Rear cylinder coughing very hard

Gents,

I've got an issue and I'm looking for information for inspiration.  Here's the low down.

(The following has the front cylinder plug wire disconnected to run only on the rear)

Rear cylinder will fire and idle for a few seconds sounding pretty normal,  Very shortly thereafter, it starts to cough and belch thick, rich smelling, light gray/white smoke.  The cadence of the coughs sound like it's only firing on every 4th or 5th revolution and with each labored fire, there's an accompanying belch of smoke.

I pulled the rear carb, tore it down, and cleaned all jets, checked the diaphragm visually, and reinstalled it.  Compression, while low at 135psi, is the same as on the (healthy firing) front cylinder.

It runs a bit without choke, but will stall and then resist turning over again.  Have to crank it and give her a lot of throttle stroking.

I swapped the coils, and trimmed a bit of the wires.  I checked for spark, and both front and rear seem about the same intensity of blue/white.

The plug is dark and oily. I've tried the hotter NGK and their Iridium plugs - same thing.

Fuel is good 91 octane and non ethanol..

It feels like fuel starvation, but I can't see how.  I'm thinking it's cylinder walls or rings.  Or valves.  Or maybe both.  Something that doesn't involve cracking the motor would be preferable!

Your thoughts will be welcomed!

Ride safe.

Capt. Willy
Rochester, NY USA

Capt Willy wrote:

Gents,

I've got an issue and I'm looking for information for inspiration.  Here's the low down.

(The following has the front cylinder plug wire disconnected to run only on the rear)

Rear cylinder will fire and idle for a few seconds sounding pretty normal,  Very shortly thereafter, it starts to cough and belch thick, rich smelling, light gray/white smoke.  The cadence of the coughs sound like it's only firing on every 4th or 5th revolution and with each labored fire, there's an accompanying belch of smoke.

I pulled the rear carb, tore it down, and cleaned all jets, checked the diaphragm visually, and reinstalled it.  Compression, while low at 135psi, is the same as on the (healthy firing) front cylinder.

It runs a bit without choke, but will stall and then resist turning over again.  Have to crank it and give her a lot of throttle stroking.

I swapped the coils, and trimmed a bit of the wires.  I checked for spark, and both front and rear seem about the same intensity of blue/white.

The plug is dark and oily. I've tried the hotter NGK and their Iridium plugs - same thing.

Fuel is good 91 octane and non ethanol..

It feels like fuel starvation, but I can't see how.  I'm thinking it's cylinder walls or rings.  Or valves.  Or maybe both.  Something that doesn't involve cracking the motor would be preferable!

Your thoughts will be welcomed!

Ride safe.

Capt. Willy
Rochester, NY USA

Grey/White smoke sounds like water in the cylinder to me, How's the coolant look? is there any Creamy like substance in the Oil filler cap?.

Capt,
I concur with Simbo, it could well be a gasket. Drain the oil and look for water.

G. Terence

First item - thank you both for your replies.

If the oil/water barrier is compromised would I not see cloudy oil through the viewing glass?  I will take a look and try to answer my own question.

It's not an unreasonable hypothesis (coolant in the cylinder) and it does fit the symptoms.  One item I failed to mention was the oil smelled of gas.  If coolant was hampering ignition, fuel from the power stroke would have to accumulate in the oil.

Any other input from you or the group would be welcomed...

Cheers,
Capt. Willy

I am not sure on the fuel system on your bike (manual or vacuum pet cock) but if you can smell fuel in the oil it sounds like the pet cock is leaking and has over filled the carb float and then run into the cylinder and through the rings and into the sump. If that is the case be careful as you could hydro-lock the cylinder causing some serious engine damage (as the petrol cannot compress). Drain the oil and check for fuel and replace. Then sort the pet-cock out.

Thanks for the note, v-twin.

If the petcock is leaking and is over filling the carb, I'd think the rear air box would be a wet gassy mess (which it isn't).  Also, wouldn't a petcock problem affect the front cylinder as well?

Regardless of the cause, I'll need to change the oil.  It's definitely smells of fuel...

CW

If the petcock is vacuum operated she could be leaking through the vacuum line directly into the inlet manifold and into the cylinder, bypassing the carb totally!!

Well, my understanding is the VS800 petcock is not vacuum driven - it has a fuel pump.  I doubt that's my issue here.

Good thought though...

CW