426 jetting w/DSP titanium exhaust?

DOes anyone have any experience with the recommended jetting for a '00 426 with the

DSP exhaust system? The guy that owned the

bike could never get the thing jetting properly so now that I have it I'd like to

try it.

Any ideas?

I work for the Factory Yamaha race team and we use the DSP titanium ex. pipe exclusively.

Unfortunately our jetting is different than yours because we have a slightly different carb. It is a FCR but it has factory mods to the accel pump. I set up Tim Ferry's stock practice bike with the production DSP ti. pipe and ran a 168 main, 45 pilot, lifted the needle 1 clip pos. and limited the accel pump spray length to .3 seconds. To limit the accel pump spray length you must drill and tap the pump cam stop and install a 4mm screw/spring combo. that will contact the pump cam. A hex socket bolt works well, approx 25mm long. Remove the sub frame so you can look down the throat of the carb and with a stopwatch time the length of the pump spray when you stab the throttle. I usually click the stopwatch at exactly the same time I turn the throttle and click it again when I see it stop. It may seem weird but it works! Also you need to adj. the pump timing screw so that the spray doesn't hit the slide when is is raising. The timing is close when the spray just misses the slide. Then set the duration of the spray to .3 seconds with the adj. screw you installed. This probably sounds complicated and it is but is the only way to get rid of that pesky bog off bottom. If you have any more questions email me and I'll try to post some pictures that may help. By the way, Timmys stock bike rips with the pipe and these carb adjustments! Brian (BK)


Brian: Are you going to come around and give some more info with your experience? A lot of guys could use your input here. I have a feeling you will get deluged with questions and requests for assistance and advice.

At .3 seconds, you must pretty much click the stopwatch and then click again right way to stop? .3 is a very short duration to try to time with a stopwatch. Do you do this several times to see if you keep getting the same duration?

Do you take back to Yamaha any of the info that's being posted here? There are a bunch of PO'd 426 owners out there.

[This message has been edited by Boit (edited 02-02-2001).]


I’d love to see a pic of the acc. pump stop. I have a pretty good idea what you are describing but I’m not willing to take a drill to my FCR until I know what I’m doing. I’ve never attempted to time it, but the stock duration must be three or four times what you guys are ending up with.

Yamaha lists different diaphragms (4 for ’00, 2 for ’01), but I haven’t been able to get any. I suspect they have longer metal inserts to shorten pump stroke which would be another way to skin this cat. Do you know anything about these?

BTW, if you read this, I also sent you an email.


the 250F owners have the same problem.

any tips for us?


Wow!! I didn't expect such a concise and indepth answer. Thx a bunch!!

Not sure if I want to drill the FCR but I may if it looks to be easy/easier or maybe I'll just send it to MXWrench and have him do it.

I called DSP and they recommended a 48 pilot and a 170 main with the needle in the stock

position. I may start here since OI'm using

only pump fuel. MxWrench, are you using race fuel in Tim'e bike? I gather that you are and if you are that should make a little difference in the jetting, shouldn't it?

It sure sounds much better than the E-Series

that was on there when I bought it and talk a

about light!!! :)

BK: Thanks for the advice, very helpful. A tip about the stopwatch. Take a video camera, hold your stopwatch in the picture (if no built in clock readout on camera), and tape the event. Play it back in slo-mo or frame by frame to get a very accurate account of event duration. I am an ME and use this trick alot with good results. I think most videos are 30 or 60 frames per second, should be plenty.

BTW, I was one of those guys that had the plug fouling problems on 426s and the Yamaha dealer tried to solve the problem with this adjustable stop that BK writes about (didn't help, problem ended up being CDI). But doing this is no big deal because the pump cam stop is not on the main carb body but on a little plate that comes off easily (3 allens, I think) on the bottom of the FCR. Correct me if I am wrong, but I think it is the same plate that the "P38" is replacing (never actually seen the P38 product). It would be best to have a drill press to get the hole perpendicular, but I would think most of us could drill and tap this screw hole with pretty good sucess.

I think I am talking to myself here. BK, I read your post again and now I think I may not have it right. You mention a screw/spring combo-what and where is the spring? I thought the accelerator pump stop was on the accel pump diaphragm housing plate on the bottom of the FCR (if that's the right terminology). Is there another stop for the cam?

Mike: Excellent tip on using a video camera. I was trying to envision me with a stopwatch trying to time something with less than 1/2 second duration...that's about the blink of an eye... :)


What response! I emailed replys to everyone that had questions and need to explain a little further.

First, this mod. is only for 00 and 01 carbs. The old type carb has a different set up. The spring needs to be around the screw like the timing adj. screw. It needs to point at the accel pump cam directly. The gap between the cam and screw usually ends up being 2-3mm. Also you need to trim the cable dust cover to accomodate for the adj. screw. You can see an area on the rear of the dust cover where it should go.

I'll bring a carb home from work and modify it and post the photos step by step if I can figure out how to post photos. I'll try to do this in the middle of next week.



I finally got to ride after installing the accel pump limit, as described by "BK." It works great and is incrediably sensitive. Just turning the screw 1/4 turn makes a huge difference. This is amazing when you consider that the screw reduces the stock pump stroke by at least 5-6 full turns!! I was skeptical that Yamaha's setting could've been that far off, but quickly became a believer. As soon as I started increasing the pump stroke (backing the adj screw "out") fron BK's recommendation the throttle response degraded severely. Thank's again Brian.

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