Backfire issues

BMF · 03-31-2018, 03:44 PM

Question, I switched out the 98 jet in my carb to a 110. I went to get my bike inspected and the guy told me my bike was running lean and causing backfire. I switched to a 105 and I still get backfire when down shifting. Should I go to a 100 jet?

roundhouse · 03-31-2018, 04:05 PM

your going leaner with the smaller jets. Mine pops a little bit but no back fires, I added a half turn out for a total of 2 turns to the idle mixture screw and it pops even less. I have a 120 jet in right now i think with the ebay exhaust. Stock exhaust on your bike?

sorry I am no expert, I just did what the other guys did before me and posted and adjusted a little here and there.

BMF · 03-31-2018, 05:23 PM

I have the ebay exhaust. I went from a 110 to 105 to 100 and it's getting worse. So you think i should go the other way and maybe try and 115 or 120?

Megadan · 03-31-2018, 06:08 PM

First thing to understand.

When decelerating, and the throttle below 25%, the main jet is having almost no effect on your fueling. Decel popping like you describe is from a lean pilot jet (and its related idle mixture setting), not a main jet.

Second, as already covered, the smaller the jet number, the leaner the jet. If you are already lean, you need to go larger.

Third, there are 3 different jets that affect fueling, but each has the greatest effect in a given area of the throttle position. As stated before, up to around 1/4 throttle, the pilot/slow jet is the jet to focus on. between 1/4 and 3/4 throttle, the needle jet and needle. and beyond 3/4 throttle, the main jet. All of these jets will have a small impact on the other two, but it is very minimal.

Is this on a stock carb or a Mikuni?

BMF · 03-31-2018, 06:40 PM

It's a Mikuni and thank you. I have been watching too many YouTube videos, i think.

I was going to shim the needle but I can get that little tiny clip out. It's not the butterfly style but circular.

Right now I have stopped because I am lost. originally I was losing power with the stock carb at 3/4 throttle. I bought the Mikuni, replaced the main jet to 110 and all seemed good. After I left the bike shop today, it started to cut out and I would get a loud popping sound.

I tried the 100, 105, 110 (the best so far) and now have the 115. From what you are telling me I need to adjust either the small jet or needle. This may be beyond me. I work on servers for a living and find that easier. One and zero's...one and zero's...(server term)

Megadan · 03-31-2018, 06:51 PM

Best thing to do would be to start from a square one baseline.

Intake and exhaust mods? What is your elevation?

BMF · 03-31-2018, 08:49 PM

I just discovered the gasket was ripped for the bowl on the Mikuni. So I took my stock carb, Dremeled the bolts so i could open it with a screwdriver, added a new jet, put everything back together and it works.

The stock carb was good but would crap out at 3/4 throttle. Hopefully the bigger jet will fix that and all will be good. I still need to test drive it. My neighbors have endured enough loudness for the day.

I am at 300 ft elevation. I have been following MotoCheese on YouTube because he is in CT and I am in NH. Pretty much the same elevation.

Thanks for your help!

Megadan · 04-01-2018, 01:22 AM

I was going to link you to that seal, but I keep running into no results.

This is supposed to be what you need, but they are unavailable anywhere. https://www.aliexpress.com/item/Pit-...652507296.html

Hopefully they will show up again.

The other option is to use fuel resistant RTV to seal the bowl, but you will need to run a small consistent bead, let it almost dry, and then put the bowl back on. This will make the bowl a huge pain in the ass to get off again though.

Ariel Red Hunter · 04-01-2018, 11:47 AM

Quote:

Originally Posted by BMF

It's a Mikuni and thank you. I have been watching too many YouTube videos, i think.

I was going to shim the needle but I can get that little tiny clip out. It's not the butterfly style but circular.

Right now I have stopped because I am lost. originally I was losing power with the stock carb at 3/4 throttle. I bought the Mikuni, replaced the main jet to 110 and all seemed good. After I left the bike shop today, it started to cut out and I would get a loud popping sound.

I tried the 100, 105, 110 (the best so far) and now have the 115. From what you are telling me I need to adjust either the small jet or needle. This may be beyond me. I work on servers for a living and find that easier. One and zero's...one and zero's...(server term)

It's not that difficult to change the pilot jet in a Mikuni. It is the other obvious brass piece in the flat bowl, quite near the main jet. I like to use the pilot jet that allows the idle screw to be at 1 1/2 turns out for a good idle. Then, when you have a rich enough main jet, you tune the mid-range by raising (to richen), or lowering the needle....ARH

BMF · 04-03-2018, 02:22 PM

It's being fixed at a local shop. I had my buddy who is a bike mechanic come by and check it and he says it's something beyond the carb. I will post the results when I get it back. I used the stock and Mikuni and got the same results.

**JerryHawk250** · 04-03-2018, 02:25 PM

Have you adjusted the valves yet? They usually come too tight from the factory.

BMF · 04-03-2018, 02:42 PM

I haven't even made it that far yet. I think it only has 56 miles on it. I just wanted to fix the 3/4+ throttle cutting out. That and the extremely low idle. idle is fixed but the mid to full throttle dies.

I will be fixed soon and then hopefully I can finally ride.

**JerryHawk250** · 04-03-2018, 02:52 PM

Valves being to tight will cause a lot of tuning issues. That should be one of the first things you should do. Many on here can confirm that. Mine had no play what so ever from the factory and the idle was all over the place. Once the valves were adjusted it idles smooth. Something worth checking.

**JerryHawk250** · 04-03-2018, 03:20 PM

Here's a good tutorial on valve adjustment. You can skip the steps on removing the tank as you can do it without removing the tank. https://www.cscmotorcycles.com/Articles.asp?ID=293

Ariel Red Hunter · 04-03-2018, 05:40 PM

Quote:

Originally Posted by BMF

I haven't even made it that far yet. I think it only has 56 miles on it. I just wanted to fix the 3/4+ throttle cutting out. That and the extremely low idle. idle is fixed but the mid to full throttle dies.

I will be fixed soon and then hopefully I can finally ride.

I would not be surprised if you have a vacuum leak causing this problem. Often people over-tighten the carb bolts when re-installing the carburetor. This distorts the flange of either the carb or the intake pipe, allowing vacuum leaks. After that, the next likely culprit is the intake pipe itself. They crack easily from too enthusiastic jiggling and wrenching....ARH

03-31-2018, 03:44 PM	#1
BMF Join Date: Mar 2018 Posts: 24	Backfire issues Question, I switched out the 98 jet in my carb to a 110. I went to get my bike inspected and the guy told me my bike was running lean and causing backfire. I switched to a 105 and I still get backfire when down shifting. Should I go to a 100 jet?
	0

03-31-2018, 06:08 PM	#4
Megadan Join Date: Apr 2017 Location: Omaha, NE Posts: 8,032	First thing to understand. When decelerating, and the throttle below 25%, the main jet is having almost no effect on your fueling. Decel popping like you describe is from a lean pilot jet (and its related idle mixture setting), not a main jet. Second, as already covered, the smaller the jet number, the leaner the jet. If you are already lean, you need to go larger. Third, there are 3 different jets that affect fueling, but each has the greatest effect in a given area of the throttle position. As stated before, up to around 1/4 throttle, the pilot/slow jet is the jet to focus on. between 1/4 and 3/4 throttle, the needle jet and needle. and beyond 3/4 throttle, the main jet. All of these jets will have a small impact on the other two, but it is very minimal. Is this on a stock carb or a Mikuni? __________________ Hawk Information and Resource guide: http://www.chinariders.net/showthread.php?t=20331 2018 Hawk 250 - Full Mod list here. http://www.chinariders.net/showpost....62&postcount=1 2024 Royal Enfield Shotgun 650 https://chinariders.net/showthread.php?t=34124
	1

03-31-2018, 06:51 PM	#6
Megadan Join Date: Apr 2017 Location: Omaha, NE Posts: 8,032	Best thing to do would be to start from a square one baseline. Intake and exhaust mods? What is your elevation? __________________ Hawk Information and Resource guide: http://www.chinariders.net/showthread.php?t=20331 2018 Hawk 250 - Full Mod list here. http://www.chinariders.net/showpost....62&postcount=1 2024 Royal Enfield Shotgun 650 https://chinariders.net/showthread.php?t=34124
	1

04-01-2018, 01:22 AM	#8
Megadan Join Date: Apr 2017 Location: Omaha, NE Posts: 8,032	I was going to link you to that seal, but I keep running into no results. This is supposed to be what you need, but they are unavailable anywhere. https://www.aliexpress.com/item/Pit-...652507296.html Hopefully they will show up again. The other option is to use fuel resistant RTV to seal the bowl, but you will need to run a small consistent bead, let it almost dry, and then put the bowl back on. This will make the bowl a huge pain in the ass to get off again though. __________________ Hawk Information and Resource guide: http://www.chinariders.net/showthread.php?t=20331 2018 Hawk 250 - Full Mod list here. http://www.chinariders.net/showpost....62&postcount=1 2024 Royal Enfield Shotgun 650 https://chinariders.net/showthread.php?t=34124
	1

04-03-2018, 02:25 PM	#11
JerryHawk250 Moderator Join Date: Oct 2016 Location: Houma, La. Posts: 11,574	Have you adjusted the valves yet? They usually come too tight from the factory. __________________ 2023 Lifan Lycan 250 Chopper 2023 Venom Evader 2022 Lifan KPX250 2020 Kawasaki Vulcan S 2004 Honda ST 1300 2016 Black Hawk 250 (sold) Keihin PE30 carb,125 main,38 slow.Pod filter,ported & decked head 10:1 CR,Direct Ignition Coil,15/40Sprockets,NGK DPR8EIX-9,De-Cat,Dual Oil Cooler,Digital Cluster 2016 Cazador180 XL 2014 Coolster150 JerryHawk250.com My YouTube Channel
	0

03-31-2018, 04:05 PM	#2
roundhouse Join Date: Sep 2017 Location: N Texas Posts: 706	your going leaner with the smaller jets. Mine pops a little bit but no back fires, I added a half turn out for a total of 2 turns to the idle mixture screw and it pops even less. I have a 120 jet in right now i think with the ebay exhaust. Stock exhaust on your bike? sorry I am no expert, I just did what the other guys did before me and posted and adjusted a little here and there.
	1

03-31-2018, 05:23 PM	#3
BMF Join Date: Mar 2018 Posts: 24	I have the ebay exhaust. I went from a 110 to 105 to 100 and it's getting worse. So you think i should go the other way and maybe try and 115 or 120?
	0

03-31-2018, 06:40 PM	#5
BMF Join Date: Mar 2018 Posts: 24	It's a Mikuni and thank you. I have been watching too many YouTube videos, i think. I was going to shim the needle but I can get that little tiny clip out. It's not the butterfly style but circular. Right now I have stopped because I am lost. originally I was losing power with the stock carb at 3/4 throttle. I bought the Mikuni, replaced the main jet to 110 and all seemed good. After I left the bike shop today, it started to cut out and I would get a loud popping sound. I tried the 100, 105, 110 (the best so far) and now have the 115. From what you are telling me I need to adjust either the small jet or needle. This may be beyond me. I work on servers for a living and find that easier. One and zero's...one and zero's...(server term)
	0

03-31-2018, 08:49 PM	#7
BMF Join Date: Mar 2018 Posts: 24	I just discovered the gasket was ripped for the bowl on the Mikuni. So I took my stock carb, Dremeled the bolts so i could open it with a screwdriver, added a new jet, put everything back together and it works. The stock carb was good but would crap out at 3/4 throttle. Hopefully the bigger jet will fix that and all will be good. I still need to test drive it. My neighbors have endured enough loudness for the day. I am at 300 ft elevation. I have been following MotoCheese on YouTube because he is in CT and I am in NH. Pretty much the same elevation. Thanks for your help!
	0

04-03-2018, 02:22 PM	#10
BMF Join Date: Mar 2018 Posts: 24	It's being fixed at a local shop. I had my buddy who is a bike mechanic come by and check it and he says it's something beyond the carb. I will post the results when I get it back. I used the stock and Mikuni and got the same results.
	0

04-03-2018, 02:42 PM	#12
BMF Join Date: Mar 2018 Posts: 24	I haven't even made it that far yet. I think it only has 56 miles on it. I just wanted to fix the 3/4+ throttle cutting out. That and the extremely low idle. idle is fixed but the mid to full throttle dies. I will be fixed soon and then hopefully I can finally ride.
	0

04-03-2018, 02:52 PM	#13
JerryHawk250 Moderator Join Date: Oct 2016 Location: Houma, La. Posts: 11,574	Valves being to tight will cause a lot of tuning issues. That should be one of the first things you should do. Many on here can confirm that. Mine had no play what so ever from the factory and the idle was all over the place. Once the valves were adjusted it idles smooth. Something worth checking. __________________ 2023 Lifan Lycan 250 Chopper 2023 Venom Evader 2022 Lifan KPX250 2020 Kawasaki Vulcan S 2004 Honda ST 1300 2016 Black Hawk 250 (sold) Keihin PE30 carb,125 main,38 slow.Pod filter,ported & decked head 10:1 CR,Direct Ignition Coil,15/40Sprockets,NGK DPR8EIX-9,De-Cat,Dual Oil Cooler,Digital Cluster 2016 Cazador180 XL 2014 Coolster150 JerryHawk250.com My YouTube Channel
	1

04-03-2018, 03:20 PM	#14
JerryHawk250 Moderator Join Date: Oct 2016 Location: Houma, La. Posts: 11,574	Here's a good tutorial on valve adjustment. You can skip the steps on removing the tank as you can do it without removing the tank. https://www.cscmotorcycles.com/Articles.asp?ID=293 __________________ 2023 Lifan Lycan 250 Chopper 2023 Venom Evader 2022 Lifan KPX250 2020 Kawasaki Vulcan S 2004 Honda ST 1300 2016 Black Hawk 250 (sold) Keihin PE30 carb,125 main,38 slow.Pod filter,ported & decked head 10:1 CR,Direct Ignition Coil,15/40Sprockets,NGK DPR8EIX-9,De-Cat,Dual Oil Cooler,Digital Cluster 2016 Cazador180 XL 2014 Coolster150 JerryHawk250.com My YouTube Channel
	0