the fuel and main are fine then I'm going to revert back to my suggestion of paying for some time on the dyno to get it tuned by some pros.

[bwamos]

I'm sorry but I beleive your main is way too lean. (Lord knows I've been wrong before though, lol)

A stock bore 8.9:1 compression w/ a full exhaust and airbox lid on should be running about a 128 main.

You have a 313cc 12:1 compression.

My 330 w/ full exhaust and airbox lid off and 300ex carb was running a 142-145 main.

I'm guessing you need to be running about 132.

Yes your plug is black.. but you said your motor will not run at full throttle consistantly so you can not use that as an indicator. All that is saying is that your rich on the bottom half where you are getting a consistent run on it.

12:1 may or may not be too much compression, but you'll find that out later once you get it jetted correctly. With the airbox restriction your dynamic compression probably won't be as high as it would w/ the airbox lid off.

I'd say just toss in a 135 and see how it runs. (will require you to probably drop the needle 1 slot too, but that's easy to tell.)

An a/f dyno run around here runs about $200-$250. Well worth it if your machine is giving you a headache.

[RichM1983]

Thanks a lot for the replies guys. Theres no way Im going to a dyno. I dont even know where ones at and besides that this thing is only worth about $600. No way Im spending a bunch to take it to a dyno.

Thanks again everyone. Ill try that fatter jet and see what happens.