carb tuning problems

Somebody help me! I'm having a heck of a time trying to get this thing running right. It's an '83 XJ750K. The carbs are clean. The fuel levels are good. The spark plugs are in good shape. The jets are clear. It has new boots. The valves are adjusted. When the bike is cold, it will start right up. Out on the road it runs ok until it's completely warm. Then it starts to stumble when you gas it. Then it gets worse from there and soon chokes itself off and dies. I can't tell if it's temperature related, fuel-mixture related, or maybe even an electrical problem. The last time it stopped working I charged the battery back up and it started immediately. Then it went back to it's old behavior. Anybody out there have similar problems? I've done just about everything I can think of. Help if you can. Thanks. DCGreer

 

You mentioned charging the battery back up, do you find it is low on voltage after it dies? If so you may have a charging problem. If you have a multimeter you can check this easily... charge your battery ( assuming battery is in good shape fully charged it will read 12.5 volts) start the bike and check voltage with meter at the battery, as you rev the engine to 2000 rpms or more it should be charging the battery at somewhere around 14 to 14.5 volts. If your bike is not charging you will eventually start to stumble and lose power and then shut off entirely... This is what happened to my bike when my stator went.

 

... voltage regulator and rotor are other culprits if it is an electrical problem

 

Does it seem to be running out of gas? If you don't find a solution with the electrical system, install a piece of clear fuel line so you can tell if it's getting fuel. If the tube is clear when it dies you have a fuel problem.

 

Well - I'm not sure what sort of time frame this all happens in - but perhaps you've got a different problem than the battery.

Perhaps what is happening is you've got a clogged fuel filter, or the floats are set too low. The reason I say this is that perhaps the battery charge didn't do anything other that let the fuel system settle back to it's rest position.

Perhaps the real problem is that rolling on the throttle consumes the gas faster than it can be provided to the carbs - starving the engine out.

The problem with this idea is I don't know why tempature would be related to it... I can see a pinched fuel line or a clogged fuel filter causing a delay in the eventual starving of fuel...

Temperature also changes the mixture that will burn... do you find the choke has any effect on this problem?

I'm not happy with the electrical ideas - my reason for that is that giving more throttle doesn't put more load on the charging system... a weak spark is a weak spark. I realize that it wouldn't be charging the battery if the charging system was bad - however - it wouldn't be related to the throttle position when the bike stalled. That's my only twig there. Now if it was a pure timing thing - then certainly that would settle better with me. It just doesn't match what he said... ( certainly my ideas don't either!!! )

hrm - can you give us more background on how many times you charged it - how long you charged it and would it run properly after cooling off without charging it?

It is possible that the temperature is changing the charging systems efficiency... a voltometer would be nice.

something to think about -- more clues please

 

just a thought: oft times gas tank caps have tiny vent holes...these get plugged, sometimes with wax....as you burn fuel, air cant get in the tank to take up space, resulting in vacuum, which keeps fuel from entering the fuel lines...

 

Something to consider, a bike that's running too rich will start easily and run better when cool. As the engine warms, running will get worse, as hot engines require leaner mixtures. Conversely, an engine that runs better when hot, but is a bugger to start and runs poorly when cold is often running too lean.

I'd check easy stuff first though, like the tank vent or coils breaking down with temperature.

 

My ten cents worth.

My bike used to run like crap. Fixed a lot of things up to get rid of similar problems to yours. I now only have one small problem. Bike runs great whiuloe riding but gets a bit of a burblr out the exhaust once running at a constant speed for ten minutes. Twist the wrist and it goes fine and settles for a while.

Biggest problem is getting caught at the lights. If sitting for a few minutes it bogs down as you start to move off. Sounds like a flooding issue. WHat I really think it is related to is carb sync. Did a sync a month ago and finally got the bike running well apart from the described problems. This weekend I plan on syncing again without the YICS tool as I think this is giving me false readings.


Plugs used to be black but now they are a perfect tan colour. Mixture is obviously spot on so the only problem I can think of is carbs not synced good enough. EG one carb letting too much fuel in at idle causing one cylinder to flood while the others are happily maintaining idle.

Either that or an air leak, but I can't find one.

From your first description I would pick carb sync and/or to rich.

Hope this makes sense and is of some help.

HG

 

Here's my experience, from a different bike. Kawasaki 550. Changed the fuel line & put in a filter. I did it cheap & used too small a hose, which kinked ever so slightly when I put the tank back on. It reduced the flow of gas to the carbs to where I could ride for 20 minutes, then the engine would surge, then burble, surge & die. Wait 5 minutes, bike would start right up & run fine, but after a while, same symptoms & stop running. With the restricted flow, the gas in the carb bowls would drop, leaning out the air/fuel mix until it stopped running. I put the right size & length fuel hose - no problems since. In line 4 cylinders need fuel. Did you check the petcock diaphram for proper functioning? Maybe the screen in the tank that is part of the petcock is clogged with gunk?

Hope this helps - Sammowry

 

Sorry I haven't responded to all the advice. I have cleaned the carbs again. The fuel lines and filter are clear. I got a new battery. I tried to check the coil resistance, but only the small wires from the coils showed a normal resistance. The high tension wires had readings only when I intermittently pulled out one lead and put it back in, but the reading was goofy and the display disappeared almost immediately. After installing the new battery the bike started but only ran for about 2 minutes. Then I couldn't start it again. The engine turns over but it acts like it's not getting any spark. I still can't figure out why it would start and then stumble and stop. I know it's getting fuel. Could it be the rectifier/regulator? I don't want to take it to a shop if I can help it. Thanks.

 

"acts like it isnt getting any spark, " or ISNT getting any spark?? pull a plug, reattache the lead, lay the plug on the head (so its grounded) & turn the bike over.....spark, or no? If theres spark, is it yellow, blue, or white??

 

Heres the quickest way to hopefully try and eliminate the charging system from the equation...
#1 Assuming the new battery was fully charged from where you bought it from ( my local yamaha dealer doesn't charge them, only if you pay them to...) verify the new battery is reading 12.5 volts before starting it again(mine always seem to read up around 12.65 volts). If your charging system is in order you shouldn't need to charge it anymore to start the bike, provided you can keep it running long enough to charge the battery each time...
#2 Start the bike and check voltage reading again, now with your meter still hooked up to the battery, rev and hold the motor just past 2000 RPM... should be charging the battery at approximately 14 to 14.5 volts. If it is, your stator, rotor, and voltage regulator/rectifier are working, and you can investigate other possibilities.

Let us know what you find... Don't get discouraged, someone here will be able to help you out, I would have never been able to get my bike fixed if it weren't for the guys on this site and the old XJ site that merged with this one. The more specific readings and info you can give the better, and let us know what the eventual solution is, because someone may have the same problem as yours, and it may help them solve it quickly.

 

Well, here's the final chapter. After all that, I realized that it had to be intake related. I looked close at the vaccum hose coming off the gas tank and there was a split at the nipple on the petcock. I replaced it and guess what? It worked! I got to ride Saturday and that's all that matters! Thanks for all the suggestions and help. The good thing is I learned a bit more about the electrical system and also that you should always attack a problem starting with the easiest fix. Thanks again, DCGreer

 

congorats :roll: greerman!

hey i took the pics already!

btw the bike also says derale any idea what it means?

 

happy trails, nothin better than yr bike up & runnin!!!! mutt

 

xj650rjseca, i don't have a clue what derale means but it's cool having my name on a bike. sorry i can't be of help, but i do know my bike is up and running, and that's all that really counts anyway, right?!