Will not idle reliably

[HoughMade]

OK, I've mentioned this issue before, but in other threads. Thought I'd give it it's own thread.

After running for a while, the bike will not hold an idle. It will hold an idle when first started and after idling a while when it is warm. After I have been riding, it slows down, then dies. This happens especially immediately after running at over 50mph for a few miles. It always restarts with the electric start, always.

Otherwise, the bike runs perfectly. It starts very easily and runs smoothly at all rpm levels.

Here is my setup. This is a '79 with a stock engine and BS34 carbs. They are clean...clean, clean, clean and I have a filter in the fuel line. Air filters are stock and clean. The idle mixture screws are 3 turns out. The valves have been adjusted and are in spec. The diaphragms are fine. There are no intake leaks.

Last week, I was setting the points gap and they were somewhat eroded (the problem was present before this). I replaced both points and the condenser (still there). The coils test within spec. cold, but I do not suspect the coils. If it was hot coils, I would expect it not to restart until cooling a bit. As it is, it will restart before I even stop.

I also do not suspect battery issues- again, it always restarts with the electric starter.

Thoughts?

 

[akronevan2]

Have you smelled the oil for gas? My bike used to do the same thing when it would get gas in the oil. It may not be the problem but it's worth a look.

 

[bcware]

What's your float level set to? When the engine is hot you're running leaner. If everything else is fine it must be a problem with the idle mix being too lean. Another thing you can do is pull the plugs when the engine is hot and has been revving through the range; this will tell you if there is any raw unburned fuel on the plugs. How do the plugs look in general? I am assuming they probably look fine. Have you tried going out more turns on the idle mix screws?

 

[cturek]

Sounds to me like your developing a vacuum in the fuel tank. Make sure your tank cap is venting properly.

 

[DanMach]

Ya it sounds to me like a fuel issue at low RPM's... maybe not enough vacuum to work the petcock or something??? Good luck!

 

[HoughMade]

No vacuum petcock. Replaced it with a manual one some time ago. Also, several weeks ago I took apart the fuel lid and made sure everything was open- even enlarged the vents- problem still present.

Thanks for the ideas,everyone.

 

[bcware]

I just fixed another significant idle issue by increasing the fuel in the bowl. My bike ran perfectly when it was cold, but after heating up it wanted to idle at 4 grand. I dropped the float height down to 23mm and the problem is solved.

 

[HoughMade]

OK- fixed it. It idling perfectly over a few rides.

How? Well, I replaced the 60w/55w headlight with a 35w/35w I had, concerned that there was not enough juice at low engine speed....that wasn't it. Still had the problem.

I took the carbs off and cleaned them again. Did not find any problems, but who knows? As an aside, if you want to make sure the low speed passages are 100% clean, screw the idlemix screw all the way in (lightly) then spray carb cleaner through the idle air jet at the carb mouth. Cleaner should shoot out the 3 holes above the butterfly forcefully. Then do the same through the idle jet in the bowl. Then loosen the idle mixture screw and do it again and the jet should forcefully come out the hole where the screw is.

That may have helped, but I don't know.

In reading through some ignition info.,I saw that an old school mechanic explained that as engines wear, they like a little more advance than a new engine. Simply put, with lower compression, more advance leads to better running.

I went back and cleaned the point and re-gapped them just to make sure they were spot on (new a couple of weeks ago), then I set the static idle perfectly to the "F" line. However, my cylinders have about 125 and 130 psi compression, so there is a bit of wear. His instructions were to warm the engine up, then idle it and advance the timing plate until the engine idles as fast at it will (idle will speed up with advance), then retard it back a bit. He suggests to ride to check for pinging and then check the plugs. I did exactly this. I only advanced it a bit, but the idle sped up and smoothed out. I tightened the screws holding the points plate and then set the idle to 1200 rpm. The bike has run perfectly since.

What did it? The additional carb cleaning or the advancing? Don't know...almost don't care. It's running really, really well.

Here is a link to where I read this and the specific page on ignition timing. That site has a lot of good info.

http://www.dansmc.com/mc_repaircourse.htm

http://www.dansmc.com/ignitiontiming.html

http://www.dansmc.com/bat_coil_ign.htm

 

[hmusket]

Congrats on the fix, great post too. I always forget about Dan's website, a lot of good tips there. Enjoy your riding, keep the shiny side up.

 

[drewpy]

interesting reading, good find

 

[HoughMade]

50 miles today...she's running sweet! Never skipped a beat.

 

[xs400neophite]

Oh my Lord!!! Dude.... this is exactly what my bike is doing!! This is why this forum is so great!!
I just went through the whole 'condenser/points/coil' thing....still not right.
I was thinking of doing the 'better battery' thing..still might when this one goes.
I was about to do the 're-clean the carb' thing.Still going to do that.
Now, I will also try the 'advance' thing too.
Search the threads...most likely your problem has come up before.
Thanks 'Houghmade'

 

[bochnak]

How much would you say you advanced it? Mine is experiencing the same problem. I have 140psi compression, though.

 

[HoughMade]

I'm afraid I can't say, exactly. I can't be more specific than "a few degrees".

Still running great.

 

[bochnak]

I'm afraid I can't say, exactly. I can't be more specific than "a few degrees".

Still running great.

OK. I'll give it a shot.

Did you notice any improvements in fuel screw adjustment? Mine are not that responsive.

 

[HoughMade]

Mine were always pretty responsive, so not really. Err on the side of making it rich.

 

[bochnak]

I advanced it 9°, and the problem still exists. My idle is pig rich, and I can completely close the fuel screws and it will still run. I have a rich stumble from 1-2k rpm, and plugs come out black. It runs great above 2k and plugs clear up. It has stock pilots, 17.5. (76' xs360)

Are there any rubber plugs that are supposed to be installed in the float bowl?

 

[xschris]

I don't think the xs360 carbs have any they are built like the bs38's could you post a few pics of what you have? If your bowls look like this then no.

 

[HoughMade]

Don't know about XS360s, but I think the plug is for newer bikes than mine and i have a '79 XS400. However, the pilot tube next to the main jet on mine has a brass screw in it with an angles opening into the main jet passage. Whenever i hear of idle mixture screws that don't make a difference, I think of tips broken off.

 

[bochnak]

I don't think the xs360 carbs have any they are built like the bs38's could you post a few pics of what you have? If your bowls look like this then no.

Thanks. My bowls look like that and I believe the plugs were added in 80'.