motorcycle engine tuning problem

[Rallispec]

I've got an old sport bike ('86 fz600) -- in the process of rebuilding it, running into engine problem.

the bike only idles when i shove a rag in the air intake to restrict airflow... if i take it out, the bike dies.... It seems to run much better at higher rpms.


any thoughts on where to start looking for problems?

 

[Jeff7181]

By stuffing a rag in it you're cutting airflow... choking it. Sounds like it's running very lean... too lean to idle. Start looking at the fuel system.

 

[woodie1]

Check the pilot jets and see if they are clogged.

 

[The Stigenator]

you are running lean.. adjust air mixture.. check float height while you are at it, clean teh carbs out!

 

[Rallispec]

carbs where just cleaned and the pilot jets are brand new -- i agree that it seems like the engine is running lean, but I can't figure out why. I have had some luck getting it to run slightly better by adjusting the mixture screws - but not enough to get the bike to run without air restriction.


what is the recommended method for checking the float height? I haven't done that yet, and I haven't synced the carbs yet ( i need to get a syncing tool)

 

[osage]

the idle enrichment circuit is most likely plugged. very common with a bike that has set for a few months or years.

the idle enrichment circuit is very small and generally need to be cleaned out with a single strand of wire...very small, very easy to not get it clean the first time. clear the passage with a wire, then blow some carb cleaner thru it. repeat on all 4 carbs.

If the carbs were cleaned by soaking them and they were not disassembled prior to soaking you may have done damage to the shaft seal as well. most carb soak stuff will eat the o-rings that seal the shafts to the body of the carbs.

yes I know it is a PTA to pull the carb on most bikes, but it must be done.


what kind of carbs are on the bike ? ?

 

[Rallispec]

The carbs are Mikuni BS30's. I disassembled them the greatest extent that I could - Removed all the jets and o rings, etc. I did not soak them, but I did use copious amounts of carb cleaner spray in every hole and crevice. I really don't know how much cleaner I could get them short of sending them off somewhere or having them cleaned ultrasonically.


I think I have an air leak on the intake side between the carb and the engine -- the rubber boots that connect the two are fairly worn out. I also have a really bad exhaust leak that I need to fix. Does it make sense that an exhaust leak could add to the lean condition?

I can fix those two leak problems this afternoon -- hopefully that solves the problem, but i'm sensing that I may have to pull the carbs again. (though not sure what i'm looking for)

 

[mooseracing]

Compressed air and cleaner works great for carbs...just be carefull.

Are the rubber boots really worn out? My '79 are orginal, all cracked on the outside, but if you take them off and look at the inside they are perfect. I would use something to help determine they are leaking, like holding a smoking cigarrete near them or maybe some wd-40.

I would more bet something is dirty inside though if it's only an off idle lean condition. Since you have multiples, have you synched them to spec after putting them back on the bike as well?