stock 400ex header question

[ExVillin400]

i hear alot about grinding down the welds on the stock header on the 400ex. what does this mean? is there hp improvment?

im getting the pro circuit slip-on anyday now and i would like to know before i bolt it on.

thanks

[Castor-426ex]

ok where headpipe bolts into head...inside the pipe some have welds that restrict exhaust flow....not much but hey....you need all you can get....get you a dremel or a die grinder with a wheel or stone....and grind those badboys down...if you do a search youll prob find a more detailed way...

[ExVillin400]

this wouldn't effect the jetting would it?

[tprender]

It might affect the jetting. Drilling holes in the airbox lid will affect the jetting. Cutting out the welds will give you about 1/2 to 3/4 increase in hp. You might have to go up 1 size on the main?

[RytM1]

Originally posted by tprender
It might affect the jetting. Drilling holes in the airbox lid will affect the jetting. Cutting out the welds will give you about 1/2 to 3/4 increase in hp. You might have to go up 1 size on the main?
I don't know about that. Jetting adjusts the air/fuel mixture prior to combustion. At the point the mixture gets to the header, it has already ignited. Grinding the welds help the exhaust gases escape easier. To my knowledge, and I have never heard of this before, jetting will not affect how quickly the exhaust gases leave your motor. Thats where things like a full exhaust system (header & silencer) come into play. Usually aftermarket headers "flow" a little better than stock, which help reduce exhaust gas back-pressure, which helps the motor work more efficiently.

[tprender]

If I read what you are saying correctly, you think that only before the gas is iginted by the spark plug only these changes will affect the jetting. If you change to a different type of air filter, taking the air box lib off, grades of fuel, changes in temp and height you might have to change jetting. But, what if I put on a different exhaust? From what I think that you said it will not change the jetting and this is after. Anything that makes a change in the flow of the air before or after will effect the jetting.

What I was trying to said was that removing the welds you will cause the air (exhaust gas) to flow a little more which might cause a need to change the jetting and it would be maybe 1 size up.
I don't think that I worded this correctly, but I hope that I mademy piont. Any change before or after will change the jetting sometimes of not all the time.

[RytM1]

You might be right TPRENDER, I was only thinking that there was no need to rejet after an exhaust mod because I did my e-series and switched to the K&N at the same time, which I knew was going to need jetting.

I guess the way to find out would be to find some of the old posts about grinding the welds, and contact some of the people who have done it to see if there was a need to rejet.