Struggling to keep a low idle

[bcware]

1978 XS400 2E

Mods:

MikesXS pods/Stock airbox
Emgo shorty mufflers

Carbs:

26mm float height (verified correct with external clear tube)
150 mains
47.5 pilots with stock airbox/50 pilots with MikesXS pods

Maintenance performed within last 2 weeks:

Timing set
Points replaced
Condenser replaced
Carbs synched
All carb internals replaced with new except diaphragms

Symptoms:

The bike runs beautifully; I ride it daily and even did a 400+ mile trip the other day through the mountains. I am still struggling to keep a reliable low idle, however. It has been very hot this summer and I do not remember having any problems earlier in the season.

The problem occurs with the stock air box and with MikesXS pod filters.

She idles best at 1,200 to 1,500 rpms, but every 10 - 20 seconds I can hear the engine miss a beat. Occasionally she will die at idle, but not often. She has only ever died when at full operating temperature while sitting at a red light. Often I can hear the idle start to get low and save her from dying by applying a tiny amount of throttle.

With all I have replaced and done should I be looking at the coils/wires/plug caps? I am still running the originals (34+ years old) and thinking I might have a weak spark.

Thoughts?

I may just replace this entire section of the ignition system for the sake of reliability. Worst case scenario it knocks a potential cause off the list.

 

[63-Coupe]

You have probably checked this but for what it is worth whenever mine starts to die at idle it's a signal to me to pull the plugs and clean them. I am running a little too rich which tends to carbon up the plugs every so often. The problem is that it is running so good right now that I am reluctant to mess with it, so for now I am just living with it.

 

[bcware]

I ordered new coils last night.

I check the plugs regularly; they look fine and do not appear to be fouled or anything. The threads on the plug are fairly dark, but the tip is only slightly darkened.

 

[domains]

My 82 was stalling on idle could not go in town with it. in higher rpm no issues.

Placed 92octane gas and was like night and day.

Kinda high compression motors.... mines 150p

Only 8.3.1 compression so I thought 87 octane would be perfect

 

[jmd_forest]

When you say all carb internals replaced I assume you include the throttle shaft seals?

 

[bcware]

The butterfly seals were NOT replaced. Perhaps those should be done as well? Do the ones on MikesXS fit? I didn't want to remove the butterflys because they look like a nightmare with the screws being pressed on.

An air leak there could mess up my idle, but I'd think it would just run lean, not outright die on me. I have very big pilot jets. Could that compensate for a tiny leak?

I only run premium fuel; 93 octane.

 

[domains]

The pilots should be plugged right? mine have rubber plugs... Even with plugs on do they work?

I read you unplug for other model of xs more power

 

[domains]

Could be your coils tho... if runs great cold

 

[bcware]

As far as I know the plugs should always be in. My 1978 uses a brass plug that screws in and, yes, they are installed.

I was thinking coils too; the symptoms of bad coils fit well. Misfires and poor idling and it seems to get worse when it's hot. It's been 90+ degrees and sunny every day here, so when I'm sitting at a red light she is definitely very warm!

 

[jmd_forest]

If I recall correctly, I bought the seals from Mikes when I rebuilt the carbs on my son's 80 xs400. and they were perfect. Since I had already torn them completely apart it seemed reasonable to spring for the extra $15 or so for the throttle shaft seals. They were not hard to remove, at least in my case. You might also want to ensure the manifolds connecting the carbs to the engine are not cracked/dry rotted and that they are tight to the engine. Also check for leaky plug wires and caps. You might try a plug 1 step hotter (BP6ES ?)

 

[bcware]

The carb boots/gaskets are new.

I am going to have to wait for the new coils to come in. That will help narrow this down.

 

[chkm8]

I had the exact same problem today. Everything else ran great. Fantastic throttle response, ran great otherwise, but died twice when stopping. It's been ungodly hot and humid here too. I was thinking coils or battery. My right side was running weaker, wet plugs and cooler, softer exhaust.

 

[chkm8]

Confirmed the right side is misfiring. Plugs are wet and when it does fire there is smoke out the exhaust. How do I diagnose the coil?

 

[Pork Chop]

My 82 was stalling on idle could not go in town with it. in higher rpm no issues.

Placed 92octane gas and was like night and day.

Kinda high compression motors.... mines 150p

Only 8.3.1 compression so I thought 84 octane would be perfect

84 Octane...? I've drunk OP Rum stronger than that. Here in Oz, the lowest I have seen is 91 RON but I only run 95+ in my cars and 98 in my bike.

 

[lhaolpa]

You weren't clear at first,but I assume that the motor dies at idle,rather than revving up. The coils can be checked with an ohmeter,just follow the instructions in a manual. Coils can be influenced by heat,but I doubt if these modern[relatively] coils shouldn't have that problem like an old style one cooled by oily tar inside them. That doesn't mean that it couldn't be the problem. The butterfly seals can be checked by shooting starter fluid,Wd-40,my favorite,propane[unlit]. In my area,and every place that I've been,the lowest octane gas I've seen is 87,and that's pretty bad. In my '88 Toyota truck,I run 93 octane alcohol-free gas from a convenience store,makes a real difference!! lha

 

[xschris]

I would say its over jetted and is flooding out at idle

 

[bcware]

That's not a bad suggestion, but the plugs just aren't very dark and I generally experience lean symptoms more than rich ones. If I turn the idle speed up a little she will hang at 3 - 4,000 rpms. I can't say that I get any bogging down.

 

[lhaolpa]

I agree that the pilot jets may be a bit rich.The pilot jet has some influence on the rich/lean condition caused by pods,more open exhaust,but less than the needle and main jets. In other words,the effect is not as pronounced at idle as higher speeds. Try the old pilots and see if that makes a difference. My $0.02. lha

 

[xschris]

I run an free flow 2-1 with mikesxs large oval pods and I run 42.5 pilots Are you sure you don't have the vm pilot jets instead of the cv stile ones? I have had people sell me the wrong ones before they almost look the same but the bike won't idle right with them.

 

[domains]

pilots are money scams

you would only need with bigbores or breathers

plus there plugged and don't work.