No power after 5 minutes at 65+mph

My 1982 xj650j runs SPECTACULAR at speeds up to 60mph with petcock set to ON or RES. Acts like it's outta gas after 5 mins or less at 65mph+, but not with petcock set to prime. Motoloco of Santa Barbara JUST rebuilt the petcock because it was leaking... A LOT. However, though it leaked, this problem was not present prior to the rebuild.

Why might it not be getting enough fuel when set to ON or RES at ONLY high speeds?

 

Also... Plugs look FANTASTIC

 

they might have lost a spring inside the switch. put a different spring but got the wrong tension and now it's too tight. not allowing enough fuel in the on or res position. the fuel switch is vacuum operated where the vacuum pulls the plunger back and forth. when the switch is in prime. it bypasses the plunger.

we're assuming the switch is proper and still requires the vacuum line hooked up to the carb intake boot.

do you have a in line fuel filter that might be plugged. OR, what happens with the gas cap off. the tank might not be venting enough.

the faster you go. the more gas is needed.

 

Bingo.

It's either a plugged gas cap vent, or they used an inferior aftermarket kit in the petcock and now it's not as "responsive" as it should be.

Assuming it's not something simple like a vacuum line problem.

 

I just put a new vacuum line in today; gonna try the gas cap open tomorrow.

The kit used was a K&N.

 

Probably K&L. I don't believe K&N makes fuel cock parts.

 

You should be able to troubleshoot this at less than freeway speeds by using a lower gear at lower speeds. 35mph in first or 2nd gear should require enough fuel to simulate the 65 mph freeway speed and not put you in harms way. Plus you should be able to test in a location closer to home. Not so far to push if it all goes wrong.

Loren

 

Tried it in 2nd at 40mph and 3rd at 55mph... Ran like it was 28 days old as opposed to 28 years. The carb drain screws are in there pretty good and I can't seem to crack them loose... I'm sure the fuel is clean though. This is aggravating.

 

Yeah, the trouble is, 3rd gear at 55mph may give you the same revs as true highway speed but you're still not pulling the same load so it's still not using as much fuel.

Wheoever rebuilt your petcock could have used a replacement spring that was too strong, or didn't get it back together right; OR your gas cap vent could be restrictive but not completely plugged.

Tear the gas cap apart (be ready to catch small flying springs) and clean it.

Or just try opening it the next time the symptoms occur.

***EDIT*** Just re-read your original post. You said that your petcock leaked, but DID NOT have this problem until this shop rebuilt it. It couldn't really be much else other than the vacuum line or the petcock.

QUESTION: "New" vacuum line? Is it REAL vacuum line? Other types of hose can collapse in on themselves under vacuum, especially once warm/hot.

 

I agree with Snowwy and Fitz; too strong of a spring.

You paid for the repair - you should take it back, but I'm sure they threw away the original parts(or did they hand them to you in a bag??)

You could use a breadwrapper tie and try to coil-bind the new spring a little, make it shorter just enough to still work.

You could also use a "T" and get vacuum from 2 & 3 , effectively doubling your vacuum signal, making the stronger spring work.

 

my trick was to simply cut the spring down.

i lost mine when i had my yamaha. used a carb spring. it was JUST long enough to push the plunger. but it could still retract from vacuum.

that sucker don't need to be very long at all. i think it was only 1/4 inch long.

 

How about just get the correct spring from chacal?

 

Well, I went to clamp the vacuum line, just in case it was leaking... and it had a bit of fuel in it. Is this bad or is it fine since it runs fabulous so long as I remember to switch to PRIME for freeway travel?

And yes, it is a REAL vacuum line.

 

Bad. Means your petcock still isn't working. The diaphram leaks and isn't being pulled open enough. In addition you're leaking some amount of raw fuel into the intake of #3 or whichever one your vacuum line is attached to.

 

That indicates a bad diaphragm, and that they did a bad job.
Take it back - - you paid a shop for a repair.

 

If the (real) vacuum line had fuel in it, something is not right in the petcock.

The "vacuum" side of the petcock should stay dry.

I know you didn't do the work, but if you're interested in what's "in there:" http://xjbikes.com/Forums/viewtopic/t=25058.html it might help you figure out the problem.

 

that would explain all your problems.

 

Here's the thing though... When I blew into it, fuel flow was good. Tried sucking and it was air tight. The problem with taking it back is that they sealed it up REALLY well; I'm told that the next step is just to replace the petcock entirely. OEM petcock assembly runs like $100, new. But again, the petcock itself seems to be working great.

Is it at all possible that the fuel came from the intake? My intake boots are definitely cracked but don't seem to be cracked all the way through (or so as results of the WD40 test tell me)

 

Correction... The intake boots are definitely leaking.

 

The fuel would have to travel through the carb and up the vacuum line all while being pulled into the cylinder on the intake stroke.

Sounds like you have more than one problem.

 

No doubt! Well, I'm waiting for her to cool off so I can apply some black rtv business to the intake boots.

The petcock isn't leaking bad at all... There was only a few drops of fuel in the vacuum line. I'll start saving for a new OEM.

 

Get an on-off aftermarket kit from Chacal for $70. Beats the $110+. Or rebuilt your old one properly.

 

rebuild kit is only $22. and comes with a new diaphragm. don't know what chacal sells it for.

 

i would definitely fix the petcock. If the boots are leaking like you say would make sense running at 65 with a lean mixture would cause it to heat up also

 

I'd prefer to spend $96 for a new Yamaha petcock assembly, if nothing else...

It's weird, though. it doesn't leak at all when it's not hooked up. I guess the engine is better at sucking than I

 

That said... Does anybody know if an aftermarket intake boot exists? Yamaha wants $70. PER.

 

you can order the boots online from chacal or other sources. the fronts are about $40 i beleive. the rears are $17.

as for the petcock itself. there's usually only 2 screws to take the handle out. i think there was 3 screws for the backing plate. on my 650. rebuilding the petcock is so easy a child can do it. there's only 2 or 3 parts to replace. maybe 4.

 

That's the problem. Rebuilding a petcock correctly takes a little more expertise.

Has anybody LOOKED at the petcock teardown/rebuild article? http://xjbikes.com/Forums/viewtopic/t=25058.html It's not difficult, but it does require care.

If the petcock "operates" fine, that is, it doesn't allow fuel flow unless it has vacuum, then it should be easily repairable. If it's shutting off, then the seat is probably fine.

I suspect that whoever rebuilt it may have gotten the diaphragm spacer plate installed back in the wrong orientation, or they didn't excercise care when refitting the diaphragm/plate. This assumes that they actually replaced the diaphragm; on an unmolested unit, fuel on the vacuum side indicates a damaged diaphragm.

snowwy is right though, the petcock is an easy part to work with. You might want to consider doing a little diagnosis on yours, and then go tell the shop what they did wrong and get your money back.

The "OEM replacement" petcocks, although "official" (and made by the same company) are NOT exactly the same as the originals, and even though I have one myself, I find them a bit obtrusive compared to what came on the bike: