Burning in muffler?

[SED]

In need of some expert knowledge here.  What is the cause of fuel burning in the muffler so bad that it discolors the chrome?

The chrome on the head pipes near the exhaust port can be OK, but it is burned further back.  I've seen this on Guzzi small blocks, Enfield Bullets with the Bazooka and it has happened, and is still happening, to my Bacon Slicer.





Excess fuel is burning in the muffler yet the carb is running stock main jet, idle jet, needle, atomizer (all new except the idle jet).  Timing is correct and the spark plug insulator is light brown though the plug base shows powdery carbon. It runs well 1/4 throttle and above, and pulls well up to 60mph or better.  It will idle at very low rpm - however idle is not very stable, it surges as steady throttle when trickling along and the transition between 0 and 1/8 throttle is poor.

I've tried richer and leaner idle jets, raising and lowering the float level slightly (2-3mm), raising and lowering the needle, advancing and retarding the timing...

I'm thinking that despite running pretty well from about 1/8 to full throttle that something there is masking a bigger problem with the transition.  So, what causes fuel to burn, not in the combustion chamber and not in the header, but back in the muffler? 

Thanks!
Shawn





 

[Sheepdog]

Look for an exhaust leak. Also check the condition of the idle mixture screw's tip. It should taper to a point, but sometimes takes on the contour of the seat. This makes idle mixture difficult to modulate.

[RinkRat II]

 Old pipe and muffler? Was there something left over inside the muffler from previous ill running motor and now it's burning out of there? My $.02

     Paul B

[Mikecocos]

Spark plugs not firing properly causing fuel to escape into the exhaust pipe that then ignites  later on causing discolouration

[SED]

Thanks for the ideas.  It is the old pipe and muffler, but the only air leaks are around the fishtail cone which is just down stream of the burned area.  The idle mixture screw is not too bad, partly because it adjusts airflow rather than fuel flow (larger than fuel metering taper).  With the correct size idle jet it runs about 1.25 turns out.

[Rough Edge racing]

 I've seen that on race bikes using open megaphones. On racers it's not ignition or jetting. They will turn blue in a particular spot...supposedly due to gas flow and turbulence. Perhaps there's a baffle in that location on your Guzzi?

[bigbikerrick]

Maybe everything is as it should be. You certainly have tried everything I would have done in troubleshooting. This is a very simple, low strung machine, almost "Briggs and Stratton" simple. My next guess is that it  possibly was caused by poor quality chrome plating, combined with the internal design of the muffler, as to the path of most heat. What I mean is that discoloration does not necessarily mean unburned fuel was actually burning, as in a "fire" inside the muffler. I have seen it on other Guzzi mufflers, mostly on tonti framed calis, like my aluminum.
Rick.

[SED]

Spark plugs not firing properly causing fuel to escape into the exhaust pipe that then ignites  later on causing discolouration

Maybe. Don't notice a miss when riding and it's a single so should be obvious.  However it does have a tendency to miss if the idle is set too high. Swapping plugs and plug wires and cleaning magneto points has not made a difference.  And it will idle without missing at very low rpm which suggests the magneto is OK. Also checked with an inductive timing light and can't see evidence of ignition failure.

I'm thinking it's a problem with the transition from idle circuit to fuel flow through the atomizer, which according to Chuck's diagram should be influenced principally by the idle circuit.  There are passages in the throat to smooth fuel flow as the slide is lifted and wonder if the problem is there?

carb_jet_circuits by Charles Stottlemyer, on Flickr

[Sheepdog]

If the old Guzzi fishtails are like the old BMW fishtails, there are no baffles. The pinched end does all the muffling. One and a quarter turns out from seated indicates that you may not have full adjustment available for your idle mixture. If it�s possible, I would suggest a size smaller on your pilot jet. You need at least two turns out from seated with your mixture screw to assure you have a sufficient adjustment range. I had this same sort of trouble with an old Beemer. Even when I got it running as well as I could, I had to raise the idle a bit to make it behave at low speeds....

[SED]

Rough Edge and Rick, I think there is a baffle there, but also know something is wrong - on the overrun it gives an occasional deep BOOM but when hot it starts crackling on the overrun like a cracking fire.  Raising the fuel level makes it boom more but stops the crackling.   

[SED]

If the old Guzzi fishtails are like the old BMW fishtails, there are no baffles. The pinched end does all the muffling. One and a quarter turns out from seated indicates that you may not have full adjustment available for your idle mixture. If it�s possible, I would suggest a size smaller on your pilot jet. You need at least two turns out from seated with your mixture screw to assure you have a sufficient adjustment range. I had this same sort of trouble with an old Beemer. Even when I got it running as well as I could, I had to raise the idle a bit to make it behave at low speeds....

Thank you Sheepdog.  It's running the stock 50 idle jet.  I tried a 46 idle jet and the mixture screw was only 1/4 turn out (it is an air strangler system).  I've also got an idle jet that is drilled to about 60 and needed the air screw out 6 turns or something.  Never found the lean drop. 

[SED]

What I mean is that discoloration does not necessarily mean unburned fuel was actually burning, as in a "fire" inside the muffler. I have seen it on other Guzzi mufflers, mostly on tonti framed calis, like my aluminum.
Rick.

Rick, don't some have it and other's don't? 

That's what I've seen and so my original question.  I've seen newish V7s that have discoloration on one side only - so what is the cause?  Too rich 1/4-full throttle?  Too lean idle? Timing failing to advance?

[Sheepdog]

Thank you Sheepdog.  It's running the stock 50 idle jet.  I tried a 46 idle jet and the mixture screw was only 1/4 turn out (it is an air strangler system).  I've also got an idle jet that is drilled to about 60 and needed the air screw out 6 turns or something.  Never found the lean drop.

Okay...I had the circuit design backward. It sounds like the bike is really rich down low. Any chance that there is a crack between the float bowl and the Venturi (or one of the ancillary circuits)? I had an Isuzu that did just that when it had been in a small collision. It ran great up top, but was miserable down low. Or perhaps a #53 idle jet...

[bigbikerrick]

I guess its hard to tell SED. A friend had a newer V7 racer, and it did the same thing, right side pipe discolored, checked on by  dealer in Tucson  that had alot of Guzzi experience, and found nothing wrong. Of course that bike is a whole different ball game with fuel injection and all....
Rick.

[SED]

Okay...I had the circuit design backward. It sounds like the bike is really rich down low. Any chance that there is a crack between the float bowl and the Venturi (or one of the ancillary circuits)? I had an Isuzu that did just that when it had been in a small collision. It ran great up top, but was miserable down low. Or perhaps a #53 idle jet...

Sheepdog, you might be on to something.  I've been thinking that there is a problem with the transition from closed throttle to just open. I know the passage is clear (or was) when I rebuilt the carb.

So been doing some reading in the Dellorto manual.  They call it the progression circuit:

      


Wonder if that passage has been drilled out? (PO worked really hard to make it run very rich)  Or like you suggest a crack or other flaw. 

Something to try out - Thanks! 

[Chuck in Indiana]

I haven't seen that one, Sean. Which manual is it in?
I have version 1.1

[SED]

I haven't seen that one, Sean. Which manual is it in?
I have version 1.1

I think it's just called Dellorto Carburetor Manual.
Here's a link: http://dellorto.it/wp-content/uploads/2015/10/dellorto_manual.pdf