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G-BRO

426 with 450 carb jetting

19 posts in this topic

01 YZ 426 with 07 450 carb-

165 Main

48 pilot

3rd clip needle

Fuel screw 2 3/4 -3 turns out ..ish..

2-4 thousand foot elevation

I've been wrestling with this problem for awhile. I'm trying to get the best overall jetting performance out of this set up. I can no longer access the jetting sticky and I'm not sure if the stock 07 carb settings even apply here

Half to full throttle is near perfect. There is a bit of hesitation from 1/8 to 1/4 throttle.

The bike starts first kick hot or cold.

Also, I think it's my lack of skill of setting the fuel screw correctly, but I also had an occasional irregular hot idle. Bike idles great without the choke while cold, but when fully warm it is sometimes irregular. It will start stumbling until it dies. I tried resetting the fuel screw today per the JD instructions I originally received with the fuel screw and ended up with 3 turns out approximately. Now from what I can tell I have alleviated the hot hiccup idle, but it is now very easy to kill the bike with a quick blip of the throttle on the stand when hot. I'd assume some amount of stumble without a load on the bike would be normal, but enough to actually kill the motor?

Tips and pointers?

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You need to either get the circuit jet adjusted to cover the sag , or change to a needle with a different diameter at the upper, straight section.  On the needle, the 4th character indicates the upper diameter.  The OEM needle in the '07 is an NFLR.  It probably needs to be a bit richer, but you should try fiddling with the pilot to see whether richer or leaner helps more.  Also might verify the accelerator pump timing.  If thew indication is that richer is what is needed, try an NFLP needle, PN 5TA-14916-LP. 

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You can try every jet option out there and never get rid of the bog. Its a design issue. The r&d powerbowl eliminates the hesitation off the bottom on these bikes. Hesitation off the bottom means lack of fuel

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You need to either get the circuit jet adjusted to cover the sag , or change to a needle with a different diameter at the upper, straight section. On the needle, the 4th character indicates the upper diameter. The OEM needle in the '07 is an NFLR. It probably needs to be a bit richer, but you should try fiddling with the pilot to see whether richer or leaner helps more. Also might verify the accelerator pump timing. If thew indication is that richer is what is needed, try an NFLP needle, PN 5TA-14916-LP.

Noted. What about the accelerator pump timing should be checked? Last time I had the carb off it appeared the fuel stream came on just as it cleared the slide (which is what I thought is the goal). It has one of those Quick shot 2 aftermarket set ups.

I have a JD jetting kit from my original 01 carb. Is it even worth trying to use one of those needles? I know the taper is more aggressive and richer down low verses stock

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You can try every jet option out there and never get rid of the bog. Its a design issue. The r&d powerbowl eliminates the hesitation off the bottom on these bikes. Hesitation off the bottom means lack of fuel

I thought bog was too rich and hesitation was too lean. My original carb had a bog because there was too much raw fuel from the accelerator pump. This feels much different then that

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My 05 had the bog or hesitation however you want to word it, the bike bogs when you grab a hand full. I would put the right model card back on your bike and buy a r&d powerbowl

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keep that 07 450 carb it is an upgrade over the early 400/426 fcr carbs, throw the original in the trash after pulling the needle out of it. I put a 07  yz450 carb on my old bike and its running great drop it in and go .if I was buying anything from R&D it would be their flexjet remote fuel screw.

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keep that 07 450 carb it is an upgrade over the early 400/426 fcr carbs, throw the original in the trash after pulling the needle out of it. I put a 07 yz450 carb on my old bike and its running great drop it in and go .if I was buying anything from R&D it would be their flexjet remote fuel screw.

Are you running the stock 450 needle in that carb of yours?

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I adjusted the fuel screw out a little bit further. The bike seems to run better at idle and off throttle. I'm at 3 turns out now with a 48 pilot jet. Seems like that setting is richer than the average.

I still occasionally have an issue with the bike idling when hot. After about 10 seconds it will start to stumble and eventually die.

What else could it be? Is it none carb related?

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keep that 07 450 carb it is an upgrade over the early 400/426 fcr carbs, throw the original in the trash after pulling the needle out of it. I put a 07 yz450 carb on my old bike and its running great drop it in and go .if I was buying anything from R&D it would be their flexjet remote fuel screw.

Have you used the power bowl? It works. Different year carbs can cause issues on different year bikes. Not very good advice

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Did you have these problems with the old carb on it?

I recall having the hot idle problems with the old carb, but then it went away. The hesitation off idle is new to this carb. I was able to tune some of the hesitation out. The idle gremlin still comes back at times

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48 pilot and bad idle makes me think vacume leak , if you have not done a leek down test its time, then you will know if its carb related or not for sure.

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48 pilot and bad idle makes me think vacume leak , if you have not done a leek down test its time, then you will know if its carb related or not for sure.

Vacuum leak is something I've been thinking about. Perhaps somewhere around the intake boot. Top end is still pretty fresh and was tested when completed

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Different year carbs can cause issues on different year bikes. Not very good advice

 

On the contrary, the carbs from '05 upward have been used on YZ400/426 models by several members here, including myself, with excellent results.  They generally run distinctly better with the newer carb than the older, and most who have done the swap think it's the best thing they've ever done to the bike.

 

To the OP: something you should look at is the seal on the vacuum release plate on the engine side of the slide.  Remove it and let it dry for 6-8 hours before evaluating its condition.  The fuel will make them swollen and wavy, but the healthy ones will return to their proper shape once dried.   If it looks torn or damaged , or excessively shrunken, replace it.

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On the contrary, the carbs from '05 upward have been used on YZ400/426 models by several members here, including myself, with excellent results. They generally run distinctly better with the newer carb than the older, and most who have done the swap think it's the best thing they've ever done to the bike.

To the OP: something you should look at is the seal on the vacuum release plate on the engine side of the slide. Remove it and let it dry for 6-8 hours before evaluating its condition. The fuel will make them swollen and wavy, but the healthy ones will return to their proper shape once dried. If it looks torn or damaged , or excessively shrunken, replace it.

I will try that. Thank you. Anybody ever heard of running anything bigger than a 48 pilot for these bikes? I feel like a pilot jet that big is only masking an existing problem.

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I will try that. Thank you. Anybody ever heard of running anything bigger than a 48 pilot for these bikes? I feel like a pilot jet that big is only masking an existing problem.

 

48 is usually the high limit of pilot jet selection, although you do see some individuals running larger ones for whatever reason.  I find that a 48 is normally only a valid choice at low temps near sea level.  45 is usually about right.

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have you confied your hot start cable has slack? and the hot start plunger works freely and bottoms? the hole on your slide faces the cylinder and is closest to the bottom of the venture? 48 is to big get it running and spray something around the intake -rpm listen for a change in the sound of the motor or dying depends what you spray. check your exhaust has no leaks its the big combination that matters .find out what others are running in your type elev and temps in the jetting threads.

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