High Idle problem with XJ650

Okay.

I went over the carbs as well as a neophyte without Chacal's special tools can, got everything connected, still wouldn't start.

I pulled the plugs, they were black and wet. I was going to replace them anyways, but talking to bike mechanic, he told me that if the plugs are or have been wet, through them. The ceramic soaks in the fuel and there is no reliable way to unfoul them. He even tried heating them with a propane torch to burn away any fuel and dry them and it was a short term fix about half the time. With spark plugs costing just over a couple bucks each (for standard NGK) it seems pretty foolish to even try. Is the time you spend trying to resurrect that fouled plug worth $10?

I gapped the plugs to .030. halfway between the .028 and .032 spec'd range.

Oh, and before putting them in, he had told me to clear the cylanders of any fuel. How to do this? Use a long lighter, like you use for starting or grill, and light each cylander up. Sure enough, each one did a nice little "poof" of flame. #1 cylander actually took several times to clear it all out.

Put the plugs in, check and make sure that the choke switch is fully opening the choke, and then try starting her up, being very careful to *NOT* use the throttle and thereby flood her out and foul another set of plugs.

Turned her over ... a couple of times ... brief start !!! Hmmm, set the petcock on PRI and try again ... off she goes, quickly climbing to over 5000 RPM. Turn of the choke. Slows down a little but still chugging along a bit under 5000 RPM.

Well, she's running anyway. Now, recently I saw a post where someone else had a problem with it idling at 5000 RPM, but I can't find the thread.

So, please, please ... how do I get this to run at proper idle.

Michael

 

may have been me. I was not getting enough fuel in the bowl. readjusted my floats and turned down the idle. Kinda the opposite of you. But now that I think of it there was another guy who had a similar problems as you here about 3 weeks ago. I'll do a little looking also.

 

here is the link to the other thread from a couple weeks ago. i don't know if it is the one you are thinking of.

http://www.xjbikes.com/Forums/viewtopic/t=9648.html

 

Try turning down the idle stop screw.

 

Thanks xyxj650, that was the one.

I checked the throttle connections and there doesn't appear to be anything impeding free movement thereof, so tomorrow will play with the Idle screw as per MiCarl's suggestion.

Mike

 

Hey, my problem was with throttle, back that idle screw way out and try again. Assuming your carbs are properly synced: There isn't a way the engine can maintain 5000R's without the throttle being open, it needs a good amount of air to climb that high. My throttle was stuck on the stupid hose clamp for the boots. Make sure the cable is routed properly under the tank. Try pushing down on the sync screws with your finger to see if they move down some as they should appear to be vertical with the throttle closed. Make sure there is slack in the throttle cable at the carb (adjusted at the throttle with the two screws at the beginning of the cable).

Somehow a lot of air is getting THROUGH the carb, it can't be a vacuum leak because it would also need a lot of fuel. That's why it has to be throttle related. Even if your pilot screws are out of tune that alone wouldn't be enough to do this. Physics demands it to be true!

 

Well, I had a chance to work with them again today. I managed to use the idle screw to turn it down, but I couldn't get it to stay completely stable. I found that I had to keep the choke on a bit or it would stall out. I took it out for a little ride ... it was *AWESOME*. Now I find that the headlight doesn't work - replaced the bulb, still no go. Grrrrrrr. All the other lights work fine.

Mike

 

Idle should be set at around 1050 and it will idle diffrent once it warms up. I would also recomend doing a sync with a guage of some kind. For your headlight I would check the fuses under the seat and take it out (headlight)and make sure the connections arn't corroded or disconnected. There are 2 connectors plugging the headlight wires together inside the light housing. The one the headlight plugs directly into and another one a few inches back down the line. Something to check anyways.

 

I've been having the same problem as tumbleweed_biff, only my bike's an '82 XJ750.

I've rebuilt my carbs, making positive to clean the enrichment circuit, etc, and have done my first attempt at carb syncing (old-school, single guage, no YICS tool, bench synced first). I'm running with pod air filters.

The bike doesn't like t idle at 1050 as per the manual... If I try setting it there, the RPMs gradually drop, and the bike will stall. Keeping the RPMs high, by playing with the throttle or turning up the idle screw keep it running, but every now and then the RPMs start climbing to the 2500-3000 range.

I took the bike for my first ride around the block today, and it ran great. Gave me a grin ear-to-ear. Just need to fix this idle problem.

I do have it set to PRIme when running, as setting it to ON seems to stop fuel flow (I'm some what new to motorcycles, so please bear with me). What's the deal with the PRIme versus ON? REServe is fairly obvious. Should I switch the petcock over to ON once the bike is warmed up, and then the vaccuum that's hooked up for syphoning does it's magic. I feel silly asking. :-/

 

you should only have to use the pri in those cases where you are syncing your carbs all at the same time or if you run out of fuel and need to get it going right away. Also make sure you synce your carbs after the bike warms up. My bike idles the same but after it warms up idles fine at 1050. Set your idle after it warms up. Hope that helps.

 

Blufish,

If the machine isn't getting enough gas in ON, then something with the carb floats/float needle isn't working right. What I found was that the fuel wasn't being properly cut off by the floats/float needle(s) and I was leaking fuel into the airbox. Unknown is whether the various enrichment circuits/jets were open to the proper diameter as I had cleaned them using brake cleaner and high pressure air from a compressor. I don't have the special bits yet to physically clear those.

If you haven't yet, get the carbs off and verify that the float mechanisms move freely and there is no varnish on them. If there is varnish or anything else other than bare clean metal, you need to soak those parts in something (brake cleaner, seafoam, carb cleaner, etc.). Be certain *NOT* to soak the float needle lest you damage the rubber tip.

I have found someone who knows these old carbs and is well respected to take it to to get it synced. I hope to do that later this week if he can fit me in. Unfortunately, he is *very* busy being one of the very few professionals in the area to work on such old bikes who knows what he is doing.

Michael

 

Thanks guys. I've been running with my petcock set to PRI... all along... I wonder if this is helping over fill my float bowls. I have removed the stock air box, and can confirm that fuel is not coming out the air intakes.

Maybe there's a problem with my petcock when in the ON position, such as the vaccuum pressure not being able to open the valve... I'll investigate tonight, and report back.

 

You should have a hose coming from the top of the boot between the carb and the motor of carb # 3 (although I believe any of them should work, assuming all are synced properly). That should be connected to the valve on the petcock closest to the headlamp. If that is not there or not properly connected, you won't get vaccum for fuel flow.

Michael

 

It's there, and it has vaccuum. Carb's a sync'd to the best of my knowledge (did this yesterday, actually), but it was my first attempt in doing so. I'm gonna check everything tonight... maybe the petcock has a bit of rust or something blocking the valve shut...? I'll take a look when I get home.

 

Well, I did an experiment where I pulled my fuel line from the carbs and left the vaccuum line hooked up to the petcock, and with the petcock in the ON position, I left the bike run for a few minutes..... not a drop of fuel came out of the petcock, ruling out a float issue there. My question with regard to the petcock now becomes is leaving it in the PRIme position a bad thing? I figure that old school petcocks were just gravity fed... with it in PRIme, isn't that what you're doing?

I also think I need to fiddle with synching the carbs a bit more. I don't appear to have any hangups around the linkage, but if I reach in and push down on the tuning screws with my hand, the idle drops to a happier place (though they don't move too far). I'll betcha I just have tuned this thing wrong. Guess thats one of the better ways to learn to do something right... do it wrong a couple times!

Could be my float levels too. I might switch back in my stock float needles ad valves, as I read today that some of the fter market kits are shite, and might contribute to my problems in the grand scheme of things.

I'm gonna keep playing with it, but I'm still open to any suggestions.

 

There are two downsides, that I know of to leaving it in PRIme:

1) if something happens with the fuel line, it will leak fuel.
2) you lose the "reserve" function of your fuel tank. When it runs out, it will be truly out. No more reaching down to the petcock and switching to reserve to get you to the next gas station ...

Mike