"cracking sound" from motor

Can anyone give me a good guess on what would cause an intermittent (randomy timed) VERY loud 'cracking' sound that I get when my 82 XJ650J is warming up? (It's like the POW or CRACK that I imagine when I'm reading a Harry Potter book aloud to my son and someone spell lets out a large CRACK! (How's that for an odd description!) Once the bike's been running and is warm, it goes away completely. It might be activated by cracking the throttle and also by backing off the throttle.

Any thoughts or suggestions are very welcome.

Thanks,
Peter Booth

 

Perhaps some of the neighborhood kids have cast a spell on it?

Is it back fire?

 

Yeah, I though neighborhood kids too. Wife said they don't have any spells.

How would I know if it was backfire? The bike has backfired twice unbelievablely loudly. (The first time my wife came out of the house which is is 100 feet away and ran out to the shed. She said she was sure I had blown myself up. My ears rang for literally 15 minutes. This occurred right after I ran the starter for 30 seconds trying to get the bike to catch. It didn't catch, I stopped pressing the starter button and then BANG!)

Anyway, would a bike backfire repeatedly like that while it was warming up? If so, what does that indicate?

Thanks,
Peter

 

It shouldn't, but could. Where does the noise sound like its comming from?

If I keep asking questions, eventually one of the carb/valve guys will show up and tell you why...

 

OK, good plan, I'll just keep answering your questions and eventually someone will take notice and bail us both out.
I would say it sounds most like it's coming from within the motor/cylinders. Maybe exhaust, but I don't think so. Wouldn't a back fire sound be coming from the mufflers?

If it is repeated backfiring, what would I do about this? (If anything. Like I said, the problem goes away once it warms up, so if it isn't causing damage I don't really care about the noise.)

Peter

 

Unusual noises rarely don't cause damage. Do all the pipes get hot equally (or nearly so) as fast?

 

thats got to be gas collecting in the pipes and exploding when conditions are right
a timing chain can make a ticking sound but nothing like you described
one or more cylinders must not be firing, putting gas through the motor and collecting in the pipes
will it hurt?...well it won't do any good

 

Works every time...

 

usually when it chime in on something one of the smart guys here will answer
and get the guy straight before i lead him too far off course
thats a good thing, problems get solved like that

 

So let's pursue "one of the cylinders must not be firing". How would I determine if this were true? They've all got good spark, I know that.

Peter

 

POW , CRACK...sounds like one or more of your intake valve may not be seating right and you are getting a backfire through the carbs. now the loud backfire from the exhaust sounds like a exhaust valve was open when there was a latent ignition in the chamber from you trying to start the bike, i am leaning towards there being a timing issue or its time for a valve job, because if the valves are not seating correctly you are pushing atomized gas into the exhaust pipes during the compression stroke.

S!

 

carefully, down under the oil filter touch each pipe, they should all be about the same temp
try this from cold after it's been running for about a minute while the choke's
still on
get some new plugs and try it again

 

Oddball: Does the fact that the sound goes away as the bike warms up make sense with a diagnosis of intake valves not seating right?

Polock: Once I do that, what will that tell me?

Thanks for the help/ideas.

Peter

 

heat=expansion 1mm opening is enough for air or gas to get through, have you ever heard an exhaust leak? ..that ticking noise...it goes away once the block and pipes have heated up and closed the gap remember i am just guessing it maybe something very simple

S!

 

if a cylinder isn't firing the pipe will be cold, or not as hot as one that is
then a set of plugs might fix the whole thing, otherwise it's going to be set the valves, take compression readings, clean the carbs, the whole nine yards
just start with the easy things first
what are you working on, 650, 750,550, it would be a shame to blow the seam out of a collector box

 

Can you shoot a video of it? I'm curious now.

 

Just for grins, check your vacuum caps on the intake boots. When I fired up my 750 after it had sat idle for a while, I heard a couple of pops from that general area. The caps had weathered and the tops blew open.

 

So it sounds like 1. check pipes for heat and if all cylinders are firing (which I think they are) then 2. check valve clearances ( would that reveal a valve not seating properly or would I need to pull off the whole top end -- oh god, please say no . ). Am I right in thinking that checking clearances is easy/cheap? Also what would be the result of ignoring the problem/noise?

 

See if you can isolate it in the exhaust or under the valve cover. IF its in the exhaust it could be be something as simple as the metal expanding (May be bad pipes or something loose inside). If its under the cover look at the chain guides and cam sprockets. See if anything looks out of place or damaged. Just my .02

 

first see that all the pipes are close to the same temp, if their not change the plugs and try again (10$)
how does it run after it's warmed up? one of the things that no valve clearance will do is make it stall and hard to start after it's hot
checking valve clearance isn't hard but it can cost a bit if you need a gasket
you don't need to pull the whole top end (head) just the valve cover

 

Cranking for 30 seconds to get it to start is an issue also. Sounds like you need to clean the enrichment jets in bottom of the bowls. Would not be a bad thing to check the valves too.

If it starts right away you shouldn't have the fuel problem you have now...

 

OK, first off, I'm overwhelmed by the number of reponses/ideas thrown out there. Thanks so much for everyone's two cents. I've shot a video of the bike starting, and it's at:

http://www.youtube.com/watch?v=rvR0CiLVlys

(I also tried to attach it to this post, but I'm not sure if it worked.)

A little background. I bought this bike ('82 XJ650J) to build a vintage racebike out of. So far all I've done is rip off plastic and put on race seat. The motor is completely stock. The fact that this bike is not an everyday runner is what causes some of my problems. For example three days ago when I started it for the first time since winter, it only started after I pulled all plugs and cleaned them and cranked and cranked and cranked the motor and played with the choke (a little... needs more... not that much... how 'bout half way...) a ton. Yesterday, having run the day before it started much more easily but still had to crank for 30 seconds before it caught and then stumbled on it's on for 20 seconds before it would take any throttle. Today, it more or less started right up. (You can see that on the video).

I'm now 99% sure the 'cracking' sound is down low in the pipes. Either right before or directly within the location where all the pipes enter one chamber directly under the motor (in the video this is where I aimed held the camera in an effort to pick up the cracking sound).

The pipes all heated up equally so I'm sure I'm running on all 4 cylinders.

Now the video. The cracking sound is nowhere NEAR as dramatic as it was two days ago (when I first started up after winter) or even yesterday. However I can still hear it when I rev the bike. Reeevvvvv... WHAP! Reeeeevvvvvvv..... WHAP! When I played the video back I can't make out the cracking sound (though I could hear it live), but perhaps people who know what an XJ SHOULD sound like will be able to pull the sound out of the video.

Still interested in any ideas, thoughts, etc etc. At this point, to my (inexperienced) mind, I think it's a backfiring. If so, my questions are 1) Is this bad, if so why 2.) What causes it and 3.) What should I do?

 

The header pipes on many of these models are 2-piece, and internal tube welded (internally) to the chrome outer pipe. If the weld breaks loose or other problems incur internally, the inner pipe may be banging against the outer pipe?

 

Peter, I listened to your video twice & I can barely make out a slite crack sound as the revs come back down. I'm no expert but I think it is in your collector on your exhaust. I do not believe it is back fire. She does sound healthy. The problem starting has got to be the enrichment circuit (choke) if it isn't clean enuff to eat out of it isn't clean enough. This will cause starting issues. Someone else will chime in on this for you as well. Just my opinion, hope it helps.
Gregg

 

What does that mean when you say it is "in the collector on the exhaust"? I'm assuming the collector is the exhaust piece that all four pipes lead into. What do you think is going on in there? (Actually what goes on in there with a bike right out of the factory?)

Where/how do I deal with the enrichment circuit? I agree that there is an issue there. Once the bike is warmed up, I can use the choke as a throttle. Choke on: engine races to 8000rpm. Choke off: bike idles nicely. Choke half-on: Bike races a bit. I can essentially rev the engine using the choke. I've never dealt with this b/c once I turn the choke off, it idles fine. As long as I never leave the bike alone with choke on as it is warming up, everything's OK. Thoughts?

Thanks,
Peter

 

Peter, the collector is under the bike where all 4 headers merge & the two exhaust pipes exit. Like Chacal said, the system is two piece, there is an inner tubing in the collector that may have broke loose & is making the cracking sound you hear. The enrichment circuit is in the float bowl of each carb it has very small pasages into a well where the brass tube pulls fuel for the bike to start. If there is any crud in the passageways or the tube you will have difficulty starting. It should start first crank or two, not have to grind it till it does start. Running your starter for more than a few seconds is hard on it. Hard starting is just a way the she tells you I need a little TLC on my enrichment circuit. HOpe this helps.
Gregg

 

There's an "inner tube" inside many of the headpipes, too!

 

If one of these inner tubes had broken loose, would the sound be a rattle or a violent pow, like something popping or slamming?

Peter

 

I can't answer that as I have no experience there but I assume it could be possible. Hopefully, someone who has had experienced this will chime in.
Gregg