... I'm trying to figure out all the different causes for it.  

So far I know that a lean condition will cause this.  However, are there circumstances where a rich condition will cause a backfire?

I just installed an HD muffler and i'm getting more backfiring before, but it's much more subdued.  I don't get the "gun shot" loud bang, just a few garbles.  However, I did notice that i'm getting flames coming out of the muffler, and when I filled up tonight, i only got 40mpgs this past tank (first tank with HD muffler).  

I know that mufflers are supposed to increase flow and thus lean the system out, but why the drop in mileage?  I didn't change my riding habits, at least not enough to warrant a 15mpg drop.

It seems that I am running rich, causing excess fuel (or at least incomplete combustion).  Fumes or excess gas is gathering in the exhaust and then igniting as I slow down and back pressure increases.  I have an 86 which from what i've read should have a slightly larger jet than later models.

I woudl appreciate some insight so that I know what areas to focus on.  As a side note I have been using 89 octane lately.  I'm also wondering if the higher octane rating could be making it more difficult for combustion and thus leading to excess fuel mixed in the exhaust.  I don't have any black smoke so I know i'm not that rich, but I just can't explain the increase in fuel consumption.

Thanks.

I appreciate the suggestion but I only really use the bike to ride from home to work (all of 8 miles each way) and I still get to work in the same amount of time, so i'm fairly sure I haven't changed my habits.  At least not enough to explain the significant drop in mileage.

T Mack 1 - FSO wrote on 12/31/69 at 16:00:10:


My understanding of that is just the opposite.  Leaning out a mixture will get you MORE power.  Increasing the air to gas ratio will increase the explosive nature of the fuel resulting in more power but at the same time more heat.  Most tuners that adjust fuel maps lean out the mixture to get easy power and increase fuel mileage.  However increasing it too much will cause too much heat that will damage engine components.

It's with that in mind that I feel I have a rich situation (lower mpgs and fire in the muffled [excess fuel]).  However, most people say that backfiring is caused by a lean condition.  How can a rich condition cause backfiring?

I do notice a small leak where the muffler and header meet.  Would that cause air to be entering at that point and the heat cause combustion?  That would mean that I have 2 problems  

If I were to adjust the pilot screw, where should I start?

If I had my choice between rich and lean I would choose rich so that's good  

From what you have said the only change is the muffler.  That said, go back and double-check that there are no air leaks in the exhaust system.

LANCER wrote on 12/31/69 at 16:00:10:

YES!  What Lancer said.  That's the place to start!

Well when I was warming up the bike tonight before I left work I held my hand where the muffler and header meet and sure enough there is a pretty substantial leak.  I'll have to tend to that tomorrow once things cool down.

How would that contribute to my issue?  What does a leak in that particular area do to the system?  I also noticed the the flames weren't shooting out until I slow down and hit ~2400RPMs and less.  I also noticed a more intese exhaust smell.

rich can cause backfire...air leak in the exhaust will too...
now,..about the mpg drop,..that's a big drop, check the vac line to the petc0ck, or your needle slide may be sticking.  Sometimes by random coincidence, an unrelated problem starts up at the same time as a change you make, and fools you into thinking it's something you did.  That big of a mpg change seems like a fuel problem and will cause sputtery backfires, bad mpg, smoke, poor running, etc.  And if it's a fuel problem, it will get worse.