R80/100 carb frustrations... (cont.d)

[Brickboy]

Hi again all,

Bloomin' old Boxers...

I've persisted with my attempt to achieve perfectly balanced 'by the book' fuelling (Snowbum, Haynes, other forums and my own 30yrs of multiple Boxer ownership - you name it: I've read it!).

After much faffing about my R80 with its R100 cylinders and twin plugs and, presumably, appropriately jetted carbs (this was done in 1990!) runs beautifully. But, having done the various adjustments and tests I have maintained a disparity between left and right. Before starting I have replaced the air filter and done the valve clearances.

I have both sides nicely balanced on the Carbtune at tickover and at 1500rpm, as per Snowbum and my own understanding of the best way to make it run smoothly. The tickover was also set using Snowbum's suggestion of shorting out each sides' plugs and listening to get a match on each side - a fiddly process with twin plugged engines. The twin plugs are run as Snowbum suggests - top plugs each side from one coil, bottom plugs from the other coil.

The problem lies with how I am achieving this nice balance.

As I understand it, as I turn in the mixture screw to its stop I weaken the mix and eventually cut off fuel supply altogether at tickover. On the rh side this is exactly what happens - the right cylinder shuts down and the left carries on thumping away.

But, when I screw in the left mixture screw the revs rise and stay up when the mixture screw hits home. I've dismantled the carb, blown through the idle circuit air jet, idle jet and mixture screw orifice, cleaned it with carb cleaner, checked it is free and fuel can flow through to the tiny ports by the throttle butterfly and it does flow! I've also replaced the o rings on the idle jet and mixture screw. I've sprayed WD40 around the carb rubbers and choke - no sign of air ingress.

So, I have a bike that runs beautifully but not the way it is supposed to. I could just ignore it quite happily and enjoy riding but it's bugging me and I'd like to solve the problem. Your suggestions are very welcome because I'm stumped!

[Kurt in S.A.]

"Tickover" at 1500 rpm seems high...should be closer to 1000/1100 IMO. Usually 1500 is near the point of checking off-idle values. Is there slack in the throttle cables at your tickover point?

Kurt

[gspd]

As I understand it, as I turn in the mixture screw to its stop I weaken the mix and eventually cut off fuel supply altogether at tickover. On the rh side this is exactly what happens - the right cylinder shuts down and the left carries on thumping away.
But, when I screw in the left mixture screw the revs rise and stay up when the mixture screw hits home.

You seem to understand the basic principle involved. There's definitely (99%) something wrong with your left carb.
One easy test is to swap the idle jet and mixture screw over to the other side and see if the problem changes sides.
Sometimes the tip of the mixture screw is worn or partly missing, and some idle jets just can't be properly cleared and need to be replaced. Swapping the complete carbs side to side and seeing if the problem follows would 100% confirm that this is definitely a carb problem, not ignition or compression related issue, but I doubt that.

It is VERY common for the idle air passage (the tiny hole in the carb mouth) to be partially blocked. To be sure it's clear, you have to remove the idle mix screw and idle jet, block those holes with two fingers, hold a thin plastic spray nozzle tube firmly up against the tiny intake hole, and with your third hand, spray brake cleaner in and confirm it's coming out (like a mini water pistol) from the both bypass holes below the butterfly. (A third hole goes to the vacuum pick-up screw but that one won't affect running and will only come into play if you hook up vac gauges.)

Other things to check include the condition and positioning of the actual butterfly, I've seen quite a few that were installed with the taper the wrong way during a previous rebuild. With the throttle stop screw backed out, there should be virtually no light showing around the butterfly when you hold the carb up to the light. The pics below are of worn butterflies (they should be perfectly round) that hindered precise idle mixture adjustment and caused some popping on decel even though the bike seemed to generally run OK.

[Brickboy]

Ah, I've failed to explain. Tickover 1000rpm, balanced at 1500 to ensure throttles open evenly ie cable free length is equal

[gspd]

"Tickover" at 1500 rpm seems high...should be closer to 1000/1100 IMO. Usually 1500 is near the point of checking off-idle values. Is there slack in the throttle cables at your tickover point?

Kurt

Brickboy stated "at tickover and at 1500rpm.
The actual point to check sync is at tickover AND just when the throttles lift off their stops. If the cables are in perfect condition the transition from resting on the throttle stops and just being lifted off the stops by the cables should be even and seamless. Anything above that will remain linear and equal. Shorting one cylinder and then the other to get the same speed (at anything above idle) is an exercise in futility, a total waste of time, and is usually only attempted those who do not fully understand exactly what's going on.

[gspd]

Brickboy -
Quick check.
Pull one throttle cable a bit, and then the other.
Do your carbs react the same?
I suspect you'll have a bog when you pull the left one and a smooth rpm rise when you pull the right one.
Am I correct?

[Brickboy]

Thanks gspd. Shorting was only for idle. Tbh, having done that and then tried it with the Carbtune I felt the Carbtune was just as good and a whole lot less faffing about. The pick up is now very pleasant on the throttle but I'll try the individual cables as you suggest and report back and also the excellent idea of swapping over the jets

[Brickboy]

Having pondered all gspd's advice I stripped both carbs again. No problem with the idle circuits on both sides - spray through from the idle air passage at the carb inlet comes out of both tiny bypass passages below the butterfly on both carbs. I enjoyed the need for various support devices to make up for only having the regulation number of fingers to stick over idle jet and mixture screw orifices whilst balancing the carb body and getting the spray nozzle in the idle air jet orifice. Limited swearing was permitted on such a special occasion!

Idle mixture screws identical with tips in straight and sound condition although one is brass and one a grey metal colour. Looking into their orifices I could see the grounding point of the tip receptacle is polished showing that both tips have grounded when screwed in. Idle jets checked again and both spotlessly clean and both are size 50. Butterflies are correctly oriented and no sign of wear I could detect.

So, a little further experimentation as the Rh mixture screw is the only one that shuts off a carb when screwed in. I also detected a slight pop and misfire when the throttle is very gently opened at around 1200-1500rpm.

Just for fun I removed both mixture screws with the engine running. It went onto one, the left, until I screwed in the Rh throttle stop a fraction. Then it ran smoothly. Perhaps a clue there.

I replaced the mixture screws, set them at 3/4 out as per the manual and tested the bike. Much better. I wonder if perhaps the suggestion that a fully closed mixture screw will stop a cylinder may be true only if the throttle stop is set to zilch.

Having got a bit bored by now, and not entirely sure I was further ahead, I balanced the idle and pick up using my Carbtune gauges and, happy that it runs and idles smoothly, have left it for today.

So, once again a beautifully running bike but a mystery I don't feel I've quite solved or understood.

[Kurt in S.A.]

Brickboy stated "at tickover and at 1500rpm.

Busted! I missed that...thanks.

Kurt

[gspd]

Idle mixture screws identical with tips in straight and sound condition although one is brass and one a grey metal colour.

hmmmm...verry interesting...
I wonder how that came to be. I doubt your carbs were born that way.
I've done hundreds of these Bing carbs over the past 5 decades and don't recall ever seeing a non-brass mixture screw.
Learn something every day. Are you sure the grey one is not just weirdly tarnished brass?
I'm curious about which one actually works correctly. I guess if switching them sides made no difference we can presume they function the same.

I wonder if perhaps the suggestion that a fully closed mixture screw will stop a cylinder may be true only if the throttle stop is set to zilch.

FACT: A fully closed mixture screw should stop a cylinder that is at proper idle speed. That's its job.
I couldn't sleep at night if mine didn't. Actually, just having 2 non-identical mixture screws would probably keep me awake too.

Did you try gently pulling on just one cable at a time while idling to establish if both carbs are picking up smoothly?