Sputtering at mid throttle

Hello Everyone,

So I own a 1975 XS 650 and I have new mikuni Vm34's that have been rejetted I have basically rebuilt everything on it. Its been running great I have been ripping around on it for 2 months about 3 to 4 times a week. I took a 30 mile ride with some friends and we were getting on it my bike was running great, then at the end of the night it was sputtering at half throttle pretty consistently and by the time I got home it was sputtering at 1/4 1/2 and full throttle. I let the bike sit for about a week ( apparently I thought it would magically disappear) I took it out tonight and and seemed to sputter minimally at 1/4 to half throttle. Its so bizarre that it ran awesome for 2 months riding it al the time at 1/4,1/2 and full throttle. I also put aluminum intake boots on it but maybe that long ride loosened up the bolts and I have a leak? Any advice would help me close out the summer in style!

Steve

First off, check the plugs. If this is an overhauled engine, the plugs should have been replaced at 500 miles. Otherwise, see if they're fouled. If so, that'll make it run bad. Clean (beadblast) or replace. If they were fouled, must find cause, like fuel or oil...

Make sure the battery is still showing 13.5 volts at 2,500 + rpm, ignitions like lots of power.

Switch back to original intake boots to see if this is a factor or not.

Leaking boots cause erratic idle and backfire. Get out the sealant and reapply.

Sputtering in the mid-range....I'd raise the needles. Since it could also do that being rich, it may get worse, requiring a reversal to lowering them. Either way, if it's ok idling and wide open I doubt the ignition theories. Don't cost nuthin' but time to play with the needle heights.

Early on drain the floats into a clean container. what do you see? a load of crap gas is always a possibility too. Plugged cap vent, as mentioned charging or ignition. Dig in!

Well I raised my needle position one slot and it seemed to help a lot the plugs are a bit rich. So I need to get some new plugs and reset the mix screw then if that doesn't help then I am just gonna clean the hell out of my jets and carbs. Thanks for the suggestions everyone!

Alright well I cleaned my carbs and all of the jets, zi took my gas cap apart and cleaned everything the vent holes had some rust. I cleaned the plugs seemed a little rich so i backed off the fuel screw. Took it out runs much better there is a bit of a sputter every once in a while at 1/4 throttle eveything else seems to be running fine......any ideas?


Steve

STVR said:

.... I cleaned the plugs ...

Click to expand...

If these were your 'break-in' plugs, best to replace them.

http://www.xs650.com/forum/showpost.php?p=278015&postcount=15

New plugs are the best way to go. Intermittent misfire sounds like a bad plug or wire. It would also eliminate plugs from being the problem if it doesn't.

all that hotfootin could have loosened crud in the tank. like gggary said, drain bows, whats in there? Are you running a gas filter? you can find some pretty small ones that will fit anywhere along the gas line, and still flow plenty. my .02
I also agree with new plugs.

Thanks for all of the tips. I ended up cleaning my carbs not to much
Crud in there. I switched out plugs seemed to help a bit but still sputtering
So i backed off the fuel screw was running a bit rich seemed to fix the problem!

Steve