What causes backfiring

[stp1stp]

Got my bike running finally after a month of rebulding. Before I started the bike was backfiring. Thinking when I change the gaskets (which were all leaking), adjusting the valves, and completly cleaning the carbs this will take care of it. It did not. I havent rode the bike on the road yet and still have to synch the carbs but I notice the low tone poping out of the exhaust after bike warmed up for a couple of minutes. Just wondering what everybody thinks I need to do to fix this.
Thanks

[Steve]

Just how intensive was your "month of rebuilding"? If it involved cam removal, go back and check your cam timing very carefully.

Could also be that the mixture is a bit rich and some of the excess is being lit in the pipes by the next exhaust pulse that comes through.

That's MY two cents. (and I expect change )


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[mark]

In general, most backfiring is the result of an overly LEAN condition. This could be caused by leaky intake boots (at the manifold), airbox that is not sealed correctly, or a carburetor that is not set correctly to name a few things.

Be more specific about your backfire. Sitting at idle? Just off idle? Full throttle? On the gas, inbetween shifts etc? Also is your bike stock or aftermarket exhaust or pods?

A good way to get started on this is to do a plug check. Normal color should be a tan or grey color.

[stp1stp]

It's happening sitting at idel. All I done was change the all the gaskets. Change #2 rings. Change all O rings. Clean the carbs. It might be the carbs are to rich. I havent adjust them yet. I turned out the screw 1 1/4 out but did nothing else so far. The intake boot were very hard to get on. It could be those. Is there a way to test those while they are on.
Thanks

[stp1stp]

I have this CARBURETOR SYNCHRONIZER KIT. Does anybody know where the needle needs to be on the dial.

http://www.jcwhitney.com/autoparts/Product/tf-Browse/s-10101/Pr-p_Product.CATENTRY_ID:2004169/showCustom-0/p-2004169/N-111+10211+600014252/c-10111

Just trying to get as much info before I get back from work.

[crc1214]

I wouldn't synch the carbs just yet. This could be about 20 different things, do the easy stuff first. Clean the air filter and put new plugs in if they are of questionable age. Clean all the connections in your ignition system wiring. Is the battery good? If the problem still persists, check the ignition timing when the bike is fully warmed up (this will require a timing light). I'm assuming your 80 has electronic ignition. If you find irradic sparking in both the 1/4 plug wires but not the 2/3 wires or vise versa, one of your coils is going out. If irradic sparking is found on all 4 wires, perhaps both are going out! If you get consistent sparking on all 4 wires but the timing light reveals your timing is off, then adjust according to your manual.

Also... do your exhaust fumes wreak of gas? Take the RPM's up to say 4,000 with the bike on the centerstand and just let go of the throttle - do the RPM's drop to 1200 or so rather quickly, or does it take a long time for the bike to return to idle?

Any exhaust/airbox mods? Stock jetting?

[mark]

I have the same synchronizer. The important thing is that they are all set the same, rather than any specific number. My '79 850 is at about 10 inch pounds per cylinder, but I have different carbs and manifolds than you do.

You can test your intake manifold boots for leaks by running the bike and spraying water around them, watching for rpm changes. Sometimes that doesn't always pick up the problem though.

You also need to check that your vacuum line from #3 to your petcock is hooked up correctly and isn't sucking any gas, that could mess you up. I also assume that your carb floats are adjusted correctly?

Perhaps the best thing for you to do at this point is to get it all back together, adjusted as best you can and then go for a ride and see how it does at various throttle positions and rpm's. Do the plug check!....maybe it is too rich.....

Good ideas in above post too that are not carb related. Other problems might be that you have a couple of plug wires mixed up (that would be a bad backfire!) or a cylinder not firing. Check to make sure all your pipes warm up evenly when you start it up.

[tconroy]

I have the same synchronizer. The important thing is that they are all set the same, rather than any specific number. My '79 850 is at about 10 inch pounds per cylinder, but I have different carbs and manifolds than you do.

You can test your intake manifold boots for leaks by running the bike and spraying water around them, watching for rpm changes. Sometimes that doesn't always pick up the problem though.

You also need to check that your vacuum line from #3 to your petcock is hooked up correctly and isn't sucking any gas, that could mess you up. I also assume that your carb floats are adjusted correctly?

Perhaps the best thing for you to do at this point is to get it all back together, adjusted as best you can and then go for a ride and see how it does at various throttle positions and rpm's. Do the plug check!....maybe it is too rich.....

Good ideas in above post too that are not carb related. Other problems might be that you have a couple of plug wires mixed up (that would be a bad backfire!) or a cylinder not firing. Check to make sure all your pipes warm up evenly when you start it up.

My bike only "pops" when I have the choke wide open,other than that it runs great.What would cause it to do that?

[GS850_Coug]

My bike only "pops" when I have the choke wide open,other than that it runs great.What would cause it to do that?

My suggestion is to move this into your own post. That way you can get answers specific to your problem. In this post the original poster has still not solved his problem, making your question more confusing to his issue, rather helping his issue. Not trying to flame, just trying to help you get the best response to your questions.

Torry

[stp1stp]

Let the bike warm up a little more today. Notice light white smoke coming from what I thought was the exhaust crossover but further inspection I think it's coming from the manifold. Maybe the boots. Thinking it's just chemicals from cleaning cooking off. Rode down the 1/2 mile driveway to see if the smoke will stop. Still smoking. Maybe the carbs are too rich. Hooked up the carb synchronizer and could not tell any difference in the needles when I turn the screws. Any suggestions and is there a noticeable difference when I turn the screws.
Back to the back firing. I have new spark plugs but they are a little hotter the my book says to get. New air filter. I do not smell any gas fumes. Disconnect the vacuum line at carb and there was some gas. Sucked on it by mouth to see if its pulling gas and nothing. Maybe I need a new petcock?
Any suggestion?
Thanks

[Steve]

Sucking on the vacuum line should NOT get any gas, at least not through that hose. When vacuum is applied to the smaller hose, it allows gas to flow through the larger hose, to the carbs. Sounds like that part of the system seems to be working fine.

What screws are you turning when you did not see any change in the gauges? If you were turning the ones on top of the throats on the engine side of the carbs, those are the wrong ones. Those are the mixture screws, not the synchronizing screws. The synchronizer screws are buried in between the carbs and are attached to the throttle shaft linkage. There is a lock nut and a slotted adjuster on each of three adjusters. There is a "master" carb, usually #3, and you can tell which one it is because the throttle cable attaches to it. The usual procedure is to adjust #2 so it matches #3, then do #1 so it matches the others. Finally, adjust #4 to match. Like Marks has said, the actual number is not as important as the fact that they are all the same, within about 1/2 inch or so. The needles will bounce a bit, so take an average reading. If your idle speed changes while adjusting the carbs, adjust it to keep it at a normal 1000-1100 rpm. Also, blip the throttle after making an adjustment to make sure it settles in at the new setting. When you are done you will notice a much smoother idle and better response just off-idle.

You may have noticed that this is going to take more than just a few minutes. Find a way to place your gas tank near the bike at about the same height, and run a longer hose to the bike. Also, have a good fan blowing on the engine to keep it from getting too hot.

Also note that carb synching is the LAST thing you do to tune the bike. Start with valve adjustment, then check ignition components and timing, verify air filter condition, THEN do the carbs. After you have them synched, go back to the mixture screws (remember them?) and adjust each of them slowly to get the fastest idle speed. Might have to readjust the idle speed screw, too, if it increases much. When you get them to where you can't improve them any more, stop the engine, screw each one in slowly, counting the turns as you go. Record this figure for each screw, then put it back where it was. Now you have a reference for next time.

Sounds like a lot, but after doing it a couple of times, I have found that it takes longer to round up all the test equipment and hook it up to the bike than it does to adjust the carbs. And, once it's done, you should not have to do it again unless you take the carbs apart.


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[stp1stp]

Hey great info. Sorry to be such a noob. I will check the ignition components tomorrow.

[Road_Clam]

Got my bike running finally after a month of rebulding. Before I started the bike was backfiring. Thinking when I change the gaskets (which were all leaking), adjusting the valves, and completly cleaning the carbs this will take care of it. It did not. I havent rode the bike on the road yet and still have to synch the carbs but I notice the low tone poping out of the exhaust after bike warmed up for a couple of minutes. Just wondering what everybody thinks I need to do to fix this.
Thanks

Aside from the backfiring, how is the idle quality?

[stp1stp]

As far as I can tell very good. Bike starts right up. RPMs stay steady. Sounds great.

[BRycraft]

Petcock

There should be no gas in the vent line, that line should be dry otherwise you can be overloading #2 carb with gas creating a rich mixture. I had fuel leaking from my left exhaust pipe by the side stand while it idled, the fuel was running in on the vent port, it fouled the plug and all that fuel was running in to the exhaust pipe.
In my opinionI would recommend replacing the petcock with a new non vacuum operated one. For about $16 for a petcock from Z1 and $12 for an adapter plate you can have a nice operation. All you have to do is measure the bolt holes on the tank that the petcock mounts in, should be like 44mm, in or around that size. The adapter plate comes with adhesive that you put on a gasket and it secures itself to the tank and petcock to prevent leaks, then the new petcock threads in the adapter plate with 3/8 pipe threads. You have to plug the vent on #2 carb as the petcock has "RUN" "OFF" "RES" so you have to remember to put it to off position after riding. Its an easy project and having to turn it to off each time is not a problem. I made a system so I wouldn't forget, petcock off, run switch off, key off, and reverse to start. Its well worth the money especially if you have a faulty petcock already.

Bill