Left Cylinder Not Firing at Idle, Fouling Plugs

[browndawg]

Hi Guys,

I bought an '81 XS650 Special back in November, and due to some titling issues, I haven't been able to do much riding other than up and down the street until just recently. I finally got the title issues resolved and got the bike out on the road.

Unfortunately, it didn't take long before the left cylinder quit firing, or firing intermittently. I pulled the sparkplug and found that it was badly fouled--a good bit of still-wet oil. I put in a new plug and the cylinder began firing again, puffing out lots of blue smoke at first, but seeming to run well. But, before long, it again wouldn't fire at idle, though it fires fine once you give it a little throttle. Also, I noticed a seemingly excessive amount of oil coming from the cylinder head breather. The crankcase is filled up to the top line of the dipstick, so this could be an issue with overfill? Also, the bike has spent a good bit of time idling over the last couple of months. I would crank it and let it idle every few days--I didn't realize that on these bikes just letting it sit idling isn't always the best.

In any case, next I checked the compression. Right was 120, left was 118. Pretty low, I know, but at least they're almost even, and I'd really like to get through the summer before doing a topend rebuild.

Can you guys give me some ideas on what my issue might be, and how to check? Perhaps excessive blow-by from bad rings? I don't have a compressor, so it's going to be tough to do a leakdown test--is there another way to check? Or could potential oil overfill account for the issue at all? Or even weak spark at idle in the left cylinder, so the blow-by oil isn't getting burned up?

I'm pretty new at this, learning as I go, and pretty much all of the ideas above--what I've checked so far and how--have come from forum searches. But at this point, I'm not sure what to do next. Currently I don't have the time or money for a topend rebuild, so I'm hoping to do whatever I can short of that to sort this issue for the next few months. Basically, I'm a fiction writer with a $900 car and a $1500 motorcycle (you know we don't make any money haha), and my first book comes out in May, and I'm trying to get both my vehicles in decent shape so I can get to all the promotional events/readings/etc through the summer months. I'd really love to have the bike in decent running order so I have a backup in case the car goes down.

Thanks so much in advance, guys!

 

[5twins]

Well, if the bike has seen quite a bit of sitting in the last few years, you may just need to run it some to re-seat the rings. Then maybe the compression will start coming back up. It will run with about 120lbs, but it's less than ideal. Your oil level is a bit high. The factory recommendation is half way between the high and low mark. That may be what's causing the smoke out the breather (if it isn't blow-by).

Since you have the later electronic type ignition with only one coil firing both cylinders, a weak spark on only one side isn't very common. That coil usually fires both or neither. But it can happen if the plug cap or plug wire is bad on one side. If your caps are originals, you might want to replace them. Trim some off the plug wire (1/4" or so) to get the new cap biting into some fresh wire.

 

[browndawg]

Thank you so much for the helpful reply, 5twins!

That's good to know about the single coil firing both cylinders and it typically being both or neither--I didn't think of that. I checked the wires and plug caps and they look good--fairly new--and everything up at the coil looks good too.

For now I'll drain a little oil, and hopefully running it some will help the rings re-seat.

Oh yeah, it isn't smoking from the breather--it's unburned oil leaking out. Don't know if that's significant, in terms of it being blow-by versus simple overfill.

 

[TwoManyXS1Bs]

Also, if the carbs aren't balanced, like if the left carb trails the right carb, the left side will pull more vacuum at idle and low throttle, pulling oil up thru the rings, and won't have enuff fuel to sufficiently fire and clean that plug.

 

[browndawg]

Thanks TwoManyXS1Bs!

Carb syncing is something I hadn't thought about or turned up in research. I'll dig into how to check on this.

Do you guys know of a good way to tell if the issue leading to the fouling is too much oil or too little fuel?

 

[Tomterrific]

I'm going to make a long distance diagnosis that you have a leaky carb float or needle and seat. This can leak raw fuel into the cylinder and wash the oil off the cylinder walls. The results are smoking out of the affected cylinder, fouled plug and a curious raising of the oil level. Do some research to correct the problems as it has happened to almost everyone who brings back old bikes to life.

Here are some tips Use hotter plugs until you get the bike sorted. I had lots of problems with my $100 XS so most of the engine running was in the garage and then close to home. I fouled the BS8ES that came with the bike so I went with a #6 heat range. The hotter plug did not foul as easy so I could figure out the other problems. Use premium fuel and install #7 before you hit the freeway.

Tom