Help! Newly-finished XS Runs like crap when it warms up

[stripingbyakin]

Hey guys, this is my first post here besides in the pic thread... but I'm in need. I bought this '79 XS as a half finished project from a buddy who had lost interest, told me it ran fine. I finally got it running on Monday night, then got all the bolts tight and took my first ride on Tuesday evening. It ran fine when I first started it up, cracked on the throttle a few times and it sounded ok (to me). So I jumped on it and took off, and as soon as I got around the corner it started to cut out with barely any throttle. I kept going thinking it may just need to be cleared out a bit, but anytime I twisted the throttle any to speak of it would cut out and die. Same thing Thursday night. It starts up fine cold, but after it warms up it won't hardly idle and if you twist the throttle it dies right away. Any ideas where to start?

 

[OakBehringer]

Nice build man, looks very clean and nice. How much of it did you do, what was done before you bought it?

In order, set: cam chain tension, valves, timing. Ride and see where you're at. Now to the carbs... start by pulling the bowls and blowing out the jets and passages, put them back on, see where you're at. Might be fine now. Might need to blow out ALL the tiny passages on carb bodies, too. You haven't told us if you re-jetted for those cones and straight pipes- you probably can't run with stock jetting.

 

[stripingbyakin]

I bought it with the hardtail finished and most of it bolted together. I moved the seat mount back, built & mounted the battery box, made the rear fender mounts, added the fake oil tank, painted the rear fender & frame, and wired it. My guess is it hasn't been jetted for the pipes and cone filters. Thanks for your help, now I know where to start. Also, by "lost interest", i mean my buddy let this thing sit for over a year.. so it seems like the carbs may be the culprit. I'm not a bike mechanic by any means, but I think i can probably get the carbs apart and blown out. As far as jetting, is there some kind of rule of thumb to go by with the mods I have?

 

[bloodthirstysystem]

your stock jets are 135 main / 25 pilot??? can't really remember, you want one size up on both, so 137.5 / 28 or something.. but that's not the problem here..

clean carbs first, and do all the things Oak said

a weak coil works best when cold and gets worse as it warms up. condenser could be bad if using points. check voltage at idle - should be 12.5 or so.. rev the engine it should be around 14 volts.. if not your rectifier, regulator or rotor is bad

 

[shotgunjoe]

A suddenly bad running engine also potentially be a fuel blockage... if its an old tank their could be rust or other gunk in it. It doesn't take much fuel to keep it idling, but above that, the bowls need to be "full" to run properly.

 

[stripingbyakin]

The tank and fuel lines are brand new. Monday was the first time fuel was run through them. I'm going to start at the carbs. Thanks for the help fellas, I like this board already!

 

[Patches]

The tank and fuel lines are brand new. Monday was the first time fuel was run through them. I'm going to start at the carbs. Thanks for the help fellas, I like this board already!

Even if they are new be sure to run some in line fuel filters.

 

[OakBehringer]

The tank and fuel lines are brand new. Monday was the first time fuel was run through them. I'm going to start at the carbs. Thanks for the help fellas, I like this board already!

No! Don't sleep on the first three things I mentioned: Cam chain tension, valves, timing. These MUST be set correctly before you can expect to tune your carbs. They're also way easier to deal with than carbs.

Also, nice work moving the seat mount back... it looks perfect right now. Maybe some taller springs so you don't slide off the back, but your seat looks to be the best distance from the tank. Drives me nuts when they're too far forward.

 

[stripingbyakin]

Good deal, I'll start there. Thanks again!

 

[5twins]

Yes, check everything mentioned but your air filters may also be adding to the problem. Take them off, start the bike, and see if it runs any better. They look to be the cheap K&N repro pods. There are several known issues with them. First, the round tapered style is physically too small to flow enough air. Second, many of these repros have a large inner rubber lip that blocks the air jets around the bell mouth when mounted. The bike will usually run OK at idle and slow speeds but goes suddenly rich at higher speeds because it can't flow enough air. Since yours isn't running at low speeds either, you do have other issues, but the air filters probably aren't helping things.

 

[Kingwj]

plugs. new plugs are the first thing to get. I'm amazed at how quickly the plugs can go from OK to crap on this bike. Then the other stuff.

 

[azman857]

Thats a fine looking ride you have there. I hope you find your problem(s) and get em' fixed soon.

 

[anomie]

I'm a noob so listen to these guys..

I don't know how much this is worth (take with a grain of salt, I have very little experience) but more info might be a good start? Would it be a good idea to pull the sparkplugs and read the condition for indicators of how your motor is firing? Pulling the airfilters off and making sure that there are no obvious problems like 5 twins says..checking cam chain tension, valves, timing like Oak says. Sometimes I find that the best way to diagnose a problem is to collect as much information as possible before tearing things apart. That would be my suggestion of a good place to start.

Nice build btw, it looks great.

 

[XSLeo]

Carbs www.amckayltd.com/carbguide.pdf
Leo

 

[stripingbyakin]

Sorry so long for an update.. I've been riding After checking the cam chain, valves, and timing, I went through the carbs lightly (throttle body cleaner and blow gun). After that, I fired it up to the same problem. Next, I pulled the tank and rewired all the connections at the coils and condensers, and voila! It fired up and idled fine for 15 minutes or so, so I took off for a ride. It's still a little poppy, but I have new jets ordered and I'm pretty sure that'll take care of the problem. Thanks again for all the help guys!!

Here are a few pics from the show I made it to that weekend:
(That's also my '64 pickup behind the bike)