Run away on start up

I bought a mechanics special on Craigslist (1980 xj650). It was torn apart when I bought it. I put a new head and rebuilt the carb. Now Im working on syncing the carbs. Once the bike starts getting warm it'll get to a point where it just runs away. Ive tried adjusting the idle speed setting and idle air mixture. The idle air mixture screws can be completely home or to their limits, it doesn't matter, a half second after it starts it just starts gaining rpm. The fuel level in the bowl is within spec. I noticed that when the bike is in the process of warming up adjusting the idle speed setting doesn't seem to affect idle speed. The throttle butterfly valves close completely, its not like a pinched cable holding the throttle open haha.

Any thoughts?

 

Massive airleaks??

Have you pulled the plugs...or will she start from stone cold, with NO choke??

 

You rebuilt the carbs how? What parts were replaced?

You do have vacuum leak(s) for certain.

 

Pcv? That seems to be a common culprit for high idle after head jobs in my experience. Also check the intake boots for cracks and wear between the carb and cylinder. Adjusting the mix won't help because it's sucking in air from somewhere else that it shouldn't.

 

A faulty PCV valve would be a solid suspect, if the XJ engine had one.

 

make sure your throttle cable is lubed and properly adjusted, sitting in the cup.
there should be some slack in the line movement in the grip without moving the "throttle plate" (where the cable connects)on the carbs.
loosen the adjuster at the grip and see if that drops your runaway idle.
as the motor warms up the idle will rise normaly

 

Hi k-moe is the PCV valve pipe the one that fits on top of the gearbox cover next to the starter motor and of so how do you clean the PCV valve. I think I have a faulty PCV valve too. My engine runs great when hot but revs very high on startup, the carburettors have been cleaned and balanced. Also it will not pull well on startup without messing about with the choke.

 

only the seca Xj 550 has a gizmo in the crankcase vent hose. could be your hose is clogged or there is crap on the inside of the airbox covering the hole.
see part 12 1nd 13 for xj650g
http://www.yamahapartsnation.com/oemparts/a/yam/50041e7df8700209bc786880/crankcase-cover


it sounds like your pilot jets are clogged this includes the mixture screw. did you replace the little tiny o ring on the mixture screw?

adding choke adds fuel as it is not really a choke it is an enrichment circuit.

 

Thanks XJ550H I replaced the o ring but I will have to check out the other things you advise on. The screw between the engine and the carburettor is the mixture screw but two of them are stuck and the heads damaged so these have to be removed and replaced with new ones. As they are stuck I don't know how many turns they are in which is a problem These could be causing the problems I have with the bike too. Any ideas on how to remove the old ones?

 

I soak them with kroil (penetrating oil) for several days then I drill and use an easy out (screw extractor) the smallest one they have at the hardware store.
I have only had 1 that was so stuck it would not come out with an easy out. if you are not comfortable using easy outs you could bring them to a machine shop and have them do it.

 

Cheers XJ550H I will try the easy outs and penetrating fluid, most important is not to drill too deep and keep the bit square I think. Two mixture screws are fine but the previous owner chewed the other two up. I will put the penetrating fluid and leave it overnight and drill them tomorrow.

 

No valve that I know of on the 550 Seca....there's just the pipe that sticks out of the cover so the breather tube can be attached. NO xj has a PCV valve that I am aware of. XJ550H, I'd like to see a clear pic of the specific valve that you're talking about. There's nothing extra shown in the diagram.

Mixture screws won't cause a runaway idle.

AIR LEAKS or a BUTTERFLY HANGING UP will cause a runaway.........most common air leaks are at the throttleshaft seals, or the intake manifold boots.

 

I read about it in one of bigfitz s posts and he said to remove it. I will look on mine to see if it is there

 

did not have one on my 82 Seca 550 but have one on my 81 maxim project/parts bike
.

and I think this is the part in my Seca catalogue

this link is about above breather tube in seca 550
550 SECA AIRBOX Tech Bulletin- Important
but does not address the "pvc" Item in photos

 

Cool! Very interesting--I'll hafta go check mine. Thanks

However, the question is for a 650........so I doubt that is causing the runaway issue

 

yes which goes back to my original statment of no pvc ....
going to pull the valve pictured and see if it has a part number.
also my other 81 maxim 550 does not have that device and was built earlier by serial number.

 

I guess that I was not clear enough. None of the XJ series has a PCV valve. They all just have a simple baffle system that can't clog up. Even if the breather were clogged it would cause increased crankcase pressure, which would NOT lead to a runaway idle. That would actually make the idle lug down.

 

Agree with all above you have a vacuum leaks , usually the intake boots are your carbs fully seated? Do your boots have any cracks in them? What type of clamps are you using ? I personally am against normal plumbing hose clamps these can cut into the rubber . The butterfly shaft seals can also be leaking although the leak can be minimal amount if all 4 carbs leak that is an issue.

 

this worked for someone else you may want to check it again

http://www.xjbikes.com/forums/threads/83-xj750-maxim-starting-problem.112652/page-3

 

My bad on the pcv, Got 4 bikes in the garage and I was at work so I couldn't verify if these xjs even have one. I'll keep my mouth shut till I have more wrench time on my xj
Cheers hope you figure it out

 

Found a few potential vacuum leak areas.
First I checked the caps over the vent on the carb boots, they look a little worn. I used some sections of 3/16 fuel hose with bolts threaded into one end as a plug.
Secondly I replaced the paper gaskets I had made for the carb boots with cork some cork ones that I also made.

While the carbs are off Im going to re-bench sync them. Is 2.5 turns out from seated the correct baseline for idle air mixture screws?

 

Just hooked the carb up after bench syncing it and making the changes I stated in my last post. It still runs away right as soon as I fire it up. The throttle cable isn't holding the valves open.

 

I replaced float needles and cleaned everything in the lower end. All the bowls are within spec (3mm +/-1).

 

The throttle cable has a bit of slack, its not holding the butterflys open at all. The idle is rising without stopping. It'll red line within 2 seconds.

 

What are the throttle shaft seals?

 

They seal the throttle shafts so that you don't have a vacuum leak. Vacuum leaks cause a runaway idle.
Your throttle shaft seals are very likely the originals, and no longer sealing.

You have some more work to do.

IN THE CHURCH OF CLEAN

 

So throttle shaft seals just got replaced I do believe I had a bad one. But.....it still runs away. could this be a result of incorrect timing adjustment?

 

I'd adjust the idle down till it stops running away. If you don't have a vacuum leak, just having idle too high can cause this. If you can get the idle to settle down, do a running sync and re-adjust idle.

 

Timing (other than cam timing) is non-adjustable, and will not cause a runaway idle.


Do what dkavanagh reccomends and ge back to us. If that dosen't solve it there are other things to look at.

 

Idle was too high. Thanks guys!!!

 

Nice to be able to ask questions here, ain't it? Ever wonder how much a shop would have charged you to diagnose and fix that?!?! Two hours would have been spent just trying to figure out how to hook it up to the computer........lol

 

I dont even want to think about how much this would've cost in a shop. At this point I've had at least an entire work week in on this thing. I can't thank you all enough for the help and parts (CHACAL--hit this guy up for everything under the kitchen sink..waaaaay more than the local Yamaha dealer even) seriously!

 

Under the kitchen sink? Heck, he's got everything under, above, in, around, and connected to, the kitchen sink.....