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Author Topic: Some carb work and a road test  (Read 1086 times)
Spurlock
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« on: March 06, 2018, 10:28:02 PM »

My bike had a slight seepage from a couple of float bowls and also had a wandering idle, so after receiving carb seals from litetek.co and float needles & seats from nrp-carbs.com I gave the carbs a thorough cleaning and seal replacement. The carbs appeared to be very clean inside, but there were cracked rubber caps on four vacuum ports which originally connected to the emissions system (the bike is a California model which had the emissions parts removed when I got it). So carb air leaks seemed to be the main problem, besides seeping float bowl seals. After some time in the ultrasonic cleaner followed by compressed air blasting and proper capping of the vacuum ports the carbs were ready to go back on.



Looking down the intake ports I was amazed at the lack of carbon on the intake valves at 13K+ miles -



After syncing the carbs and warming up the bike the idle was much smoother. It still wanders very slightly up and down but much less than before. I expect fine tuning the pilot screws will settle it down. But of course after all the work I had to do a thorough road test! Luckily today's weather was perfect for a 190 mile loop of favorite twisty roads. Just a few pics I took along the way....
























-Bill
« Last Edit: March 07, 2018, 12:14:26 AM by Spurlock » Logged

1975 Honda CB125S2, 1989 Honda NX250, 1989 Honda GB500, 1989 Honda CB-1
a_morti
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« Reply #1 on: March 07, 2018, 02:03:10 AM »

Time warp bike mate. Enjoy it!
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« Reply #2 on: March 07, 2018, 08:22:25 AM »

That really is a lovely bike you have there. The countryside & weather looks fantastic too
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See Bee-Won
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« Reply #3 on: March 12, 2018, 04:20:55 PM »

You're a man who isn't afraid to tackle unpleasant jobs, I'll give you that.  I used gas proof o-rings instead of the (very expensive at the time) preformed ones for the float bowls when mine leaked. Just give them a light coat of silicone grease and push them into the groove as you work them around the corners.  Still holding years later.

The slight wandering idle may be do to typical emission leaness. I found 3 turns out to be the sweet spot for a nice steady beat and off idle throttle response but with factory exhaust you may find 2-1/2 to 2-3/4 to be better.  If you didn't grind off the stop tabs from your float bowl covers while you had them off you're limited on how far you can turn the idle screws in or out.
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Spurlock
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« Reply #4 on: March 12, 2018, 07:44:18 PM »

....The slight wandering idle may be do to typical emission lean-ness. I found 3 turns out to be the sweet spot for a nice steady beat and off idle throttle response but with factory exhaust you may find 2-1/2 to 2-3/4 to be better.  If you didn't grind off the stop tabs from your float bowl covers while you had them off you're limited on how far you can turn the idle screws in or out.

Thanks, yes the idle is much more steady after tweaking the pilot screws. Not perfect, but much improved. And defeating the pilot screw limiters is standard practice any time I work on carbs so equipped.  Wink

To adjust the screws I like the "idle drop method" described in Honda's Common Service Manual, at least as a starting point. For those not familiar, below are screen shots of the manual describing the procedure. First off, it requires a tachometer sensitive enough to detect a 50 rpm difference. I use a cheapo hour/tachometer meter from ebay. This one reads in 10 rpm increments so is plenty sensitive enough. Since the ignition coils are shared by two cylinders, every plug fires once per revolution so step 1 is to set the meter to one spark per revolution-






In the case of the CB-1 the "initial opening" of the pilot screws referred to above is 2-1/2 turns out (2-1/4 for CA models). The "final opening" is one turn out, which means after step 8, turn the screws 1 turn further CCW. Of course it might be that some other final adjustment amount might give a better transition from idle to off idle, but what I like about this system is that it customizes the adjustment for each carb which helps to account for minor differences between carbs. And in fact the final positions of all my screws ended up being slightly different. If I had set them all to the same number of turns the mixtures would not have been as consistent for each cylinder. Before doing this procedure my idle wandered over a range of about 70-80 rpm and now it wanders just slightly at 10-20 rpm. 

-Bill



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1975 Honda CB125S2, 1989 Honda NX250, 1989 Honda GB500, 1989 Honda CB-1
See Bee-Won
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« Reply #5 on: March 13, 2018, 01:31:10 PM »

Yes. With a 4cyl bike like this you have to find the sweet spot on each cyl.  Gotta think of it as 4 individual 100cc engines sharing a crank. The carbs have to be tuned individually and then synchronized with each other using a mercury meter so they work in unison to do an equal share of the work. Can lead to a lot of back and forth until you get it right.

 In a perfect world we would each have a spare set of headers with O2 sensors on each pipe and a synchronizer hose on each carb so we could tune for the perfect fuel/air ratio throughout the rev range but as it is we have do it with a little old fashioned science and back yard mechanical skills.

 Some folks like to use an infrared thermometer on the headers and fiddle with the carbs until they get the temps to show  + or - each other within a few degrees. As the bike warms up do these differences change? Back to the drawing board.  Angry

When you blip the throttle does your tach needle slowly drop back to the set idle?
 You're too lean.  Does it drop below the set idle before coming back up? You're too rich. Some guys like to "read" the color and amount of carbon on the plugs.  There's a lot of little tricks like that. Most will have you running in circles if you're a perfectionist. My next bike WILL have EFI.  Cheesy
« Last Edit: March 13, 2018, 01:34:08 PM by See Bee-Won » Logged
Efreeman55
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« Reply #6 on: March 14, 2018, 11:51:26 PM »

Now that you've mastered the CB-1, you need to get a CBX so you can synch all 6 carbs!  Yes, I have one of those...

Eric
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Spurlock
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« Reply #7 on: March 15, 2018, 10:57:54 AM »

Hah! I had the pleasure back in the late '70's while wrenching at a Honda shop. That was enough....

-Bill
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1975 Honda CB125S2, 1989 Honda NX250, 1989 Honda GB500, 1989 Honda CB-1
Dash
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« Reply #8 on: March 17, 2018, 07:08:36 AM »

Now that you've mastered the CB-1, you need to get a CBX so you can synch all 6 carbs!  Yes, I have one of those...

Eric

Was that the one where they bolted a turbo on, for good measure? Tongue
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Efreeman55
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« Reply #9 on: March 17, 2018, 02:44:48 PM »

No, that was the CX 500/650 that had it's two cylinders arranged like a Moto Guzzi.  My 1979 CBX is an inline six cylinder, 1047cc, twin overhead cams, 24 valves and six carbs!  Absolutely smooth as glass and plenty fast as well.

Eric
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