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Discussion Starter · #1 ·
Hi. I've got an 04 ltz400. I just rebuilt the engine top and bottom. I put a 440 big bore kit in it. The bike has an hmf slip on exhaust and I'm running an open air box.

When I fire it up it idles fine but when I try to accelerate it pops and backfires. If I accelerate to quickly it dies. I cleaned and fully rebuilt the carb. The float measures within spec. It was originally running a 130 main jet. I replaced it with a 142, 146 and 150 with little difference. It smells like it's running rich with the 150 installed. I've played with the mixture between 1 and 3.5 turns on each jet change in 1/4 turn increments. I'm running a 27.5 pilot jet. The valves have been adjusted and shimmed within specs.

I did notice that the needle jet that the main screws into was bent when I rebuilt it but I replaced it with a new one. There is also a small exhaust leak where the slip on exhaust pipe connects to the header pipe. I would think if that was going to cause any issues it would only be on deceleration though. My issues are definitely with acceleration.

I bought this bike with a broken case due to a thrown chain. I never saw it run so I don't know if it was having issues before hand but given that they were riding it when the chain broke the case I would assume it was running good. Help would be greatly appreciated. Thanks
 

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When you reassembled the engine did you make sure you installed the neutral safety switch correctly? Sounds like your hitting the rev limiter based off your description.
 

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Do you have an adjustable needle? Try going down with the clip. pilot jet is for idle, needle is 1/4-1/2 throttle and main jet is 3/4 to full throttle. knowing this should help in identifying what circuit you are dealing with when the popping/backfire occurs. Popping/backfire on accel is a lean condition.
 

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Discussion Starter · #6 ·
When you reassembled the engine did you make sure you installed the neutral safety switch correctly? Sounds like your hitting the rev limiter based off your description.
I believe so. The spring and plunger are in place and the switch bolted in the only way I think it can go. The bike only starts in neutral. When I put it in gear it won't crank. That leads me to believe it's working right. Is there another way to check it? Thanks
 

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Discussion Starter · #7 ·
Do you have an adjustable needle? Try going down with the clip. pilot jet is for idle, needle is 1/4-1/2 throttle and main jet is 3/4 to full throttle. knowing this should help in identifying what circuit you are dealing with when the popping/backfire occurs. Popping/backfire on accel is a lean condition.
I don't have an adjustable needle but I have shimmed it. I have the white spacer and then added two washers under it. It didn't seem to make a difference. I agree it seems like it's running lean but when I put the 150 main jet in it smells really rich. The plug also has a lot of soot which leads me to believe it is running rich. I really haven't seen much of a difference between the 142 and 150 mains other than that. I don't have any larger jets but I suppose I can get some. What size do you think it should be running? Or do you think I need more shims? Thanks
 

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Hi. I've got an 04 ltz400. I just rebuilt the engine top and bottom. I put a 440 big bore kit in it. The bike has an hmf slip on exhaust and I'm running an open air box.

When I fire it up it idles fine but when I try to accelerate it pops and backfires. If I accelerate to quickly it dies. I cleaned and fully rebuilt the carb. The float measures within spec. It was originally running a 130 main jet. I replaced it with a 142, 146 and 150 with little difference. It smells like it's running rich with the 150 installed. I've played with the mixture between 1 and 3.5 turns on each jet change in 1/4 turn increments. I'm running a 27.5 pilot jet. The valves have been adjusted and shimmed within specs.

I did notice that the needle jet that the main screws into was bent when I rebuilt it but I replaced it with a new one. There is also a small exhaust leak where the slip on exhaust pipe connects to the header pipe. I would think if that was going to cause any issues it would only be on deceleration though. My issues are definitely with acceleration.

I bought this bike with a broken case due to a thrown chain. I never saw it run so I don't know if it was having issues before hand but given that they were riding it when the chain broke the case I would assume it was running good. Help would be greatly appreciated. Thanks
I recommend maybe running bigger jets, same thing was happening to me when I installed an HMF Competition Series Slip On. It’d run fine at the low revs but when I was romping it in the high revs it was quick to die and it was falling on its face, I installed 155 jet and took the air box lid and it’s very quick, it even runs a bit lean tbh but it’s not that bad of a problem
 

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Discussion Starter · #9 ·
I recommend maybe running bigger jets, same thing was happening to me when I installed an HMF Competition Series Slip On. It’d run fine at the low revs but when I was romping it in the high revs it was quick to die and it was falling on its face, I installed 155 jet and took the air box lid and it’s very quick, it even runs a bit lean tbh but it’s not that bad of a problem
I'll run over and pick up a couple of larger jets today and see what that does. Thanks
 

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I believe so. The spring and plunger are in place and the switch bolted in the only way I think it can go. The bike only starts in neutral. When I put it in gear it won't crank. That leads me to believe it's working right. Is there another way to check it? Thanks
Sorry I'm not certain if you can or not. However it does sound like its working if the quad only starts in Neutral.
 

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Discussion Starter · #11 ·
Hey everybody. I wanted to update you all. The quad is running terrific now! I went ahead and replaced the cdi box with an aftermarket unit that increased the Rev limiter by 1,000 rpms. I was hesitant to spend the money. I was convinced that the carb, jetting, intake and exhaust systems were tuned properly. It ran great at idle and up to about half throttle so I didn't really think it was the cdi either but I had no where else to go. So following advice from this forum and my son who said all along that it had to be the problem I went ahead and replaced it. Runs perfect now. Thanks for all the advice. You were all very helpful.
 

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27.5 might be a little lean still. I recommend buying the next couple of steps and trying them out. I was running stock everything just delided airbox and k&n, and I had to run a 32.5 pilot 152.5 main
 

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Hey everybody. I wanted to update you all. The quad is running terrific now! I went ahead and replaced the cdi box with an aftermarket unit that increased the Rev limiter by 1,000 rpms. I was hesitant to spend the money. I was convinced that the carb, jetting, intake and exhaust systems were tuned properly. It ran great at idle and up to about half throttle so I didn't really think it was the cdi either but I had no where else to go. So following advice from this forum and my son who said all along that it had to be the problem I went ahead and replaced it. Runs perfect now. Thanks for all the advice. You were all very helpful.
What cdi did you put I'm having the same problem and would like to also get a cdi just don't know what to put
 

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What cdi did you put I'm having the same problem and would like to also get a cdi just don't know what to put
In alot of other discussions on here most recommend a dynatek cdi. From what ive read most of the other cheaper cdis are hit and miss on quality with most of them being made in the same place with different stickers. I have put a dynatek on my 2003 and its been amazing.
 
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