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Backfire

Chuck Stone

Just got it firing!
Joined
Jul 26, 2017
Messages
10
Location
Virginia
Good day. I have an '83 LM3 with 14K which has slowly developed a new problem.

Bike starts and runs great, dash volt meter showing 11.5-13v w/o headlamp on at 4K, less than 11v with headlamp on. Checked voltage at battery, 14v running, 14.2 at 3K w/headlamp on. All appears well and dash voltmeter assumed to be way off.

After riding 20-30 miles at 3-4K and stopping, starting out, losing cylinder, popping back (loud) missing badly until up to 3-4K again (charging) and ignition comes back and all is fine until next situation requiring idle and symptoms repeat.

Battery is flooded wet cell, 1 year old, kept on trickle charger. No problems until battery appears to run down from lack of running charge causing ignition failure(?). Once again, on center stand, all indications are that charging is taking place from voltmeter readings, headlamp brightening, etc.

I'm merely asking before I dig into checking the rotor, regulator, etc. as if someone has experienced the same.

Thanks,

Chuck

IMG 2970
 
My suggestion is to clean all the contacts associated with the charging system. CRC makes a good aerosol cleaner available at most auto parts houses. Yes you do have to get some RPM to excite charging, but your case is extreme. It also helps to tin the three spades that come off the stator to have a better connection. Those connections carry a bunch of amps and any resistance will generate heat. I've seen the harness connector melted at the stator on some bikes. Let us know if that helps and what type of ignition system you are running.
 
Thanks John,
After taking your suggestions (no melting observed), as well as checking rotor, stator resistance. Found battery ground at frame underneath rear mc loose and oil coated (eureka!), cleaned re attached and on my way after improved starter rate. Good ride until, 10 minutes in, dies. Push to clear area, starts, backfires and revs up sufficient to get home. Put on center stand to observe idle (12.5v at battery with lights on). Eventually starts missing, give some throttle, backfires excessively.

Now, wait for it...... Remove gas cap, start, idles like a champ with no problems, throttling up, etc.

Ordering new cap from MG today. Congrats to all contestants who guessed correctly. When you think ignition, it's fuel. Think fuel, it's ignition.

Chuck

PS: Dyna
 
You probably didn't need a new cap, just to flush out the present one with solvent (gum cutter/carb cleaner). It's likely gummed up from evaporating fuel and isn't allowing the tank to vent. Happens a lot this time of year.
 
Ok, update. Gas cap apparently not the problem. 10 miles into ride today after 4-5k run and cruising a little at 3k, miss/backfire intermittently returned. Was able to power through it and it disappeared.
 
How old are your fuel lines? Old lines could be breaking down and allowing trash into the carburetors clogging jets that may or may not eventually clear. Old lines were not ethanol resistant and could be breaking down.
 
Now it sounds like it must be an ignition problem, the backfiring must be unburned mixture in your exhaust.
Something doesn't like to get warm.
1-Can it be that your battery doesn't hold a charge so it depletes quick whenever your rpm's drop?
2-Or can you try another set of coils?
3-What state are your contact breakers and or capacitors in?
4-When your bike has stalled, how do your spark plugs look? Wet/dry/black?
5-Can it be that your chokes are not closing properly and your bike is drowning in fuel?
6-Can you replicate the stalling when the bike is warm by turning the handlebars from side to side?
7-Just to eliminate your kill switch as an unlikely problem, you can disconnect the connector of the right hand switch gear after you start the bike and go for a ride(mind you; this will also disconnect your starter switch, so when the bike stalls, you will have to reconnect it first to be able to restart it).
8-You could duct tape your voltmeter to your tank to see what the battery voltage does when you have the symptoms.
9-I have heard of people that had brand new spark plus that didn't work. Unlikely, but worth a check I suppose.
Hope this will get you somewhere with trouble shooting.
JR
 
JR,

All good suggestions. I guess I need to focus on voltage depletion while under way. The overall question I have is, with a Dyna ignition, will a roll off of voltage precipitate the symptoms of losing a cylinder and loud repeated backfires when throttling up to get underway?
 
JR,

All good suggestions. I guess I need to focus on voltage depletion while under way. The overall question I have is, with a Dyna ignition, will a roll off of voltage precipitate the symptoms of losing a cylinder and loud repeated backfires when throttling up to get underway?

How old is the Dyna unit and how many miles have you used it? As with any electronic system it can begin to fail. If voltage is not dropping, do you still have the points plate and capacitors to see if that stops the problem?
 
Took a look in the file (I've owned for 8 years, 8k miles). Prior owner had ticket with bad idle/backfiring and shop rebuilt carbs and replaced dyna coils with be ones. So, Dyna unit is more than 8 years 8k miles old, probably significantly.
And no, unfortunately I don't have points,etc. Bike came this way.
 
Do you have the green Dyna coils or the black ones? There have been issues with the green ones. I'm currently running a Dyna III that has been in my LeMans for over 20 years and 25K miles (Bike has more miles that that) that is still running flawless. The Dyna unit may be OK and it may just be a bad coil, there have been reported issues with the green ones. If you can't find a points plate nearby to borrow, send me a PM, I think I have one I can loan.
 
How old are your fuel lines? Old lines could be breaking down and allowing trash into the carburetors clogging jets that may or may not eventually clear. Old lines were not ethanol resistant and could be breaking down.

Along the same lines as John L.

Have you checked fuel is flowing properly from the petcocks?

If your bike sat for a while the fuel could have evaporated leaving a thick brown goo in the bottom of the tank, gumming up the petcock filters.

May not cause the backfire but could lead to stalling if the fuel isn't filling the carbs.

BTW,

I'm curious, most of the time I hear bikes popping and backfiring on rapid deceleration, i.e. engine breaking.
Isn't deceleration usually a result of reducing the flow of fuel into the engine?

I never understood how suddenly reducing the fuel in the air / fuel mixture causes exhaust gas to ignite in the exhaust system.
 
I never understood how suddenly reducing the fuel in the air / fuel mixture causes exhaust gas to ignite in the exhaust system.

When fuel is reduced, exhaust temperature rises. Any previously unburnt fuel in the exhaust may then ignite.
 
Do you have the green Dyna coils or the black ones? There have been issues with the green ones. I'm currently running a Dyna III that has been in my LeMans for over 20 years and 25K miles (Bike has more miles that that) that is still running flawless. The Dyna unit may be OK and it may just be a bad coil, there have been reported issues with the green ones. If you can't find a points plate nearby to borrow, send me a PM, I think I have one I can loan.
 
John,

I have the green ones. Wondering if someway to check them while replicating problem......

Not that I know of. Check everything else first (including using a points plate) and then go for a pair of the black ones if you think that a coil may be the issue. FYI, coils are heat sensitive and may break down when hot which could give you some of the symptoms, but then again it could be something breaking down when hot in the Dyna controller. Did you check the Dyna connections? They are tiny and fragile and need to be clean and tight.
 
One thing I forgot to mention, if you will be having the Dyna plate out of the "distributor", it would be a good time to replace the advance springs. They do tend to weaken over time.
 
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