Bogging ? and Carb squirt!

At the hare scramble sunday, I was greeted at the end by someone on a KTM 400. He asked me if I experienced a bog down low when the throttle is whipped open. As soon as he mentioned it, I recalled a few times in the woods where I DID have a 'blurb' on the gas. This is running in 2nd gear mind you, using the low end of the bike. I referred him to this site, but I think I will ask anyway....anyone know how to get rid of that? My bike is bone stock in every way. Gearing? Exhaust system? Jets? I kind of like the bog, as the bike still pulls, but it sort of keeps me from breaking the rear wheel loose. Just tap the clutch if you don't want it at all, like coming out of a berm....

ALSO, I always see on the YZ side, everyone talking about a BK mod for their carbs, lowing the carb accell pump squirt to like .3 seconds.

Well, a few months back when I was cleaning the bike up good to ship it to CA, I noticed my carb squirt is CONSTANT! I can twist the throttle, and it will just keep right on shooting that fuel in! I held the gas wide open for about 20 seconds, and it never stopped....(but she still fired in one kick smile.gif , YZ guys would be sitting for 3 months!) Whats the deal with our bikes having constant squirts, and the YZ guys having 2.3 second squirts, and needing to make it .3 seconds? Why don't we need to?


'01 KTM 520 SX

'95 KX 250 FOR SALE!!! -asking $2000 with decent amount of plastics/decals, parts!!

AMA Dist 6 Member

Hare Scramble Class B #523

All four strokes are prone to this "blurb" it takes a while to get all that stuff in the engine moving when you twist the throttle.There are things you can do to minimize it but it will always be there.On the KTM's you need to turn the idle up to 1850rpm.Make sure that the accel. pump is clean and working. Yours sounds like the diaphram or rod is dirty. It should not squirt for 20 seconds.On the Yamahas they shorten the stroke of the pump to make a less rich mixture trying to eliminate the bog.



I am curious about the BK fix on the KTM also. I met a guy on a 250f who said it made his bike bog less with no other mods. His bike did seem to respond instantly to the throttle just like a two-stroke. My 400 sure doesn't nor does my sons 250f. I think I may try the fix on the both bikes and see what happens.


Steve W.

01' 400exc

01' YZ250F (son's)

From what I know, the 00, 01, and 02 YZ426 and 250F use the second generation FCR carb that has a vertical slide. The first generation FCR has a tilted slide. Most everyone I have read about is using the BK mod on the 2nd generation FCR.

With that said, the overall theory should apply to the first generation FCR. The acc pump is the same. I think the newer carb body makes the BK mod easier. Most people are using between .3 and .5 sec duration on the pump spray.

Unfortunately, I had to sell the Great Pumpkin (sorry gang, but I needed a house more than a bike) before I discovered the BK mod, or I would have tried it. I would love to have access to a KTM to see if it would work, for future reference (heh).

Don't all the racing four strokes use the same carb? YZF 400, 426, 250, and all the KTM RFS's? If so....I guess the BK mod should function as well for ours....not that they need it mind you, I get ripped off the back as it is, WITH my bog smile.gif


'01 KTM 520 SX

'95 KX 250 FOR SALE!!! -asking $2000 with decent amount of plastics/decals, parts!!

AMA Dist 6 Member

Hare Scramble Class B #523

I have been having problems with the bike dying when I wick the throttle, found that the pump diaphram was ripped, replaced it, and still having problems, the pump does throw gas as long as I have the throttle open, it squirts for at least 5 seconds, is this normal??? I have about 800 miles on it, I am going to adjust the valves today to see if that helps.

[ November 03, 2001: Message edited by: katoom300 ]

I did the BK on my yz250f and what a difference. If the squirt stays on there is a problem. Not only did my bike start easier and pull harder, my gas mileage got amazing. If your carb does not have the tab for the mod, you can buy an accelerator pump cap called a p-38 which also stops the acc pump rod at the right spot. About $80.

Do a search on the yz426 site for bkmod or p-38. Some of the threads even have photos of the mod.


I wonder if the P-38 would work on the KTM version of the carb?

ok, totaly cleaned every part of the pump, sprayed carb cleaner thru every opening, and then blasted with air, when the carb is off the bike and i open the slide, the pump works well, it goes thru the complete stroke very quickly, when i put it on the bike, and it has gas, it squirts right away, but takes a few seconds to go thru the stroke and hit bottom. If I assist the pump with my finger, it squirts faster, is this supposed to squirt slow??

ok, seems I got the wrong plunger from ktm, the stock one had a "stop" on the gas side to prevent it from compressing all the way, and the replacement did not, which explains the continuous stream of gas (it kept pushing the plunger down till it bottomed)

both had the same part # NOK 055 (same as YZ/WR 400/426)

now to see why ktm doesn't have the correct part.


The P-38 for the KTM works great. I'm using one on my 02 520EXC. It shortened the pump stroke from 0.118" to 0.068". The pump stroke and accompanying fuel stream into the throat of the carb is too long and under certain situations mainly woods riding or tight motocross work with lots of small opening and closing of the carb the prolonged fuel squirt can lead to loading up and sluggish response. I spent a lot of time investigation this on my 99WR400 which has the same carb.

The 00,01,02 Yamahas have a unique for Yamaha only version of the FCR which in addition to having a vertical mounted slide also has a completely revised Accelerator(AC)pump linkage. The revised linkage, with a little owner modification, alows for the installation of a set screw and stop nut so you can externally change the pump stroke. This is the so called BK mod.

With the FCR on the KTMs the P-38 works fine.


Katoom, mine is still completely stock. WHen I had her apart for shipping to CA, I noticed a stream as long as I had the throttle open. I think it was in the range of 20 seconds or more, before I finally decided that was enough. BIke was 2 months old at that time. I wonder if I should get it checked out, or test it again and see if it is still doing it.......

With the FCR on the KTMs the P-38 works fine.


Thanks Clark. Where can I get the P-38? Is it very difficult to install?

[ November 07, 2001: Message edited by: RidinDawgDoc ]

The P-38 is manufactured by Factory R&D and is specific to each bike.

Factory R&D

13825 Alten Parkway

Suite A

Irvine, Ca. 92618


Just call them they will take care of you.

Installation is straight forward. Remove seat and tank, losen the clamps holding the carb to the intake manifold and the air box boot, rotate the carb to the right to gain access to the bottom and remove the three screws holding the accelerator pump bowl to the bottom of the carb. Carefully remove the stock pump bowl, spring and diaphram, (note orientation of diaphram0. Be careful NOT TO LOOSE the TWO "O" rings which seal between the bowl and the bottom of the carb, they are small and for some reason easy to loose quickly!!

While in there clean off any crud tham may be on the top of the diaphram. Reinstall the Diaphram (get it right side up), the spring, use the TWO stock "O" rings and install the new P-38 pump bowl. Make sure the bowl is lined up correctly so the holes in the bowl edge (where the "O" rings seal are lined up with the holes in the carb bottom.

Rotate the carb back to straigh up, secure the carb mounting clamps, reinstall the seat and tank and you are done.

Clark :)


In your photo the diaphram on the right the one with the "button" in the center is the one I have seen in all the carbs on Yamahas and KTMs. I know that there are a number of different part numbers for the diaphram and have speculated that perhaps the difference was the lenght of the "button" which would control the pump stop point but never confirmed it.


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