New to me 91 ST1100 blowing blue smoke

I purchased a 93 1100 yesterday with 60k on the clock that is in really nice shape. However, after riding it over 120 miles home in 90+ degree heat, it suddenly starting blowing smoke the last half mile. It appeared to be worse when I let off the throttle then accelerated vs. steady throttle. I don't know if this could be ring failure, valve guide seal failure or something simple like PVC system failure...hopefully the latter. Hoping someone can steer me in the right direction, my local shop is honest and does good work, but they are not a Honda shop and may not even want to deal with it.

Thanks

I assume by 'blowing' you mean out the exhaust, or did you just see smoke in your mirrors and you're not sure exactly where it came from?

There are coolant hoses under the carbs, which can fail and spill coolant onto the hot engine, and you'll see smoke in your mirrors. Depending on how big a leak you have that might explain your symptom. Another possibility of sudden smoke is coolant in your exhaust, which will have a sweet smell if you stand behind the bike while its smoking. That would be relatively bad news as it would likely be caused by a failed head gasket if its coming out the exhaust. A failed head gasket could also allow oil into the cylinder as well, so excess smoke in the exhaust could be from oil.

The first thing I'd do is check the coolant level at the radiator (not at the overflow tank). If its low, then I'd fill it back up and run the bike in the garage and look for any signs of coolant leaking onto the engine. If you see coolant dripping off the back of the engine, then you have the typical coolant leak under the carbs, and you can get a lot of advice for doing that job here. If you see smoke coming from the exhaust, and it smells sweet, then your coolant loss could be from a bad head gasket, but those failures are far less common.

So first step is to determine if the smoke is coming from the exhaust or not, and if so, does it smell more like coolant or oil? If not, then see if you have signs of a coolant leak anywhere.

The OP said blue smoke. That is oil, as coolant in the exhaust would be white.

From the initial description - "It appeared to be worse when I let off the throttle then accelerated vs. steady throttle." - this would indicate valve seals passing oil. Bad oil rings generally cause smoke all the time. If the bike sits overnight and then fires up with a cloud of blue smoke, but stops shortly after, that also indicates valve seals.

I doubt coolant is involved, but check your oil for coolant contamination by looking at the filler cap. Coolant in the oil will cause a whitish/brown mayonnaise like substance to collect there.

Also check what kind of air filter is in the air box. It may be possible that the PO used a K & N and had recently cleaned and maybe over oiled it, causing restricted air flow and more crankcase air/oil particles being sucked into the carbs.

One other note . . . valve seals and oil rings don't just "fail" all of a sudden. A ring could break, but that would likely cause an engine seize. These items wear, to the point of passing oil, over time, and poor lubrication, whether from poor quality oil, lack of oil changes, or a bad oil pump is the main reason.

Bush said:

The OP said blue smoke. That is oil, as coolant in the exhaust would be white.

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agreed, but when I saw the coolant plume behind me in the mirrors for a few seconds I can't recall if it was pure white or a little bluish, so I figured I'd cover a few possibilities in one post. Like you said, oil related seals don't generally fail all at once, unless its something like a head gasket. Given the small number of head gasket failures reported here, and large number of hose/elbow failures, it would seem that a sudden burst of smoke behind the bike could possibly be coolant related.

I am pretty sure it isn't coolant, it was definitely blue and oil smelling, not sweet like ethylene glycol. Although I will check the filler cap and the coolant as suggested, never rule anything out until I know the answer. If it is a head gasket is it best to do both while the engine is apart...and is it a real PITA while its in the frame?

Thanks for all the replies.

OK, I checked the oil cap, clean as a whistle although the sight glass appears to show overfilled and possibly contaminated due to color...but sight glass could be the cause (hate that design). The coolant appears normal also, so I fired it up just to be sure and it is blowing blue oil smoke for sure. There is slight wetness on the right head from the exhaust ports down, so I assume that could be from a bad head gasket? I just don't know if I can afford to have them replaced...bike is only worth so much.

bonth123 said:

There is slight wetness on the right head from the exhaust ports down, so I assume that could be from a bad head gasket?

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a slight wetness on the engine can be a minor leak from the head gasket, but that leak wouldn't necessarily get into the exhaust to create the smoke you're seeing. Head gasket leaks come in different forms, you can get coolant into the cylinder, oil into the cylinder, coolant/oil mix, or combinations of those three. Depends on what part of the gasket failed, and what passages are adjacent to the failed section.

I'd adjust the oil to the correct level and check the plugs periodically for signs of oil fouling, and monitor the sight glass for signs of coolant in the oil. But the fact that you had an instantaneous burst of smoke start occurring all at once is not a good sign.

edit: if you did have a ring failure, one of the symptoms is excess crankcase pressure. On a car this often causes the dipstick to get blown out of its housing, but on a bike with a sightglass and no dipstick I'm not sure what symptom you might find.

Well, I don't know how I can check for excess crankcase pressure, I suppose I can pull plugs and see their condition, might at least narrow down the location of the issue. Does anyone know if you can pull one of these heads without having to pull the cams? I am hoping you can just remover the timing belt and pull it as an assembly without disassembling the cam shaft units.

bonth123 said:

Well, I don't know how I can check for excess crankcase pressure, I suppose I can pull plugs and see their condition, might at least narrow down the location of the issue. Does anyone know if you can pull one of these heads without having to pull the cams? I am hoping you can just remover the timing belt and pull it as an assembly without disassembling the cam shaft units.

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to pull the head you will need to remove the carbs, the timing belt, and the cams. On the ST1100 pulling the cams is very easy because they're gear driven off of the timing belt. I think the head bolts are underneath the cams, so you need to remove the cams to access the head bolts.

Ok, I was afraid of that, well I ordered the honda so manual so I can get started, sooner I get this done the sooner I can be riding

STOP !! Please get back to the basics... Pull the plugs and "read" them to determine what may be going on in a particular cylinder. When the plugs are out you can also see down in the cylinder's with a strong flashlight. Borrow or buy (they're cheap enough) a compression / leak down tester. Perform a compression and leak down test to check the over-all health of the engine. As mentioned by above posters, it could be something very simple. This bike is new to you and you don't know it's history. You won't know anything until you peel off all the plastic and start off with all the simple diagnostics and maintenance checks....

I'd first check the air box to see there is no build-up of oil from the crankcase breather, which would get sucked in during acceleration. That taken care of and the condition continues, the symptom of blue smoke appearing after decel, then accel, would indicate valve seals. A compression and leakdown test would reveal any possible head gasket issues or rings. It would be helpful to know how much oil you're losing after a thousand or two miles..... hard to judge severity...... is it a lot of blue smoke or just a wee bit?

+1 on the comments about doing further diagnostics, you haven't really identified the problem yet. Pulling the head at this point wouldn't make a lot of sense.

Is it possible that the bike was always prone to smoking when you did the decel/accel, and you just happened to notice it at some point in your ride and assumed it had just started smoking? In other words, were you riding the exact same way prior to the smoke appearing, and you're sure there was no smoke symptom prior to that under the exact same riding conditions?

I'd keep riding/checking for a while to see what your actual problem is. If all you have to do is top off the oil occasionally, then you may not want to do anything at all to the engine at this point.

regarding the compression tester, AutoZone will loan you one for free.

bonth123 said:

Ok, I was afraid of that, well I ordered the honda so manual so I can get started, sooner I get this done the sooner I can be riding

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Did you do an oil change with the proper oil installed yet? Remember start with the simple things before anything major!!

OK, I think you are all right...I need to back off the drastic measures and remember the basics. The amount of oil smoke is enough that I am not sure I can ride safely the three miles to the local dealer, it may burn it all before I arrive...it is a LOT of smoke. There was absolutely zero smoke prior to this occurring, and when it did it was very obviousl. The only change was that I sped up slightly to roughly 5k on the tach for the last mile, which is when this occurred, but I had it well past that before this on several occassions...although never near redline. I will pull the plugs later today and look them over, after I remover all the plastic.

Thanks for keeping me on track...I tend to always go for the worse case scenario, but this could be much simpler.

Remember Occam's Razor: "when you have two competing theories that make exactly the same predictions, the simpler one is the better."
At one time I had smoke on the right side of engine (not from muffler).
Turned out a small leak from valve cover dripped onto exhaust pipe.
Re-seated gasket fixed it. Getting the plastic off was the hardest part.
Aulia

Doesn't the 91 model have the rubber hoses running from the oil filter pedestal to an oil cooler? Has there been any reports of those oil lines breaking and leaking oil onto the exhaust?

Or even something simple as a loose oil filter?

This smoke is most definitely coming from the exhaust chambers, not from burning off of a pipe. I will start disassembling the plastic in a while and see what I can find. If nothing else I can get it all really shiny while its off