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

[Waylon-2]

Well, the bike has carbs from a cb750k. I've cleaned them out and synched "by ear." Runs like a champ until about 7000 rpm, then starts missing.

Also, once the bike heats up it wants to idle for a while at 5000. I can turn down the idle screw but it'll do it again as soon as I pull in the clutch approaching a stop. When I pulled the carbs I noticed the throttle plates on 1 and 2 were covering the secondary (or primary?) jet holes and the plates on 2 and 4 were not. So I adjusted them so that all 4 were set behind the second jet. Could that be the reason for my high idle?

But what I really wanna know is where is my high-end power?

many thanks in advance!

[CB750Winger]

  I would'nt try to sync by ear. Sure, you can get them fairly close but a little off can cause what you are seeing. Have you changed main jets, air box, exhaust or are they the same? Try to get access to some gages and sync the carbs should do the trick.

[cbRider]

Carb syncronisation does not affect high-speed performance.
It is important at low to medium speed and critical at idle.
It sounds like a lean condition.
Follow up on CB750Winger's suggestions.

Karl

[CB750Winger]

  Like Karl says, it sounds like it's lean at high speed. If you put carbs from a "K" on your "F", you may need a bigger main jet, not sure. You could also have something partially plugging a jet. You need to get a good sync to start tho.     Paul

[cbRider]

waylon, you haven't answered any Paul's questions.
The answers would certainly help getting a handle on your problems.

Karl

[impaxg]

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.

[Waylon-2]

hey guys--thanks for the input. I've been "off-line" for a few days, but still on the road!

But the bike is stock--new stock air filter and all. I'm pretty sure the carbs are off a DOHC, as the model number on the side is the same that Clymer says came on the 1980 K. The PO told me the carbs came from another bike. The pipes are 4 into 2 Sankei 2048. Pretty sure those are stock. The primary jets are 102, the secondary jets are stamped 68. I haven't checked the timing, but it's now on my list. Thought the timing could wait until the winter, when I plan to tear her down and put her back together all pretty. I'll check that this week.

In the meantime I set the butterfly valves on the throttle so they cover the second set of air jets and she idles down proper. Much less to be embarrased about at intersections.

Also, I'm not sure where to plug the lines in for a carb synch. I know you have to have some nipples plugged somewhere into the carb, but I don't know where they plug in, or even what they're called. I plan on doin the ATF and snapple bottle rig. If anybody knows what those nipples are called or where I can find a plan for the homemade synch tool, that'd help me along nicely.

Many thanks and happy trails!

[cbRider]

On the cylinder head, just in front of the rubber boot to the carburater on each cylinder is a philips head screw. Take them out and replace with nipples, get four vacuum gauges (very reasonable at Harbour Freight) hock one to each nipple with some plastic or rubber tubing. I found the nipples on ebay. They come in 5mm or 6mm so you nee to take one screw out to measure.  Good luck.

Karl