xs1100 sputters and backfires under 4000 rpm's

[MidnightCowboy]

Hey guys new to the site. I bought a 1980 xs1100 midnight special 2 months ago when it was cold from an older guy who owned it for the last 17 years. He told me it was cold blooded and I took his word for it, shame on me, anyways, i have cleaned the carbs, put in fresh gas, checked to see if the vacuum advance is working (it is). It spits fuel and backfires until it gets to 4000 rpms.To get it there you have to feather the throttle or it will choke like its getting to much fuel and die. Any help would be great, I really want to get this black and gold beauty on the road. Thanks for any help in advance.

[DiverRay]

I think you have something wrong with the carbs. If you pull them and pull all the float bowls, you may find one or more of the main jets sitting in the bottom of the bowl. Once you have EVERYTHING together and tight, recheck the float height, and DON'T FORGET TO BENCH TEST before you re-install the carbs.

[geniusloerts]

clean them again while you are at it. I had to do it to mine

[TopCatGr58]

Hey Cowboy,

"HOW" did you clean the carbs....in other words, how "THOROUGHLY" did you do this??? You will have needed to remove ALL of the jets, the Pilot screws=mixture screws on the front top of carbs along with the little metal and rubber washers that are down in the pilot screw tower, the center Main needle jet=emulsion tube, the floats, needles and seats, and O-ring under/around the seat and replaced with NEW, the Vacuum slide/rubber diaphragm...checked for pinholes, and then squirted liberal amounts of spray cleaner thru ALL orifices of carbs, ensuring that you saw it spray out from Pilot JET tower, thru Pilot screw tower, and the 3 holes in the front upper section of the engine side of carb throat. There's more, see the TECH TIPS!!

Sounds like a clogged Pilot jet circuit, or the PO had it cleaned and serviced years ago with a K&L kit which has history of wrong sized pilot jets. Once you hit 4k rpm, you're getting into the mains!
T.C.

[planedick]

Sounds like weak spark....

Had that symptom once. Spark was weak and wouldn't fire under pressure and lots of fuel. Only could accelerate very slowly and giving it much throttle would only quit.

[fvincent]

What was causing?

What was causing your weak spark?

Floyd
-80G

[planedick]

PO

PO replaced the wires to the plugs with a set meant for cars that had a carbon center instead of wire. I went back to the stock set up and the spark was much hotter.

[fvincent]

Thanks!

Thanks!

Floyd
'80G

[MidnightCowboy]

I REMOVED AND CLEANED ALL THE JETS AND TOOK THE CARBS ALL THE WAY APART. LOOKING AT THE NEEDLE VALVE I CAN TELL IT HAD A REBUILD KIT, I WILL TEAR THEM ALL THE WAY DOWN AND TRY AGAIN, I DID CHECK THE SPARK LAST NIGHT AND 2,3,AND 4 WERE VERY SOLID, 1 WAS NOT AS STRONG BUT LOOKED STRONG ENOUGH NOT TO CAUSE THE PROBLEM. THANKS FOR ALL THE ANSWERS SO FAR.

[DiverRay]

LOOKING AT THE NEEDLE VALVE I CAN TELL IT HAD A REBUILD KIT, I WILL TEAR THEM ALL THE WAY DOWN AND TRY AGAIN,

You need to check the parts!! The PO MAY have put in a K&L kit. If so, I would look to mikesxs.com for new jets! Some of the kits are good, some have the WRONG SIZE JETS. You need to be sure of what you have.