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Discussion in 'Thumpers' started by laramie LC4, May 31, 2008.
I'll never look at my carb the same again.
New guy here. I have a 2001 Duke II that sat for over a year. Since it had constant throttle fueling issues when I put it away and had sat full of gas for so long, I figured now would be a good time to pull the carb apart to clean and start messing with jetting and such. I ordered some jets, a new airbox boot to replace the disintegrating one on the bike, and dug in using the info in this thread (very helpful BTW). Aside from the incredible pain in the ass that removing and reinstalling the carb is on these bikes, it all went pretty well. With one exception. Being the unfortunate combination of noob and dumbass that I am, I tossed the float weight in the containter of carb cleaner to soak with the rest of the jets.
The results were predictable. The little rubber conical tip kind of melted and deformed. I figured I'd put it all back in the bike anyway to see how it acted. Bike started right up with the new battery that I bought, and idled and revved great. Then started pissing gas out of the float overflow. So it looks like I need a new float weight. But I don't feel like paying $50 for the entire needle valve assembly. Does anyone have a source where the float weight can be purchased separately? Once I get one, I'll recheck my float level too, to make sure that isn't contributing to the problem. Any help would be greatly appreciated. Thanks guys.
Working on jetting a '97 620 RXC with the stock BST40 carb. Waiting on main jets but today I drilled the slide holes to .125, set the needle clip to the 4th position from the top and adjusted the fuel/air screw to 2.5 turns out. Bike did not need much choke and the idle was too low. Adjusted the idle screw to compensate, I am assuming that is the correct thing to do.
The bike seemed more responsive and wheelied in 1st easier than before and pretty easily in 2nd which was more of an effort before the changes. Seems to be hunting (surging) a little at steady throttle at lower revs, not sure what that is - any ideas?
The fuel/air screw was at 2 turns and I went to 2.5. Does that make a drastic difference? I am assuming that is what effected the idle. It is hard to tell if the needle is better in the 4th position because the 142.5 main is definitely too lean. Ordering 150 thru 160 mains tomorrow so we'll see. Have the airbox modded and the stock Supertrapp that has been cored and has 8 discs. Any feedback would be greatly appreciated. thanks,
So everthing seems to be going great, almost. Bike starts and runs with no gas leak, however I still get idling problems...not all the time but about half the time the bike dies in idle. It also seems not as peppy as it used to. The bike also starts easier when the choke is off, which seems odd to me. I'm 100 percent sure all the parts are in order. The only thing that isn't where it used to be is there was a washer on top of the throttle valve spring that popped out and I couldn't get back in. The washer seemed to compress the spring more and from looking at your bible you clip your spring so I figured I accomplished the same on accident. Any suggestions on getting it not to die in idle?
The important parts order is at the jet needle, washer and clip arrangement so that the needle hangs at the right length, also please double check that you tightened the rubber boot that holds the carb to the engine. Adjust the idle speed screw so that the engine idles at 1500 rpm, now adjust the idle fuel screw for fastest idle(around 2.5 turns open), now reset the idle to 1500 rpm. Ride.
If this doesn't work check the valves.
Stock jets should run perfectly at idle and through the range with the needle in the 4th position from the top.
sorry. not that i know of but you may try some the larger carb suply shops like sudco or carbparts.com
what kind of solvent where you using to clean?
Most of the bucket dip type carburetor and parts cleaners contain combinations of methylen chloride, cresols, xylenes, and ethyl benzene. Almost any type of rubber or plastic used in a carb will be weakened or destroyed by at least one of these ingredients. If you make the mistake of dipping a float bowl, rubber tipped float needle, o-ring, or gasket in this stuff you are probably best off going for a replacement part. After the swelling goes down, the part may look OK but it won't last long.
There are two ways to avoid the probem. 1) Be real careful not to dip your rubber tipped parts in places where it could get injured :eek1 , or 2) move to California where the helpful bureaucrats ensure the carb dips you can buy are harmless to you, your rubbers, and mother earth.
trying to control the population here bub...
Got my main jets and tried the 155 first. Definite improvement there. Also checked my needle and realized that the plastic spacer was not snapped right up against the clip so fixed that. Next went to a 157.5 main and put spacers under the stock airbox lid and adjusted the fuel/air screw to 2.5 turns out. Idle was better and popping on decel is almost gone. With 16/40 gearing it wheelies instantly in 1st gear and easily in 2nd gear. 3rd gear takes a lot of effort but the bike rips now. May try the 160 main but want to get it in the woods first. If I can find a needle shim I may try the clip at 3 1/2 instead of 4. Thanks to everyone who gave feedback and all of the long time members who contributed to the index and thanks to Laramie for creating the BST bible!. I ended up at almost the exact same set up as many others with similar mods have. great to have the combined knowledge and experience of so many riders. THANKS!
So, where do I get replacement needle? Stock is the only option we have?
If I go to high elevation (WMRS) should I go back to 3 clip position? I run 4th at sea level.
the easiest thing would be to lower the clip and turn in the fuel/air just a bit. if you are going to be at high elevations for a long period you may consider dropping back to the smaller idle jet (assuming you installed the larger 47.5).
most likely though, if you are just going to be up and down, i wouldnt worry about it too much. this carb is VERY good at taking elevation changes without any or much adjustment.
Last time I went to Bishop for WMRS(6K at camp site and many rides go up to 10+k) my bike had a lot less power and bad gas mileage. I didn't want to bother with it and used 87 and it actually helped.
I'll be there only for 3 days, but now i can rejet it easily since I took my carb out few times and have loaded screw installed.
So, the only source for spare needles is from the dealer? There are no "performance" needles like JD for BST?
im sure if you did some hunting you could find other needle for the bike but i personally have not heard of anyone using one. i know there are several bikes that run this carb, so im sure there are others available.
when you hit 10+ in elevation it reasonable to think that you will loose some power. adapting for it though isnt that tough. you can easily access the needle, the main jet (remove the cap on the bottom of the carb), and the fuel screw. thats all you really need and all of it can be done in under 10 mins. i would go (depending on mods) to the 3rd clip, low 150's MJ, and turn in the air screw just a touch (maybe not at all if you change the others) but you wont know till you try.
I cannot recommend one, but here is Factory Pro's LC4 kit:
And here is a chart some mofo put together (ferget who):
Keep it simple.
It's not going to make much difference what you do, just live with a good average. Fuel injected bikes suffer just as much and they auto compensate.
The factory in Mattighofen is 40 miles from the Alps and they tested the bikes from sea level to Alpen high every day, why do you think the BST is on the bike?
They could have put on any carb. The BST compensates the best of any, BMW used diaphragm carbs on the Airheads for exactly the same reason.
Guys, tore my carb apart to rebuild it for third time at 35K miles, frikkin jet needle almost sawed in half cause the hole in the slide is so worn and it must have rattled causing the wear, the rest of the slide is fine, anyone know a cheap fix to make the hole round again?
Guess I have to replace it otherwise.
I live right at 5,000' and have rode my 640 from Baja to passes in CO without re-jetting.
I recently pulled the cover off the side of the air box and raised the needle one clip and it gave me a noticeable boost in power, but we were in some water last weekend and it felt like it was sucking some at times so I will have to either buy the modded cover or do something myself.
The BST is not perfect, but it will work without being jetted perfect. YRMV
that sucks man, the new slide is not cheap. :dog
since your other one is ruined though, you might as well play with it and see what you can do. you may try some JB weld and use a little dab to fill the hole, let it harden, then re-drill the hole. the trick is gonna be getting it centered. if your off it will bind in the needle jet and may cause some sticking and other issues. other than that i cant think of much. sorry.
you may also think about pulling your carb a little more often. its not hard (as you have figured out) and allows you to keep an eye on issues like that. i used to pull mine every time i did an oil change.
DJ do a needle/jet kit for the DR650.
Just to be sure those working with their BST-40 on an LC4 have all the important information, I'd like to remind everyone of Creeper's excelent LC4 setup thread available here.
It touches on many of the same topics as here but expands the conversation to include more detail on the whole intake/carburetion/exhaust system and fine tuning. You'll find an approach to setting the pilot circuit that you won't see anywhere else and it works better that the normal way.
Now go out and get an adjustable air screw and quit burning your fingers.