won't start, but will back fire

[brock]

I did the unthinkable. I let her sit for a couple weeks without starting. What's worse is right in the end of the first week, it got really cold.

Week later, i started it up with alot of trouble, it would idle at 1000 at full choke, and then stall after a minute or two. I try to start it again, and after a couple revs, it backfires LOUD.

At that time, i happened to notice that all the pipes were cold except number three, so i assume that's the only cylinder firing.

Now it'll turn over fine, but won't start, and after about 5 tries, i get a puff of black smoke, with a backfire that sounds like a shotgun blast.

I figure it sat around, and got really cold, and the gas gummed up the carbs. I'm gonna take em off saturday and give em a good cleaning, and hopefully that'll fix me up.

But is the carb thing consistent with everything else i said? It's sat for a couple weeks before, and always started right up. I promise I'll never let it go more than a couple days again after this.

[randy]

Really cold? In Florida?

I used to leave mine out all winter in NY state, started her once every couple of weeks, and never had the gas 'gum up' from the cold weather!

Is it more likely you got water in the gas? Try draining the tank through the bowl drains and using fresh gas.

HTH

Randy

[brock]

I thought the same thing. it got cold, down in the 40s for a couple days, which is much colder than it's used to. but still, it shouldnt gum up just from that. that's what i was worried about.

could be some water in the tank, tho i don't know how it would have gotten there. I did drain the tank, but i'll try a fresh gallon before i tear the carbs off. Thanks.

oh, but why the backfiring? and why did only one cylinder run that first day?

[ratbyk]

Pull your plugs first and see if they are wet or dry. If wet you likely/may have a flooding problem. A petcock or two or if a Special the vacuum control in the octupus may be allowing fuel flow constantly and the needle/seats are not holding back the flow.
If the plugs are dry then the opposite. Not enough fuel reaching the cylinders. Although I doubt this with the black smoke and the backfire. The backfire is caused by a build up of unburned fuel in the cylinders and exhaust system and then one cylinder fires igniting the excess in the exhaust.
From what you are describing I really suspect a fault in the ignition system that once and awhile allows a spark to reach a plug.
After checking plug condition pull the left engine cover off and check the pick up coil wires. Refrer to the Tech Tip Section on pick up coil wires and follow the "how to".
Ken/Sooke
78E Ratbyk
82 FT500 "lilRat"
92 ZX11 Temptress

[brock]

I did notice a bit of fuel leaking out of the octopus, and some of it even ran down and collected in the air box. I pulled the plugs. They weren't really wet, but did appear a little black and oily. Not really oil, but sorta smudgy.

the ignition thing would explain the backfire better to me, a build up of fuel and then an occasional spark. it was so loud it scared some kids playing across the street.

What do i do if the octopus is letting fuel pass to one side of the carbs?

[Snow]

Originally posted by brock


What do i do if the octopus is letting fuel pass to one side of the carbs?

You could turn the petcocks to the 'off' position while not in use or you could rebuild the petcock.

You should make sure you're actually getting spark - pull out a spare plug and hook it up to a sparkplug wire and let the plug rest on a metal piece of the engine. Then, crank it over a couple seconds to check for spark. Do this for all wires to make sure you are getting good spark.

It should be just visible in daylight and really bright at night. Make sure not to touch the sparkplug while turning over the engine.

[John]

[QUOTE]Originally posted by brock
[B]I did notice a bit of fuel leaking out of the octopus, and some of it even ran down and collected in the air box.

Collected in the air box? Hmmm...the only way it can collect in the airbox is if the petcocks, or the octopus, if you still have it, is bypassing, and flooding the cylinders with fuel. Check your oil level, and smell it to see if it smells like gas. If it has a real high oil level, and smells like gas, the DO NOT run it until you have changed the oil. The fuel will dilute the oil to the piont where it will no longer withstand the loads placed upon it at the crankshaft and WILL RESULT in bearing failure, and you don't want that. Many of us with Specials have just eliminated the octopus anyway, and that has cured the flooding problem. The only catch there is having to turn the fuel on and off all the time.

[brock]

actually, i could see where it was running down the outside of the airbox, and leaking through the place where it mates up. i don't think it was leaking back through the breather, since there was no gas on the inside of the filter.

but there was alot of gas leaking. so much so that i just drained the tank. but i also changed the oil just to be sure, and it was fine.

[TopCatGr58]

Hey Brock,

If the gas tank wasn't full, and was outside, it could have heated up from the sun, then as the weather turned cold, water could have condensed on the inside of the tank, just like DEW on the grass.

Also, with a drastic change in weather, a borderline battery can be caused to fail, and so even though you were able to get the bike to turn over, it could have not been providing all of the required juice to the ignition box!?

With the Octy leaking, the fuel got to the carbs, and if into the airbox, then the floats and needle seats weren't working as they should. You may want to put some Seafoam or Techroline in the tank, but since you're going to tear down the carbs, you probably don't need to add the cleaner then!?

Take off the vacuum advance hose and then try starting it, if it runs and all 4 cylinders get warm, then plug the hose back into the advance unit, and if it starts backfiring, then the flexing of the timing plate is causing intermittent breaks in the pickup coil wires, a very common problem with these old XS's! Repair it!
Good luck in trouble shooting!
T.C.

[skids]

The stock airbox should have a small, filtered drain hole in the center at the bottom. If your carbs are leaking backwards into the box, and not forewards into the engine, the filter is likely soaked. Fuel soaked filters can cause such a rich mixture, that it will not burn. Once in the pipes, it can ignite causing an afterfire. But...you leaked a whole tank??? Did it take a long time?

Originally posted by brock
actually, i could see where it was running down the outside of the airbox, and leaking through the place where it mates up. i don't think it was leaking back through the breather, since there was no gas on the inside of the filter.

but there was alot of gas leaking. so much so that i just drained the tank. but i also changed the oil just to be sure, and it was fine.

[prometheus578]

I agree with topcat about checking for water. Drain the carbs into something where you can see the gas. I use a piece of aqaurium hose at the carb to drain the fuel into a container. You have other probs, though. Leaking fuel means faulty float needles and seats. Last time it ran sort of decent, was actually running rich due to too much fuel. Fuel fouled plugs. You mentioned they were dry, but black and sooty. Spark is shorting and running along the insulator... but ocasionally sparking and firing all the fumes in the exhaust. Throw in new plugs. If it fires right up and runs fine for a while and then floods out and fouls, redo the needles and seats.

[dbyers]

EXACT SAME ISSUE as BROCK

I have the exact same issue as brock. I worked all summer getting my XS running and idling right only to not have it start after letting it sit in the garage for a couple of weeks. What was worse is that my Dad was in town and I was trying to impress him with my first bike overhaul...I also get the same shot gun blast sound but now all it does is turn the started and doesn't even back fire. I removed all the plugs and they looked fine. I also checked the wires with a volt meter and had good numbers there. I run 4 single air filters and not the stock air box. I removed the filters and sprayed carb cleaner in them hoping they were just dirty or maybe a little wet. Still nothing. I'm going to try draing the tank and the carb bowls and will let you know.

[Snow]

I agree with what Topcat says about the pickup coil wires, but I think if the wires are damaged enough it won't even start with the vacuum advance tube unplugged.

Back to basics, you need 3 things in order to fire: Spark, Fuel, and Air.

If the the sparkplug tests shows no spark, it's an electrical problem. If it sparks but is wet or sooty, it's a fuel or air problem.

Of course, all of those basics go out the window when you are actually riding due to dynamic conditions and uh...stuff..

[brock]

went out today, and drained all the carbs, tried it, nothing. changed all the fuel lines. tried it, nothing, drained and replaced the gas. tried it, nothing. changed the oil, tried it, nothing.

changed the spark plugs, which were sooty and a little oily, and it started right up. i'm gonna take it on a longer ride in a couple hours, but first i have to charge the battery.

but wtf? i fully expect it not to start tomorrow.

[ratbyk]

Won't Start

Think positive, think good thoughts and have a fully charged battery. It will go. Happy motoring.
Start finding out why the plugs got wet in the first place.
Ken/Sooke