engine backfiring

[greatwhiteshark]

I bought my 1980 xs1100 special about 5 months ago it was not really running when I purchased it recently I took it into a shop to have it brought back to life this is what they have done so far to get it running they took off the carbs and rebuilt them and cleaned them out, put in new plugs they got it running but said that it was backfiring and figured that it was probably a tight valve so I said ok do the valves and they did they said a couple were tight and a couple were loose, so I went to pick it up and I said I wanted to hear it running first and when it was fired up it was still backfiring not backfiring out of the pipes but it sounds like it is in the carbs or engine somewhwere it pops and sputters a little at idle but if you rev it up to about 3000 rpm and hold it there it will backfire consistently so now they are telling me that it could be a burnt valve and that they want to do a compression test ,or where the header pipe meets the muffler there is some leaking and that could cause it but in me experience if the exhaust is leaking it will backfire out of the pipes but only when you are rolling back the throttle and the other possible cause is the rubber tubes between the engine and the carbs they are cracked pretty good and the mechanic told me that one of them he thought might be leaking , so thats where it sits right now any help or ideas would be greatly appreciated also the bike itself has only about 20,000 miles on it and before I bought it it had been sitting for about 7 years ....thanks....mark

[MartyA]

I'm thinking you should find another dealer first. They did a 'valve job' and THEN suggest there is a burnt valve? They 'think' the carb boots are leaking? How about a compression test? Did they think of that?

Second thing I would check is the pick-up coil wires. Then cracked plug wire, anything else electrical, and keep searching this site for help.

[mitch]

Just my thoughts

. Id bet they never cleaned the carbs right or synced them as well as checked the pick up wires. You have a great guy living in BC that would no doubt help you if you asked. I think your getting taken , get it out of thier hands fast !!..........MITCH

[Ken Talbot]

Hey, Mark - I think it may be in your best interest to see if you can hook up with one of the XSives on the south end of the Island. The closest, if he is available, would be Ratbike Ken in Sooke, and he really knows these bikes inside and out.

[skids]

A sticking valve or bad exhaust valve will cause some backfiring and I think that would be in the pipes. A hole (usually a cracked factory weld) in the pipe underneath the heat hield can cause a lean condition that can make it back fire through the carb(s). Forget about tuning the carbs until you resolve the exhaust problem, if indeed you have one.

[fusionking]

How much do you owe them now??

[greatwhiteshark]

they did the carb work, adjusted the valves, rebuilt both front brake calipers and the bill stands at 650 big ones , the bike is definately running better and there is a lot of leaking from under the heat shields , I have a new set of mufflers that I got off of e bay they should arrive any day do you think I should do the exhaust work first before I let the shop do any more messing around with it or should I have them go ahead with the compression test .....thanks...mark

[oldnortonrider]

.........Gad! > I wouldn't let 'em do anymore work on it, if they offered to do a $100.00 worth of additional work, for $1.00 .

[skids]

I guess you know what I would do (read prev. post above). If your muffs are slip-ons and there is a crack under the heat shield at the weld, how are you going to handle that? Jst wondering, because I don't know where slipons attach...

Originally posted by greatwhiteshark
they did the carb work, adjusted the valves, rebuilt both front brake calipers and the bill stands at 650 big ones , the bike is definately running better and there is a lot of leaking from under the heat shields , I have a new set of mufflers that I got off of e bay they should arrive any day do you think I should do the exhaust work first before I let the shop do any more messing around with it or should I have them go ahead with the compression test .....thanks...mark

[greatwhiteshark]

I'm hoping the new mufflers will fit better than the ones that are on there now which don't seem to fit well and are quite loose but if there are cracks I will need to have some welding done I guess

[POOLBOY]

Now I am confused??Exhaust leak causes lean condition??

Can someone explane to me how a leak in an exhaust system can cause a lean condition on a 4-cycle engine?I can see it on a 2-stroke,where you depend on back pressure, or a tuned pipe for scavaging, but the 4-stroke is a "pump" at that point.

[Jerry]

This is going to be a short explanationn of a complex subject....

4-stroke engines produce pulses in the exhaust system. While they do not have the same impact as on 2-strokes, the pulses have a definite effect on engine performance.

There is a time in the engine cycle where both the intake and exhaust valves are open, a condition known as valve overlap. In an ideal situation, the pulses of the exhaust create a vacuum that helps pull the next air/fuel charge into the cylinder. This incoming charge has a second effect: it cools the spark plug tip.

If there is a condition that changes the pulse characteristics of the exhaust, such as a leak around a gasket or hole in the collector or post-collector system, the incoming charge is affected. Either the amount of the charge is reduced, or the plug tip does not get cooled, or both.

Abnormally high plug tip temperature causes pre-detonation, that is the air/fued mixture ignites before it should. If this happens while the exhaust valve is still open, a backfire is the result. In worse cases, the intake valve may be open as well, and the pre-ignition will cause a rise in manifold pressure, causing either a loss of power or 'spitting back' through the carburators.

So yes, if there is a leak or hole in the exhaust system, the exhaust scavenging is affected, causing a loss of power and an increase in cylinder head / plug tip temperatures that can lead to pre-ignition, or backfiring.

First things first....before trying airbox or carb mods, make sure the exhaust system is in top-notch conditon, and your sprark plug are the correct heat range. (NGK BP6ES or equal is the factory recomendation.) Carbon build-up can also affect pre-ignition, as the carbon tends to retain heat and increase your cylinder temperatures.

[Hobbit]

backfire

Perhaps we need to clarify the term "backfire". Jerry's example is correct for backfiring out the carbs.

Some people call the popping out of the exhaust backfiring. This normally occurs during closed throttle deceleration. If there is extra fuel coming out of the combustion chamber and a leak in the ehaust which allows oxygen to mix in the heat of the pipes, it ignites and "pops".

[POOLBOY]

Great explaination.Here is my problem.

What I have seem's more like a lean condition "spit back" from the carb's.Let me set the scenario.I had done a basic carb cleaning. The bike hasn't run for a couple of year's, so I thought I would "pop" it off to see what other problem's I have.The original owner said it was running fine when he parked it, except for a clutch slipping condition.(he has no reason to lie, he gave me the bike).I had an aux gas tank connected to the carb's through a "Y" connector, and the air box completely off.The temp was cold(60f) and I was using a 32/1 two stroke gas mix to provide some top end lube.After a short crank, with the enrichener lever pulled full on, it started.It would "spit" out of all four carb's about the same when you tried to come off idle, but rev to 4k or so ok.I let it warm up as much as I felt comfortable, but could never completely push off the enrichener.The "spit back" got better as it warmed up, but didnt go completely away.There was a crack completely through the left muffler pipe where it join's collector,(held together with a hose clamp, now welded solid).There is a small hole(1/8") on the right side muffler too,back by the end. I have a X3 cleaning comming up on the carb's,and new kit's to install. No broken off tip's in the idle circuit.I just dont want to be chasing a demon exhast leak, if it is the carb's, and I dont want to replace the exhaust as a "guess".Is there any way I can determin a lean pop as opposed to an exhaust backfire without expensive guages and such?I intend to do a bench sync with the bread tie wire after the rebuild's.I already have the new rubber manifold's in stock, mine are cracked, but not all the way through.I am starting to get some real money invested in this project, and I need to know if there is a light at the end of the tunnel, or is it the oncomming train !!
*************************SPLASH*******************

[ratbyk]

Pop Back

Poolboy, your symptoms tell me that 90% plus of your problem is leanness in the carbs.

Possibilities: low float level in all carbs.
:Blockage or restrictions in the idle circuit.
:Blockage or restrictions in the pilots or galleries.

First thing I would do is drop the carbs back off. Remove float bowls, carefully measure what the float level is at the moment. Remove the floats and then chase through the idle and pilot circuits. Reset the float level 1/16" higher. Or even more if your recheck measurement showed that they were low to spec. at the recheck measurement.

Forget about repair to the exhaust at the moment. Get the engine going vrooom vrooom first.

Also, with the carb air-horns wide open without the partial restriction of the air-box/filter etc. you will be just a bit lean. You should still be able to get good throttle response.
Ken/Sooke
78E Ratbyk
82 FT500 "lilRat"