Do I need to re-jet when putting new exhaust on my 426?

Sorry, I know this has probably been asked several times. I am trying to learn more about jetting, suspension, etc. I have an '02 YZ426 that was basically bone stock with very low hours when I bought it in Texas last May. I immediately moved to the Vegas area and I have been riding it here in the desert and having a blast. It has run great and I have no complaints with the motor or the way it runs. A friend out here had a DSP Titanium exhaust system with a spark arrestor built into it that he had previously had on his 426 (which he sold), so he just gave it to me. I put it on last week and didn't notice much difference in the noise level. I haven't ridden it enough yet to see if there is a noticeable power difference. In just the couple of minutes I rode it, I seemed to notice an increase in the amount of backfiring on deceleration (I didn't remember having any backfiring before). Now maybe it was just that I didn't have the bike warmed up yet or the temp was colder outside, but maybe I need to re-jet to match the exhaust characteristics better? Incidentally, I didn't re-jet when I moved to Vegas, but the altitude here is about 1150 feet. I was at about 700 feet in Texas. Any ideas? Also, the same guy gave me a weighted flywheel that I am going to try, as I still manage to stall out a couple times during each ride in the desert. Am I going to notice a significant decrease in acceleration?

Can help you with the flywheel Q but you'll need to go richer on your jetting. More air=more fuel. The popping is an indication of a lean condition.

You may try to step the main up a size to see how it responds. The free flowing exhaust may will most likely benefit from the larger jet. The popping on decel could be a sign that you need to play with the fuel screw or pilot size. When jetting, only make one change at a time so that you can se the result of each.

With the aftermarket vs the stock exhaust being the only change, I would bump the main a size as my first step.

Las Vegas is mostly at around 2200 feet, Henderson is at 1600. It depends on where you ride.

On decel backfire, check for air leaks first:

http://www.thumpertalk.com/forum/showthread.php?p=3710599#post3710599

You may or may not need to re-jet. Run it and see.

You will most likely notice no significant loss of acceleration with the heavier flywheel, just some mellowing of the snappy response to throttle in the lower gears.

Gray makes a good point. That DSP exhaust is a slip fit if I remember correctly. If it is sucking air at that joint, you will get backfires in the exhaust that are not jetting issues at all.

Thanks everyone. I will run it as Gray suggested and see how it does. It does have just a slip fitting at the middle junction. I thought this was strange as the stock pipe has a clamp, but I guess they just decided it wasn't necessary? If I do have an air leak there, is it possible to purchase a clamp somewhere and clamp that junction point just like the factory pipe?

I have used high temp silicone on the slip fitting. It will burn away, but it leaves a residue that will usually seal the small gap that is present.

Besides which, using a clamp would require splitting the larger pipe 2/3-3/4 down the joint so the clamp can tighten something.

Thanks everyone. I will run it as Gray suggested and see how it does. It does have just a slip fitting at the middle junction. I thought this was strange as the stock pipe has a clamp, but I guess they just decided it wasn't necessary? If I do have an air leak there, is it possible to purchase a clamp somewhere and clamp that junction point just like the factory pipe?

atropine - what part of town are you in? I have had to do some major re-jetting ('01 426) compared to where the jetting was in the summer. I will be gone this weekend, but if you still need help nex week, PM me and I will help you get it dialed in! :lol:

Depends on the Altitude and Temp. I put on a new White Brothers E2 series during the summer and didn't re-jet. No problems. With the winter - Temp change going colder, I have noticed a little pop on decel. Nothing major but I probably need to jet to weather.

- RCD

Thanks again everyone.

LVThumper-

I have had to work this weekend, so I won't get a chance to test it out until next week. If I am having a lot of popping after it is warmed up, I'll be asking you some questions. I live out in Logandale, and our elevation is a little lower than Vegas. We are almost as low as Lake Mead here on the valley floor (Mead is at 1128 feet right now, and we are at about 1200 something I believe), and the hills all around us probably average from 1500-2500 feet. I didn't notice much difference from the summer to a couple of weeks ago prior to my changing the exhaust. Hopefully it will still be OK once I get the bike warmed up.

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