Perfect carb tune

Tater

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Hey fellas how are ya I'm new to this forum but did surf the 650 one quit a bit last year and got some great advice. I have a 1980 xs 400 with a 81 special engine along with seat gas tank and side covers.. I had a heck of a time wiring the bike because I had it all apart and painted the frame.. I'll post a pic tomorrow.. Anyways I'm trying to get the carb tuned I made my homemade sync tool and did that I bought an carb kit from mikes xs that came with a number 30 drill bit and drilled the air lift and put in 45 pilot jet 145 main I have my needle set on 3 third from bottom. I tried dead cylinder method with my fuel mixture screw but it seems to only work on the left cylinder and the bike stalled when giving it a little throttle but runs better once warm, and back fires like crazy. Any help or ideas would be much appreciated
 
I just synced my carbs again now it fires right up and runs good when I give it quarter to half throttle but bog's down when three quarter to full I drive it down the road and it misses around 5000 rpm's I'll check it again for sure
 
I took my mac muffler of and ran open pipes and it seemed to run better. Should I be able to see light through the other side of the muffler when looking trough it, cause I cannot on my mac muffler like I can on my emgo shorties?
 
I have my needle set on 3 third from bottom.

You have notches on your '81 needles? I forgot which year they stopped doing that, but I thought it was only on older bikes. I could be wrong though!

I believe stock position is 4th notch from the top. If I recall correctly you always count notches from the top down. Whenever I altered my needle clip position it always caused problems; I ended up leaving mine stock.

I run emgo short mufflers and pod filters and only went up +2 on the mains. What is your setup? You may be too rich.

Do you have brass floats? What did you set your float height to?
 
I remember I was having backfiring problems with carb issues. check Float height, Clean again, once I had it all figured out, the backfiring went away. hope that helps. (I had a bad float) day 1 after cleaning, it would flood the air box, and engine comp. Good luck, tobie
 
When you guys are talking about the needle and adjusting it, what is top? the blunt wide end? my needle has 5 positions, and the clip is on the middle, so thats 3 either way but I am confused as to which is "up" and which is "down" and which makes it leaner/richer. thanks for any help on the matter

i apologize for my stupidity.
 
Consider how the needle is oriented when it is installed in the carb. The point is at the bottom, and the notches are at the top. If the clip is moved to a lower slot, the needle will be raised, and this will enrichen the mixture. The opposite also applies.

For my particular setup, I had to raise the needle (lower the clip) by 1 notch from stock.
 
Thats kind of what I was thinking but I've been known to assume before and end up making mistakes. Thanks for clearing that up for me!
 
I have been working on getting my carbs tuned for a while now and I cant seem to get it right. I have also been reading all I can on this site and others. Found some useful stuff but I still am at a loss so I am going to explain the problems I am having and see what your expert opinions are.

first, awesome write up on carb tuning: http://home.everestkc.net/malsin/Motorcycle/carb/mc_carb_theory_101.htm

I've been trying to follow the guide from that link in fig. 7

problems:
- jumpy/cutting out/surging from 7/8 to full throttle when riding at high rpms/speed
- bogs down, backfires and pops when open throttle at idle
- rpms hang around 3500 at idle
- rpms hang around 4000 when riding and switching gears/stopping

current setup:
-145 main jets
-47.5 pilot jets
-needle clip moved from 3rd to 2nd position (or 4th....to make it richer)
-air mixture screws 4 turns out
-idle set to about 1200 rpms (almost...almost dies when coming to a stop and rpms come down)

I just cant figure out if the cutting out at full throttle at high speed is a rich or lean problem, and is the hanging rpms at idle a not a lean problem? I thought adjusting the needle and turning air screws out would help, and also the bogging down and backfiring when its idling and given full throttle would be a...rich problem?

any and all help will be greatly appreciated. as I said I'm at a loss at the moment and really tired of pulling the carbs off and reinstalling them. (and my wife is starting to complain about me smelling like gasoline)
 
Look at your mufflers and spark plugs. Lean is clean. Rich is dirty.

Sounds to me like you are still running lean. Double check that you don't have any vacuum leaks, especially throttle shaft seals. Then check fuel filters and float levels.
 
Nico, Check and make sure your pilot mix needle tips are not broken off in the carbs:doh:
 
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