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Backfires on de -acceleration??

  • Thread starter Thread starter KapitalKat
  • Start date Start date
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KapitalKat

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I have a 1984/85 GSX750ES KATANA that has a muffled like small back fire when backing off the throttle as in de-accelerating when stopping/slowing . As the bike has a fixed ignition pick up backing plate with no adjustment am I to assume it is carb problem and not ignition. Not too sure on this as would have thought that the ignition was advanced or retarded to cause this. It's not a big explosion just a muffled sort of exhaust note. Any suggestions would be greatly appreciated.
Thanks, Rob.
 
The usual cause for backfiring is an intake or exhaust leak, most likely an intake leak.
 
I had this EXACT thing happening to my GS1100E. It was so annoying. There is a screw in the center of each carb right near the intake from the air filter. It is basically a stop for part 34 in this picture:
http://images.powersportsnetwork.com/fiche/images/Suzuki/1982/Motorcycles/2156_16.gif

It probably has a small aluminum cap with a tiny whole. If you get a drywall screw you should be able to gently start it into the hole, then pull the cap off. I would take some white out or other marking device and mark where all the screws are currently in case you need to set it back. Unscrew them a little at a time (all the same number of turns). Count the turns and write them down so you know how far you went.

I think it took like 6 full turns before it would stop for me. Basically all it's doing is letting in more air. Not sure how this effects gas mileage, but just keep an eye on it.

Also, you may not be able to get the caps back in. I wouldn't worry too much about it since the factory puts them there because of emission control laws. If you really want to put them back in and stay you may need to tap them with a hammer lengthwise to alter their shape slightly. I put mine back in and one vibrated out somewhere on one of my rides. I have no Idea where it is :)
 
Thanks for the replies, I would have been looking in the wrong place. Will try those screws on the carbs and see if that fixes the prob. Am running a locally made 4/1 exhaust" FORMULA" brand on it, which is nice but a bit loud for my taste.:)
 
If you do remove the caps, just leave them out. It is one of the first steps of carb tuning. And yes, it was for emissions. They are called the fuel mixture screws and are used for adjusting idle and low rpm cruise. In the even older days, they were in the open for everyone to see.
 
Mine popped / backfired on decel until I synched the carbs, now it seems to growl like a beast. Carb synch may be another thing to consider.
 
KapitalKat said:
Thanks for the replies, I would have been looking in the wrong place. Will try those screws on the carbs and see if that fixes the prob. Am running a locally made 4/1 exhaust" FORMULA" brand on it, which is nice but a bit loud for my taste.:)
Click to expand...

When the 4-1 was fitted, did you fit new exhaust gaskets at each header? If not you probably have a leak there which will cause the backfiring on decel. Also check for any cracks on the welds around the collector area.
 
An interesting issue with Suzuki fours, like a lot of others, is that the ignition is timed off the crank where most 4 cycle engines are timed off the Camshaft which turns at 1/2 engine speed. This means that they will also be firing at the end of the exhaust stroke as well. Or in other words they fire at a few degrees before TDC on every stroke. Not a problem because the exhaust gas shouldn't burn though you are doubling the use of your spark plug, still not a big deal. However if you are running rich, you will have unburnt hydrocarbons (fancy word for fuel) present in the exhaust and they will be ignited, especially during overrun (throttle closed high RPM). Whilst we generally point the finger at lean mixture or gas leaks for the 'popping' in the pipe, it can also be a symptom of a rich mixture...! Confusing, it is and we need to look at other symptoms such as plug condition to figure which way to go.
 
On mine it was a lean condition that was causing it. I went up on the pilot jets and it cured my problem. I run flatslide carbs.
 
Many thanks fella's for the input. Seems to have corrected the problem. Wasn't too sure which way to go on this one. Keep the oil hot!!:)
 
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