Big Gun Jetting

Can someone help me i dont know what to jet my 400 i have a full big gun w k/n and outerware with no lid i just cant get it to stop back frieing by the way i have a k/n jet kit and only have a 42 piolet in stock any sugestions will help please fellow big guners

[MIKE400EX]

When I had that set-up it ran great with the 42 pilot, air screw out 2.75 turns and with a 165 dynojet main. Ran good from sea level to about 2000'.

[Quadfather]

Try a 165 MJ

thx allot guys i have a 42 but thought it was too big was there any spots in the power band flat and what elevation are you at? thx

[ChadEXer]

I have heard from many people that the Big Gun pipe is the hardest to jet correctly. Im not being mean but if it was me I would sale the damned thing!!!!!!! they have those little bitty headpipes and they hold back power!!!

[MIKE400EX]

I don't see any design reasons why the Big Gun should be harder to jet. I've never had a problem rejetting after making changes. Maybe it's the full system I have, but the head pipes are larger than stock - and plenty big enough for a healthy 440. The header is stepped up to full size, where as many aftermarket pipes are full size from flange to collector. I read somewhere that earlier full systems had smallish head pipes? No flat spots once jetted correctly.

[ChadEXer]

Get some stock headers and compare them to the Big Gun. They are NOT bigger than stock. Big Gun headpipes are the smallest headpipes I have ever seen on any pipe. I took my Big Gun off and put a Yoshi on and it made a HUGE difference. It was like going from stock to aftermarket!!! Next to Engine builders,,,right now WB and Yoshi are the best and have the biggest headpipes!! I heard this from the engine builders Ive talked too!!

[MIKE400EX]

The Big Gun headers I have ARE larger than stock!!!!!!!!!!!!!

[ChadEXer]

They must have sent you the wrong ones:D Ask anybody,,,,,Big Gun has the SMALLEST headpipes of any pipe:rolleyes:

[VegasEx'r]

Here is what I have found. The stock pipe diameter is smaller than the BG diameter. The BG diameter is smaller then most other headers I have seen at rhe exhaust ports, but the BG header does grow larger as it gets closer to the silencer. Big Gun says that this improves flow, and I don't know if it does or doesn't (reding their explanation of it on their website, it does seem logical), but that is what Big Gun says. The diameter of the Big Gun header is larger than the stock exhaust port on the head, so unless you have had some head work done, I don't think it really matters. I have also noticed that the Big Gun is more of a top end pipe, with lesser gains in the bottom to mid-range. But it is a good exhaust, I have been happy with it. I haven't had any problems with jetting, and have found that a 42 pilot/165 main work well for most applications. Since I have removed the airbox & choke assy. completely, I stepped up to a 170 main. This looks to be a touch rich, but it will help with cooling, so I have left it. If you are unsure how to jet, call Big Gun and ask them. That's what I did when I first bought it, and they were very helpful.

WOW you guys are real big downers on big gun personally I like the looks of them the best and yes the head pipes are a touch smaller however the stepped pipe they use is made for top end and thats what i really want so all of you dissin big gun step:mad: thx for all of you that have gave me help and no thx for you have not. I tried a 42piolet 65main and 2/3/4 out on the ajustment screw and the damed thig still backfires If I go up one size to a 170 will it stop im about ready to take it to a shop.

I have a question too does the WB rev limiter help with a pipe? thx

crap sorry im at about 1500ft to 0 thats the places where i ride thx

[VegasEx'r]

Is it backfiring when you get off the throttle? A 42/165 jets setting should work well, unless you are below sealevel. Which slot is your needle set at? Have you tried moving it down one? How does the plug look?

boy thx for the quick reply well yes after WOT it backfires Im using a K/N Jet kit and a K/N needle 3rd notch down is that right?

im at 1500ft above sea lvl and i ride at sea level too

A lot of times, backfiring when you're off the gas is caused by an exhaust leak. Did you replace your crush gaskets at the exhaust port when you changed pipes? I have the Big Gun and even before I changed the carb I was very happy with the performmance of it. And it never backfires. I would look for an exhaust leak. When you get off the gas it can suck air into the leak and cause an explosion when mixed with the unburned gas of deceleration.

shoot i hope not the crush ring i replaced those could it be at the joint i havent tightend the clamp too well actually not at all i check that could it be that?

Yes, any where air can get in can cause it to backfire.

wow thx allo do you use a clamp betweent the two?

Yes, mine came with a clamp. FYI I also ran mine with a 160 main jet and a 42 pilot and it still ran a bit rich, fouled the plug sometimes. It improves the low end so much that I could easily pull 5th gear wheelies across the field I ride in and pass people who were down on all 4.

fat ill try it tonight thx for the info now i wont have to spend 50$ at a shop to geta clamp tightend HAHAHAAH

Good luck, let us know if that works.

Now that I think about it, don't forget to use some high temp silicone in there at the joint and let it set up overnight.

[MIKE400EX]

Also check that your idle speed is not too low. Vista Cruiser, you're right, most normally aspirated four strokes like to be fat on the bottom and lean (a little) on the top. Doesn't take much of an air leak to make it backfire.

thanks for the info no it backfires a ,little when warming up. but pretty much fixed i think. but i have another problem now when i went to start it the pipes started glowing red hot im scared what has happend?:(

[K400EX&A250EX]

I don't race and certainly don't know which pipe gives the BEST performance increase, but I will say that I'm VERY happy with my Big Gun Race full system. I've been to the dunes twice and have REALLY enjoyed the power of the pipe. And...by the way...the head pipes ARE bigger than stock...I don't care what anyone says.

[ChadEXer]

Sorry I got crazy on yall. The OLD Big Gun pipes had SMALLER headpipes than stock or any pipe for that matter. The newer Big Gun pipes have bigger head pipes than the old ones. I didnt know Big Gun started making different headpipes!! I definately had the OLD Big Gun pipe on my bike. It had to be the OLD one,,, it was all rusted all to he!!
Sorry for the misleading info,,,it will be OK guys just settle down,,,I know you love your Big Guns:D

[VegasEx'r]

Originally posted by victor7279
i have another problem now when i went to start it the pipes started glowing red hot im scared what has happend?:(

I have heard people say that this is normal, but I don't agree. The only time I saw headers glowing was on an old Mustang I restored, when it was first fired up & being tuned. That car was running WAY lean, so you might be, too. Check your plug & see if it looks white. It should be a tannish color. It seems wierd that this would happen, but that would be the first thing I would check.