Honda CBR125R 2006 Carburetor Dilemma

Recently bought K+N air filter and Black Widow exhaust along with a stage 2 jet kit to improve the performance of my bike. However while stripping the carburetor I attempted to count the turns of the fuel mixture screw as a starting point before I installed the stage 2 kit and found the screw was rock solid. Due to this in my efforts to try and free the adjustment screw I broke one side of the "flat head" screw rendering it irremovable unless I were to drill it out. The jet kit doesn't come with an additional mixture adjustment screw so I was forced to put the carburetor back together as it was when I removed it with the stock jets. The bike runs acceptably but just rich I've gathered, and between 5 and 7k RPM sometimes the odd "chug" if I linger at those RPM at a consistant speed. I can accelerate hard completely fine and it idles fine from cold. I'm just wondering now what the best move is; to ether go ahead and buy another carburetor for my bike which is hard to find from my efforts, to find another carburetor that would fit my bike and the stage 2 jet kit I bought originally for the stock carburetor or to drill out the existing mixture screw to replace with a new one but risk damaging the carburetor in the process.

My plan was originally to install the exhaust and air filter, clean and install the stage 2 jet kit into the stock carburetor and then adjust the fuel mixture to compensate for increased airflow, additionally I was planning on swapping out the stock drive sprocket of 15 teeth for one of 14 teeth to improve acceleration. Any advise and constructive criticism is greatly appreciated.

Many Thanks

Michael

Put it back to standard and just change the gearing.

Put it back to standard and just change the gearing.

 

I'd rather not as I've spent a substantial amount of money on these parts and the aim is to improve the performance as much as possible as I'm going to have the bike for at least another year and a half; also learn along the way.

You should be able to get another screw quite easily and an easy-out should sort the extraction.

You might want to soak it in WD40 or ATF/acetone before you try though.

Even if you knack the thread you can re-tap or helicoil it.

This is a pilot air screw and is only responsible for slow running and idle above that and the main jet kicks in . If you couldn't move it it's a fair bet that it is set at roughly the right number of turns out . I know that Dynajet instructions specify a certain setting but how close is this to the standard setting ? So if the bike runs ok you might be ok to just leave it alone for now . If you do decide to have it drilled to accept an extractor then make sure whoever you take it to really knows what they are doing. This is a pretty delicate process requiring great precision that might be beyond the scope of an ordinary mechanic.I can recommend this company but please bear in mind that it is a one man operation and Graham is now semi retired.http://i.imgur.com/4Pt892E.png

Put it back to standard and just change the gearing.

 

I'd rather not as I've spent a substantial amount of money on these parts and the aim is to improve the performance as much as possible as I'm going to have the bike for at least another year and a half; also learn along the way.

I can't see why you didn't go ahead and install the dynajet kit anyway, most of what you pay for is in the shape of the needle and the appropriate jet might cure rich running. As I have said , the pilot air screw might not play as big a part as you believe.

I will go ahead and fit the new jet kit and see how it runs and get back to this post.

Put it back to standard and just change the gearing.

 

I'd rather not as I've spent a substantial amount of money on these parts and the aim is to improve the performance as much as possible as I'm going to have the bike for at least another year and a half; also learn along the way.

I can't see why you didn't go ahead and install the dynajet kit anyway, most of what you pay for is in the shape of the needle and the appropriate jet might cure rich running. As I have said , the pilot air screw might not play as big a part as you believe.

 

put the stage 2 jet kit in, it runs without chugging now at all RPM ranges from my test ride this evening. However when i rev the bike it takes almost double the time for the revs to return to idle. must be mixture again but i cant do anything as the screw is totaled. it idles consistently but sometimes it sounds a little choppy other than that its fine and a usable bike.

I'd rather not as I've spent a substantial amount of money on these parts and the aim is to improve the performance as much as possible as I'm going to have the bike for at least another year and a half; also learn along the way.

I can't see why you didn't go ahead and install the dynajet kit anyway, most of what you pay for is in the shape of the needle and the appropriate jet might cure rich running. As I have said , the pilot air screw might not play as big a part as you believe.

 

put the stage 2 jet kit in, it runs without chugging now at all RPM ranges from my test ride this evening. However when i rev the bike it takes almost double the time for the revs to return to idle. must be mixture again but i cant do anything as the screw is totaled. it idles consistently but sometimes it sounds a little choppy other than that its fine and a usable bike.

Don't be jumping to any conclusions just yet have a good read of the Dynajet troubleshooting guide . I'll have a look to see what sort of carb it is before offering any further suggestions.

Ok, so its a CV carb . I've read the dynojet instructions . Did you drill out the little hole in the slide ? When you assembled the slide , needle , diaphragm and spring did you ensure everything was properly seated and all in the right order ? If you're not sure , do it again . A good test is remove the air filter and if you can get your hand inside the airbox , push the slide up with a finger and let it go . It should snap back down again pretty smoothly and quickly but not instantly. The reason I ask is that I suspect that the engine is not returning to idle due to an air leak, an obstructed slide, a kinked spring or the needle not seated . Give all this a go before blaming that mixture screw.

I can't see why you didn't go ahead and install the dynajet kit anyway, most of what you pay for is in the shape of the needle and the appropriate jet might cure rich running. As I have said , the pilot air screw might not play as big a part as you believe.

 

put the stage 2 jet kit in, it runs without chugging now at all RPM ranges from my test ride this evening. However when i rev the bike it takes almost double the time for the revs to return to idle. must be mixture again but i cant do anything as the screw is totaled. it idles consistently but sometimes it sounds a little choppy other than that its fine and a usable bike.

Don't be jumping to any conclusions just yet have a good read of the Dynajet troubleshooting guide . I'll have a look to see what sort of carb it is before offering any further suggestions.

 

Its a Sigma jet kit, ill have to have an in depth read but your input would be greatly appreciated.

Ok, so its a CV carb . I've read the dynojet instructions . Did you drill out the little hole in the slide ? When you assembled the slide , needle , diaphragm and spring did you ensure everything was properly seated and all in the right order ? If you're not sure , do it again . A good test is remove the air filter and if you can get your hand inside the airbox , push the slide up with a finger and let it go . It should snap back down again pretty smoothly and quickly but not instantly. The reason I ask is that I suspect that the engine is not returning to idle due to an air leak, an obstructed slide, a kinked spring or the needle not seated . Give all this a go before blaming that mixture screw.

 

I didnt drill the hole as i dont have a drill in my workshop currently. and thank you i will give these things a try. Im sure everything was seated again properly, i checked the slide before i reinstalled it and it appeared normal.

Ok, so its a CV carb . I've read the dynojet instructions . Did you drill out the little hole in the slide ? When you assembled the slide , needle , diaphragm and spring did you ensure everything was properly seated and all in the right order ? If you're not sure , do it again . A good test is remove the air filter and if you can get your hand inside the airbox , push the slide up with a finger and let it go . It should snap back down again pretty smoothly and quickly but not instantly. The reason I ask is that I suspect that the engine is not returning to idle due to an air leak, an obstructed slide, a kinked spring or the needle not seated . Give all this a go before blaming that mixture screw.

 

I didnt drill the hole as i dont have a drill in my workshop currently. and thank you i will give these things a try. Im sure everything was seated again properly, i checked the slide before i reinstalled it and it appeared normal.

Hold on , don't drill any holes ! Unless the Sigma kit also says that you need to . If it does then there should be a drill BIT in the kit . I looked at the DYNOJET instructions so the Sigma kit could be different. To be quite clear, if the Sigma kit says to drill a hole to make an existing hole larger and you didn't do so that could explain the slow drop in revs. If you do have to drill use a HAND drill not an electric drill and do it carefully . Good luck.

Ok, so its a CV carb . I've read the dynojet instructions . Did you drill out the little hole in the slide ? When you assembled the slide , needle , diaphragm and spring did you ensure everything was properly seated and all in the right order ? If you're not sure , do it again . A good test is remove the air filter and if you can get your hand inside the airbox , push the slide up with a finger and let it go . It should snap back down again pretty smoothly and quickly but not instantly. The reason I ask is that I suspect that the engine is not returning to idle due to an air leak, an obstructed slide, a kinked spring or the needle not seated . Give all this a go before blaming that mixture screw.

 

I didnt drill the hole as i dont have a drill in my workshop currently. and thank you i will give these things a try. Im sure everything was seated again properly, i checked the slide before i reinstalled it and it appeared normal.

Hold on , don't drill any holes ! Unless the Sigma kit also says that you need to . If it does then there should be a drill BIT in the kit . I looked at the DYNOJET instructions so the Sigma kit could be different. To be quite clear, if the Sigma kit says to drill a hole to make an existing hole larger and you didn't do so that could explain the slow drop in revs. If you do have to drill use a HAND drill not an electric drill and do it carefully . Good luck.

 

it said to drill the hole but it was usually only for race applications and it could change behaviour drastically. so i didnt bother atleast not yet, so what do you think my best bet for the slow rev drop is along with the other stuff you mentioned?

I agree , drilling the slide is stated as optional in the kit contents so probably best not to bother. As I say just go through everything again just to be sure the slide can rise and fall smoothly. If its ok then I guess there's nothing else for it but to get that screw extracted and replaced with a new one. The only remaining thing you could try is to lower the main needle one notch by re positioning the circlip up a notch .

I wouldn't drill the slide either.

Revs "hanging" after throttle blipping is a lean condition due to the mixture screw. You'll either have to sort that screw out or live with it.

I agree , drilling the slide is stated as optional in the kit contents so probably best not to bother. As I say just go through everything again just to be sure the slide can rise and fall smoothly. If its ok then I guess there's nothing else for it but to get that screw extracted and replaced with a new one. The only remaining thing you could try is to lower the main needle one notch by re positioning the circlip up a notch .

 

mine didnt have any circlip. as indicated on the instructions that came with the kit it said if your carb doesnt have a circlip then add 2 washers to raise the needle manually, so ill just have to remove the washers or just trial and error etc. worst come to worst ill have to replace the needle/carb. thanks for your input its greatly appreciated and ill get back and let you know how i get on and if i fix it and if so how

I wouldn't drill the slide either.

Revs "hanging" after throttle blipping is a lean condition due to the mixture screw. You'll either have to sort that screw out or live with it.

 

my bike pops and visibly backfires on deceleration, could this be rich in high revs and lean in low maybe? im still trying to learn

my bike pops and visibly backfires on deceleration, could this be rich in high revs and lean in low maybe? im still trying to learn

 

Yes, absolutely.

Here is a really good guide for carb tuning using the arse-cheek-dynamometer.

If it really pops and bangs on the overrun, check that there's no leaks on the headers......or, if it has secondary air injection, that's not been disabled......