Interesting problem

The bike idles beautifully at just over 1000. The last two times i've ridden it I let it idle for 5-10 minutes before riding. Both times I made it to almost the exact same place on the road before it starts to hesitate. The rpm's were bouncing and it would lose sufficient power for a moment and then catch it again. Both times I made it back without the bike dying. When I get back I find that my fuel filter (recently installed, could this be the cause of my problems?) is not FULL of fuel. It merely has some residual left inside. Could this be fixed by nixing the fuel filter? I'd rather not. Could it be a vaccuum issue? The weather is beautiful this weekend and it's getting late in the season. Your input is greatly appreciated as always. Thanks.

 

sounds like the vacuum petcock is not opening properly, try running it on the PRI setting to see if you still have problems, if you do, the screens inside the fuel tank may be plugged up, pull the petcock and clean them out good.

 

Idling while cold causes plugs to foul.

 

Hmm, i'll try cleaning them out and see what happens.

 

holy cow there are alot of small pieces that go to this petcock lol. I got it apart and i'm swabbing everything with Q-tips. What should I be looking for? The screen on the tower that sticks into the tank appear good. I pulled the vacuum side of the petcock off and the screen seemed a little dirty but the q-tip handled everything fine. To empty the tank I left it on prime and the fuel flowed out easily. This leads me to believe that the petcock is fine. How could I test the vacuum line? I'm beginning to think that i do not have enough vacuum to supply fuel at higher rpms.

 

pretty sure vacuum levels tend to go up as the rpm's do... i would guess that bubble in your filter is causing issues. I had to take mine out for a while, before I found a filter with a 90 degree elbow in it, because the filter was a high spot, wouldnt fill completely, and then caused flow issues. could try re-positioning the filter lower to make sure the flow is all "down-hill" so to speak, if possible.

 

thats a good point switch. Everything on the petcock seems good to my newbie eyes. I'm gonna pick up some fuel line and try to bypass the filter until I find one that will work. As it is the filter is too large to position anywhere other than it's current position.

 

yeah, thats the problem i had too, tommy. i just didn't use one for a while. not good, i know, but better than not riding =)

 

the filter being higher than the outlet would be more of a problem if it was a gravity fed system. Holes in the diphram can be tiny & difficult to see, mine was up against the centre pin, (see pic) test the petcock by sucking on the vacuum pipe, with the fuel line discharging into a can, suck should get you flow, but you should not taste fuel.

 

I just sucked pretty hard (lol) on the vaccuum line and nothing came out of the fuel line. Does this mean I have a bad diaphragm? Is there a fix or must I order a new petcock? Will the bike run on prime?

 

also, is it possible for the diaphragm to be bad and still give enough gas to idle? Like I said earlier, twice I have let it idle for 10 minutes or so....

 

strange! bike will run on prime, remember to switch back to ON after riding, or you could get fuel in the crankcase.

 

If the diaphragm was bad in that it would not open due to leaks you should have tasted some gas. I'm concerned you may not have gotten it back together right after inspection. You may want to double check. From the initial description it sounds like your filter placement.

The gas is mostly a gravity feed. Some feel the diaphragm works somewhat like a pump. I'm not convinced of that. I could be wrong but it does not look to have the same dynamics of vacuum pumps I have disassembled before.

 

Ok. I just filled the tank and retested and everything worked perfect. Back to the drawing board I guess. Gotta see if there is a shop open late that sells fuel line. Anyone know ofhand what size fuel and vacuum line goes on an 82 Xj750J seca? thanks

 

I bought a small mower filter at autozone - works great. I think most of the bikes use 1/4 inch line

 

correction, yes this is gravity feed, it's just a vacuum actuated tap.

 

Ugh. I bypassed the filter and just went straight from tank to carbs. I just got back from my test ride. I did not let it warm up for the full 10 minutes and I still ended up witht he same problem. I switched over to prime to see if that would solve the problem. It did not. I'm gonna try starting and doing the whole ride on prime, perhaps that will help. Is there a set time your supposed to leave the bike on prime before riding? Anyone have any other ideas? I'm running out of things to test.

 

same problems when it is run on Prime. I'm accepting all ideas.

 

Normally you do not need to use prime unless you pulled the carbs so the bowls are empty. There is enough gas in the bowls to get it running for quite a while and the petcock should open once started.

That said you should try to run it on your ride set to prime - will not hurt anything. Just make sure you put it back to the normal position at the end of the ride. If any of your needle valves leak you can get fuel in the oil issues if left on prime.

It sounds like a fuel issue. Are you running fast when you have the problem? If so it could be float height issues.

 

Not that fast, well under 50. Plus, rickomatic set my float heights so i'm positive they are dialed in. We rebuilt the carbs in his shop so i'm pretty sure it's not a carb issue, though it could be I suppose. But like I said earlier, I tried running the bike while on prime and it still had the hesitation. Unfortunatly, rick is away for the weekend otherwise i'm sure he'd be able to get me heading in the right direction. Could it be a bad air filter? Or would that be a problem all the time and not just after a certain period of riding?

 

Well if Rick helped you the carbs are right for sure. Have you checked for air leaks? I can't see the air filter doing that.

 

No air leaks. I've checked thoroughly. I really feel like it's a fuel problem. But the petcock works. Running it on prime did not help. The Petcock screen was clean. Is it possible that the petcock is simply not allowing enough gas into the carbs? I feel as though the fuel in the bowls should be enough to last a few miles.

 

It's very odd indeed. Fuel in the bowls can get you a ways for sure but I don't think you could warm up 10 min on bowls with no fuel supply. You may be getting lower flow from your petcock. I would put a tube on the petcock and start the bike and see what kind of flow you get into a can or other container.

If you get good flow I would suspect some junk in the fuel rail - despite just cleaning. Maybe something n the line that entered after cleaning?

 

wow. that just confirmed my worries. Man these bikes are not easy. But for 250 bucks what can you expect really. I'll test fuel flow tomorrow. Is there a way to clean the fuel rail without removing the carbs?

 

Not that I know of - sorry. Maybe someone has another idea but if the floats are right and the carbs are clean and the petcock is right and no air leaks....

I can't think of anything else. I'm ruling out pistons and leaks in the piston diaphragm etc since you and Rick did the carbs we can be sure all that was checked properly.

Sorry Tommy I'm out of ideas beyond some kind of blockage....

 

Yea, your right. Tomorrow i'll have to pull the rack again and do some checking. wish me luck lol.

 

could be a coil breaking down.

 

is there any way to tell which one? Or to test that theory?

 

with your multimeter on ohms scale test the low tension leads, should read 2.5 ohms.
meter to k ohms test h.t. leads (no caps) should read 11 k ohms.

 

i tested the coils last week and they ohmed out fine. I thought you were saying they were only failing at high rpms.

 

it could be that, the only way to test is to swop out with known good ones.

 

FOR US NOOBS... COULD YOU EXPLAIN ALL THESE POSITIONS ON THE
FUEL SWITCH AND WHAT THEY DO, AND WHERE THEY NEED TO BE FOR
PARKED, DRIVING, WARMUP, ECT...

THANKS !!!!!!!!!!!!!!!!!!!!!!

 

Straight up and down is the normal or on position. In this position the petcock vacuum valve will open with the bike running and close with it off. 99% of the time you want to leave it in this position.

To the front is reserve. This is a lower intake than the on position. It is controlled by the vacuum petcock also. Basically the on position leaves about .7 to .8 gallons of gas. Turning to reserve allows access to that gas. Since most of us do not have gas level gauges this keeps you from getting stranded.

To the rear is prime. this is an open flow that is not controlled by the vacuum valve of your petcock. Normally this is used when first putting the carbs on after cleaning or after running completely out of gas as it will allow the bowls to fill. It should be very rare to need to use this setting.

If left on prime and one or more of your needle valves leak then in theory your entire gas tank can be drained to the carbs and through them to the cylinders and into the oil. In practice it is a slower leak and would take a while to empty the tank but the result is the same.

So assuming your petcock works properly set it to on and forget it.

 

Could your gas cap vent be plugged and not allowing the fuel to flow. Try driving with the cap off.

 

A while back someone had a similar problem. Turns out his gas tank was getting vacuum locked. His gas cap didn't vent correctly. I'm not too sure of the safety or ease of doing this, but try opening your gas cap when you have this problem.

 

Just make sure this doesn't happen.... 8O

http://hibernia.jakma.org/~paul/edwards/

 

Would this still be possible on prime though? I'll give a try tomorrow. I made a complete mess out of my bike trying to do my brakes today lol. I may be in need of new calipers. I've got 1 piston that will not come out of the caliper and is now loaded with grease and the other one has a piston that will not go back into the caliper lol. Total and complete idiot. There is nothing more demeaning than having a grease gun literally stuck to your brake caliper. But yes, i'll try driving with the cap off. I did paint the tank myself so i'ts possible i messed that up to. Being the bumbling idiot I am.

 

you will still get a vac lock on prime.
crikey, have you pumped grease into the bleed nipple?

 

i'm embarassed to say yes. lol. Major brain fart. I got if off though. I read in the posts that people were getting frozen pistons out with the grease guns so i figured that would work. no dice. However all is now well, i had a hell of a time getting my piston rings back in the first caliper. the second caliper has yet to be cleaned and lubed and what not but i'm not looking forward to those rings either. Is there a trick to getting the dust ring back in easily? my was so stretched out, despite my best efforts, that it was too big to fit in correctly. what do you guys do to get to your master cyclinder? Mine is tucked in quite possibly the most inconvenient place possible. I only have another removal or two before the screws are stripped and i'm trying to think ahead.