Need jetting change with new header?

02 yz426f

My carb is setup as follows:

165 main

45 pilot

Needle in stock position

Bike currently has FMF powercore 4 on it, and I intend to replace the stock header with the FMF power bomb. Will the main jet need a changed to accommodate the header or will it most likely be ok?

More than likely will change nothing.  But you can tell by how it runs.

More than likely will change nothing. But you can tell by how it runs.

Ok.

Do you believe I will gain anything by hanging the header? I have always heard that having a dent in the header is a real power hindrance because the tube is so small, my stock head does have a dent in it, so that's why I'm planning on changing it.

Depends on the dent; a lot of factors involved.  The main advantage of the PowerBomb is a little wider, flatter torque curve, not more power.

obviously an extremely different type of engine, but dents are nowhere near as important as some people think.

obviously an extremely different type of engine, but dents are nowhere near as important as some people think.

I was always under the impression this comparison was apples to oranges because the cylinder in a dirtbike is much smaller, and so is the tube, so even the smallest dent in the pipe will choke the engine. Wouldn't be the first time I've been mislead though.

Alright guys, while we are on the topic of jetting, I've been battling this hesitation in my bike for a while now. I thought I had it figured out when I put in a bigger pilot, but it still seems to do it. It's like a hesitation if you try to snap the throttle while lugging through a trail or something similar. It's not really a bog , but a hesitation, kinda hard to explain. Anyone have any ideas, I'd try to get video while riding, but I'm afraid it wouldn't be noticeable in a video

It seemed to start doing it after a top end rebuild. Not sure it's correlated or not. It also seems to be worse when the engine is cooler, but that could be my imagination as well.

Edited by 0bigsilver6

Alright guys, while we are on the topic of jetting, I've been battling this hesitation in my bike for a while now. I thought I had it figured out when I put in a bigger pilot, but it still seems to do it. It's like a hesitation if you try to snap the throttle while lugging through a trail or something similar. It's not really a bog , but a hesitation, kinda hard to explain. Anyone have any ideas, I'd try to get video while riding, but I'm afraid it wouldn't be noticeable in a video

It seemed to start doing it after a top end rebuild. Not sure it's correlated or not. It also seems to be worse when the engine is cooler, but that could be my imagination as well.

 

This is not a chat room, so you do not piggy back on other peoples threads......

 

Pilot jet is for idle circuit only.

Read this and do this to fix quick throttle bog issues:

 

You have to:

 

change leak to 50

changet he apump linkage spring to a stiffer version  http://tokyomods.com...arburetion.html

replace the pump diagragm with a 2008 version/upgrade kit  http://www.crfsonly....roducts_id/3932

verify the squirt quality and pressure visually

verify the squirt never hits the slide, and adjust the timing with the timing screw under the throttle wheel cover

 

Read this for more details and basic maintenence and tuning for the FCR:

 

http://www.thumperta...p#entry11143305

 

http://www.dirtrider...r_carb_rebuild/

This is not a chat room, so you do not piggy back on other peoples threads......

Pilot jet is for idle circuit only.

Read this and do this to fix quick throttle bog issues:

You have to:

change leak to 50

changet he apump linkage spring to a stiffer version http://tokyomods.com...arburetion.html

replace the pump diagragm with a 2008 version/upgrade kit http://www.crfsonly....roducts_id/3932

verify the squirt quality and pressure visually

verify the squirt never hits the slide, and adjust the timing with the timing screw under the throttle wheel cover

Read this for more details and basic maintenence and tuning for the FCR:

http://www.thumperta...p#entry11143305

http://www.dirtrider...r_carb_rebuild/

First of all, this was my post. So thanks for that. Second there is no leak jet on my bike. Third I already upgraded the accelerator pump diaphragm and the linkage spring.

First of all, this was my post. So thanks for that. Second there is no leak jet on my bike. Third I already upgraded the accelerator pump diaphragm and the linkage spring.

 

then best of luck

then best of luck

Would you think I need to go back down to the stock pilot size? Could that help?

That second link seems to have something odd going on. http://motoman393.thumpertalk.com/tech/carb.html hopefully this one will work. Mine seems to run well but when I end up in the carb I should take a look at if it's been done.

This may be a silly question, but how do you go about timing the pump spray and setting the duration without completely flooding the engine?

Also, could having the larger pilot make it more difficult to start when cold? It's not really all that hard, but it takes 4-5 kicks when cold. Usually 1-2 when hot. I guess I just need to get everything setup properly with the bk mod and the squirt timing and jetting and see how it is.

Edited by 0bigsilver6

You never need to change jetting with exhaust upgrades. Anyone who says different does not know how a carb works.

Opening up your air box or a less restrictive air intake is another story and is only affected under wide open throttle and high rpm's

Idle circuits are unaffected

Altitude changes are the only occasions jetting of both will need to be changed and if your jetting is spot on for your altitude it likely won't need to be changed unless your altitude changes by 5,000 feet

Otherwise you'll be "close enough" unless you race and are looking for optimal performance

I was always under the impression this comparison was apples to oranges because the cylinder in a dirtbike is much smaller, and so is the tube, so even the smallest dent in the pipe will choke the engine. Wouldn't be the first time I've been mislead though.

 

The bore on that engine is more than likely 4" (101.6 mm).  The bore on a YZ450 is 95 mm.  Not much different, and as the header diameter on small block Chevrolets is typically 5/8", that's pretty close, too.  In any case, the smaller per cylinder displacement means that there is less exhaust coming from the YZ450 than from any single cylinder of the mouse motor, so the header tubing doesn't need to be as big, anyway.  It's a lot less a fruit salad comparison than you think.

 

 

You never need to change jetting with exhaust upgrades. Anyone who says different does not know how a carb works.

 

 

Don't say never.  My son once hung a White Bros R4 on his YZ250F, and we ended up having to make several changes to the pilot and needle.  The pipe made the bike run excessively rich.  I know that's backwards of what folks commonly believe, but there it was.

The bore on that engine is more than likely 4" (101.6 mm). The bore on a YZ450 is 95 mm. Not much different, and as the header diameter on small block Chevrolets is typically 5/8", that's pretty close, too. In any case, the smaller per cylinder displacement means that there is less exhaust coming from the YZ450 than from any single cylinder of the mouse motor, so the header tubing doesn't need to be as big, anyway. It's a lot less a fruit salad comparison than you think.

Huh, never thought about it like that I guess. You're a very smart guy. Can I get your input on my last post with the new questions??

1) dismount the carb from the intake port.

 

2) Cold engines need to be artificially richened, thus the need for a choke.  A pilot that's too large to run correctly when warmed up will actually make it easier to start cold, and harder to start when warmed up.  To start cold more quickly, twist the throttle once or twice first, then kick.

So I went back to my 42 pilot jet today, timed the ap squirt, installed the bk mod, and set the squirt duration. Holy cow, it feels like a completely different bike, on the stand anyways. didn't have a chance to ride it yet, but I'm sure it will be great. Will ride it Wednesday at the first mx practice day here

Ok, new question. This is a problem that has seemed to arise since I did the top end rebuild, and changed jets in the carb the first time. Now that it's back to what it was before I thought it might be fine, but here's my issue.

On a cold start with the choke out, when the bike fires up, it won't have a very high idle until I blip the throttle, then it will idle like the choke is on. Why? I believe this is what makes it take a couple kicks when cold to start

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