Need help. Right cylinder fires intermittently at idle.

CoastsideXS650 · Oct 19, 2019

Hi guys. I’m looking for some suggestions as to where to look for a intermittently firing right cylinder at idle.

The basics:
1980 xs650 motor with fresh bottom end rebuild, installed 700cc pistons and fresh top end (valves lapped not re-cut). Stock bs34 carbs and air boxes, black box ignition unit which works properly, dual output green monster coil.

The bike starts fine from cold. Runs very well when choke is on. Once warm the right cylinder drops off at idle speed and the whole thing runs on one cylinder.

I have checked the following and don’t seem to have anything wrong:
-swapped out carbs off perfectly running bike and no change in behavior.
-intake and exhaust have no leaks (carb cleaner spray check method)
-both plugs are firing (used inline spark plug light to check)

I haven’t checked the compression or done a leak down test. My instincts tell me it’s electrical of some kind.

Any ideas or suggestions out there from people? I’m getting pretty stumped and could use some help.

5twins · Oct 19, 2019

It does sound electrical. Your coil fires both plugs at the same time so you could swap the plug wires side to side and see if the problem follows. That would narrow it down to a bad plug cap, plug wire, or the connections between those parts. When I install plug wires into caps and coils, I take an extra step to insure I get good contact and a good connection between the wire and part. I strip off about 1/8" of insulation on the wire and fan the wire strands out in a radial pattern like so .....

This pretty much assures I'll make good contact and a good connection between the wire and cap or coil. Last step is a little dielectric grease on the end of the wire before making the connection.

CoastsideXS650 · Oct 19, 2019

Good call on swapping plug leads. I always forget that trick as my previous bike had a rephase and two separate coils.

gggGary · Oct 19, 2019

Recheck cam chain (just because) and valves?
and yeah check compression.
Torch dry plugs, swap sides.

CoastsideXS650 · Oct 19, 2019

I know everyone on this site always mentioned camchain and valve lash (I do it too). But remember the motor runs great at higher than idle RPM and I would not expect such behavior from a machine with poorly adjusted valves. Even when they are loose they still run just a bit noisy.

Can you elaborate on torching dry plugs? I’m not sure I understand what you mean. Like take a blowtorch to them?

gggGary · Oct 19, 2019

Yeah just worry a bit about a valve being tight.
A plug that's got a bit of oil on it may clean out work fine at higher RPM not so much at low RPM
Anytime I have a miss or hard start motor I put the plugs in the vice and burn any dark off the insulator with propane torch till it's white all around.
But what do I know? I 'm fighting this exact same problem with WRL (points, stock early carbs) always cleans up and runs great at higher PRM a cylinder drops out, at idle. Just did complete carb work and changed plugs, caps, wires. Same as you, spark tester looks good at all RPM. On mine I'm opening valve gaps to early specs. and working over points, timing next. Is your charging, battery up to snuff? try running without headlight on? ( I did that test too)

CoastsideXS650 · Oct 19, 2019

Yeah I get what you’re saying now. I tested everything with the charging system disconnected and the battery always sits on a tender so it’s probably not that. Will check the plugs too with some freshies when I get back at it

Moabite · Oct 20, 2019

...maybe pilot jet clogged, happens all the time here with such fine sand in air...
...in fact, yesterday honda cub ride bike bogging off-idle, 99% sure it carb...again...
...tiny grit, but tinier hole in pilot it can not pass through...

CoastsideXS650 · Oct 20, 2019

I don’t think it’s a clogged jet. If you read the first post I took carbs of another bike to test and that bike runs flawlessly. The carb swap didn’t change the symptoms so it’s safe to say that it’s not a carb issue I think.

GLJ · Oct 20, 2019

CoastsideXS650 said:
Once warm the right cylinder drops off at idle speed and the whole thing runs on one cylinder.

Based on this and other things you have posted I would give the right carb a wif of carb spray when it was idling. No change ignition problem, idle increase would mean slight air leak. Manifold or gasket.

Jim · Oct 20, 2019

GLJ said:
Based on this and other things you have posted I would give the right carb a wif of carb spray when it was idling. No change ignition problem, idle increase would mean slight air leak. Manifold or gasket.

I'd also suggest replacing the cap on the right manifold vacuum port. Might have a crack in it... or just be hardened and a loose fit.

nj1639 · Oct 20, 2019

On capping the manifold ports,
I run gravity fed aftermarket petcocks and then link the two vacuum barbs together with about a one foot length of vacuum hose.....liken to think it equalizes manifold pressure lol.

grizld1 · Oct 20, 2019

An acquaintance of mine chased the same symptom (one cylinder dropping out at idle) for weeks. The source of the trouble turned out to be a new but bad sparkplug--an NGK BP7ES, not a Brand X. Rare, but it happens.

CoastsideXS650 · Oct 21, 2019

CoastsideXS650 said:
I have checked the following and don’t seem to have anything wrong:
-intake and exhaust have no leaks (carb cleaner spray check method)

GLJ said:

Based on this and other things you have posted I would give the right carb a wif of carb spray when it was idling. No change ignition problem, idle increase would mean slight air leak. Manifold or gasket.

Click to expand...

Already tried that. Intake boots are good.

CoastsideXS650 · Oct 21, 2019

Jim said:
I'd also suggest replacing the cap on the right manifold vacuum port. Might have a crack in it... or just be hardened and a loose fit.

I have JBM Industries boots, no vacuum ports and no need to cap them or connect with a hose.

CoastsideXS650 · Oct 21, 2019

grizld1 said:
An acquaintance of mine chased the same symptom (one cylinder dropping out at idle) for weeks. The source of the trouble turned out to be a new but bad sparkplug--an NGK BP7ES, not a Brand X. Rare, but it happens.

Spark plugs and coil/HT leads/plug caps are the next things I am going to check. Im thinking the issue lies here more and more. I just cannot check them till the weekend when I Can make it down to my shop.

5twins · Oct 21, 2019

Give each plug cap a gentle tug. If it comes right off the wire in your hand, you may have found the issue. I found that to be the problem on one of my bikes one time after it started mis-firing.

Need help. Right cylinder fires intermittently at idle.

CoastsideXS650

Princeton Motor Works

5twins

XS650 Guru

CoastsideXS650

Princeton Motor Works

gggGary

If not now, When?

CoastsideXS650

Princeton Motor Works

gggGary

If not now, When?

CoastsideXS650

Princeton Motor Works

Moabite

XS650 Addict

CoastsideXS650

Princeton Motor Works

GLJ

Never go faster than your guardian angle can fly.

Jim

Beyond the edge is the unknown. Here be Dragons

nj1639

Paleo-simplistic

grizld1

Grumpy old man

CoastsideXS650

Princeton Motor Works

CoastsideXS650

Princeton Motor Works

CoastsideXS650

Princeton Motor Works

5twins

XS650 Guru

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