jetting problem came up out of nowhere

[mscottiel]

i have a ltz434 with a fcr40 mm carb. altitude is 750 ft.

motor mods are

tc full porting using his cams

full rossier exhaust

white bros. cdi

athena 434 kit

fcr 40mm carb jetted as follows

pilot jet 42

main jet 160

ocemn needle 3rd clip down from top

leak jet 55

fuel screw 2 turns

i established this jetting using advice from burned and tweaking it a bit after testing.

this quad ran great for months with no problems and no bog and at a recent race it all of a sudden developed the dreaded off idle bog. this bog was so bad that it would sometimes die if the throttle wasn't backed off. i adjusted the fuel screw and it helped a bit but not much. i checked the ap fuel squirt and it still looks good. its not hitting the slide. it runs great when i roll the throttle on but not when i open it quickly. when i got home i experimented with the fuel screw by screwing it all the way in and it did not effect the idle. i put in a 40 pilot jet but it didn't seem to help any either. i removed the carb and totally cleaned all jets and passageways including the air mixture jet. i am certain there is nothing foreign in the carb. i am confused why all of a sudden it would not be working right when for so long it did. it thought it may be the heat but a month ago when it was just as hot it ran great. could it have something to do with the float level. i haven't messed with it but am not sure how or where to set it. i know this is the most common issue for fcr carbs but i have tried all solutions with no progress so any help would be great. i need to get it going so i can go racing again. here is a link to the thread that burned helped me with to get it right the first time.

https://www.thumpertalk.com/forum/showthread.php?t=284268

[Wayward Son]

does that carb have a hot start?

[mscottiel]

if the hot start is located above the choke then yes but it is blocked off. i removed the hot start and choke and made sure it was clean and not stuck open. hot start is ok as well as the choke. if i open up throttle quickly it will die if i don't let off. if i let it die it is difficult to start without opening up the throttle so i think it is flooding out but i am not sure. is it possible that i need to go down with the pilot jet to a 40. like i said it will not respond to the fuel screw with the 42. i raised the clip one notch but that didn't do anything. thanks

[mscottiel]

i decided to go down to a 40 pilot like the jetting guide for pilot circuit suggests since i cannot get the fast idle to respond to the fuel screw but even with the 40 in it will not respond to the fuel screw. what am i missing?

it used to run great with the 42 pilot, and burned suggested a 45 pilot when i first set it up. my only guess it that it is pulling fuel from somewhere but i cannot figure it out. help please.

thanks

[Wayward Son]

sounds like the needle jet is worn out.

[mscottiel]

that makes sense burned. it has to be pulling fuel from somewhere.

what would be the best needle and needle jet to get for this. i have the ocemn but since i am going to replace the needle jet i am going to get a new needle too seeing as they are both brass and both are probably worn out. also where should i get this from. is the stock needle in the 02 - 04 crf 450r carb ocemn?

thanks

scottie

[duke2au]

Just wondering if you got this fuel screw problem sorted out? Mine is also shut and it still idles... ?

[mscottiel]

well i did a compression check and it was very low on compression so i put in a stroker crank and piston but have not got it back together yet but it will be soon. i will update it when i get it all done.

[mscottiel]

i finally got this back together and since today was dry and about 70 degrees out i decided to test it and break in the new piston and crank. it runs good but it still has the problem i ran into before i tore it down. i have a 40 pilot in with a sunline adjustable pilot screw. OCEMN needle and new needle jet and a 160 MJ. i still get a bog off idle while riding and the idle doesn't react to the fuel screw if it is turned all the way in or all the way out. where is it getting the extra fuel from? i have helped all my friends figure out their carbs and i thought i was getting good at it until this came up!!! ?

[Dave Hopkins]

I don't understand why it was OK, then went down hill, Yes needle jets do wear but very slowly, I was questioning engine compression as I read my way down this but it sounds like you fixed that. I have found best throttle responce by having the mixture transition from lean to rich as you twist. Your OCEMN is fairly rich in that first stage of the needle, just off idle thru 1/4 throttle. The last letter is that first stage and each letter down the alpebet is1/10mm leaner and they skip "O" (one letter is not worthwhile to change) so the next mini step is "P". I would seak out a OCEMQ. IF that one is not available OCEMR is still in reasoanable range but "R" is as far as I would go.

[mscottiel]

so dave

do you think fuel is being pulled up the MJ at idle and that is why the fuel screw will not change the idle?

[Dave Hopkins]

Yes/partly, some fuel is coming up the main, between the first stage of the jet needle (I call it the shank of the needle) and the needle jet at idle. An OCEMN is rich in that shank area.

Before we too far into diagnosus, you need to ask your self a question as you ride. If you roll the throttle on at a slower rate will it runn proper? But when you whack it it bogs? That is combo of AC pump and how it interacts with jetting. The pump is flooding the engine, you can help the engine burn the fuel by;

1: Raising the combustion chamber temp by making the shank of the needle bigger, thus leaner, thus hotter combustion chamber. Thus my "lean to rich transition"

2: Reducing the amount of time the AC pump squirts. Now I do not have an explanation, wish some one did but with too long of a squirt duration it will bog instantly, yet you shorten the duration say from 3 seconds to one and it improves, you increase the fuel pressure* of the squirt and it improves.

3: Raise the idle speed

4: Ride in loose soil? Just had to toss that in but the AC pump is needed from the time you twist to the time the engine RPM is "spooled up", which lets guess is 3,500 RPM. Thus tall geared 100% traction pavment guys need about a 3 second squirt where low geared wheel spining dirty guys need 1!

*fuel pressure at the AC pump is raised by putting an Oring around the linkage, this acts as an additional spring to press on the pump

Go to https://www.thumpertalk.com/forum/showthread.php?t=464299

and read the carb section I posted including AC pump 101

[James_Dean]

The pilot jet has 2 passages that feed the venturi. Your fuel screw only meters one of them, allowing the other to continue to fuel your motor when the fuel screw is closed.

The needle jet/needle also supply fuel to the motor at idle too.

Try going back to your original jetting and wrap a wire around the accelerator pump linkage. The pump could be partly clogged and causing your problems. This would be a test to start with.

James

[mscottiel]

The pilot jet has 2 passages that feed the venturi. Your fuel screw only meters one of them, allowing the other to continue to fuel your motor when the fuel screw is closed.

The needle jet/needle also supply fuel to the motor at idle too.

Try going back to your original jetting and wrap a wire around the accelerator pump linkage. The pump could be partly clogged and causing your problems. This would be a test to start with.

James

ok james that is new info to me. i was unaware that there were two passages that fed the venturi. where is the other passage and is it controllable? i have always believed that if the fuel screw was turned all the way in that the bike should die if the right pilot jet is installed. is this one of those "yes except" situations. i have a tach and am going to set up the pilot circuit per the recomendation in a ThumperFaq. i will try the wire/oring and see what happens. i believe all passageways in the carb are clear but i could be missing something. the carb has a new AP kit (cover, spring, diaphram,pushrod, orings) on it too. hopefully i have not been chasing my tail by setting the idle to low and expecting to much from the FCR.

Dave

your statement is correct. if i roll the throttle it pulls excellent with no bog and runs as it should. if i whack it open, as if coming hard out of a corner or throttling through a tricky set of whoops it has a bog. usually the bog will recover and rpms will increase but occasionally and if i open it up fast enough it will die. i am reading the material you linked and it is helping some. also why is the aluminum fuel screw so bad?

thanks for your help so far.

[James_Dean]

The second passage is closer to the needle jet. The 2 openings are about the same size as shown in the picture below of a disassembled carburetor. There is no adjustment of the second passage other than changing pilot jet size.

James

[Wayward Son]

your idle needs to react to the fuel screw.some will idle with the fuel screw fully closed but the idle at least needs to be clearly poor.