can't figure this out

[thebottle]

My bike runs good through town riding where I am stopping and going and changing gears a lot. The problem comes when I am on the highway for 10-15 minutes going a constant speed (60-70mph). Even then it runs fine up until I let off the gas, then if I go back to the gas it starts backfiring and loses power. If I pull the throttle all the way back it just barely keeps its constant speed so it seems like it is a lack of gas to the engine. Eventually it will start backfiring a lot and eventually die if I am riding for a long time on the highway. Then I have to pull over and choke it to start even though it is hot... so again it seems like a lack of gas.

I have removed the fuel pump and put the lines directly to the the carbs (fuel pump membrane was shot but it didn't resolve the issue). I have checked to see if the tank was locking and opened up the gas cap. I have checked the vacuum advance line on the pickup coil and that looks good.

I think I have narrowed it down to being either the carb boots to the engine or maybe particles in the bottom of the tank plugging it? What do you guys think? The carb boots do have some cracking on the outside but when I had them off when I painted the engine I checked them and they weren't all the way through the other side... would that cause the problem?

I bought another engine because the one I have has a crack in the case that leaks oil and I want to figure this out before I make the swap so I can take care of it.

[xsroadrunner]

I've had similar symptoms with my XJ and its always when the main part of the tank has drained and its time to switch both petcocks to reserve. It might take 10 seconds but then the above symptoms go away...time to fill the tank. Have you switched your petcocks to reserve???

[petejw]

might not hurt to check ur pick up
coils for broken wires, theres heaps
of threads if u do a quick search.

[mack]

Gas Cap

I had a similar problem that just started out of no where. I blew my gas cap clear and it would come right back again on the next tank full. Eventually I took the cap apart completely and found that the small filter cloth under the spring had rotted and was falling apart. Everytime the tank started to draw air it was pulling in the little bits and re-plugging the vent holes. I cut another out of a spounge I had and recleaned all the vent holes, it has worked well ever since.
However, I second the opinion on the pick up coils wires. Id also put the advance under vacumn with a midi vac and check the action.

[thebottle]

So yeah, check the fuel supply. Fuel tap filters clear? In-carb filters clear? Got in-line filters? Are they clear?
When did you last clean your carbs? Clean them again, eh?

The lines are clear, inline filters are clear. The only thing I can think it is are the filters in the tank or the carb boots... maybe the carb needs to be cleaned but this is the only time that it runs poorly. I would think if it was the carb it would run poorly all the time... or at least more than just this one scenario.

The wires on the pickup coil are fine, I replaced them a couple years ago. The vacuum line is fine. No cracks. And of course gas was the first thing I checked people... But yeah, when I referred to the fuel pump it was the fuel cutoff valve.

[crazy steve]

The lines are clear, inline filters are clear. The only thing I can think it is are the filters in the tank or the carb boots... maybe the carb needs to be cleaned but this is the only time that it runs poorly. I would think if it was the carb it would run poorly all the time... or at least more than just this one scenario.

The wires on the pickup coil are fine, I replaced them a couple years ago. The vacuum line is fine. No cracks. And of course gas was the first thing I checked people... But yeah, when I referred to the fuel pump it was the fuel cutoff valve.

There's also small filters on the float valves, if crap is getting past the tank filters that could be your problem. Even if the upstream filters are working, there can be 'stuff' in the carbs by the fuel tees that can cause problems. It's not the boots; that would cause the bike to run bad all of the time. Running on the highway at a steady speed is when you need the most fuel flow, and if it's restricted that could be it...

To check your gas cap vent, next time it does this open the cap; if the problem goes away, the vent is at least partially plugged. And it wouldn't hurt to at least look at the pick-up wires; depending on how the repair was done, another failure isn't uncommon.

[thebottle]

There's also small filters on the float valves, if crap is getting past the tank filters that could be your problem. Even if the upstream filters are working, there can be 'stuff' in the carbs by the fuel tees that can cause problems. It's not the boots; that would cause the bike to run bad all of the time. Running on the highway at a steady speed is when you need the most fuel flow, and if it's restricted that could be it...

To check your gas cap vent, next time it does this open the cap; if the problem goes away, the vent is at least partially plugged. And it wouldn't hurt to at least look at the pick-up wires; depending on how the repair was done, another failure isn't uncommon.

Yeah, already tried the gas cap, mentioned it in my original post. I looked at the pickup coil wires today and they look good. I soldered the wires together and used heat-shrink tubing. I am going to check the tank tomorrow after work, a girl interrupted my progress tonight. Thanks for the advice everyone. I was thinking that the carb boots might have been getting hot and then the crack would open more but now that I think about it, you are right... it would keep doing it after it was hot so that couldn't be it.

[cywelchjr]

Yeah, already tried the gas cap, mentioned it in my original post. I looked at the pickup coil wires today and they look good. I soldered the wires together and used heat-shrink tubing. I am going to check the tank tomorrow after work, a girl interrupted my progress tonight. Thanks for the advice everyone. I was thinking that the carb boots might have been getting hot and then the crack would open more but now that I think about it, you are right... it would keep doing it after it was hot so that couldn't be it.

You can't just look at them, you have to use a meter and tug on them to make sure they didn't break on the inside again, as it's not uncommon for that to happen and they LOOK just as good as new. If you have inline filters, I would check them too, cause if they get clogged that's pretty much how they will act, pretty much on any vehicle actually. And if you still have the screens on the tops of the needle valve seats, those too can clog and cause symptoms very much like you describe (DAMHIK ).

[oldyam80sg]

crazy steve -you are correct in those screens.If they are clogged fuel wont flow smoothly.I had that issue several years ago-It would run fine around town-as soon as i would venture a mile or so bike would feel like i was running out of gas.I tried to open the fuel cap-no good.I sprayed the pipes, some were hot and 1 wasn't as hot as the others.I finally found the issue after taking apart the carbs.I simply could not fully do a thorough cleaning at that time.When the carbs were totally clean,i was not afraid of getting stuck like before.Also i checked pick-up wires and decided to replace them as i wanted to eliminate them as an issue.

[thebottle]

Hey, thanks very much for the advice... I will recheck the wires tomorrow. If it isn't that then it is most likely the carbs like you mentioned. I will check the tank just to be safe but at least it is narrowed down now. If it is the carbs then I will just give those a good go through while swapping engines this weekend. I didn't want to tare the carbs apart if I didn't have to but it looks eminent.

[oldyam80sg]

I think a lot of people shy away from carb cleaning because of the float pin removal-scary at first but once you remove them and you take out the float and the other part(cant remember name)you will see the screen and it probably is clogged.Clean them (i use carb cleaner and a piece of wire) and you will feel the difference!

[ViperRon]

Seems like all things technical have already been suggested but I will tell you a little story.

When i was young and in the Navy there was a practical joker who was always messing with other peoples cars. Given he got every everyone, after working the night shift one morning it was my turn. I go out start my hemi GTX put it in gear and no movement. There are several in the parking lot laughing as my drive shaft was taken out and on the ground. All pretty funny but he let the wrench slip when taking out the bolts and messed up my yoke. I went threw hell.

One month latter he bought a brand new Ford Granada and was going on vacation. When he got back he was so mad seemed when the car got warm everytime he stepped on it (like to pass) a minute or two later it would start sputtering and cut off acting like it was out of gas. He took it to several ford dealers. In the shop they could not find a problem or recreate it. It was great. I let it go on for a month before I gave him a new gas line in front of everybody. Seems the gas line to the pump had the inner part of the line removed and as soon as it got hot and that fuel pump sucked hard on the hose it sucked it shut. As soon as the car cut off all was back to normal except cranking like hell to get it started again. In the shop they did not put a load on the hose like on the freeway so it could not be dupicated. That was the end of the Games!

If you have a fuel pump a hose can be sucked shut. Also a vacume line to a fuel petcock can be sucked shut. Either way you are out of gas.

[gui_tarzan]

Great story and a beautiful color on your bike.

[shovelhead]

I think a lot of people shy away from carb cleaning because of the float pin removal-scary at first but once you remove them and you take out the float and the other part(cant remember name)you will see the screen and it probably is clogged.Clean them (i use carb cleaner and a piece of wire) and you will feel the difference!

you can get a little spring loaded punch that makes removal/installation of the float pin not so scary...about 20 bucks. some feature an adjustable spring preload.

theyre also fun for breaking glass/****, and poking holes in stuff like your friends!
no tool box should be without one.

[81xsproject]

you can get a little spring loaded punch that makes removal/installation of the float pin not so scary...about 20 bucks. some feature an adjustable spring preload.

theyre also fun for breaking glass/****, and poking holes in stuff like your friends!
no tool box should be without one.

There is one thing that should be added to this. I love the spring loaded punch. But, if it is pointed, you should slightly grind the tip down so it is flat(ish). I had some stubborn pins on one set and the point on the center punch actually caused one pin to expand a bit which made removal impossible. Flattened the tip after that "duh," and haven't had a pin that couldn't be removed since (dozens of pins since).