Update...left side carb backfire on Le Mans III

[sdcr]

So the issue is a strong backfire through the left carb, at low rpm.

I've cleaned the fuel system, and left Carb jets, and same symptoms persist.

So, at someone's suggestion, I switched the leads on the coils, so the left coil, is firing the right cylinder, and vice versa.

And after that switch,......the right side carb, is now backfiring, exactly the same as the left one did. The left side does not backfire with this switch.

So, does this indicate a bad coil, and not a fuel system issue?

[yogidozer]

sure sounds like it. try looking for a spark jump in the dark. look in the swap meet section for a coil

[lucian]

Wait a minute, you just switched the plug leads? Or did you switch the coils ?  Or did you switch the left coil lead to the right cylinder?

[sdcr]

This.

.......  Or did you switch the left coil lead to the right cylinder?

[lucian]

Do you still have dual points? 

[sdcr]

Yes, stock, dual point set up. Coils are original 35 years old too.

Do you still have dual points?

[lucian]

When is the last time you  replaced points and condensers?   sounds like it may be time. Or better yet convert to Dyna pick up and be done with it.

[TOMB]

Are the card manifold and gaskets tight? Air leak ?
TOMB

[Chuck in Indiana]

And after that switch,......the right side carb, is now backfiring, exactly the same as the left one did.

I've said many times that carburation problems are generally ignition. 

[canuck750]

I fought tuning the Le Mans MKI carbs on the bike I just finished rebuilding, lots of 'carb' problems turned out to be the ignition ….

after I modified the points plate as per Greg Fields article on "This Old Tractor Tonti Frame section - modifying the distributor" I was able to properly set the LH points and timing and the backfiring, hard starting, sputtering was gone!

I highly recommend you modify the points plate so that BOTH the RH and LH pints can be properly timed.

[Groover]

Check for any arcing in the points when running (best to do in a darker environment). Sounds like an easy fix, I'd start with cleaning and setting the points. If you have excess sparking between the points, then it's likely the condenser to that circuit. Start with the cheap and easy, then move up towards the coils. You can do the Dyna as suggested above, but nothing wrong with a well maintained points system IMO.

Edit: Also check the spark plug wires are tight and locked in both the spark plug cap and coils. I've seen coils that have a broken copper pin/screw and the wire arcs in there eventually widening that gap.

[jacksonracingcomau]

Sound like you have proven it is the coil, points and condensors both sides work.
By switching coil, you only switched the coil, ergo the mis followed the coil
Process of elimination, solved
I would buy a pair, keep your good one as spare

[Antietam Classic Cycle]

I fought tuning the Le Mans MKI carbs on the bike I just finished rebuilding, lots of 'carb' problems turned out to be the ignition �.

after I modified the points plate as per Greg Fields article on "This Old Tractor Tonti Frame section - modifying the distributor" I was able to properly set the LH points and timing and the backfiring, hard starting, sputtering was gone!

I highly recommend you modify the points plate so that BOTH the RH and LH pints can be properly timed.

I've found that this is not always necessary - it wasn't on my Convert.

[lucian]

Sound like you have proven it is the coil, points and condensors both sides work.
By switching coil, you only switched the coil, ergo the mis followed the coil
Process of elimination, solved
I would buy a pair, keep your good one as spare

But he didn't actually switch the coils, only switched leads to opposite sides. Makes me think it's  the left side points . My guess is they are arcing from a bad gap, burned points or condenser on that side. Left coil is working .

[jacksonracingcomau]

But he didn't actually switch the coils, only switched leads to opposite sides. Makes me think it's  the left side points . My guess is they are arcing from a bad gap, burned points or condenser on that side. Left coil is working .

Read it again, repeated for prosperity

So, at someone's suggestion, I switched the leads on the coils, so the left coil, is firing the right cylinder, and vice versa.

And after that switch,......the right side carb, is now backfiring, exactly the same as the left one did. The left side does not backfire with this switch.

[sdcr]

Let me try to lend some clarity to what I did with the coils. At the junction, where the spark plug wire, enters the coils, I simply removed  those wires, and switched them.

If there is a another way of testing the coils, please share.

    As you probably can tell, I am not a technician, just trying to get the old girl back on the road.

   I very much appreciate the help that you guys are providing.

  I am putting new coils on, since at 35 years old, it cannot hurt. I will take a look a t the points. I don't know how old they are, but at least 6 years, and about 8000 miles.

[lucian]

You have a bad spark plug wire or plug cap on the side that is backfiring. The coils are fine as they both work when you switched the leads.   I would start with some new copper core HT plug lead and caps from any auto store. I couldn't for the life of me figure out how it started by just crossing the plug wires from side to side unless you had a wasted spark ignition. But I have never seen one with twin coils. Sounds like your onto it.   

At 8,000 miles since last tune up I would throw plugs and points at it as well, they are cheap  enough.  Also to test for a bad coil by the switching method , you have to switch the primary low voltage wiring to the coils with a dual point style ignition or you will be inverting the firing order which is never a good thing. What you did is test the Ht leads by only switching them and in this case identified the problem.

[Groover]

I don't think there is a clean way to test coils other than flip-flopping them like you did. So you changed the wires going in the coils only? (green and red on G5, not sure what color they are on the LMIII), I think the running test would be to flip those two spades then also flip the sides on the spark plugs (just swap sides if they reach) but both need to happen to test the coil I think keeping the correct spark timing for the cylinders (off the top of my head).

Also, new coils aren't necessarily better. Those old red cap Bakelite Magneti Marelli coils are pretty durable if that is what you have. If you have the later black cap coils, then maybe not so much. Some of the new coils out there aren't as reliable as the OEM ones. I tried new wax-filled ones, they leaked, then went back to some old OEM oil filled ones and all is well. Maybe the batch I bought new was just a bad batch, so take my experience as maybe an isolated incident. My point is, replacing an old working part may backfire (no pun intended..  )

 

[sdcr]

Lucien,

 The backfire issue was prevalent before I replaced the plugs, plug wires and caps. I get the same running/ backfire condition, after changing to new plugs,wires and caps.

Groover,

  Here is a pic of the coils that have been on this bike at least since I have owned it. I believe these are the OEM coils.




On the coils in the picture, each coil has three spade connectors. The coil on the right side, has 3 wires into the spades.
The coil on the left, just has just  2 wires, and one bare spade. Is this setup correct? I don't see a missing female wire end anywhere, that would connect to the bare spade.

[Groover]

Those appear to be OEM coils. I don't thing they have a brand stamped on them, so probably a transitional supplier for Moto Guzzi (or Magneti Marelli) at the time. Those are the later ones used I mentioned, I think maybe starting around 1980. Right before that were the red smooth MM caps. My bike ('81 G5) had 1 of those which I think was original to the bike, and I've also seen those on 1980 SP models.

The red and the green wires should be plugged in the + terminals, and I think the extra spade you see off the - may be for a kickstand safety switch which you may or may not have. I don't recall of the top of my head, and looking at some photos I have on my computer I kept missing somehow that detail. I'll keep looking.

Edit: I can't seem to locate a Le Mans III wiring diagram, but should be similar. The spades are correct. You should have:

Coil 1 + to the red wire (right cylinder points)
Coil 2 + to the green wire (left cylinder points)

Coil 1 - to Coil 2 - (jumper, looks black on yours), then the extra spade off the - of either coil 1 or 2 will go to the safety switch. That would probably leave an open - spade on the other coil - I could have this confused, but your problem is not here as anything wrong here would be an all or none deal, not just left side or right side.

[lucian]

If you only switched the leads and the backfire changed sides with the swap, the problem has to be with the one lead that is with the side affected. Check both the coil and plug cap ends of that lead to make sure it is connected properly. If you can spare a small amount of slack I would re trim the wire ends and re install cap and connect to coil.

[lucian]

I just took a look at my MKIII and your coils look like the originals. Your connections are fine as Grover stated. 

[jacksonracingcomau]

Let me try to lend some clarity to what I did with the coils. At the junction, where the spark plug wire, enters the coils, I simply removed  those wires, and switched them.

If there is a another way of testing the coils, please share.

   

You could use a meter but method you used is perfect, repeatable and absolute.
I like the easy ones but swap it back just for your own piece of mind.
Don't fudge with anything else, worms come out of the can

[izzug otom]

Hi John, thanks for dropping in on my post http://wildguzzi.com/forum/index.php?topic=96703.0;topicseen . Yes certainly looks like a similar problem, best of luck, and I'll be sure to let you know immediately if I find an answer to my issues, and likewise, please do the same.

I do hope I get my Cali sorted soon, she's my only mode of transport and the UK summer has been really cracking too.

I have the digiplex electronic timing, so no points here, but how did you manage to test the coils without changing the green and red wires over along with the HT leads? If it wasn't for that niggle in my mind, I'd say it must be the left coil.

Cheers, Martin

[John A]

usually power goes to the coil + with a jumper between coil positive terminals. The trigger, be it points or digiplex, are the red and green that go to the coil negative without a jumper

[Old Jock]

I'm sort of following this on and off and I am thoroughly confused........... ...that does not take much though

Can you tell me when swapping the HT leads, if I am understanding correctly, you took the HT lead from the left coil and put it into the right coil & vice versa and not change anything else?

So effectively you now have the RHS cylinder getting a spark when the LHS cylinder is calling for one and vice versa. For the life of me I cannot see how the bike would run unless you also swapped the commensurate negative primary connection (the negative low voltage connection)

If it's anything like the LM 1000 I have then then one coil has 2 positive connections on the LV side and that simply carries a jumper across to the other coil.

It's the negative connections that matter regarding cylinder timing

To test a coil I remove all the wiring from HT and LV swap the coils over and reconnect all the wires back, it's important that the LV negative and HT wires are not crossed over, in other words having the Right HT and the Left negative LV and vice versa

I still cannot figure how the bike would run that way with a stock points setup, but I'm willing to be educated for sure

[jacksonracingcomau]

I'm sort of following this on and off and I am thoroughly confused........... ...that does not take much though

Can you tell me when swapping the HT leads, if I am understanding correctly, you took the HT lead from the left coil and put it into the right coil & vice versa and not change anything else?

So effectively you now have the RHS cylinder getting a spark when the LHS cylinder is calling for one and vice versa. For the life of me I cannot see how the bike would run unless you also swapped the commensurate negative primary connection (the negative low voltage connection)

If it's anything like the LM 1000 I have then then one coil has 2 positive connections on the LV side and that simply carries a jumper across to the other coil.

It's the negative connections that matter regarding cylinder timing

To test a coil I remove all the wiring from HT and LV swap the coils over and reconnect all the wires back, it's important that the LV negative and HT wires are not crossed over, in other words having the Right HT and the Left negative LV and vice versa

I still cannot figure how the bike would run that way with a stock points setup, but I'm willing to be educated for sure

He swapped ht AND lt leads, basically swapping coils. Easy coil test, problem followed coil
Reading first post would have helped you understand,

[quote author=sdcr link=topic=96780.msg1530308#msg1530308 date=1530135755

So, at someone's suggestion, I switched the leads on the coils, so the left coil, is firing the right cylinder, and vice versa.


[/quote]

[sdcr]

Ok, I may have explained this incorrectly in an earlier post.

I did not move the the wires on the spade connections, just the main spark plug wire, when I did the switch.
 The bike did not run when I did this, but did backfire through the right side carb.  From Old Jocks explanation, I believe that I need to move  the spade connection wires also to actually test the coils, correct?

  I am sorry for the confusion.

[Old Jock]

Heh no problems sdcr

Yes the low voltage connections matter, the positive doesn't really but the negative is important.

To save any confusion just swap all the wires and you'll be good

Oh and a big thanks jacksonracing for having a pop when I (and others in the thread) had interpreted the posts correctly.......... ..............Shees h some folks

[sdcr]

Old Jock,

 Thanks  for setting me on the right path here.

I don't think it is anyone elses fault , other than mine for any confusion. I mis-explained what changes I made.

So, I will have another go at the " switch"

Thanks again.