Main Jet too big?

[Nathan91]

I went to begin the jetting process on my bike and started with a 155 main and 42 pilot, but the bike was still backfiring on decelerate very bad so I continued to go up on main jets and it still backfired all the way up to the 180 main. At that size it has stopped backfiring, but that seems way too big for the few mods I have on the bike. I was just wondering what you guys thought. Thanks for any help or suggestions

[NacsMXer]

Backfiring on decel would indicate a lean pilot, so changing the main is going to have little to no effect.

You are running a stock carb I assume? I would put the main back to a 155 and up the pilot one size at a time until it stops backfiring on decel. If it's cold/winter where you are testing (don't know your location), then it probably wants a larger pilot.

Once you get the pilot correct, then you can move to the needle and main circuits. Remember to only make one change at a time when testing carb settings.

[Scro]

Also, make sure that you don't have exhaust leaks anywhere. In particular, the crush gaskets between the header pipe and head, and where the header and muffler meet. This can cause it to backfire consistently. Go ahead and check your intake boot as well.

[2001400exrida]

yeah like scro said check boot. that happened to me and i couldn't figure it out until i noticed that my air filter was just laying in the airbox.

i don't think you need to go any bigger than a 42 pilot, that's big, mine was flooding with that pilot, but i do have less mods than you. if u tune the pilot screw make sure you have it set at 2 and 1/2 or 3 turns out.

[Nathan91]

I just checked the exhaust and air filter, both look good. I live in Southern IL so this whole time i have been tuning is been less than 32 degrees most of the time less than 20 and we just got 5 more inches of snow yesterday. I didn't see any exhaust leaks and I am running a stock carb.
So here's where Im at, I haven't chopped the plug yet to check on the amount of fuel on it cause of the cold weather and snow. I know that's what i probably should do next, im wanting your input on if the 180 main is too big. The backfiring has stopped with it, so should i mess with getting a bigger pilot or just leave well enough alone.
Thanks everyone for the help

[2001400exrida]

as long as it's not bogging down.

[Crazy Bob]

Hey Nathan,
Like the others have mentioned, the Main jet has no effect on backfire
during decel as the Carb slide is all the way closed. I live in Missouri and have built quite a few motors for friends, from mild to wild. The others have mentioned things to check like exhaust port seal rings/ other exhaust leaks at header/muffler.
Air boot crack do not have to be very large because motor is at high vacuum and low flow during decel. Yeah, it is cold out! so decel pops are more common during very cold weather.
At our altitude and your motor mods a 42 pilot jet should be right.
With your motor mods a 180 main jet is WAY too big. The bike will not run it's best and will invariably cause ring sealing issues soon.
A 155 Main jet should be right for yoour motor.
Here is a tip I use. In stead of removing or cutting airbox lid. I unclip the back two clips, lift the back of the lid and push clips forward ,then sit back of lid on top of the clips. This gives about a half inch gap all around the back of airbox for more air flow. If I run into Mud, Water, Or other SLOP, I just set the lid back on the airbox, clamp it down, and GO!!
Bob

ok on my 400ex stock carb with moose racing jet kit i ran full sparke pipe a 165 main 43 and it ran good idk thou it would it help my brother 400ex he got a kit with sparke and they sent a full pipe and 155

[Nathan91]

Thanks for the tip Crazy Bob i tried taking the lid off but got too much mud and leaves in the box so i'll have to try that. I figured 180 was too big but i didn't know what to do. I also though that the the cold might be playing a large part in the backfiring. I guess I'll just go to the 155 or so and see what she does and also chop the plug and check it.

[2001400exrida]

if you're not bogging down and the backfiring as stopped then i would leave it.

that goes against the theory that was prevoiusly mentioned that backfiring is not affected by the mainjet, but whatever.

My quad used to backfire on decel and it was my needle jet. I had a 42 pilot, it was too rich, so i got a 40 pilot and took my needle down a clip position, then it ran perfect, no backfire.

i have uni filter/lexx pipe with a 40/152 setup.

[blacklabel117]

you may not need the main jet when i jetted mine out i couldnt get the damn thing to run right at all until i took out the main jet and just ran the pilot thing runs like a dream now

[Nathan91]

Originally posted by blacklabel117
you may not need the main jet when i jetted mine out i couldnt get the damn thing to run right at all until i took out the main jet and just ran the pilot thing runs like a dream now

Well that's a new and bold strategy, I have never heard of anyone doin that. Has anyone else had luck with this technique? So u just didn't put a main jet in right? Im gonna be tuning mine tomorrow hopefully.

[NacsMXer]

Originally posted by Nathan91
Well that's a new and bold strategy, I have never heard of anyone doin that. Has anyone else had luck with this technique? So u just didn't put a main jet in right? Im gonna be tuning mine tomorrow hopefully.

You can run it that way, without the main, but only temporarily. I found out by accident one time when rejetting. I took off to test it out, was running good until I got to about 3/4 throttle then it started sputtering and cutting out something terrible. I didn't know what happened until I took the float bowl off the carb and found the main jet laying at the bottom :p

So just try to stay at about 1/4 throttle and below and note how it runs with each pilot jet change. Not sure if this is the best way to tune the pilot circuit because I have heard that the main jet has a very slight effect on low speed operation.

[DragonGunner]

A 180 main, with open airbox an K&N filter in winter I can see...your bringing more denser air an need more fuel...in your case I would adjust your air screw out a half turn at a time, if you get past 4 turns you need a bigger pilot...its amazing what a half turn can do. Remember your main is only for 3/4 to full throttle. If your running stock filter an closed box like you said, 155 should be ballpark.

[Nathan91]

Well i messed around with it last night and today so here's where im at. I got a 155 main 42 pilot and the screw about 3 turns out. Luckily its warmed up too so its about 45 degrees out.
Its idling good and pretty constant, seems to be pulling harder too.
The only thing is when i go from idle to WOT real quick it bogs down and dies.

So im debating getting a larger pilot or should i try and go another turn out on the screw? After going 3 turns out the stick on jetting said to go up a size in pilot but im afraid that the bigger will give too much fuel.

Thanks for all the help everyone!

[Crazy Bob]

One thing that can cause stumbling and bogging when going from idle to WOT quickly is a blocked or malfunctioning accelerator pump squirt.( I know, silly name) It’s pretty easy to check and see if the accelerator pump squirts fuel into the throat of the carb when the throttle is cranked to WOT.
The little hole on the backside of that brass fitting sticking up from the bottom of the carb throat is VERY small and can get blocked by the smallest amount of dirt or dust or any debris at all that gets into the fuel tank. Cleaning this little bitty hole and its port routing is very tough and requires the ability to completely dis-assemble the carb and use various methods of trying to blow high pressure air backwards through this tiny hole in order to clear any blockage. The way we ride and getting on and off the throttle many times a minute, this extra squirt of fuel when going from idle to open throttle makes a big difference.
Best of Luck… Bob