CB750F1 Carb. Balancing ....

[theriddler]

Hi all. Last year I got a carbtune manometer setup (with metal floats rather than mercury...). At that stage I had 2 goes at using a carbtune setup  ... initialy got it running pretty good (with a friends mercury setup) but felt that one carb was a bit out ... so had another go with my own metal float setup (shouldn't have bothered). Managed to get the balance levels better but in doing so the idle speed is now higher than it should be...
My questions ... is there an order you should balance them (like ... 1&4, 2&3 ...)? Does it matter which carb is the one you set the others to? I am aware of the need to balance once all the other bits are checked ... oil, air filter, plugs etc. I will have to reset the throttle linkage stops anyway as they are now quite a bit away from standard... Last question ... is the metal float  carbtune unit  as good as the mercury unit(the reason I ask is that the first time I balnced carbs I used a friends mercury unit & 2nd time I used a metal float setup that I had accquired...)?

[K2-K6]

Usually it's carb number 2 that's fixed and you "never" adjust this carb as it achieves nothing. Just balance all others to that fixed carb (or whichever one is named as such) to get them all the same.

But adjusting that fixed carb will do as you describe, fast idle, if you get it wrong.

To correct it now you need to re-establish the relationship of the fixed carb as an offset from the throttle cable and lift mechanism..  It's not difficult but you just need to be aware of what's needed. 

If you just for a moment consider just carb 2,  then raise the tickover (engine not running) by using the main single adjust screw,  then go back to no2 and lower the carb slide with its own adjuster. You should be able to lower that carb slide to close off effectively (and so set reset the tickover).  Now bring each of the other carbs down to similar level for approx match,  then resync them with gauges by only setting 1,3 and 4.

This should correct the tickover by putting all of the carbs back into normal adjustment range for the tickover screw.

[theriddler]

Thanks for confirming that. I think I started off not touching no. 3 but then forgot and adjusted them all.

[Bryanj]

If they have adjusters you can adjust them all, on the ones with a "fixed" card there is no adjuster on that one to move so you have to treat it as a base carb. It is usually the PD carbs where this happens, 750 F2 and 550 K3.

[Liam]

My f1 has Keihin round top carbs.  Number two carb isn't fixed. 

I adjusted mine using a carbtune balance and it was mint.  As you adjust carbs and the revs increase (or decrease) just adjust using the single screw on right side of lifting mechanism to bring it back into line.

[CR750]

How often guys do you reckon carbs need balancing ? especially PD carbs , or are they best left alone unless you know or suspect you have an out-of-balance

[Bryanj]

If you keep the standard service items spot you should rarely need to balance carbs HOWEVER if you dont run it for 4 weeks you may have to strip them and clean all the jets and pathways.

[philward]

In my experience - when I used to do decent mileage on my Honda Fours in the 70's - was that they didn't go out much and the amount they did change, when adjusted, made little difference in everyday running

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[CR750]

If you keep the standard service items spot you should rarely need to balance carbs HOWEVER if you dont run it for 4 weeks you may have to strip them and clean all the jets and pathways.

I agree in part, you certainly have to run the engine quite often during winter lay up.
I have had my CB750 for almost 5 years, it is my 1st Honda, it was completely rebuilt at the time. I have had issues with the carbs initially in my learning curve with this bike, I learned that you must keep a good quality petrol in the bike [no ethanol if possible]+ a Stabiliser, also keep the tank at least 3/4 full at all times specially during a lay-up & then during the winter period start the bike at least once a month & allow the engine to really warm up,i've never had to strip the carbs as yet I got away with occasional carb cleaner...also having the correct spark plugs is important, I now run with the NGK D7EA & all is good so far, previously tried many others including iridium

[Arch stanton]

I'm sorry but I could not disagree more.
The most wear on a engine occurs in the first 5 mins of running. Also filling the exhaust with corrosive condensation.
Plus contaminating the oil.
I have 14 bikes 2&4 strokes & never have a carb problem.
I only use BP ultimate petrol. And prior to laying up for winter.
All I do is. Switch of the fuel. & open the carb drains screws.
& leave them open. Never a problem.
Sean.

[Bryanj]

Arch, the problem is particular to the lean running PD carbs in which the pathways and jets block for a pastime. When working full time on Hondas it was first noticed on 900 twin cam and the wet dreams which if left s/hand on the showroom floor for more than 4 weeks were a real swine to start and get running properly.