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Hello all, new here but not to bikes. I recently picked up a 08 crf100 in need of a lil tlc. As far as I could tell when purchasing, everything seemed ok. It ran good, compression was fine, fired up first kick hot or cold so I bought it for $950. Seemed like a deal, but now not so sure... Only weird thing was the choke position didnt change the way it ran, but it started great so I figured whatever I'll figure it out. Whenever I get a bike I like to go through the basic stuff; plugs, carb, air filter, tires, etc. So I pulled off the carb, and noticed the butterfly was sticking open when actuated by the choke lever. So I soaked the body, bowl, and jets in some berrymans chemdip (worked great on all my other bikes), sprayed compressed air through all passageways and jets, and put it all back together. I went to fire it up, and now itll only run with the choke on. Nothing changes between position 2 and 3 (1 being closed; 3 being completely open). So I pulled the carb back off, and re checked the jets for clogs... everything was ok. I resprayed compressed air through all the passageways and could feel compressed air coming out, so at this point I can rule out a dirty carb. I checked for air leaks with some electrical spray, and couldnt find any on either side of the carb. Mind you this whole time Ive been fiddling with air/fuel screw on bottom and the idle screw on the side because I figured it was a tuning issue but at this point I think not. So I checked the plug by taking it out and putting it against the engine... very good spark. This is very frustrating as I have no idea what could be causing it at this point. I inspected all the rubber components (including a/f seat) and they were all in good condition. Some things I noticed; with intake boot off (exposing butterfly and 2 holes on bottom) and choke open , I can see a fine mist being generated when I open and close the throttle, this carb doesnt have an acc. pump but whatever it has instead I'll assume is working; I can get it to idle ok with the choke on by adjusting the idle and a/f screws; it will run without choke if I play with the throttle, but it runs like crap; also with the choke open, intake boot off when I plug the left hole on the bottom it will begin to die, and if I let it die, the right hole will spit out a little bit of fuel. I plan to eventually do valves/cam chain tension but I highly doubt that those would cause it to not idle at all. Any wisdom/pointers very appreciated. This ones got me stumped... and Ive seen it all lol.

You need to start over. It ran fine before and now it don't. Assuming you have fresh fuel, clean air filter, good fuel flow. You have already verified no air leaks, good ignition and spark.
Start with identifying that the jets MJ & PJ are the correct ones. Then float level is correct. It sounds like you have already verified the fuel screw is fine and the o-ring, spring and screw are in the correct orientation. Also, you cannot try to trouble shoot jetting with the air box removed or filter out. Jetting requires the exact same conditions while trying to evaluate what is wrong and that means the system has to be in the condition that it will be in when being used. It sounds from your description that you have something wrong in your primary circuit. That is where you need to be looking.

You need to start over. It ran fine before and now it don't. Assuming you have fresh fuel, clean air filter, good fuel flow. You have already verified no air leaks, good ignition and spark.
Start with identifying that the jets MJ & PJ are the correct ones. Then float level is correct. It sounds like you have already verified the fuel screw is fine and the o-ring, spring and screw are in the correct orientation. Also, you cannot try to trouble shoot jetting with the air box removed or filter out. Jetting requires the exact same conditions while trying to evaluate what is wrong and that means the system has to be in the condition that it will be in when being used. It sounds from your description that you have something wrong in your primary circuit. That is where you need to be looking.

It was running fine before because the butterfly was stuck open, acting as choke all the time regardless of the choke lever position. its got fresh fuel, I cleaned the tank replaced the line and added a filter. The jets were ok, I soaked them then ran my carb wire tool through them and looked through to ensure they were clear. The float bowl was sagging slightly so I bent it up a little but that didnt change anything. You said the primary circuit could be problematic? At this point what should I do to further clean that? I already soaked it in chemdip for 48 hrs then blew out with compressed air and could feel it coming out the passageways... Appreciate the pointers!

Sorry for the books lol, I found this carb on eBay for $35 shipped. Not bad considering just the body of one of my carbs on my 450 is worth almost $500. Anyone have experience with this carb or any aftermarket carbs. I’ve been taught to work with what you got but when a rebuild kit is 75% the cost of a new carb it may just need replacement. If it’ll run like crap I’m not interested but if $35 and some tuning fixes my issues I’m all for it.

Have to agree with Neil that it is in the idle circuit.
Remember there are two passage ways in an idle jet, one vertical one horizontal. Well the jets I have do.
I would confirm the idle passage way is free and replace the idle jet to be sure.

Not saying this is the cause but in the past there was a post and the choke lever was being used back to front. They had assumed it was the same as a previous bike.

Is your carb like this one
http://fiche.worldofpowersports.com/yel ... CARBURETOR

Some of the description of the carb sounds a bit off and don't know if is the carb or just a difference in terminology we all use.

Have to agree with Neil that it is in the idle circuit.
Remember there are two passage ways in an idle jet, one vertical one horizontal. Well the jets I have do.
I would confirm the idle passage way is free and replace the idle jet to be sure.

Not saying this is the cause but in the past there was a post and the choke lever was being used back to front. They had assumed it was the same as a previous bike.

Is your carb is like this one
http://fiche.worldofpowersports.com/yel ... CARBURETOR

Some of the description of the carb sounds a bit off and don't know if is the carb or just a difference in terminology we all use.

yeah my carb is like the one you linked... what do you mean choke was being used back to front? On the jets, are the horizontal holes on the bottom of the shaft that slides into the carb body? if those are what you are talking about they are all clear. I dont want to throw money at it unless its an entire new carb as a pair of jets is like more than half the cost of a whole new carb. does anyone have a diagram of the passageways for the carb I have so I can get a better understanding of what im cleaning for the idle circuit? is there a carb you guys recommend as an aftermarket replacement? this mikuni carb looks almost identical to mine apart from the choke, but can anyone confirm it works good? https://www.ebay.com/itm/OEM-Mikuni-Car ... 1438.l2649 I dont want to make this thing run like crap with some knockoff Chinese crap, but if that or another one works I'm all for replacing.

The other poster had the choke fully closed (lever up) which he had thought was the open position , which is actually lever down in a crf.
He had a previous bike where the lever had to be down to close choke butterfly.

The other poster had the choke fully closed (lever up) which he had thought was the open position , which is actually lever down in a crf.
He had a previous bike where the lever had to be down to close choke butterfly.

When you push the choke lever down, the disc opens; when you push it back up it closes. I'm not quite sure what your asking but if thats how its supposed to be then we're good there. I ended up ordering this https://www.ebay.com/itm/MIKUNI-Carbure ... 0570972265 as all the reviewers who put it on their crf100 or xr100 gave it 4 stars+. looks good to me! will update when its here with how it runs.

The other poster had the choke fully closed (lever up) which he had thought was the open position , which is actually lever down in a crf.
He had a previous bike where the lever had to be down to close choke butterfly.

When you push the choke lever down, the disc opens; when you push it back up it closes. I'm not quite sure what your asking but if thats how its supposed to be then we're good there. I ended up ordering this https://www.ebay.com/itm/MIKUNI-Carbure ... 0570972265 as all the reviewers who put it on their crf100 or xr100 gave it 4 stars+. looks good to me! will update when its here with how it runs.

So i got the carb, its a Chinese knockoff which i presumed due to the low price. However the bike is not running right with the carb. It starts ok choke on or off, but it uses a different choke mechanism than the oem carb (a plunger not a butterfly). When i open the throttle as fast as possible, the bike would bog and die, and it only ran close to right with the needle in the richest position. So I tried sucking on the old carb, through the intake side with the choke closed, covering all openings and it was a very hard pull. I had already soaked this carb for 4 days in berrymans chemdip a second time so if that wont get it im not sure what would. I ordered a new oem carb. And even with this brand new carb it runs the way the old carb ran! Dies as soon as I turn off the choke, acts exactly like a killswitch in the way it dies. If i rev it a little with choke on and then turn it almost all the way off (holding it ever so slightly open) it will rev and idle seemingly normally but as soon as it gets to the closed position itll die leading me to believe its an air/fuel issue. The needle on the oem carb has only one shim setting, I ordered a carb kit for like $10 with needle with multiple settings. I also checked the float height and its exactly where it should be on both oem carbs. I've already checked for air leaks all around the intake boot to the engine and there are none, if theres anywhere else I should check someone please let me know. This little "simple" dirt bike is more difficult to diagnose than any of my other bikes. Really beginning to think i got ripped off.

So with the new carbs what are the main and pilot jet sizes and have you adjusted the idle fuel mixture screw. You can't expect a brand new carb to be jetted or adjusted for your bike no matter what the add says. Just because it has standard jets doesn't mean they will be right for your engine.

As you said definitely running lean at idle so either there is too much air from a leak somewhere or jetting and adjustment is out.

Be aware you can't just open the throttle and expect it to rev as there is no accelerator pump on these little carbs. Should work by rolling the throttle on from idle.
Since it was running when you bought it even with the choke stuck the problem is something that has happened since then.
Have you confirmed fuel flow from the tank is actually getting to the carb.

Other than that I would check compression and valve adjustment. Do compression test before adjusting the valves. Doesn't matter if it is cold it will give you a baseline.

So with the new carbs what are the main and pilot jet sizes and have you adjusted the idle fuel mixture screw. You can't expect a brand new carb to be jetted or adjusted for your bike no matter what the add says. Just because it has standard jets doesn't mean they will be right for your engine.

As you said definitely running lean at idle so either there is too much air from a leak somewhere or jetting and adjustment is out.

Be aware you can't just open the throttle and expect it to rev as there is no accelerator pump on these little carbs. Should work by rolling the throttle on from idle.
Since it was running when you bought it even with the choke stuck the problem is something that has happened since then.
Have you confirmed fuel flow from the tank is actually getting to the carb.

Other than that I would check compression and valve adjustment. Do compression test before adjusting the valves. Doesn't matter if it is cold it will give you a baseline.

thanks for the advice. its running the same as the old oem carb, still running ok with choke on, but once choke is off its like a killswitch the way it dies. the carb i bought was an oem honda part keihin carb. I would assume that its got the 98 main and 35 slow jets as that is oem spec. the a/f screw it came with was non-adjustable so I replaced it with the adjustable one from my original carb and played around with it in spec (1.5-2.5 turns out). Ive got a tusk cold start jet kit on the way already (was hoping to wake this things up a bit) which comes with 100 main and 38 slow jets. Ive also played around with my new adjustable needle but it doesnt seem to want to run without the choke. Im thinking this is an air issue as the choke only seems to affect the amount of air going in. Where specifically should i check for air leaks? i already sprayed electronics cleaner around the intake boot for the engine side of the carb and no change in idle; and this is a new carb so I wouldnt assume theres any air leaks on the body.thanks again well see how jet kit helps.

The only other place it could leak air into the engine is where the boot bolts/ clamps to the cylinder head. Either that or the boot itself.
I know it is a pain but might be worth removing from bike for a closer inspection. A pinhole in the boot can really open up under a vacuum.
There is a gasket and an o ring at the head end, perhaps one is missing.

Since you have tried 3 carbs it points to an air leak between the carb and the engine.
I have never found spraying the boots with anything other than a combustible to really work and don't recommend doing it either.

The only other place it could leak air into the engine is where the boot bolts/ clamps to the cylinder head. Either that or the boot itself.
I know it is a pain but might be worth removing from bike for a closer inspection. A pinhole in the boot can really open up under a vacuum.
There is a gasket and an o ring at the head end, perhaps one is missing.

Since you have tried 3 carbs it points to an air leak between the carb and the engine.
I have never found spraying the boots with anything other than a combustible to really work and don't recommend doing it either.

yea on the new carb it had the o-ring engine side, and to put the chineese carb on i had to take off the intake boot. because of this i ripped the insulator so i had to replace so i got a 2pck and put one on the carb side too. this was a little while ago, didnt change the running condition. I'm really stumped on this one. I dont feel like its a jetting issue because it would just run like crap if it was but it would defiantly idle unless the jetting was way off (which is currently stock). where else should i check for air leaks? I also replaced the gasket in the cup that attaches to the cable that screws onto the top of carb. I feel like ive covered all the bases and im not getting anywhere just emptying my bank account we'll see if its jetting when it gets here in 30 minutes

The only other place it could leak air into the engine is where the boot bolts/ clamps to the cylinder head. Either that or the boot itself.
I know it is a pain but might be worth removing from bike for a closer inspection. A pinhole in the boot can really open up under a vacuum.
There is a gasket and an o ring at the head end, perhaps one is missing.

Since you have tried 3 carbs it points to an air leak between the carb and the engine.
I have never found spraying the boots with anything other than a combustible to really work and don't recommend doing it either.

yea on the new carb it had the o-ring engine side, and to put the chineese carb on i had to take off the intake boot. because of this i ripped the insulator so i had to replace so i got a 2pck and put one on the carb side too. this was a little while ago, didnt change the running condition. I'm really stumped on this one. I dont feel like its a jetting issue because it would just run like crap if it was but it would defiantly idle unless the jetting was way off (which is currently stock). where else should i check for air leaks? I also replaced the gasket in the cup that attaches to the cable that screws onto the top of carb. I feel like ive covered all the bases and im not getting anywhere just emptying my bank account we'll see if its jetting when it gets here in 30 minutes

So i tried the jetting, nothing. a little quicker though and it smells richer. I adjusted the valves, they were out of spec, about .07-.08mm when spec is .05. Still nothing. Top end is a lot quieter. Also I did the cam chain tension before the valves and that didnt help either. I really dont want to throw more money or time at this thing, so does anyone have any final advise for me before I drill a tiny hole in the flapper in the old carb? The bike will run if i hold the flapper ever so slightly open leading me to believe a hole would allow a little more air in making it run even with choke off. I know its redneck but when the number youve invested into the thing is getting close to what its worth some things just arent worth it.

Seems to be a bit of difference in terms still so humour me so I know we are communicating correctly.

Choke on equals butterfly closed, restricts airflow from aircleaner side into carb
Choke off equals butterfly open, no restrictions in carb except throttle slide.

So choke on to start from cold
Choke off to start hot, sometimes might have to be part choke when only warm due to small idle jet. Same position for normal running.

Is that your understanding.

So are you finding that the choke needs to be almost closed for the bike to run properly at any rpm? Or just to accelerate from idle.
Does it run ok at idle, does it run ok at riding engine speed.
What are you referring to as a flapper.
Have you had a chance to confirm compression pressure.

Sorry it is a bit long just trying to get all the issues straight in my head.
I am sure you know what that's like at the moment.