Topic: 75 750 wont idle

[01vic99]

ok here goes. i got a 750 ive been messing with for a few months. so far ive cleaned the carbs and put new gaskets in. while i was in there i noticed that the bike had 150 mains installed (bike had cheapy pods and 4-2 turn outs) this seemed way too big and a quick plug check confirmed it. some on here recomended trying 120s so i did. got the bike to run by cleaning the plugs. it runs but wouldnt  idle on its own and revs great until 4000 rpm and then falls on its face. so ime thinking tham means its now too lean. so my next step was to go to a stock box with a k&n. i was thinking this set up was going to get me close. wrong. now i get alot of poping and banging and when it does run i have to hold the throttle 1/4 open with half choke just to keep it running. im going to try new plugs tommorrow but im wondering where do i go from here. i know they need to be synced and i have a guage coming but i wont be able to sync them until i can atleast keep it idleing. any suggestions would be greatly appreciated, im about to drop this project and start on the old 2 stroker

[01vic99]

oh one other thing i forgot. where do the two vacuum line that come off of the center carbs go?

[KB02]

So you've got the 120's in there now, right? Just to be on the safe side, drop the bowls and double check that your idle jets are clean. I've been working on a bike where I am on the fifth cleaning of the carbs right now (grumble, grumble, Grrrr....).

As for your plugs, my first thought was to trash to old ones and get new. NGK D8ES I think is the plug you need (Someone will correct me if I am wrong). Do, that and make sure your boots are tight and not leaking allot of air and report back.

Oh, yeah. And performa search on the boards here for "Bench Syncing" Carbs. Might be usefull for you to make sure you're in the right neighborhood.

Hope this helps.

[TwoTired]

oh one other thing i forgot. where do the two vacuum line that come off of the center carbs go?

Those are fuel tank vents.  I'm not certain about your model, but on the Cb550s they have hoses that route back and down between engine case and swing arm pivot.  Anyway they should route to the same place the carb cowl overflow hoses go.

As far as your jetting goes, you have four things in each carbs to tune, Idle air bleed screws, Idle or slow jets, Throttle valve needle height setting, and main jet.

These all control mixture at specific throttle settings.  See chart below.  Without an exhaust gas analyzer, reading the spark plug after operating under load at a specific throttle setting is a way to tell what combustion conditions you have and which way the settings should be changed.

Fouling plugs is usually caused by running way too rich.  However, it is possible to set up carbs to run rich near idle and lean on the throttle valves and rich at WOT.

Other things that can effect carb settings are exhaust and intake restriction and their magnitude mapped over the RPM range or air velocity range.

Cheers,

[01vic99]

ok put the new plugs in yesterday. i can only get her to fire with ether and it still wont idle but as long as you keep reving it it runs like a champ. it may be a touch lean but i can deal with this later. im thinking the next step is to pull the slow jets. to me it seems logical that the hard starting and unwillingness to idle would be caused by the slow jets. thanks for the help and as always any other advice is appreciated.