Re: high-speed missing, high idle once warm--1980 cb750F

Posted by impaxg on
URL: http://vintage-and-classic-hondas.81.s1.nabble.com/high-speed-missing-high-idle-once-warm-1980-cb750F-tp460585p460586.html

SOUNDS LIKE A LOT OF "ADJUSTMENTS" HAVE BEEN MADE. I ALWAYS START BY CHECKING THE TIMING AT 5000 RPM AND AFTER THE TIMING IS CORRECT SYNC THE CARBS WITH VACUUM GAUGES. MAKE SURE ALL THE NEEDLES ARE IN THE STOCK POSITION, THEN ADJUST BY USING VACUUM GAUGES TO SYNCH ALL 4 CARBS. YOU DO NOT SAY WHETHER THE CARBS ARE FROM A SOHC OR A DOHC. FROM THE SOUND OF THINGS , IT SOUNDS LIKE YOU HAVE DOHC CV CARBS. IF THEY ARE NOT SYNCRONIZED PROPERLY THE IDLE WILL ALWAYS GAIN RPM WHEN WARM BECAUSE A HIGH VACUUM IN THE CARB DOME(S) WILL OCCUR WHEN THE OUT OF ADJUSTMENT CARB(S) IS PULLED TO A HIGHER VACUUM AND WILL HOLD THE SLIDE OPEN ON THAT CYLINDER(S) CARB CAUSING HIGH IDLE FROM MAIN JET FUEL BLEED. IT IS IMPORTANT THAT THE TIMING BE CHECKED AND ADJUSTED IF NECESSARY BEFORE SETTING CARBS WITH VACUUM GAUGES. THE STOCK MAIN JET ON A 1980 CB 750F IS #100 AN IS #102 FOR THE K MODEL. IF THERE IS AN AIR FILTRATION OR EXHAUST CHANGE OR MODIFICATION, A REJETTING MAY BE NEEDED FOR OPTIMUM HIGH END PERFORMANCE. HOPE THIS HELPS SOLVE YOUR PROBLEM.