Backfiring out of right cylinder

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knucks

XS650 Enthusiast
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1979 XS650 w/ 2 sets of points and coils.

Seems to be only at idle, but not great.perhaps even too slow. I haven't connected my inductive tach but the tach on the bike shows less than 1000RPM.

It backfires out of right cylinder. Right exhaust is also quieter than the left.

I haven't checked valve timing yet, but I just adjusted the chain

I played with the mixture screw a bit but it only seems to feel better when I turn the screw in but then the right exhaust is very quiet...

 
Carbs in sync?
What do the plugs look like?
Are the carb boots cracked or brittle?
Check the exhaust for leaks and loose hardware.
Sounds like dirty/clogged pilot circiut though.
 
sprayed some carb cleaner around the RIGHT carb boot, RPMs increased. Right seems OK.

Couldn't find the right boot on MikesXS, help?
 
Is the backfire coming through the right carb? I had that same problem and pulled the carbs, bench synched them, installed new carb boots now good to go. I have had good luck finding carb boots on ebay. Just make sure they are the right ones for your bike.
 
XSsesive50 said:
Is the backfire coming through the right carb? I had that same problem and pulled the carbs, bench synched them, installed new carb boots now good to go. I have had good luck finding carb boots on ebay. Just make sure they are the right ones for your bike.
Click to expand...
I am not sure that it's coming through the carb..It doesn't seem like it, I only hear it from the exhaust
 
I checked the right side valves, I think I did it right, the exhaust valve was not to spec (.006") so I changed it to that.

I think? my exhaust on the right is now at least the same temperature as the left, but still backfires. I pulled the air/fuel mix screw, sprayed some carb cleaner in there, that didn't help.

The left and right do not 'hit' the same, I am guessing thats indicative of the carbs not in sync?
 
.006 out is prob not going to give you a backfire, yes it sounds like the carbs may not be synced. Whats plugs look like ? not that is your issue, but could be a contributing factor. If you have still not addressed the issue with the high idle when you spray around the boot, that is more then likely the culprit
 
incidental extra air from the boot is the equivalent of adding more air to the combustion, essentially leaning it out, marginally as to give an occasional backfire pop. I may be mistaken, but I think that is a classic lean situation.

Alternatively a pop can come from the exhaust if the exhaust does not have a good seal at the flange from a gasket, extra air in ignites within the exhaust itself as a secondary fire. I think this is also possible from too rich a mixture when things get got, kind of an external detonation.

I am also hunting down a crackling sound from one cylinder, unfortunately mine is none of these more basic causes. Mine may be caused by not having rubber passage plugs on my pilot jets possible sucking up a ton of fuel and spewing it into the left cylinder resulting in weird detonation. Anyway, I digress. My boots from MikesXS have been decent, I am not sure what the difference is from the other vendors but I think it was revised after some failures and I no longer have a leak at mine.
 
I *think* I've resolved the issue at the boot. I took off what was there on the right and made myself a gasket out of fel-pro. Seems to be doing the job.

The backfiring continues - I want to test the coils next to rule those out?

I am waiting on my carb sync setup to dynamically sync the carbs. Perhaps tonight, in the least, I will bench sync them.

Plugs look to be the same to me, neither one looks to be saliently lean..
 
I believe you can swap the coil leads from Right to left, but the coil as a whole can be partially shorted leading to two weak sparks. This could, in theory, make a marginal cylinder bad and a good one OK.

There is a lot of anecdotal evidence on this site that the coil replacement and sometimes ignition can fix a lot of this unusual gremlins. I am not really sure that applies to you though because you don't have evidence of rich plugs.

Posted via Mobile
 
What I did last night was:

Took the wire/boot from the left i connected on the right plug.
The wire/boot from the right i connected to the left plug.

bike backfired and would not start. switched them back to normal, and it started.

Should I switch the actual COILS next to test if there is an issue with one of the coils? Am I doing this right..?
 
Both coils fire with each stroke, there is one wasted spark per cycle. I may be mistaken but that sounds unusual. My bike when swapped made no difference but observing the spark I believe both are weak.

Posted via Mobile
 
Do you have one or two coils. usually the 79 has two sets of points and two coils.
In your first post you say a 79 with TCI. The TCI uses just one coil with two outputs.
On a TCI ignition you can swap plug leads, On points you can't.
When you adjust the idle mix screws you need to "kill" one cylinder while adjusting the other.
On points that's easy just unplug one plug wire. On the TCI you need to use another method.
Put the petcock on PRI pull the vacuum hose to the petcock of the vacuum port on the carb holder to "kill" the left cylinder, adjust the right. Pull the cap of the right side vacuum port on the carb holder to "kill" the right, adjust the left.
Once you "kill" the cylinder back off the idle speed till the engine barley runs. The lower the idle the easier it is to tell what the mix screw is doing.
Turn in till the bike stumbles. Now turn it out till it stumbles. Turn to half way between these two points then richen the mix 1/8 turn. once both sides are done reset the idle speed to normal.
 
am I the only one who thinks it sounds like a compression leak ...a leaking exhaust valve or head gasket ?:D

If the valve lash is correct I'd do a quick compression test to eliminate a compression leak first.
 
Unfortunately I do not have a good compression gauge. The one I have was a HF purchase and it never seems to read right. It's always read low, even on engines that run well :(
I pulled my carbs off, held throttle open, kicking and kicking. Takes a good 10kicks to even get to 90, I really would like to try a better gauge because this one I do not trust..

Things I've done at this point:

Checked/reset timing. Cleaned points. Bench synced carbs (they were, already, actually).
Swapped coils entirely.

I noticed that both my points are arcing. Bad condensers? I can use something from an old muscle car, 2 of them, yes, as a replacement? Would something like that cause backfiring..?

The backfiring continues on the right side. I think I noticed some backfiring through carb on the LEFT but I think that is resolved as of me re-timing the bike.

My next step is to take the carbs apart and see if I can run them through my friend's ultrasonic cleaner..

As far as leaks at the head, I am not sure honestly, I've sprayed carb cleaner and can't tell if revs increase or not. I really hope all is good on the topend :/

What compression reading is 'ok'? 130?
 
I'm guessing it's a failing condenser, i think it's cheap enough to try to get 2 new condensers to see if the arcing goes away.

On another topic, I noticed there is air escaping at the muffler/header connection. I also noticed that the brackets are broken/cracked around the muffler.

In pulling off the right muffler, I noticed there are no gaskets in there. The PO cut the header in that the tip angle up and there is an 'adapter' of some sort.

Can I run the bike without mufflers for now? Will that increase my troubles?

FYI this is what my end pipe looks like:

1385923_10103158011910633_1050599383_n.jpg
 

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