85 FJ1100 flat spots at 5k an 6k revs when accelerating hard

[MarkFJR]

Recently purchase an 85 FJ1100 with 32k miles. Replaced the battery and sorted out the charging, had the carbs rebuit and all new carb kits installed. The bike runs great if you don't thrash it but if you accelerate hard the bike has 2 flat spots around 5 and 6k revs. Is it bad coils, ignition or a fuel issue? The bike feels like it goes dead for 1/2 a second and then picks up again.

Thanks for any help.
Mark

[SlowOldGuy]

What do the plugs look like?  Are you hammering it WOT when this happens?  If so, is it only a problem when you hammer it WOT?

If it only happend on hard acceleration, stumbles badly, then picks back up and acelerates hard after the stumble, and the plugs are black, then my guess is the fuel level is too high in the float bowls.

Although it wouldn't hurt to check/clean the 2 coil power connectors under the front of the fuel tank.

DavidR.

[MarkFJR]

Only happens at WOT.

[RichBaker]

Stock exhaust or aftermarket? 4-1s cause a flat spot in that area. It is a well-known effect.....just one of with the characteristics of that type exhaust system.

[andyb]

What jetting?  A grossly too big main jet will cause almost exactly that behaviour.

[MarkFJR]

It has a 4 into 1 exhaust. No idea what jets are in. I have to get some other work done on the bike (rear Givi rack mount is cracked), I will ask the shop to check the jets.
Thanks
Mark

[racerman_27410]

my 4/1 supertrapp had a terrible flat spot at 5 grand until i swapped out the air jets...IIRC they were 155's


Kookaloo!

Frank