hanging idle

[a.graham52]

hey all. iv got an 86 fj12. i should first start off by saying that my first ride, i had missfiring just off idle. it was pretty substantial too. that was the ONLY problem i had at the time. since then, iv removed the the carbs. cleaned the air filter, cleaned the jets, uncapped the idle mix screws, changed plugs and replaced one spark plug wire boot. honestly the bike run pretty well all around but idle i cant figure out. idles at about 1200 rpms but if you give it any throttle the idle will hang up at 2500 solid like. iv verfiied the throttle blades are returning closed. i did noticed that if i hold the brakes and let out on the clutch to force it to idle speed, it will in fact idle 1100 rpms and stay there until any more throttle is applied. it just seems odd that if you can force it down to idle, it will idle then. any ideas?

[IceFJ]

I'd guess that you have vacuum leak somewhere between the carbs and engine head....

[movenon]

I would remove the tank and take a good look at the throttle cables. Make sure the carbs are returning to the stops like they should every time.
If that is good then I would check for a vacuum leak but if it was a vacuum leak it would probably be high all the time. Sounds more like cable binding or adjustment to me. Let us know what you find out. 

While the tank is off it wouldn't hurt to check the carb boots are on properly and tight. Check all the vacuum hoses just because.  Stuff happens, I have been around carbs all my life and recently I had a tuning problem, turned out that I got distracted while putting my carbs back on and didn't tighten the intake boots down. 

More remote but there are some o rings that go between the intake manifolds and the cylinder head, they can leak. But my money is on the cables. what money I have anyhow......
George

[a.graham52]

Ended up being the vaccume advanced. Removed the hose and capped it and it's all good now. Doesn't run perfect. Almost perfect accept right off idle. If u try a steady cruise, more so with a bit of a down hill grade, u can tell once cylinder see isn't doing all of its fair share of work. I though I synced them fairly well but I probably aut to recheck before I start to worry. Iv put my first 40 miles on it. Made two trips to the dealer for an inspection but all in all... It's rideable until I get my bandit done. Then I'll tear back into the fj carbs and check float levels and etc.

[movenon]

IMO leave the vacuum advance plugged.  It is only an emission thing.  Advances when you let off the throttle.  Sounds like you are close.  Do some "blip" testing.  If your fuel air screws are close you should be able to run a 3-4000 rpm steady state in second gear with no surging or running rough. You should be round 1/8 -less than 1/4 throttle.
George 

[a.graham52]

Good info. My mixture screws are all at 2.5 turns out at this point. I have noticed it backfires when coasting down hill in gear. It's got F1 slip ons too.

[Harvy]

Good info. My mixture screws are all at 2.5 turns out at this point. I have noticed it backfires when coasting down hill in gear. It's got F1 slip ons too.

Backfires as in out through the carbs, or as in pops through the exhaust?

Popping out the exhaust is not a problem - just unburnt fuel igniting it the pipe, but backfiring out the carbys is something to be looked at as it is symptomatic of  a valve timing issue.

Harvy

[a.graham52]

Good info. My mixture screws are all at 2.5 turns out at this point. I have noticed it backfires when coasting down hill in gear. It's got F1 slip ons too.

Backfires as in out through the carbs, or as in pops through the exhaust?

Popping out the exhaust is not a problem - just unburnt fuel igniting it the pipe, but backfiring out the carbys is something to be looked at as it is symptomatic of  a valve timing issue.

Harvy

to be perfectly honest it does both. but backfire though the carbs is very seldom. id did check the service history from the dealer today and its been 20k miles since the last valve adjustment. NEEDS to be done.

in my experience, backfiring through the exhaust under decceleration is also a symptom of too lean of idle. i had a carbed 4.3 in my 85 pickup that i forgot to adjust the idle after putting on a new edelbrock. backfired horribly until i properly set the idle. the bandit was the same way. tomorow im going to richen the idle just a tad and see how things act.

[oldktmdude]

   Back firing through the exhaust is also caused by one or more poorly sealed header gaskets. Air is drawn into the system and back fires when mixed with unburnt fuel. It may be time to tighten header nuts or replace gaskets.   Pete.

[a.graham52]

   Back firing through the exhaust is also caused by one or more poorly sealed header gaskets. Air is drawn into the system and back fires when mixed with unburnt fuel. It may be time to tighten header nuts or replace gaskets.   Pete.

Those were done last year when a new collector was installed. I'm not getting any exhaust tick when the engines running either

[a.graham52]

I turned all te idle screws out to 3 turns and the problems better. What's a good window for the idle screw adjustment?

[Arnie]

2-3.5 is a reasonable range.  If you need more than that you should probably get larger pilot jets.

[a.graham52]

All at 3.5 turns now. Took it for a good 40 mile ride. First thing I did was put some gas in it. After the gas station the engine got quieter, smoother and the bike was all around better to ride. Still a bit raspy just off idle but for the time being I'll deal. Amazing how much of a difference fuel alone can make.

[FJ Flyer]

When was the last time you sync'd the carbs?

[a.graham52]

I didn't a quickly sync when I first got it back together. I really should recheck.