elluded by a runaway idle... Can you help me brainstorm the problem?

[Snikare]

I've been doing a ton of work to my bike and have found one problem that I just can't seem to get over.
I'd really like to have some feedback from people that may be able to help.

Here are my stats-
I've got a 1979 xs650 special. There are 10,000 original miles on it. It's running BS38 carbs and a 2 into 1 Delkevic exhaust header. I've sync'd the carbs and tuned the jetting to match my exhaust as best as possible. I've also replaced every seal and gasket in the carbs after thuroughly and meticulously cleaning them several times over.

My issue is that I have a runaway idle. Things were great with my cutoff stock head pipes. I had a small baffle in them and the bike ran well. Then I bought the new headers and put a light baffle in them. The new pipes though are much bigger and needed tuning. I ran through all the jets and found some good ones that matched the needed mix with the new free flowing exhaust. But I immediately found a runaway idle wich I atributed to a lean mix. So I kept moving up in jetting to richen it up, the problem was not being fixed though. I ended up replacing my throttle shaft seals and thought that may have been the final culprit as they were the last thing needing to be replaced, but no changes.
Eventually I put my stock cut off headers back on and it helped... But didn't fix the problem. It's not running away so often now, but it does run off once in a while when I'm blipping the throttle while in my workshop.

I'm figuring that by working up a new custom exhaust and properly creating the right restriction, I'll probably be able to get it to perform more like how it should. But I'm still worried about the innermitent runaway problem. I'd like to get rid of any chance of it doing that. I'm at a total loss though! What am I missing? What else would cause a runaway idle? I've checked the timing, the tappet clearances, carb jetting, and all the seals. Nothing seems to be wrong with any of it...! So what's causing the speedy idle?

Thoughts? Any help would be greatly appreciated!!! As soon as I can get it running right, I'm gonna tear it down for paint and then start enjoying it.

 

[grizld1]

Two causes. 1. Intermittent failure of the advance mechanism to return to full retard at idle, due to lack of lube on ignition cam rod and/or weak bobweight springs. 2. Vacuum leak at throttle shafts and/or carb mounts. Much has been written on these topics. Seek and find.

 

[Snikare]

Thanks for the input. I've done a ton of searches for this problem and found all the threads and topics helpful for my process of ilimination, but nothing has solved the issue. I've checked both of those things. I replaced and lubed the springs in the advance several weeks ago. It moves free and smooth and returns nicely. The throttle shaft seals were just replaced in my last rebuild of the carbs. I've also got new intake boots and gaskets leading to the head and checked for leaks. It's all tight..!

 

[fredintoon]

Hi Snikare,
Grizld said check THREE things.
Bobweight springs? Yes.
Carb vacuum leaks? Yes.
But didja check that the shifter shaft was properly lubed and turned freely inside the camshaft?
You didn't say.

 

[Snikare]

Actually no, I haven't checked that. Thank you for clarifying. I'll go give it a try.

 

[Snikare]

So, I'm actually a little at a loss as to what I'm checking for on the shaft. The bob weights move and return freely. Is there something more that should be moving though? The small collar that the weights join with rotates when I pull on the weights, and then returns with the weights when I let them go. What else am I supposed to be checking for movement on?

 

[fredintoon]

Hi Snikare,
the advancer mechanism on one end of the advancer shaft is supposed to turn the cam plate on the other end of the shaft to advance and retard the spark.
It can't do that if it's seized up inside the camshaft and can't turn.
Check that the cam plate turns on the one side as you move the advancer on the other.

 

[Snikare]

ok. So if I'm understanding correctly, when I move the weights it turns the shaft that also moves the cam lobe on the points side. That is moving nicely. But I'm not sure what you mean by the 'cam plate'...

 

[fredintoon]

Hi Snikare,
cam plate, points plate, the thing the points are attached to, call it what you will.
Well, of course it ain't the plate that moves; it's the points cam that moves relative to the camshaft.
Except that if the advancer shaft turns inside the camshaft it ain't seized so it ain't causing your problem.

 

[TwoManyXS1Bs]

Try this quick experiment. While it's doing its high-idle thing, plug the upper crescent-shaped vents on the carbs with your fingers or thumbs. Report back if the idle changes...

 

[grizld1]

Yup, and one more thing. Sometimes an intake leak will only open up when the motor is hot, so do your checks after a ride. One semi-treacherous cause of intake leaks: extruded edges on the screw holes in the head, due to frisky tightening by Og the Cavewrench. If you find that, wrap a flat file (one that's actually flat!) with 200 to 320 paper, and do the needful.

 

[pamcopete]

Sync the carbs. One of the carbs could be on the edge of working / not working and when it suddenly decides to work, the revs go up.

 

[XSLeo]

On the advancer rod when you open the weights and release them do the weights snap back instantly or do they return slowly?
They should snap back. If they are even a bit sluggish the timing won't return as it should. This can cause your runaway idle.
If sluggish remove the advancer rod. I do it by removing the nut on the advancer side that holds the slotted disc. Slide the rod out the points side. Now you can clean the rod.
On the advancer side look for any lines on the slotted disc , there should be one that points to a line on the advancer. Also note the alignment pin so you can get the disc back in the right way. Now pull the disc out. This gives you room to clean the inside of the cam.
Do you or a friend own guns? If so use a large rifle or small shot gun brass cleaning brush to clean the inside of the cam.
Once clean you can see into the end of the cam. You will see what Yamaha calls a labyrinth seal. It is a bushing with grooves in it. Fill these grooves with a moly base grease. Put a thin coat of grease on the rod. Reinstall the rod and see how easy it spins, it should spin freely. If not the rod or bushing may have a bit of corrosion that is binding it.
If so remove the rod, clean out the grease. I use valve lapping compound to coat the rod where the rod rides in the bushing. Install the rod, use a socket on an adapter in a hand drill to spin the rod. Use a finger tip on the other side so you can push the rod back and forth. Polishing the rod and bushings. Do it just a bit, remove clean, lube and check for free spinning.
I had to do this when I put the new rod and bushings in an 82 engine. They were too tight and bound up so I got no advance.
After all this your advance should snap back. You say you replaced the advance springs, if you got them from Mike's they may be a bit weak. If after the cleaning and fresh lube the snap back is a bit sluggish take one spring and mark it with a Sharpie. Clip off the loop on one end of this marked spring and bend up the first turn for a new loop. This may be enough to get the proper snap back. Marking the spring is so if you need to clip the other spring you know which is which.
Now test run the engine, use a timing light to check timing, at idle and rev the engine up as you watch the timing. It should move over to the full advance mark but not past it. It should reach full advance about 3000-3200 rpms. If the springs are weak it reaches full advance too soon, like 2500-2800 rpm, if to strong, full advance comes late, like 3500 or more rpms.
It can be a bit of fiddling to get the springs right but worth it.
Leo

 

[Snikare]

Great input! Thank you, I'll be checking these things today and I'll report back soon.

 

[5twins]

Here's the "labyrinth seal" or bushing w/ grease grooves in the end of the cam. There's one at both ends. This is mine after 5 or 6 years, some grease left but could use a refill .....

..... and here it is refilled and ready for re-assembly, good now for another 5 years or so .....

This is a maintenance item but not one Yamaha ever mentioned in any of their manuals. It doesn't need doing often but it does have to be done. If you've never serviced yours, I can pretty much guarantee you're going to find it grease-less and dry.

 

[Snikare]

I pulled the advance rod this afternoon and it ended up being pretty dry in there. I gave it a healthy dose of grease and reassembled. Then started it up. I haven't been able to give it a good test yet because of the jetting and my carbs are now out of sync. But, the little I have just run it, it seems to have improved. I hope that solved the runaway idle issue.

And now there is another issue. I'm new to carb syncing and I think I've thrown them further out of whack. The right cylinder is popping back through the carb. I think it will solve itself though when I get it rejetted.

 

[Snikare]

I'll report back as I get more time to see if that had worked. Thank you all for the input!

 

[TwoManyXS1Bs]

A single-point, double lobe cam.
For other's reference, Snikare's rework of the cam is in here:

http://www.xs650.com/forum/showthread.php?t=12814

 

[Snikare]

Try this quick experiment. While it's doing its high-idle thing, plug the upper crescent-shaped vents on the carbs with your fingers or thumbs. Report back if the idle changes...

So, I tried this. I did it on each carb separately. I found no changes in the idle when I did it.
Should there have been?

 

[TwoManyXS1Bs]

No, no changes is good.
It's a way to test if the starter plungers are seated or leaking.
Scratch that off the list...