Kaw FD620D cylinder imbalance

[JohnF]

First, an anecdote:

Last winter met with some 6X6ers in WY for some snow fun. Unloaded an my machine was running really bad. Tim (Obsessed) tossed a tiny bit of snow on the exhaust pipes and showed that the back cylinder wasn't running. He checked the spark plug wire. I put my hand down in to follow the circuit just to see what was down there and all of a sudden it started working again. It hadn't faltered like this before and hasn't since. When I got home I tried to get it to do it again. No joy.

Forward to today. I thought I would try again and see if I could get it to fail. Started it and during idle pulled the plug wire off each cylinder. There is definitely a larger RPM drop when the back cylinder is unplugged compared to the front cylinder. Thought it might be valve lash and adjusted that. Didn't fix it.

So, is this normal? If not, what do I check next?

[Rock Doctor]

Just tossing ideas out

Swap the SP plugs to opposite cylinders see if the problem moves to the other cylinder. Maybe try the same with the Coils.

Compression test cylinders

RD

[JohnF]

Oh yeah, tried the SP plug trick - no change.
Hmm, funky coil even fits the anecdotal freakiness.
Gotta see a friend about borrowing a compression tester...

[JohnF]

swapped coils - no joy.

Also thinking vacuum leak around intake manifold on that cylinder. I think WD40 sprayed around the gasket area uncovers that one. Will still check compression.

If those check out then I'm going to assume this is normal... ?

[JohnF]

cold engine, throttle and choke open:
cylinder 2: 177ish
cylinder 1: 182ish

found no indication of vacuum leak.

So I guess this is normal for this engine.

[wfo]

fuel ?....carb ? try spraying just alittle bit of carb cleaner down intake when running and
see if that changes the way it idles, use something that is very flamable could be a
cyl is not getting enough fuel, or still could be not getting a good spark ,my .02

[Rock Doctor]

fuel ?....carb ? try spraying just alittle bit of carb cleaner down intake when running and
see if that changes the way it idles, use something that is very flamable could be a
cyl is not getting enough fuel, or still could be not getting a good spark ,my .02

GUNK
"Carb Medic" is a good product. Comes in a Yellow Can.

RD

[JohnF]

One barrel carburetor. I would think that if there were a carburetor issue it would show in both cylinders.
Following the fuel path is how I figured it could be something in the intake manifold on the cylinder 2 side.
Spark actually looks (and feels ) good and snappy.

Could be that timing is just a bit off on cylinder 2. That's set by a pickup coil off the flywheel I think.

It runs well. Sort of. I use vicegrips and held rpms at 1440. Pull cylinder 1 plug wire and it drops to 1320. Pull cylinder 2 plug wire and it drops to 1280.
I think the cheapy tach I have has a resolution of 20 (??) so that would be 2 resolutions difference.

[JohnF]

Dug into the service manual a bit. There's an "ignitor" that also separates the cylinders. Maybe this thing is doing something. It just looks to be a big capacitor but I don't really know what it does. I would think I would see a difference in spark if this were bad and I don't.

Maybe the intake manifold gasket sticks into the fuel/air flow a bit in cylinder 2.
Maybe a bit of wear/flat spot on cam lobe/tappet for intake valve on cylinder 2.
The contribution test above at 1440 RPM (this is in the range of the "book" low idle for this engine) indicates a drop between 21 and 40 rpm difference.

The engine runs well. I don't think I would have known if I hadn't popped the spark plug wires off one at a time just out of curiosity.

Out of curiosity, if you happen to pop the plug wires off one at a time on your engine - let me know what it does.

[gimmegreens336]

I will followup with better information, but mine will low-idle with 1 of the plugs out but will shut off with the other plug out. Noticed this while testing for spark after she heated up. Can't remember which cylinder it ran better on. Will let you know.

[sparkola]

Were compression tests done with both plugs removed? If the adjecent cylinder had the spark plug in, it will throw off the readings. If you had both plugs removed while testing the cylinders and only came up with @ a 2% difference in compression readings I wouldn't worry about it. Once you get more than 10% difference, it's time for a repair. Those are pretty good readings for a cold engine by the way.

[JohnF]

Yes, both plugs removed.

I thought that was good cold. Thanks for the confirmation. It's my understanding that cold readings are sort of worst case? (I'm no compression reading expert)

And the fuel pump was unplugged. But that's really a story later when I couldn't for the life of me get the thing started.