400ex carb on 300ex

[beaubs]

here's what I have
over bore
high comp piston
over size valves
ported polished
cam
shaved head
pipe
rev box
you know all the go fast stuff.

Well it's now time to feed the beast.
Has anyone done this before?
I'm looking for more than "jet it and ride"
My motor tells me I need to run 110 or better, but I'm curious as to will the stock 400ex jetting even be close?

[bwamos]

What's your bore and compression?

I have a 80mm piston (330cc) w/ 11:1 compression. Similar setup, (oversized valves, cam, sparks pipe, port & polish, etc.. ) but I am also running no airbox lid.

I'm running a 165 main & 42 pilot in the 400ex carb. Should be a good starting point for you.

[beaubs]

so you think I'm too lean at 148?

[bwamos]

dunno. What's your bore (displacement?). Stock displacement you may be close. Big bore, you're too lean.

[beaubs]

bore is 74.25 11:1 comp piston
compression is 220-230
the 148 doesn't allow me to throdle up, it sneezes at me and dies.
I'm dropping it to 138 tonight and we'll see what that does.

[bwamos]

If it's dying on you at around half throttle, then you need to adjsut the needle.
When I was doing mine I got jets in 5 increments. 145 150 160 etc.. got them 3 at a time so I could take them back to the dealer and swap out for the next three until I got it close, then I'd hit narrow up the selection to fine tune it.

That's one thing about the 400ex carb on the 300ex. It's really picky. Either it runs great, or horrible.. lol. Kinda makes it easier to tune, lol.

[beaubs]

yeah, that's how the 300 carb was, I had a small wind that boy when you were in it you really new you were in it.
I wish I could get to half throttle, I think I'm dieing at the squirt, thanks for your help.

[Honda250XRacer]

hey I have a 400ex carb forsale , It was set up for my motor which was .80 12:1 compression piston hmf with white brothers header oversized valves and a big cam , it worked perfect no throttle lag , bogging, cuttin out an such.. had an awesome response , I actually put it on before I did all my motor work and all I had was just the pipe. but let me know if you interested

thanks, Trent

[beaubs]

Thanks but I'm good. I bought one last week, settled in on adding ten to my 300 jet size now i'm just fiddling with the needle. Although we did run it this weekend and holy @#$% did she wake up. If losing a hole shot by a foot to a 660 is any indication, I think i'm good...

Thanks for all your help

[Bolinracing]

Hey guys,
I am new to the 300's and have a question. What is involved in putting a 400 carb on a 300? Are there any modifications or does it just fit right in. Thanks for any help you can give.

[bwamos]

Fits right up with some fiddling. Basically the same carb just larger.

Put the 300ex throttle cable on the carb.

Heat up the intake boot and stretch it over the carb flange. It takes some patients.. lol.
Kinda like trying to put on a tupperware lid that's 1 size too small.. lol.

Rejet, and ride.

When working on the carb I leave the intake boot (between the carb and airbox) on the carb and disconnect it from the airbox. Makes life much easier. ;)

[beaubs]

yeah, just like putting girdle on a fat lady, sorry to any of you fat lady’s

Yeah the cable on the 300 will work great.
I took the carb boot off first just to see how tight it really was, she's tight.

I've found with the carb boot on the bike,
With the bike in gear
Place the bottom of the carb in the carb boot first
This way you can see the boot
the non aggresively push the carb in, it gets easier the longer it's in the boot too.

I tell you what, that carb done did wonders for my 300.
Now I'n thinking of trying the 300 pumper carb on my blaster

[Honda250XRacer]

ttt

[Honda250XRacer]

ttt

[jeffdmason]

i have a 300ex that i am working on modding and so far the only things i have done to the motor is added a new hmf slip on exaust which for a while went well until i really tried riding it hard and it would die around half throttle it would just have nothing left at half. i looked into it and at first thought it was my rev box or perhaps i needed to have it jetted but it is very unusual sometimes running better or sometimes running worse and dieing right from the start, please someone give me some advice!!!

thanks