Backfire on Deacceleration

jsouthard · Sep 1, 2010

Mighty Norton Knowledgeable,

Recently my 74 850 has started backfiring on normal deacceleration. I haven’t changed anything (this week).

Everything else is great. Idles fine, Pulls great, starts in one kick etc. etc. Point me in a good starting direction?

highdesert · Sep 1, 2010

First thing I would suspect is that your timing has changed somehow, become more advanced.

If electronic ignition, maybe the stud nuts holding the plate came a little loose?

Put a strobe on the timing easy enough anyway.

Otherwise I would suspect a too lean carb setting, but you said nothing has changed in their recently.

ebowling81 · Sep 1, 2010

First question, is it actually backfiring through the intake manifold and carb. This would more then likely be a timing issue. Do you have an electronic ignition or a points setup on your bike. If points you could have an issue with you auto advance sticking which could possibly cause this issue. If electronic ignition, it could be that the control module is failing.

Second, or are you hearing the sound in the exhaust and silencer? If this is the case, it could be a carburetor issue. Having to much fuel in the exhaust, running rich, can cause detonation of the unburnt fuel in the exhaust on deceleration when pressure drops.

Those would be the first two things I would look at.

jsouthard · Sep 1, 2010

The backfire is coming through the exhaust not the intake.

I'm running a single mikuni with a boyer elec ign.

chris plant · Sep 1, 2010

hi joe, check the exhaust pipes at the head joint,sounds like they might be coming loose

Flo · Sep 1, 2010

Well, I was going to write the following, but you have a foriegn carb., so I don't know.

Check your air/pilot screws have not vibrated outwards. It happened to me a couple of months ago, backfired good & proper on deceleration. Put a couple of new o rings on them afterwards.

BrianK · Sep 1, 2010

Any problems idling? Popping on decel usually means too lean in the slow circuit. Had this recently on an airhead that had sat for a few months - idle jets gummed up.

If yours is truly "backfiring," this could be the reason. Or not.

ebowling81 · Sep 1, 2010

jsouthard said:
The backfire is coming through the exhaust not the intake.

I'm running a single mikuni with a boyer elec ign.

You are either running to rich on the carb then, or you have a leak near the head. Check to make sure that you have both exhaust collars tight. If they are not tight, that is probably the issue. This is probably the case if it is only occuring on one side or the other, since you are running a single carb set, so that make diagnoses easier. Otherwise, I would say that you are running rich, and should check your carb settings.

jsouthard · Sep 1, 2010

BrianK said:
Any problems idling? Popping on decel usually means too lean in the slow circuit. Had this recently on an airhead that had sat for a few months - idle jets gummed up.

If yours is truly "backfiring," this could be the reason. Or not.

No idling issue, 1,000 all day long -

ebowling81 said:
jsouthard said:

The backfire is coming through the exhaust not the intake.

I'm running a single mikuni with a boyer elec ign.

Click to expand...

You are either running to rich on the carb then, or you have a leak near the head. Check to make sure that you have both exhaust collars tight. If they are not tight, that is probably the issue. This is probably the case if it is only occuring on one side or the other, since you are running a single carb set, so that make diagnoses easier. Otherwise, I would say that you are running rich, and should check your carb settings.

Exhaust collar is tight. I thought of that and had found a crack on the exhaust header. I replaced both the headers, peashooters, and bought the new CNW header nuts.

I will check the Mik again. It just isn't operating rich and doesn't smell gassy or foul the plugs?

ebowling81 · Sep 1, 2010

jsouthard said:
BrianK said:

Any problems idling? Popping on decel usually means too lean in the slow circuit. Had this recently on an airhead that had sat for a few months - idle jets gummed up.

If yours is truly "backfiring," this could be the reason. Or not.

Click to expand...

No idling issue, 1,000 all day long -

ebowling81 said:

jsouthard said:

The backfire is coming through the exhaust not the intake.

I'm running a single mikuni with a boyer elec ign.

Click to expand...

You are either running to rich on the carb then, or you have a leak near the head. Check to make sure that you have both exhaust collars tight. If they are not tight, that is probably the issue. This is probably the case if it is only occuring on one side or the other, since you are running a single carb set, so that make diagnoses easier. Otherwise, I would say that you are running rich, and should check your carb settings.

Click to expand...

Exhaust collar is tight. I thought of that and had found a crack on the exhaust header. I replaced both the headers, peashooters, and bought the new CNW header nuts.

I will check the Mik again. It just isn't operating rich and doesn't smell gassy or foul the plugs?

You may not be running to rich then, but maybe just enough to get some popping on decel. Hopefully just a few easy adjustments on the carb will be enough to cure it. If the bike is running smooth and has no other issues, from what you are saying, there is nothing else that I could think of it being the would be simple. Things that always seem to get me on the old bikes are the one easy thing that I forget to check. :lol:

jsouthard · Sep 5, 2010

Still trying to work this out.

I have changed the plugs, Pulled apart and cleaned the carb, tried a small pilot jet (35) then went back to the standard 40, tightened the headers and sprayed carb cleaner to see if anything was getting in (air tight) and i still have the backfiring.

It got even worse when i used the smaller pilot jet. Back to normal with the size 40 jet. I would imagine the 45 would be to rich.

I bought a strobe light but don't know how to properly use it. I hooked it up and shined the strobe at the ignition and when i got the revs up to 3,000 the strobe stopped (not sure what that indicated).?. Any direction on checking the timing will be appreciated. At the same time the boyer plate is screwed in as tight as the day is was installed.

Any other suggestions will be appreciated.

Thanks,

highdesert · Sep 5, 2010

Joe, at this point I suspect as I said earlier, that you timing is perhaps too advanced.

You can clearly see the degrees marked off on the scale inside the cover, right? If you take the plugs out and on the centerstand put it in fourth gear and move the rear wheel forward to
rotate the motor, then watch until you can see the mark of the alternator rotor come into view and line with with the 30 degree mark, this is what you want to see at 5000rpm.

Ok, plugs back in now, bike in nuetral.

You have a timing light now you say. Hook the leads up to your battery. Is it the kind that that has a plastic clip that encircles the spark plug wire, if so does not matter which plug wire
you choose. You have the forward little primary inspection plug out so you can shine the light in there, is the light flashing, working, at this point?

If so, then have an assistant bring the rpms right up to 5000 while you keep the light shined in there, at 5000rpm the boyer is fully advanced so this is when you should clearly see
the mark on the rotor line up with about the 31-32 degree mark on little built in scale on the inside of the primary cover.

If the timing light cut out before at 3000 rpm it may be simply because the vibration caused either one of the batter leads to come loose, or the thing fell off your plug wire.

Try it again, and let us know what you can see it is timed at now.

Norton-Villier · Sep 5, 2010

I think it was L.A.B. who said you should put a rubber pad under the center stand and the bike wont move around so much Phil

jsouthard · Sep 5, 2010

Thanks Guys,

I just tried to get the RPM's to 5k and the bike was dancing like Roy Orbison is back in town. The Neighbors are having an outdoor dinner so i'm done for the night. I have a much better understanding of timing the bike now. I'll try again tomorrow.

I will let you know what the results are.

Thanks,

hobot · Sep 5, 2010

Door mats collected or couple layers of rug or folded carpet tame rev dancing.
Its not recommended to use bike battery to power time light d/t voltage issues.
But its likely not really an issue if battery topped and normal size as charging up on rev ups. I find it handy to lean over seat to both stablize me and bike but to look down ward with light into the peep hole. Also leaves me on side to diddle timing before laying across my throbbing baby again. Line flickering location tends to wonder a bit from cam drive and throttle control. Easiest is just blip right past 5000 somewhat and see where the wandering line stops wandering at. Diddle as indicated but suggest going by ear and feel more than objective reading.
It was not object but subjective sensation that revealed the issue and will tell when its fixed in the end.

ILLF8ED · Sep 5, 2010

jsouthard said:
Mighty Norton Knowledgeable,

Recently my 74 850 has started backfiring on normal deacceleration. I haven’t changed anything (this week).

Everything else is great. Idles fine, Pulls great, starts in one kick etc. etc. Point me in a good starting direction?

Hi Joe,

Another possibility...check your carb fuel bowls. If they get loose or leak due to the joint face not being flat you will get the symptom described. Mine was backfiring the weekend before last. It got worse and I pull over to see what was wrong. One of the screws on the right carb bowl was missing and the bowl was dropping down. After getting home I pulled the bowl and found the other side had stripped treads and the surface needed lapping. After a heli coil and a new gasket, had it out today for a 160 mile short run. Everything back to normal. If your not using Amals, disregard.

BrianK · Sep 5, 2010

If not an air leak at intake or exhaust, then idle mixture too lean.

NOT too rich. Too lean.

jsouthard · Sep 6, 2010

Is it possible that it's electrical?

I noticed this morning that one of the two wires coming out off the alternator / rotor has cracked and is partially disconnected. The green and yellow wire is partially broken.

Also my warning light had stopped working just before the backfiring had started. Could it be that at idle it's ok but with higher revs and deceleration the vibes force the wire apart causing the timing to advance?

hobot · Sep 6, 2010

YES its Entirely possible back connections not liking the back off vibes but more likely the voltage dropping on lower revs if only partial charging to Boyer.
I've followed a Cdo that an alternator wire connection bad, would back fire so bad it threw rusty glowing embers in my face. Thought the worse but half night searching in motel parking lot found the reason and rode off fine next day.

ewgoforth · Sep 6, 2010

jsouthard said:
Still trying to work this out.

I have changed the plugs, Pulled apart and cleaned the carb, tried a small pilot jet (35) then went back to the standard 40, tightened the headers and sprayed carb cleaner to see if anything was getting in (air tight) and i still have the backfiring.

It got even worse when i used the smaller pilot jet. Back to normal with the size 40 jet. I would imagine the 45 would be to rich.

I bought a strobe light but don't know how to properly use it. I hooked it up and shined the strobe at the ignition and when i got the revs up to 3,000 the strobe stopped (not sure what that indicated).?. Any direction on checking the timing will be appreciated. At the same time the boyer plate is screwed in as tight as the day is was installed.

Any other suggestions will be appreciated.

Thanks,

Backfiring in the exhaust indicates too LEAN a mixture, not too rich.

I'd suspect an air leak between the carb and exhaust manifold or between the manifold and the head.

-Eric