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Discussion Starter #1
Hi Guys,
First, thank you to all the contributors here!
I picked up a new-to-me KLR a few weeks ago. 2015 with 3400 miles on it.
The bike runs great and pulls strong but pops and backfires on decel very badly, and occasionally between upshifts. The excessive popping and backfires only happen during deceleration at about 3K RPM and below. It's bad enough to where I don't like engine braking in traffic or residential areas and just have to grab the clutch.

The PO installed a Yoshimura RS2 exhaust and cut an opening in the top of the airbox. Air filter is stock (and clean) as far as I can tell. It's definitely not a K&N or anything fancy.

So far I've tried:
Deactivated the AIS system and plugged off the carburetor.
Replaced the exhaust gasket at the head (this helped a little, but not enough)

I could tell with my finger that someone had removed the plug over the air/fuel mixture screw, so I suspected that the carb had been played with. I pulled it off today and found:

KLX (I think?) needle with clip on 2nd position. No washers except the large one above it.
Main jet marked 165
Pilot jet marked 40
Slide has been drilled to 7/64"
The air fuel screw was backed out 2.25 turns

I've never worked on a bike before, but I'm fairly mechanically inclined. The last carburetors I worked on were big Holley double pumpers 30 years ago.

From reading posts here, I suspect the pilot jet might be my problem. What do you guys think?
Pertinent photos:






 

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Discussion Starter #2
Oh, and is it weird that my needle is flat at the base? The photos I saw when searching other threads shows the taper coming to a point.
 

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What you probably are going to need is a #42 pilot jet.
 
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My 2012 KLR has a modified airbox (several one-inch holes drilled in the top) and a KLX jet needle. My main jet is #140 with a 0.013" shim beneath and installed on the second groove from the top. The pilot jet is #40 with the pilot screw two-turns out from lightly seated.

Even with my pilot screw out only 1-3/4" turns I have no backfiring on trailing throttle, so I'm surprised that you're getting popping with your pilot screw out 2-1/4" turns.

It sounds to me like you have an air leak somewhere. Also your main jet seems way rich; what's your gas mileage?

But having said that, perhaps your Yoshimura is flowing much more than my stock muffler and causing your lean condition. So, perhaps Tom's suggestion for a #42 pilot jet is the answer.

Jason
 

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a 42 pilot can't hurt (try 1.75 turns out) but you are also running way too rich on the main as Jason mentioned.

The needle appears to be a Dynojet needle and thier jetting recommendations are far too rich.....I imagine you're getting crap mileage too. Keep in mind that DJ jets and KH jets are numbered differently, but regardless, that's far too rich. For comparison; I'm running the DJ needle in my 2000 KLR with airbox mods and silencer and I'm using a KH142 main, second clip, no washers, 40 pilot at 2 turns out.


Dave
 

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I'll ask if the air hose to or from AIS control valve is corked off? If the only thing you did was dis-connecting the vacuum hose from the carb & capping the carb nipple, I think that the AIS is still flowing air into the exhaust port.

I think that normally under trailing throttle (high vacuum) the applied vacuum to the control valve closes Off the incoming air to reduce the popping.

The air-cut off diaphragm on the LH side of the carb is also activated by high vacuum to reduce trailing throttle after-fire (popping).

Maybe you need this? https://www.eaglemike.com/Air-injection-blockoff-kit-AIS.htm?categoryId=-1

And your Keihin 165 main jet is way to large, imo. I'm surprised that it will even pull near 6500rpm, much less 7500.

Oh, and is that clear air filter Thoroughly Oiled?? It is the oil that catches the dust.
 

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Discussion Starter #8
a 42 pilot can't hurt (try 1.75 turns out) but you are also running way too rich on the main as Jason mentioned.

The needle appears to be a Dynojet needle and thier jetting recommendations are far too rich.....I imagine you're getting crap mileage too. Keep in mind that DJ jets and KH jets are numbered differently, but regardless, that's far too rich. For comparison; I'm running the DJ needle in my 2000 KLR with airbox mods and silencer and I'm using a KH142 main, second clip, no washers, 40 pilot at 2 turns out.


Dave
Mileage has been about 50, all street, but I've also only had the bike long enough for 3 fill-ups.

Do I understand correctly that a #40 pilot jet that's backed out more turns functions the same as a #42 with fewer turns? So rather than swap in a #42, maybe just leave the #40 and back it out an additional .25 to .50 turns?
 

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Discussion Starter #9
I'll ask if the air hose to or from AIS control valve is corked off? If the only thing you did was dis-connecting the vacuum hose from the carb & capping the carb nipple, I think that the AIS is still flowing air into the exhaust port.

I think that normally under trailing throttle (high vacuum) the applied vacuum to the control valve closes Off the incoming air to reduce the popping.

The air-cut off diaphragm on the LH side of the carb is also activated by high vacuum to reduce trailing throttle after-fire (popping).

Maybe you need this?

And your Keihin 165 main jet is way to large, imo. I'm surprised that it will even pull near 6500rpm, much less 7500.

Oh, and is that clear air filter Thoroughly Oiled?? It is the oil that catches the dust.
Oh man. I'm gonna feel REALLY dumb if that's the case. Correct, I disconnected from the carburetor and capped the vacuum port and left the rest intact, assuming the the valve was closed by default. I'll pick up a block-off plate from EM, but for the time being I'll plug up the hose.

The air filter is clean and oiled.
 

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Discussion Starter #13 (Edited)
THANK YOU PDWESTMAN!

I tested the hose from the AIS diaphragm to the head. You were right on the money. Bike runs great and no popping or backfires. I'm embarrassed that I didn't catch that.

The 165 main jet still concerns me. As does the #40 pilot jet. Feels mismatched. I may go ahead and scrap them, and the needle, and get an Eagle Mike kit.

Thank you all for the help!
 

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I’ll pile onto the MCP kit as well. I’ve tried both KLX and MCP, and like the MCP more. Good luck with whatever route you choose. Report back with what you use.


2018 KLR 650
Mods in Progress
Clovis NM (till the AF moves me again)
 

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Stop it, y'all are making me waste perfectly good coffee through my nose!

Good thread, it's incredible you're getting such great mileage with that setup! I dropped significantly with KLX needle from stock, but man the bike feels right now. I'll be interested to hear what changes for you.
 

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First off that is a Keihin main jet — note the stylized “K” before the 165
That is not a Dynojet needle either and certainly not stock
Eagle Mike sells a that seals off the AIS at the head. I bought one and it does it’s job. It also solved all my popping issues.

A dynojet Typical setup would be:
Running Dynojet 3rd clip, 155 main jet, stock pilot, 2.25 turns on the mixture
FMF Powercore with K&N filter and air box door removed
Running 17/43 50mpg (95% highway use)
 

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First off that is a Keihin main jet — note the stylized “K” before the 165
That is not a Dynojet needle either and certainly not stock
Eagle Mike sells a that seals off the AIS at the head. I bought one and it does it’s job. It also solved all my popping issues.

A dynojet Typical setup would be:
Running Dynojet 3rd clip, 155 main jet, stock pilot, 2.25 turns on the mixture
FMF Powercore with K&N filter and air box door removed
Running 17/43 50mpg (95% highway use)
You might edit that to read, 155DJ main jet.

Which I think is still too rich for most KLR's.
 
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