Looking for ideas on 350X big bore jetting

[atctim]

Hey guys, I just installed my Sprock / Barton Racing 400cc big bore motor in my 350X last night. Much to my delight it started right up once installed in the bike. Here is what I have run into. I realize that I have gone up 12.5% in the displacement department. Now, how does that translate to fuel / air into the motor? I run a Cobra head pipe with an Xcellerator shorty muffler (by DG) and previously with the 350cc motor I was jetted a hair lean with no air box lid. I have always had slight back firing during engine braking (deceleration). But now, if I rev it up while sitting idle, it backfires crazy big. It actually shoots a flame about a foot long out of the muffler when stabbing the throttle and letting off it.

I assume this is a lean condition, however isn't what I am seeing unburnt raw fuel? Thus leading to a rich condition? I'm confused which way to go - fatten her up or lean her out????? What causes backfiring?????

To anyone who runs this 400cc motor? Would I be better off just to throw a 400EX carb on it and be done with it????????

[ATC-Eric]

Garry and I threw a 400ex carb on his big bore a couple years ago, and absolutely hated it. Thing almost ended up in the trash. Come to think of it, I dont know what we did with it lol. We have never figured out why they are worshiped by the X guys around here. The excelerator pump on it just interferred to much with it from what I remember.

Yeah your lean for sure. What happens when you "stab" it? Just absoluetly try to kill it with fuel? Does it just rev right up, or fall on its face?

[atctim]

It revs right up - no hesitation or falling on it's face.

[ATC-Eric]

Yessir, fatten her up some and go from there. If your still playing with it later tonight, I can see whats in mine when I get home.

[keister]

Garry and I threw a 400ex carb on his big bore a couple years ago, and absolutely hated it. Thing almost ended up in the trash.

Not what I wanted to hear. I bought a brand new in box 400EX carb with the intention of mating it to my 400 kit. Crap!

[ATC-Eric]

Maybe we can get someone to chime in here and tell us about their 400ex carb experience.

Garry and I jetted, and jetted, and jetted, and just couldnt get the danged thing to GO! Way to sluggish on bottom no matter where we put it.

Stock carb has done everything Ive ever needed, and wanted that machine to do.

[atctim]

We are gonna try the 400EX carb. My 400 is bone stock except for exhaust - valve train is all stock, so maybe it will react better tot the stock 400EX carb. Will keep you updated.

[ATC-Eric]

Please keep me posted. We put it on a Super Trapped stage one hotcam big bore.

[Dirtcrasher]

Whats your plug look like Tim?

Have you tried putting the lid back on? If it's better, your lean.

Some guys are running the CRF250 carb; Honestly, I just dread the jetting. I tell ya, you change jets 6 times and you just want a Bud Light Lime

My 350X is plenty fast for me but I would like to send out another NOS head to be ported along with larger valves. Then I'm sure I will want the CRF carb.

[atctim]

Did't look at the plug yet - but that is obviously a great idea Dirstcrasher - I'll have to do that. This thing runs so good - such good throttle response. I tried putting the air box lid on - she falls on her face at 1/2 throttle. BTW - this bike wouldn't run past idle with the air box lid on before the motor transplant - so it is jetted to have the lid off. It's just that it back fires on decel - other than that the power is there from start to finish - the entre way! The jetting must be pretty close because of it running so good.

I have heard the CRF carbs are mint. Maybe I'll try one.

[Jason Hall]

Tim does the head pipe have any place that could be leaking??? Like around the head, or where the head pipe meets the muffler? If It does, It will backfire wildly. Once you are sure you have no exhaust leaks Anywhere, then the usual rule for backfiring Is - If It backfire's out the exhaust It's Rich, If It backfire's out the carb It's Lean. Lean backfire out the carb means the engine Is trying to burn the gas before It even reaches the engine, and out the exhaust means you have to much unburned fuel In the exhaust.

A 400EX carb Is Great on a 350X once you realize the X wants a HUGE shot from the accelerator pump. I know you would think different, but the 350X's Intake port make's all the difference because It's a low velocity port (This Get's hard to explain behind a key board). If you do Install a 400Ex carb, You need to open the Intake port and boot up to make the EX carb worth It. My suggestion Is to add alot of taper from the beginning of the port & boot (for the bigger carb) then blend It Into the port as It goes farther Into the head. I have learned over the years experimenting with FRC carbs on all kinds of different engine's that this Is the case lol. I will go this far to say that most every bike / atv engine that uses a FCR carb has a funnel (high velocity) port, and when using the FCR on an older low velocity engine you need A lot bigger shot from the accelerator pump to keep the engine from stumbling because It takes time for the slide (And throttle flap on the 400Ex carb) to open and get the gas pulled Into the engine. Hope this help's, and does'nt add confusion lol. I keep refering to the FCR because It helps you to understand the difference In the Intake track of the modern 4 stroke. From my experience The EX carb act's the same on the 350X for some reason.

[beets442]

+1 plug.How far is your airscrew out? Does it decel smoothly besides backfire? I've had some backfire in rich condions too.
No intake or exhaust leaks?

[atctim]

First off - thank you Jason - you explained alot to me. I do have leaky head pipes at the head (new crush rings are ordered, but my old ones are in there now - this might be where my backfiring is all coming from - never thought about that. I need to pull the plug and look at it - I did not do that yet. It does idle down normally after stabbing the throttle. I can say that this bike always backfired on deceleration while it had the old 350cc motor in it as well. Once I get the pipes sealed up at the head, and if my plugs look good - will a bit of backfiring on decel hurt anything???? I guess the plug will really tell the story.

I really don't want to do any head work on this bike - so that rules out the 400EX / FCR carb.

[CRAZY70MAN]

First off - thank you Jason - you explained alot to me. I do have leaky head pipes at the head (new crush rings are ordered, but my old ones are in there now - this might be where my backfiring is all coming from - never thought about that. I need to pull the plug and look at it - I did not do that yet. It does idle down normally after stabbing the throttle. I can say that this bike always backfired on deceleration while it had the old 350cc motor in it as well. Once I get the pipes sealed up at the head, and if my plugs look good - will a bit of backfiring on decel hurt anything???? I guess the plug will really tell the story.

I really don't want to do any head work on this bike - so that rules out the 400EX / FCR carb.

x2 on the exhaust leak Tim. Lets's not forget the 2 ft. flame I shot out of my 350 at Jamestown indoor ice race after the studs chewed through the header on a crowd wheelie! lol.... The smallest leak on mine anywhere and it will do the same. Hopefully it is as simple as that.

[atctim]

OK - all exhaust leaks fixed. That fixed my back fire problem. now - I'm gonna run it another 15 minutes or so and see how the plug looks. With exhaust leaks - it was running terribly lean. Plug was pretty much on the white side.

I'll report back with my findings.