Question about 1988 Yamaha FZR 1000

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Back firing at idle

My project bike is nearly ready to hit the road but I have not been able resolve the back firing problem. It is bad at idle, I found that if I turn the idle up to 1500 or so it clears up. Thinking I could live with this so I take the bike for a test ride. Well it's impossible to get the bike moving without it back firing. It runs perfect once it gets past that. I have maticulously cleaned the carbs. I do not know the history and set up of the carbs. The jets are unmarked (I assume Dynojet) but I do not know to what stage. The carbs were in pretty bad shape when I took them apart, 2 of the slides were stuck, there was corrision on all the needles and junk in the bowls. The popping or back firing comes from one cyl. in particular. Could the problem too lean in that one cyl.? How could I correct this. Idle mixture???? Carbs out of sync? Don;t know how to sync. them. Exhaust gaskets are good.

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A backfire comes from unburned fuel getting into the exhaust manifold and igniting.The cause is normally poor ignition. So it can be either your spark plug gap is too big, dirty, faulty, etc or your coils are problematic, but as 1 coil is allocated to 2 pots I would doubt this. The other is that the fuel mix of the cylinder is way too high and it can not burn all the fuel. balancing the carbs will help fix the problem. Your idle RPM should be around 12-1400.

To balance your carbs you need 4 vacuum guages. You will plug these into the carbs (1 per carb) by removing the screw in the manifold just above where each carb attaches to the engine. Set your idle screw to get the engine running at about 2-2500 rpm and adjust the butterfly position till you get all the vacuum gauges at the same reading (IE max that you can get for the pot).

Hope this helps

Posted on Sep 17, 2009


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