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jalarson

Severe bog on acceleration - any advice

7 posts in this topic

I have a 04 yz450 that i picked up a year or so ago. It was having some severe bogging when accelerating which is the exact same thing my yfz was doi g right before it seized. I decided to tear it down and had the shop put a new top end with 13:1 comp piston and new bottom with crank berrings and some new shift forks. I just got the bike put back together and am still having severe bogging when accelerating. I am at roughly 4500' with 95 degree temperature and I have tried rejetting with 155, 158, 160, 162, and 165 main jets. I have also tried 38, 40, and 42 pilot jets. Still having the same issue. I bought an external air/fuel screw and have tried everthing from 1 - 2.5 with no luck. The bike has a pro circut pipe on it so i checked for leaks by pluggi g the pipe with a rag to create backpressure and it doesnt seem to be leaking. I read on one of the forums that removing the tps can help but no luck there either. When i had the engine apart i had them check my valves and they seats looked prety good and all the clearances were in spec. I am really out of ideas and not sure where else to go. I built this while bike in hopes to be ready for this weekend but all the shops are slammed. Does anyone have any advice or thoughts of what could be wrong? This is not just a mild bog but severe enough that it will kill the engine under throttle. Any suggestions are appreciated! Thanks in advance!

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I'm no expert but are you easing into the throttle or just hammering it open when it bogs? I would check the diaphragm on your AP. Might try the "O" ring mod if you haven't already.

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1. 165 main / 45 pilot / NCVS needle on 4th, 50 leak jet (0 - 8000 ft). 175 main if you are full flow exhaust

 

2. You make no mention of the accelerator pump. Has to work 100% or it will bog. 

 

Do this and read this; you have to clear, clean, tune, and test the apump. It's a pain, but you have to do it. It is probably corroded.

 

http://www.thumpertalk.com/topic/1053317-06-crf450r-still-bogs/?hl=%2Baccelerator+%2Bpump#entry11143305

 

If you have the long or medium rivet apump diagprahm you have to change it to the 2008 specs:

 

http://www.crfsonly.com/catalog/product_info.php/cPath/138_147/products_id/3932

 

3. Have you done a stator test, a ground path test, a high tension lead test, or a leak down test on the head? One slightly leaky exhaust valve would do the same thing.

 

4. You must put ethanol treatment in your gas, every time, or the carb will corrode again.

Edited by KRANNIE

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1. 165 main / 45 pilot / NCVS needle on 4th, 50 leak jet (0 - 8000 ft). 175 main if you are full flow exhaust

 

2. You make no mention of the accelerator pump. Has to work 100% or it will bog. 

 

Do this and read this; you have to clear, clean, tune, and test the apump. It's a pain, but you have to do it. It is probably corroded.

 

http://www.thumpertalk.com/topic/1053317-06-crf450r-still-bogs/?hl=%2Baccelerator+%2Bpump#entry11143305

 

If you have the long or medium rivet apump diagprahm you have to change it to the 2008 specs:

 

http://www.crfsonly.com/catalog/product_info.php/cPath/138_147/products_id/3932

 

3. Have you done a stator test, a ground path test, a high tension lead test, or a leak down test on the head? One slightly leaky exhaust valve would do the same thing.

 

4. You must put ethanol treatment in your gas, every time, or the carb will corrode again.

 

First off, none of the NC** series needles are appropriate for the YZ450; These apply only to the "smog" carb used on the WR.  There is usually no reason whatsoever to change to an alternative needle in the '04 model, although some do like things better with an NFLQ or NFLP (5TA-14916-L1 or 5TA-14916-LP) because of the smaller upper straight diameter, which richens the transition from pilot to main circuit.  All YZ450's are "free flow" exhausts (the poster is speaking from experience with WR's), so while the 45 pilot is a valid suggestion, don't go bigger than 170 on the main.  Note that the main won't affect the stumble or bog anyway.

 

Second, "some" of the information in the posted link to the CRF forum is applicable, but only some.  It's pretty much completely unnecessary to change from the stock AP diaphragm, and the jetting info given ignores the fact that your carburetor does not have a leak jet in the AP circuit.  You're far better off in most cases to simply adjust the AP timing according to the manual, which can be downloaded free.  The key is to have the discharged stream of fuel "just miss" striking the slide as the slide lifts.

 

Finally, ethanol treatment in the fuel has never been necessary for me at all.  Ever.  And no corrosion in the carb, just a varnished pilot jet if I let it sit too long through the summer.

 

Cranedaddy may be more of an expert than he thinks, because a lot of long time two-stroke guys do fall into the habit of banging the throttle open as instantaneously as possible, and that will make a correctly tuned four-stroke stumble.  The throttle should instead be "rolled" open.  It's actually almost a two step maneuver, wherein you quickly open to only a quarter or so, allow the engine maybe half a second to react, then open the rest of the way.  The response sounds and acts as instant as if it had exploded. 

 

If you made it a focal point, you could build the carb in such a way as to be able to break the throttle from idle to full in an instant without a single bit of bog, but then, especially without a leak jet, you'd have to deal with a bike that runs dirty, blubbers during low speed maneuvers, ans stalls more often, also fouling plugs more rapidly. 

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First off, none of the NC** series needles are appropriate for the YZ450; These apply only to the "smog" carb used on the WR.  There is usually no reason whatsoever to change to an alternative needle in the '04 model, although some do like things better with an NFLQ or NFLP (5TA-14916-L1 or 5TA-14916-LP) because of the smaller upper straight diameter, which richens the transition from pilot to main circuit.  All YZ450's are "free flow" exhausts (the poster is speaking from experience with WR's), so while the 45 pilot is a valid suggestion, don't go bigger than 170 on the main.  Note that the main won't affect the stumble or bog anyway.

 

Second, "some" of the information in the posted link to the CRF forum is applicable, but only some.  It's pretty much completely unnecessary to change from the stock AP diaphragm, and the jetting info given ignores the fact that your carburetor does not have a leak jet in the AP circuit.  You're far better off in most cases to simply adjust the AP timing according to the manual, which can be downloaded free.  The key is to have the discharged stream of fuel "just miss" striking the slide as the slide lifts.

 

Finally, ethanol treatment in the fuel has never been necessary for me at all.  Ever.  And no corrosion in the carb, just a varnished pilot jet if I let it sit too long through the summer.

 

Cranedaddy may be more of an expert than he thinks, because a lot of long time two-stroke guys do fall into the habit of banging the throttle open as instantaneously as possible, and that will make a correctly tuned four-stroke stumble.  The throttle should instead be "rolled" open.  It's actually almost a two step maneuver, wherein you quickly open to only a quarter or so, allow the engine maybe half a second to react, then open the rest of the way.  The response sounds and acts as instant as if it had exploded. 

 

If you made it a focal point, you could build the carb in such a way as to be able to break the throttle from idle to full in an instant without a single bit of bog, but then, especially without a leak jet, you'd have to deal with a bike that runs dirty, blubbers during low speed maneuvers, ans stalls more often, also fouling plugs more rapidly. 

 

My WR450 has a stage 3 ported head (way beyond stock YZ) '07 YZ ex cam,  and '05 450 YZ carb, and runs the NCVS needle, which gets me 14.0 AF ratio in the mid throttle range.

 

My YZ carb from ebay (2005) came with a middle depth apump diaphragm and short push rod, so both needed upgrading to '08 CRF specs to eliminate the WOT whack bog.

 

My YZ carb has a leak jet provision.

 

My YZ pipe flow less than the Dubach racing, allowing me to go to 175 on the main.

 

You seem to be very fast on the critisism...........

 

I'm just trying to help, and keep the threads on topic.

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'05 and later FCR-MX carbs on YZ450's do have leak jets.  Previous models do not.  This is why I always recommend the '05-'09 carbs for swaps onto older bikes like teh 400/426 models, and not the '03-'04 carbs.  The presence of leak jets in the early FCR-MX was by-model, as the '03 YZ250F did have one, while the 450 did not.

 

One of the other considerations in this case, considering the age of the carb, is the possibility that some of the corrosion the OP found has intruded into the space between the upper and lower carb body.  This joint is separated by removing the four screws found inside the float bowl on the "roof" of the bowl cavity.  DO NOT remove these screws until and unless you have a replacement gasket in hand, and be advised that, as far as I know, there is only one source for the gasket: JD Jetting.   The original is "rubber" like a complex formed "O-ring" seal, but it's rarely reusable. 

 

Also worth looking into on a carb this old is the condition of the slide's vacuum release plate and the seal between it and the slide.  The "square end" with the small hole near the edge goes down.  Look for cracks or chips at the corners, or a large amount of wear on the plate.  As to the seal, it will come out looking swollen and ruffled form exposure to gas, but most of the time they will return to near normal shape if you dry them in air overnight.  If it won't do that or is otherwise damaged, replace it.

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My YZ pipe flow less than the Dubach racing, allowing me to go to 175 on the main.

 

 

And of course, you have an actual flow bench test to prove that. 

 

Note that the change from a stock '06 pipe to a DRD stainless on either of the two I had required no change to the jetting we had in place.

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