backfiring

[400exridin121]

i just rejetted for a full sparks system 42 pilot and 158 main and i didnt mess with the fuel screw, whenever i let off the throttle it backfires? is it the fuel screw or should i change the main? 900 altitude about 60 degrees

[400exridin121]

and it like puts out a puffing noise or somethin when i turn it off after ive rode hard, but that could just be because it needs repacked

[Eviltanker]

Try putting your fuel screw at 2 1/2 to start and work it up or down to get a spot where it won't do it. Also where is your needle at? you are a little higher in elev. than me and I had to go to a 160 main(Keihin) Start with the fuel screw. I would suggest a plug run at WOT then cut the plug to see if you are good on the 158.Alot of this is trial and error. Just buy a few extra plugs, you'll need them for plug chops

[underpowered]

try turning your pilot screw out. backfiring is a sign of a lean condition. you may actually have to go bigger on your pilot jet. i would try going out a bit more on the air screw first before going a size up on the pilot.

[400exrider707]

Originally posted by underpowered
try turning your pilot screw out. backfiring is a sign of a lean condition. you may actually have to go bigger on your pilot jet. i would try going out a bit more on the air screw first before going a size up on the pilot.

The pilot screw should remain seated at all times. Try adjusting the air screw. 1/4 turn at a time until it goes away. With your 42 pilot it should just be a simple air screw adjustment.

[underpowered]

Originally posted by 400exrider707
The pilot screw should remain seated at all times. Try adjusting the air screw. 1/4 turn at a time until it goes away. With your 42 pilot it should just be a simple air screw adjustment.

sorry, i use the term interchangibly. pilot screw=Air screw=mixture srew=air/fuel screw


but you may need a bigger pilot. i actually had to go all the way up to a 48 pilot with my stock carb. running a 55 with the FCR.

[400exridin121]

i dont want to take the carb out again so ill try that when i get the right screwdriver....oh and i think i had it at 2 1/2 already. i dont have the airbox off either....tried that yesterday and when i ramped and hit the throttle it didnt respond and it nose dived, so should i try 3 turns, and is the air screw right by the choke? i saw it yesterday and it was tight not turned, could that be the problem?

[krt400ex]

Originally posted by 400exridin121
i dont want to take the carb out again so ill try that when i get the right screwdriver....oh and i think i had it at 2 1/2 already. i dont have the airbox off either....tried that yesterday and when i ramped and hit the throttle it didnt respond and it nose dived, so should i try 3 turns, and is the air screw right by the choke? i saw it yesterday and it was tight not turned, could that be the problem?


popping could also be a sign of being WAAAYYYYY too rich too...but i dont think that is your problem.

i think you are running lean on the pilot circuit

[400exridin121]

so what should i go with?

[krt400ex]

i would try a 45

[GPracer2500]

Don't go crazy richening up the pilot circuit in a quest to reduce backfiring. A 45 strikes me as going too far. The pilot circuit shouldn't really be tuned by listening for decel backfires anyway. It can be an indicator, but there are much better ways to properly tune the pilot circuit.

Did you replace the header gaskets when you installed the exhaust? A small air leak at the joint where the header bolts to the head can cause backfireing on decel.

[400exridin121]

no i didnt mess with the header gaskets should i try tightening the bolts that hold it to the head? or do i just need new header gaskets?

[400eXr1d3rZ]

Originally posted by 400exridin121
no i didnt mess with the header gaskets should i try tightening the bolts that hold it to the head? or do i just need new header gaskets?

I suggest you change them, you should change them if you ever take off your exhaust, I do.

[400exridin121]

are they hard to change?

[400eXr1d3rZ]

Originally posted by 400exridin121
are they hard to change?

Their easier to get out than in, just use a pair of small pliers to get them out.

[400exridin121]

i found a set on ebay its a cometic head gasket and base gasket, thats what i need right?

[400eXr1d3rZ]

Originally posted by 400exridin121
i found a set on ebay its a cometic head gasket and base gasket, thats what i need right?

Make sure it's for an 85mm piston, because they make different size gaskets for different size bores.

[GPracer2500]

Originally posted by 400exridin121
i found a set on ebay its a cometic head gasket and base gasket, thats what i need right?

You can just get the exhaust gaskets by themselves. 18291-MN5-650 is the OEM part number. Couple bucks a piece, I think. The same gaskets are used on a few dozen different Honda machines. Most any Honda dealer should have them in stock, I would think. I'm not sure what's available aftermarket but I probably wouldn't order a whole top end gasket kit just to get the exhaust gaskets.

They are copper crush gaskets and they're designed to only be used once. They're little round suckers--about the same diameter as the headpipe. They usually stick into the head and can be a pain to remove. Once they're crushed into place, somtimes it's hard to see that it's even a seperate piece and not just part of the head.

I've reused old ones successfully (just left them in the head) but am always on the lookout for leaks if I do that.

[400exridin121]

this is them right?

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&ih=019&sspagename=STRK%3AMEBI%3AIT&viewitem=&item=290093179788&rd=1,1

[GPracer2500]

Originally posted by 400exridin121
this is them right?

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&ih=019&sspagename=STRK%3AMEBI%3AIT&viewitem=&item=290093179788&rd=1,1

No. Item #1 in this picture:

http://www.servicehonda.com/hard%20parts/atv/trx400ex%2099-04/muffler.jpg

[400exridin121]

oh ok thanks ill get them from our honda

[400exridin121]

will it harm the engine or anything not replacing them? our honda has bad hours and i couldnt get there and am going ridin tomorrow.

[GPracer2500]

Assuming that the backfiring is caused by a header leak, then no--it shouldn't harm anything. You could try just tightening the header bolts a little and see if that helps. But don't overtighten.

Is the backfire a soft burble? Or is it more like sharp "machine-gun" style pops that come in rapid fire? The former is nothing to be concerned about. The latter should be addressed.

Some backfiring on a closed throttle (especially from high rpm) is normal. Bikes that don't have ANY closed throttle decel backfires (softish, mild ones) are usually jetted too rich on the pilot circuit.

In my estimation this is a common circumstance on 400EXs because so many people want rich pilot circuits for easier starting and warm-up. I'm getting a little off the subject here, but the 42 pilot jet has become the de facto pilot jet for the 400ex on these boards. I'd guess that's really too much jet for most folks (cold starts notwithstanding). I don't speak up about it much because I think it's mostly a fair compromise. The 400ex is cold blooded and a rich pilot ciruit helps that. But a rich pilot circuit is what it is--rich.