Carb adjustment - sort of...

[Fraggle]

Hi,

I posted a while ago about the really poor performance of my 1980 r65 (I've got the date right now...)

I noticed that the throttle was very notchy and when I looked the twist grip and pull machanism were worn. I was also not getting full throttle.
In addition, I had about 1/4 turn free play in the throttle twist grip (a personal hate of mine is free play in the throttle - not sure why and understand the need for it but I still don't like riding with it.)

Anyway, replaced all the bits (not the cables) and aligned as I think it should be according to the haynes book of wisdom. Then no freeplay at all and a tickover of 3500 rpm.

So I've slackened the cables off to give a little bit of free play and now the tickover is OK. But when I put it on the carb balancer it's way out at half throttle. If I adjust the cables to balance then the slack in the cable goes and I have the tickover problem back.

OK more slack in the cables then... not possible, adjusters are screwed right in and when I do open the throttle I can see that I'm not getting full throttle.

Any suggestions?

[Bob_Roller]

you synchronize the carbs with the idle and mixture adjustments, not the cables .

To adjust the carbs, the throttle and chke cables need to be slack .

To properly adjust the cabrs, you really need to use a manometer, just doing it by ear, usually won't work very well .

[Fraggle]

I have a manometer.

At tickover the carbs a balanced at half throttle they're not. This I take to mean that the butterflys are not opening the same amount.

How do I adjust that out if not with the cables?

[Barry]

How do I adjust that out if not with the cables?

Yes with the cables.


1. Check there is 1 to 2 mm free play at the throttle cables.

2. balance the carbs for equal vacuum at idle using the throttle stop screws.

3. Once you have good balance at idle open the throttle slightly and check for equal vacuum balance at 1500 - 2000 RPM. This is checking for equal pull on the 2 throttle cables. Once finished ensure both cables have 1 - 2 mm free play with the throttle closed.


On the lack of full throttle there are marks on the twist grip gears to indicate where they should mesh. They are not exactly highly visible but they are there.

[Bob_Roller]

The throttle tube also has an index mark, that lines up with the line on the cam gear .

[montmil]

The full Bing carb synching ritual begins with slack throttle cables and the idle adjust screw just clear of the butterfly arm.

Most all idle issues can be traced to mis-adjusted and/or clogged idle circuits.

The very last item of business in a carb synch is the cable balance at approx 3500 revs.

Confirm the twist grip gears are correctly meshed. Look for a hairline factory mark to synch the teeth. Check that the throttle cable/s at the twist grip are fitted in their proper recess position.

With all tension off from the throttle cables and zero contact of the idle adjust screw and butterfly lever, then you can successfully adjust the idle revs. If you cannot do so, it's time to dis-assemble and clean out the idle jet circuits.

PITA it is but you'll be happy with the eventual results. Go for it.

[Ed Miller]

The cool thing about these bikes (or the carbs, anyway) is you don't have to do it very often.  Like, not for tens of thousands of miles.  I just shake my head over the guys with later model Meriden Triumphs that came with Bings, that want to get rid of them.  

[Fraggle]

Thanks for the advice - it's a cable thing I think.

Anyway, am replacing the bars for higher ones and will need new throttle cables for that so will sort that in the next day or so when I change the bars. So will follow the last posters advice about slack etc.

Next silly question, how does the rubber instrument condom thingy come off - is it just the retaining ring around the ignition switch that holds it on?

[Barry]

Next silly question, how does the rubber instrument condom thingy come off - is it just the retaining ring around the ignition switch that holds it on?  

Yes the retaining ring and also 2 spring clips underneath which are located in metal strips moulded into the "condom thingy" .  Release them with a flat bladed screwdriver. You don't absolutely have to release these clips as they will pull out on their own but the metal strips then get bent in the process.

The cover is likely to be a bit stuck around the instrument bezels.  If you have a fully intact instrument cover take care pulling it off as that's how they get split.

[montmil]

Remove the ignition switch locking ring -the one with the two holes 180 degrees apart.

Two sheet metal screws on either side of the instrument cluster housing. Watch for the "J" nuts that accept the screws... they may disappear into a parallel universe.

[Fraggle]

just finished putting higher bars on the "bitch"

Riding position is greatly improved as is stopping ability - this down to the fluid change (rusty water isn't that great as hydraulic fluid...) and also the hand position on the bars gives a much more comfortable pull at the lever. Same applies to throttle, with less weight on my hands I get better feel and the thing seems to cruise better.

Slow speed control greatly improved with the additional leverage the wider bars provide.

[quixotic]

The very last item of business in a carb synch is the cable balance at approx 3500 revs

Would a synch at 3,500 rpm affect what has already been done at 1,500 rpm?  Or vice-versa?  

[nhmaf]

Well, there can/may be some interaction of course, as all one can do is to adjust the cable lengths +/- for the cable balance.   I try to mitigate the differences, allowing for a little imbalance just off from idle to try to achieve closer balance at 3500 rpm, if necessary.   Others may choose to do differently, but I figure that I spend more time riding the bike at 3500 RPM than I do in the transition period when the engine is just accelerating off from idle from a dead stop.   As long as things aren't too far out of synch at 1500, and they are balanced nicely and idling well with throttle off - I'd call that good enough, IMO

[montmil]

The very last item of business in a carb synch is the cable balance at approx 3500 revs

Would a synch at 3,500 rpm affect what has already been done at 1,500 rpm?  Or vice-versa?  

Read through the pdf. info I posted on balancing throttle cables at 3.500 revs. It is not anywhere as difficult as the initial idle circuit work can sometimes be. What it does do is create less engine vibrations at cruise speeds by insuring both carbs are pulling the same vacuum/manifold pressures.

Well worth the effort and you can actually feel the difference in smoothness at cruise.

And no, the cable balance at high revs will not effect the low rpm balance work. Remember, throttle cables should have just a wee bit o' slack during the mixture adjustments. At 3500 revs, there's no slack in the cables. Pay close attention to the instructions regarding threaded cable adjusters and which way they need to be turned -clock or counter- during the work.

[Barry]

Point taken about 3500rpm being a more typical riding RPM but the reason I balance the cable pulls at 1500 RPM is because the carbs produce a stronger vacuum signal just off idle and the balance procedure is therefore more sensitive. I also think it helps to make the off idle transition smooth. All things being equal it will still be in balance at 3500 RPM.  I balance at 1500 and then rev up to 3500 or more. If the vacuum stays in balance, I'm happy.  That said balancing at 3500 RPM under no load conditions is not the same as riding at 3500 RPM as the throttle openings will be different.