Jetting advice for a new carb for the FXR

[hmurphy]

Hey guys! I have a new carb on the way for my FXR. It's apparently identical ish to the CB250 carb that everyone uses on their FXR's (I think it's the CB250RS carb). So... what kind of jets are you fellas running? I will update what carb I am getting for mine soon, forgotten the model of it for the moment. I have a friend helping me out. Cheers!

[F5 Dave]

You are hoping for a "Jam a 360 in it" type suggestion then forget it. Unless you are running the same motor with same inlet & exhaust & no other mods & someone else has put on the same crab with the same needle jet, airjet then there is too many variables. You can only go try it & try some other jets & record how it goes. No real shortcuts. Try small steps. See if you can borrow some jets but you need to know what type they are. Have a look on sudco or 'show&go' website or similar to identify.

[hmurphy]

Yeah sure I'll try that. I was told to ask about what jetting is used for the CB carb on an FXR to get a starting point that might turn out to be pretty damn close. Just going off advice I have been given by the guy who is sorting my carb for me. He asked me to post on here and see what others are doing and then I can go from there with my testing. Cheers Dave

[Rick 52]

135 main is a good starting point

[hmurphy]

Awesome thanks mate

[richban]

Hey guys! I have a new carb on the way for my FXR. It's apparently identical ish to the CB250 carb that everyone uses on their FXR's (I think it's the CB250RS carb). So... what kind of jets are you fellas running? I will update what carb I am getting for mine soon, forgotten the model of it for the moment. I have a friend helping me out. Cheers!

Did some google on the carb. Looks like a 30mm pumper. Could be a good option but a little big. But probably better than your current setup. I would put it on and see what happens. If it runs ok then do some plug chops and see where you are. The thing to remember is to get an accurate reading on the main jet is you need to be on full throttle for around 20 to 30 seconds. Bring it along on Sunday.

[hmurphy]

Ok so I have a few jets now. The jet that was in the carb originally was a 165. I have a 135 in it now and a 140, 155 and a 160 to test with. The bike runs ok with the new carb just idling, but likes lots of choke (even when warm I think). It can run with no choke but I have to be careful with the throttle or it stalls when trying to rev. Basically I can't wind the throttle really quickly or it bogs/stalls.

So.... am I ready for a plug chop test? Where should I go from here? I will need to use a home made airbox of some sort to fit the carb. Does anybody know websites that have really helped them? (Just so I don't keep posting here with questions that are already answered elsewhere). I have had a look at some websites but I have heard slightly different things from other people who have done this testing with their FXR's so I figure it's best to ask the pros.

Cheers guys, I will update any progress and let you know if this carb goes well with the FXR.

[Pumba]

mate have a read through this thread that I put up recently as I was going through a simular excercise and this was my first time doing such a thing.

http://www.kiwibiker.co.nz/forums/showthread.php/121837-Jetting-advice-VM29ss-on-a-FXR150

Biggest advice I can give is mark up your throttle so you can easily identify where in the circuit you are having issues, have a read of some of the links in the thread I linked.

[koba]

Ok so I have a few jets now. The jet that was in the carb originally was a 165. I have a 135 in it now and a 140, 155 and a 160 to test with. The bike runs ok with the new carb just idling, but likes lots of choke (even when warm I think). It can run with no choke but I have to be careful with the throttle or it stalls when trying to rev. Basically I can't wind the throttle really quickly or it bogs/stalls.

So.... am I ready for a plug chop test? Where should I go from here? I will need to use a home made airbox of some sort to fit the carb. Does anybody know websites that have really helped them? (Just so I don't keep posting here with questions that are already answered elsewhere). I have had a look at some websites but I have heard slightly different things from other people who have done this testing with their FXR's so I figure it's best to ask the pros.

Cheers guys, I will update any progress and let you know if this carb goes well with the FXR.

It is probably the same carb as Hells has.

If so its on the big side but should go OK.

It sound like you need to fiddle with the screw on the engine side under the float bowl, Hels bike was quite boggy and crap until I got that adjusted better.

Not sure in the super technical stuff but that seems to affect the mixture just off idle.

I played with the pumper but have left it disconnected as once set up it seems to run alright without it, maybe lots of fiddling would see some gains but so far it has worked better without pumper.

We have leaner main jet than 135 at the moment, but standard exhaust and a filter etc, Ricks advice sounds on the money.

The problem we have had is a real heavy throttle, would be a good idea to make up another cable to convert it to push/pull and run fuck all spring. (Daves brains there, not mine)

[hmurphy]

Just took it out for a quick spin with the new carb (it's my day off). It went ok I guess. It haaates quick throttle movements. If the revs are low and I pin it, it just bogs and eventually seems to sort itself out then revs like mad. I think it's pretty good up top to be honest. Seems to want to keep going up to the rev limiter (stock carb doesn't like rev limiter, no power near it), but that could just be me not getting a good chance to ride it properly. Didn't need any choke. Home made air box that only allows static air to be sucked into the carb has been fitted. 135 jet and needle was on highest setting and is now on lowest setting.

So:
-What would be the best thing to play with to eliminate how unhappy it is with quick throttle movements (from mostly anywhere in the revs)?
-The pumper isn't on yet so that might help I guess? (Should still work reeeasonably well without it though right?)
-Plug chop time?

Cheers

[F5 Dave]

Yes pumper should come into play when you open throttle. Ideally you should have it so that it is lean enough for best power on constant throttle, but likely too lean for acceleration. Whether the pumper is sophisticated enough to compensate is the sticking point. If it isn't then you may need to richen it either in the transition (raise or change needle) or main. Experimentation.

[Pumba]

My guess without hearing it and based on my own limited experience is that the pilot jet is rich or lean.

While stationary (only because I cant think of a way of doing it when running) get a bit of card limit the amount of air getting to your pilot air jet. Rev it, if it is better you need a richer pilot jet, if it is worse you need a leaner jet. It should make any difference above 1/4 throttle.

[hmurphy]

Ok.... carb is very close to being all sorted! Up to 3/4 throttle I am getting a really decent throttle response and quite a fair bit of power. It's actually really similar power to the stock carb and I'm not even full throttle yet. The throttle response is absolutely immediate (well, definitely compared to the stock carb!) but the main jet definitely needs some work because 3/4 throttle to full throttle is not very good.

The 135 jet that I was trying was pretty terrible. It would just splutter and surge and jump around. According to a bunch of sites I was looking at (I have done a fair amount of research on setting up carbs now) I was running the bike too lean. So I chucked in a 140 jet and wow the difference was really noticeable! It was spluttering less but still far from perfect. I thought I would try chucking in the 165 jet I have and seeing what would happen then, but sadly a van swerved off the road and smashed my carb bowl and tools that were sitting just off the road.

That's not the end of the story haha. I now have a new carb bowl so I can start the testing again.

I was wondering if anyone would be so kind as to lend me a few jets of theirs to test with? I will return all jets and even pay for the one I need. If you aren't willing to sell me any, I would really appreciate the chance to borrow jets and then go out and buy the one I need (or buy the replacement for the person who lent me theirs so they can have the new one).

1 other thing, spark plugs are cheap for FXR 150s right? To do more accurate plug chop tests (once the bike is running ok) I guess I will need a new spark plug.

The pumper on my carb will be one of the very last things I will bother playing with but I can see how it might be very valuable. Although the throttle response is really good and it hardly bogs at all down low, I think it might be running a bit lean near the pilot circuit or when the needle is reasonably low. An extra squirt of gas might make things more interesting!

Cheers guys!! Any help is muuuuch appreciated!

[F5 Dave]

you will need to tell people what types of jets you are after, ie Keihin & Mikuni both have a couple of options. Find a pic on a website & post it.
Plug chop on a 4 stroke on unleaded might not tell you too much unless you are running it for ages. New plug will be harder to read.

[hmurphy]

It's a Mikuni carb and the main jet looks like this: (Part number MI-4/042)

http://cycletreads.retail.sprint3.com/package/webservice/kernel_resources_render.asp?strResourceCode=21.752

Yeah I have been given advice about the plug chop test. Apparently I need to have it full pin in top gear for at least 20 seconds to get a decent reading. I will be able to do this, although it's gonna make a hell of a noise.

[F5 Dave]

but where can you hold it pinned in top gear for 20 seconds? Just jet it so it goes best without farting & then compare against other bikes & minor changes next kart day.

[hmurphy]

Ah fair enough, I will give that a go. Well it isn't legal but it's responsible where I have been testing. There are hardly any cars around at all and no houses also. So I am out of mostly everyone's space. Also it only does probably 90km/h max with the gearing in it at the moment so I don't go far in top gear with it pinned for 20 seconds haha. Hey cheers for the help Dave. Just need jets to test with now haha

[hmurphy]

Sorry for digging this thread up again, but I need help still haha.

Basically a van ran over the bowl of my carb while out testing a while ago and I have finally got a new carb bowl and it's all fixed. Long story, but some people must not like bikers or something..? This is why it has taken so long.

Testing was going well and now I have the bike running again, I just have no time to really test it and get things sorted. Too many assignments etc. How easy would it be to take it to a shop and get them to sort it? Would it cost much? All it needs is tuning (jets, needle, etc). Where is a decent shop in Wellington for this sort of thing?

At the moment it doesn't start easily at all (it did before the van hit the bowl) but once it is running, up to half throttle it's pretty good!! Main jet is shit and it doesn't like it when I wick the throttle fast. It was actually really close to being sorted before a van ran over the bowl (the rest of the carb was in my hand).

Also, it seems to only start with the choke, even when hot and I have to give it a really decent crash start. So it's fair to assume it needs more gas on start up?

Cheers

[Pumba]

well if you are talking about taking it to a shop you are talking Dyno time. Defently one of the best ways to tune the bike but it costs.

Wellington motorcycles is the only dyno shop down there I know of.

[F5 Dave]

I wouldn't know of any shop who has the experienced staff to do a from scratch jetting. They all seem to leave. It will end up being more expensive than you would stomach. If it doesn't start easily you don't need a dyno (won't help) to fix that. Get that sorted first. Check pilot jets aren't blocked. Or float height isn't crazy.

[richban]

I'm with Dave. There used to be people but they seem to have disappeared. Still might cost you $300 with the right person on the Dyno. Go See Andrew A and ask to look at his carb. I think it is the same as yours then at lease you will have a good start point. Failing that get yourself to the track and jet the main first. Work back from there. I would be happy to help out one weekend but I am away from this weekend until the next round. So know good to you really.

[hmurphy]

WAAHOO!! Found a major problem. The air fuel mix screw was completely missing! The bloody thing was running far far too lean for starting and idling. I have shut that whole valve off (I know it's not correct, I will have to find a proper valve) and it idles mint now, even when hot. It might be running a tad too lean though on the pilot circuit still, so I should really get a proper pilot jet but hey, it's not running too badly off the pilot. Also, I have the needle pretty spot on I believe and I just went out and tested the main jet.

THIS IS THE JET I WANTED TO TEST BEFORE A F*CKING VAN RAN OVER MY CARB BOWL!!

After this test, the carb is pretty much ready to race with. It was running MINT!! It pulls better for longer up top for sure. I think that I need a bigger jet still though. I am currently running a 165 standard Mikuni carb hex jet (the common one... MI-4/042) but I would like to know if anyone would want to lend me some others for testing? Maybe 170 ish?

I will most likely bring this to Slipway for testing, might as well.

Cheers!