Revs holding temporarily

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DeftQuiver

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Hey Everyone,

I'm currently in the process of fixing up a 1980 XS650G, and all has been going pretty well up to this little gremlin.

Background:
About 4 weeks ago I adjusted the cam chain tensioner to spec, cleaned the carbs, checked the float height, installed new throttle shaft seals, new carb boots, and set my valve clearances. Then about a week ago, I threw on some drag bars, and swapped out the brake lines, master cylinder, and clutch cable, leaving the throttle cable stock.

After all that, it was running pretty strong for about a week (maybe a little rich, a faint squeaking noise that I'm trying to pinpoint, and some top end noise that I'm pretty sure is normal). So just last night, I was riding and the bike kept holding Revs at around 3-4k RPM's and would stay up there for about 3-5 seconds.

I went home and sprayed some carb cleaner at my carbs and boots, and didn't find any leaks. I also checked the cable and lever operation, and it appeared that the cable and the throttle lever were snapping right back into position. I then put it on the center stand to see if maybe the clutch was not disengaging, but that wasn't the case either. It had fouled a set of plugs before I made the adjustments listed above, so I've been pretty diligent about checking them, and they've been a bit carbony, but when I pulled them last night they still had carbon on one side, but the other side was bone white. I'll post pictures when I get home.

Any other thoughts on what this might be? I always thought that holding revs was a lean condition, which may be the case, but my plugs have been pretty carbony for the past week.

Thanks for any insight,

Deft
 
It sounds like you just about covered everything, but the one thing you didn't mention is syncing the carbs. Did you?

Also, you didn't mention what ignition you have, so I am assuming stock TCI. If not, then what?
 
Is it a points system? I have the same issue but i know its my throttle cable. Mine "Appears" to be snapping all the way back to idle, but when I check its position on the carb it is not. I pulled the cable out from under the tank and now it snaps ALL the way back.
 
Do the TCI equipped bikes have advance units? I know nothing about TCI, but if they do have the advance mechanism it might be sticking.

I had a similar problem last year, and I found that the bob-weights/springs on my timing advance weren't returning to their normal positions after I released the throttle, and would hold the revs at 3+K RPM.

You'd just have to pull the cover on the advance side (opposite the points cover) and see if the weights return to the "idle position" after you blip the throttle.
 
I have been looking at the lever on the carb, and the cable is not binding (the lever on the carb is snapping right back to the idle screw). My suspision is that it is an issue inside the carb. I'm not extreemely familiar with these carbs, but does anyone have any ideas as to what kind of internal carb issues could be causing this?

I could possibly see it being something to do with the ignition, but I'm having trouble conceptualizing why an ignition issue would cause the revs to hang for 3-5 seconds... In my brain, it doesn't make sense that having the spark go of incorrectly would cause the revs to hang without air/fuel getting into the cylinder.

Anyway, I'm going to shoot some video later today and post pictures of the plugs.

Thanks again,

Deft
 
I only asked what ignition you had on the chance that it was NOT the stock TCI, in which case it could be an advance problem. Since you do have the TCI, then it is most likely the carbs.

It sounds like you covered everything. My best guess is an intake leak. Get the carb cleaner back out, and check EVERYWHERE, completely all around the boots, and all around the entire bodies. Do it when the problem is worst, which is probably when the engine is nice and hot.
 
Ok, I'll test it out again when I get home and let you know how it goes. Could something like this be caused by leaky butterfly shaft seals? How about a leaky O-ring in the carb?
 
retiredgentleman said:
Any air leakage that can be drawn in by the intake vacuum is a possibile source of a hovering high idle.
Click to expand...

Exactly what I needed to hear... I'll be doing the carb cleaner test when I get home, and if nothing turns up, It looks like I'll be pulling the carbs.

Is there anything you guys can think of that I should keep an eye out for while I'm in there? ie. anything that commonly leaks air.
 
Remove and plug the petcock vacuum line run it on prime see if that changes anything.

If the left carb was the "fluffy" one that might point at a leaky vacuum petcock. And it could be crud on a float valve seat or a bad float valve o-ring.
 
Sounds like its running lean on the white crust cyl. and staying above idle sounds like an vac leak. Sounds like its drawing air after the throttle is closed I would check the carb holder gaskets and vac nipples, hose, as well as the petcock
 
gggGary said:
Remove and plug the petcock vacuum line run it on prime see if that changes anything.

If the left carb was the "fluffy" one that might point at a leaky vacuum petcock. And it could be crud on a float valve seat or a bad float valve o-ring.
Click to expand...

Sounds like a plan... After reading your posts... I should probably be a bit more clear about the plugs. I wish I could upload pictures from my phone, but I'll put them up when I get home. The best way I can describe it is that one individual plug has carbon on one side, and is bone white on the other. I guess to me, it looks like the bike may be going from rich (and fluffing the plug with carbon) to lean (and burning it off) which i'm assuming would imply an air leak.

The petcock does leak a little when it's off the bike, but it will stop once I quit sloshing the gas around in the tank. Thanks for the advice. I'll try this when I'm home as well.

Deft
 
Ok, so I just got home and ran some tests... I haven't yet done the carb cleaner, but I can rule out the petcock.

This is with the vacuum petcock ON

This is with the vacuum petcock set to prime

I'm really hoping I find something with the carb cleaner... otherwise I'm taking them off this weekend.

Deft
 
And here is the picture of the plug I promised...

These are both from the right cylinder.

full


full


I also forgot to mention, that in the first video in my previous post, you can here a bit of a squeaking when the RPM's are coming down. Anyone have a clue what that is?
 
Here is a hint on the squeaking anyways

Sundie
Jabroni



Join Date: Feb 2010
Location: Toronto Ontario
Posts: 965


icon1.gif
Re: carbs are giving me a headach? can use some help.
lol - 155 is *way* too big. run 30's on the pilot, 135's on the main, and put the needle in the middle, *possibly* the second from the bottom. Going bigger than 140 on the main just ensures you're wasting fuel. and, since i find that open pipe guys *rarely* run wot and over 6k rpm, 135 will do you fine.

oh, and if your bike is squeaking at idle, pop your choke bodies off and *very very lightly* skim coat the seals with 1194 and re-assemble. They're problematic in their warping.
 
On your plugs the white side is toward the intake valve. The carbon gets washed off by the incoming air/fuel.
If the black side is dry, that indicates a rich mixture. If wet, an oil leak.
www.amckayltd.com/carbguide.pdf is the carb bible.
Leo
 

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