Would premium fuel make my '79 XS11 run rough?

[grossmad]

I just purchased a 79 xs11 and when i got it, it ran perfectly the other day, but today it's running rough. I got it with a 'full tank of premium gas'. The next day, after riding a while but not running the gas out, I filled it back up with premium gas and noticed no difference in performance. Today, the fuel was almost gone and i had to refill from almost empty and now it's giving me a lot of backfires and has trouble idling. Could it be that i'm supposed to be using regular fuel and this premium stuff is throwing off my compression?

[Silent]

Did you turn off the petcocks when you parked? If not, check your oil level and see if you have fuel in there. My SF will run like crap if I leave the petcocks open for a couple hours. The fuel works past the seals in the float bowl and floods the bike out. I'd also install an extra set of fuel filters if you don't have a set already.

[grossmad]

I was told by the guy who sold it to me that they were vacuum sealed and so having it on RUN was fine while of... i thought that was strange but no i've left it on RUN the past few days.

[grossmad]

So i just went out and double checked it... i'm new to this bike, and bikes in general, it's only my second one. i'm assuming you're talking about the fuel petcocks, and mine only have the Reserve, Prime, and On options, i was told the Prime lets the fuel flow freely and that reserve and on stop it with a vacuum seal. Shouldn't i be leaving it on "On"? Or am I just thinking of the wrong part altogether...

[dbeardslee]

Gross - Yes, leave them in the 'on' position. Those vacuum petcocks are notorious for leaking - check your airbox and see if there is fuel in the bottom of it. If your petcocks leak and your carb's fuel valves aren't sealing, you can get it leaking past the valves - into the airbox on one side and into the cylinders, past the valves and down into the oil. Gas-thinned oil can do more than make it run bad - it can make it spin a bearing. You can tell if there's gas in your oil by putting a stick down into your oil and see if it lights easily with a match. If it does, change it.

On the fuel, some members like premium, and it shouldn't cause a problem unless you got a bad/watered tank of gas. Even then the water should go to the bottom of the tank so unless you're running on reserve, that shouldn't cause much of a problem either. It's a good idea to run some seafoam through it periodically to dry the tank and keep the carbs clean. Speaking of which, are your carbs clean and properly adjusted? Backfiring could just be an indication of out-of-synch carbs. Could also be valve clearances or timing related.

Personally I prefer regular gas. The 79's have a 9.2:1 compression ratio, but with these old motors your probably not getting as much compression as when new. The octane rating on gas is actually a measure of the compressability of the gas, so the lower octane gas is actually more 'explosive' than the higher octane. I run regular in my 79 all the time and haven't had any problems with pre-ignition.

You might also want to pull your plugs and take a look at them. They're supposed to be cleaned every 1k miles, and clean plugs make a huge difference in the way they run.

[Silent]

So i just went out and double checked it... i'm new to this bike, and bikes in general, it's only my second one. i'm assuming you're talking about the fuel petcocks, and mine only have the Reserve, Prime, and On options, i was told the Prime lets the fuel flow freely and that reserve and on stop it with a vacuum seal. Shouldn't i be leaving it on "On"? Or am I just thinking of the wrong part altogether...

My bad, for some reason I thought you had a Special. Special petcocks have an off, Standards don't.

[CatatonicBug]

Welcome to the forum, and the the world of XS1100's! You have found the right place for answers to your bike issues. The carbs on these bikes are the most troublesome part on them, especially for beginners. Take a look through the tech tips portion of this site, and read everything. Even if it's not something you are dealing with now, it will probably help you later. If you don't have a manual yet, the link in my signature will take you to a page where you can download both versions if it.

You can check the petcocks by removing the fuel lines from the carbs overnight, and set the open ends in a cup. If there is fuel in the cup, rebuild the petcocks. Rebuild kits are available on ebay from a seller named georgefix. If they are not leaking, you know you can look elsewhere in your diagnosis.

These engines were designed to run on regular fuel. Using premium won't usually cause a problem, but it also won't help anything. You'll just be wasting your money. As was mentioned, backfiring can be a symptom of your carbs being out of tune. Look for tips/threads on how to sync and colortune them. Idle should be set right around 1100rpm.

Also, it would help if you put your model and year in your signature, so people know what bike you're working on. There are subtle differences between them that change what you might need to look for in certain situations. Looks like you have a '79 Standard (aka, '79 XS1100F). The Standards have a vacuum controlled petcock, with no "off" setting, while the Specials (XS1100SF) have what we call the "octy", which acts as a vacuum-operated valve between the petcocks and the carbs. The Petcocks on the Specials have an "off" option.

BTW, we like pictures!

[b.walker5]

.... The octane rating on gas is actually a measure of the compressability of the gas, so the lower octane gas is actually more 'explosive' than the higher octane......

Weeellll, Same thing I guess but actually the Octane rating is a measure of the fuels resistance to premature ejaculation, oops, pre ignition, while the fuel is being compressed, and exposions are to be avoided. Engine knock, or pinging, or detonation is the result of the compressed fuel igniting too soon (exploding) and expending it's energy in an uncontrolled fashion. Lower octanes ignite at lower pressures and have slower burning rates, and the engines intial compression ratio and ignition timing design is calculated around the octane of the fuel to be used. Theory has it that an engine designed to run (develop maximum power at a controlled burning rate) on lower octane fuels will generally not perform better on higher octanes, may actually produce less power, and cause damage, due to faster burning rates than the engine is designed to handle, however my experience here in NZ is that my bike performs better and runs smoother on 95 than it ever did on 91, and gives better mpg. It might be something to do with the difference between the English and US methods of classifying the rating. Our 91 here is little more than mud and lawnmower fuel.

[grossmad]

Thanks guys... I'll check it all out... I know that the guy who sold it to me did some work on it and its carbs were cleaned, jetted, and sync'd 6 weeks ago. it's not so much of a loud explosion for the backfires, and it's not engine knocking at all... it's juts like, out of the exhaust pipes come several pops, almost like fuel isn't being burned in the engine and is getting ignited slightly out of the pipes... that's why I questioned putting premium gas in. I think the guy put in 91 octane gas mixed into the existing regular i'd imagine... then i put a whole tank of 93 in it yesterday and it started doing this. I've got some seafoam i can run through it and i just got a siphon to change out the gas and put regular in to see if it helps. My suspicion is that the higher octane gas isn't firing in my engine. I'm thinking the compression isn't high enough for the 93 octane gas in it currently. Does that sound right?

Other than that, yeah, I'm gonna check the oil, etc, see if it's leaking at all. Thanks!

[oseaghdha]

I wouldn't bother draining out the gas, unless it has dead cats in it or something.

It won't make it run crappy, just won't run better with premium.

Check the oil window, I'm betting it's over full.

[dbeardslee]

You might try putting a digital multimeter on your ignition coils and make sure your primary and secondary resistances are correct. Failed or failing coils can make them backfire as well. You might also want to insure that your pickup coils are properly gapped as well. On the XJ your pickup coils don't move, but it never hurts to check the resistance to them just to make sure you don't have any problems in their wiring. And speaking of wiring, have you gone through your electrical connectors to insure they are all clean and serviceable? I doubt seriously that gas is causing your problem, but I guess it's possible.

[Mr. Schweer]

I WISH we had 93 here in CA. 91's the best we get at the pump. I prefer running 91 over the 89 or 87 available here. I don't have any problems and my emissions sticker says 91 octane minimum.

My factory exhaust was sh*t when I got the bike and would pop constantly. When I first bought it, I was afraid to go over 3K because the noise sounded like a rod knock. After synching the carbs and adjusting the valves, the rod knock sound went away but the popping persisted. The inner exhaust tubes in two of the head pipes were broken inside. Couldn't tell from the outside, but removing the pipes from the bike, you move the inner pipe around with your fingers.
Changed the exhaust and the popping went away.

Hope that helps FWIW

James

[DGXSER]

Another County Heard from

Well...to add fuel to the fire....so to speak...I would say you have a sticky petcock perhaps or a vacuum leak in the vacuum to the petcocks perhaps. Sounds like a lean condition perhaps as far as not idleing and popping.

I will also add that my bike has stock pipes but a PO drilled out the baffle plates on the end. Every dimple is now a 1/2" hole. So my bike is a little louder and it will pop on decel or engine braking. Not loud, but noticable. Done it since I bought it no matter how I tune the carbs.

[grossmad]

Petcock and fuel tank issues!

Long story short, I still have the same bike, but i let it sit all winter... it died last year and i couldn't figure out what it was... turns out a fuse blew and i didn't see it, so that threw me off while looking for my original problem... anyway, I got more into it this summer and found that my petcock filters are torn to bits and the fuel tank interior is in dire need of cleaning and re-sealing... so, i found the petcock filters for sale online but they're like 30 bucks for each side... anyone know where I can find some to replace mine with after the cleaning is done? I looked at Georgefix's ebay store and he didn't have any it looks like.