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General Category >> Rubber Side Down! >> I know there are plenty backfire threads, but...
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Message started by toddlamp8 on 10/04/07 at 17:26:58

Title: I know there are plenty backfire threads, but...
Post by toddlamp8 on 10/04/07 at 17:26:58

... 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.

Title: Re: I know there are plenty backfire threads, but.
Post by rigidchop on 10/04/07 at 17:34:13

maybe you are riding differently and just don't realize it. i can't imagine putting on a new part and riding the same way i did before, at least at first any way. you wanna listen to how different the bike sounds, runs etc.maybe give it a little time then see what happens.

Title: Re: I know there are plenty backfire threads, but.
Post by toddlamp8 on 10/04/07 at 17:38:58

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.

Title: Re: I know there are plenty backfire threads, but.
Post by T-Mack1 on 10/04/07 at 17:51:15

OK.....back to some basics. Best energy efficiency is at correct fuel to air mixture. That means the most power for fuel consumed. Most power means farthest distance traveled per gallon.

Too rich is dumping gas away. I think everyone understands that.

But, too lean means you're not getting an efficient burn. Not as much energy out. So, you have to open up the throttle more to get same response. But, more throttle is just dumping gas into a poor burn condition. You aren’t going to get the same energy out, so the gas won’t move you as far. Means poorer MPG…..

Even though you think you are driving the same, you really are using a tiny bit more throttle.

I hope this is understand-able.

Title: Re: I know there are plenty backfire threads, but.
Post by toddlamp8 on 10/04/07 at 18:02:55

T-Mack1 wrote:

But, too lean means you're not getting an efficient burn. Not as much energy out. So, you have to open up the throttle more to get same response. But, more throttle is just dumping gas into a poor burn condition. You aren’t going to get the same energy out, so the gas won’t move you as far. Means poorer MPG…..

Even though you think you are driving the same, you really are using a tiny bit more throttle.

I hope this is understand-able.

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?

Title: Re: I know there are plenty backfire threads, but.
Post by barry68v10 on 10/04/07 at 18:20:29

Yes, a rich condition can cause what most people would call "backfiring", especially if you have an exhaust leak. If it's rich enough AND hot enough (with no exhaust leak) gas vapor will exit the muffler, THEN BURN. This is pretty spectacular...especially with a big v-8!

Title: Re: I know there are plenty backfire threads, but.
Post by toddlamp8 on 10/04/07 at 18:25:09

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 ;D

Title: Re: I know there are plenty backfire threads, but.
Post by barry68v10 on 10/04/07 at 18:27:12

toddlamp8 wrote:

Hmmm...With a given timing set-up there is an ideal air/fuel mixture for both power and MPG and they are different. If you go too lean, the mixture won't even burn. If you lean the mixture to the 16-18:1 range AND retard timing you can increase mpg but heat build-up can become a problem. A given richer mixture requires more advanced timing and a given leaner mixture requires less. Got a way to change timing on your bike that's easy?

Actually, if you re-gap the spark plug you can effectively change timing somewhat. Smaller gap will act more like retarded timing...hotter/larger spark will act like more advanced timing.

Title: Re: I know there are plenty backfire threads, but.
Post by LANCER on 10/04/07 at 18:33:49

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.

Title: Re: I know there are plenty backfire threads, but.
Post by barry68v10 on 10/04/07 at 18:35:41

toddlamp8 wrote:

I would follow Lancer's tuning procedures listed in the tech section. The KLR650 guy lurking around here also posted a very effective tuning procedure specifically for our style carb (if it's stock.) The concept is, start with the main jet and work your way down to the pilot screw to tune the entire range.

One other thought, a muffler has specific flow characteristics for both volume and exhaust speed. The stock muffler is very hard to beat for low RPM torque. It actually creates a siphon effect which enhances low RPM flow vs. a more open exhaust. As RPMs build it will begin to restrict flow vs. your harley muffler. The size and length of exhaust header needs to work together with the design characteristics of the muffler for optimal broad RPM performance.

Title: Re: I know there are plenty backfire threads, but.
Post by barry68v10 on 10/04/07 at 18:37:36

LANCER wrote:

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.

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

Title: Re: I know there are plenty backfire threads, but.
Post by toddlamp8 on 10/04/07 at 18:49:32

I will do that thanks.

Title: Re: I know there are plenty backfire threads, but.
Post by toddlamp8 on 10/04/07 at 20:16:50

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.

Title: Re: I know there are plenty backfire threads, but.
Post by islandwahine on 10/05/07 at 12:23:26

LANCER wrote:

double-check that there are no air leaks in the exhaust system.

Thanks LANCER for that thought! I have non-stop backfiring when I reduce throttle or shift, since I did the carburetor.
I really catches everyone's attention, so maybe it's a safe thing to have happen! But I myself find it really annoying.
How would you go about finding an air leak?

Title: Re: I know there are plenty backfire threads, but.
Post by serowbot on 10/05/07 at 17:14:51

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.

Title: Re: I know there are plenty backfire threads, but.
Post by toddlamp8 on 10/06/07 at 11:25:30

Thanks. I am running the petc0ck on prime because I thought I was having vaccuum issues. I'll have to break-down the carb again just to be sure.