Adjusting carbs on Triumph Thunderbird (but are bonneville carbs)
Jeff Williams
I was helping a friend solve the electronic gremlins on his 66 Triumph
Thunderbird 6T (it's freakin' beautiful!). It is not an original, and
he insisted on the dual carbs. (not a nice fit between tank and where
throttle cables go into carb). But, then again, just about nothing is a
nice fit on that bike... Ask me about the nacelle....
Anyhoo, after replacing a shorted zener, the thing fired up on first kick
(what's all this about them being hard to start? Man, this thing worked
great!). However, it's idling waaaay too high. Not a good idea for a
zero hour motor which was lovingly rebuilt. So, how do you adjust the
idle? There's a throttle stop screw, which goes in diagonally from the bottom,
the mixture(?) screw, which comes in right on the side, and that's it.
I saw on the blow up drawing that there's a clip that can set the throttle
cable
in one of 3 positions, I think. Would you set it on the lowest and then
tighten up the adjustment on the top of the carb to open up the throttle piston
to get it to idle? What's the role of the trottle stop screw? Does it limit
max throttle or min? I'd suspect min, based on where it looks like it comes
out at. What is the clearance of the opening formed by the piston and the
bottom of the trottle supposed to be at idle? What's the tolerance s'posed
to be between the two carbs?
Colin Richmond
Jeff Williams wrote in message <647pnr$i...@piolet.mxim.com>...
>
clip
However, it's idling waaaay too high. Not a good idea for a
>zero hour motor which was lovingly rebuilt. So, how do you adjust the
>idle? There's a throttle stop screw, which goes in diagonally from the
bottom,
>the mixture(?) screw, which comes in right on the side, and that's it.
>I saw on the blow up drawing that there's a clip that can set the throttle
>cable
>in one of 3 positions, I think. Would you set it on the lowest and then
>tighten up the adjustment on the top of the carb to open up the throttle
piston
>to get it to idle? What's the role of the trottle stop screw? Does it
limit
>max throttle or min? I'd suspect min, based on where it looks like it
comes
>out at. What is the clearance of the opening formed by the piston and the
>bottom of the trottle supposed to be at idle? What's the tolerance s'posed
>to be between the two carbs?
>
1) The throttle stop screw sets the tickover, if it's still racing I'd check
the cable for being too tight, you want a bit of play there.
2) The clip probably sets the needle not the throttle cable and it sets the
higher rev range mixture.
3) The clearance at idle should be resting on the throttle stop screw.
Dunno about twin carbs, try the triumph owners club, they have a web site (I
think).
Other causes of racing?
1) Air leak somewhere, usually around the carb-head joint.
2) Mixture far too weak
3) Tight throttle cable
Good luck
Colin
Paul Matthews
On 10 Nov 1997 12:10:35 -0800, je...@macs.mxim.com (Jeff Williams) wrote:
>idle? There's a throttle stop screw, which goes in diagonally from the bottom,
They are the ones you want.
>I saw on the blow up drawing that there's a clip that can set the throttle
>cable
>in one of 3 positions, I think.
Is that not the needle position? tweaking that will adjust the mixture.
>to get it to idle? What's the role of the trottle stop screw? Does it limit
>max throttle or min?
Min. If you look at the expolded pic of the carb (description makes it sound
like a monobloc) the screw alters where the slide can drop to. adjust it while
looking in the carb mouth and you will see what I mean.
> I'd suspect min, based on where it looks like it comes
>out at. What is the clearance of the opening formed by the piston and the
>bottom of the trottle supposed to be at idle?
Whatever gives the right idle speed.
> What's the tolerance s'posed
>to be between the two carbs?
>
As close as possible.
--
Paul Matthews
http://www.triton.u-net.com
paul...@triton.u-net.com
Tony Williams
In article <346a2d51...@news.u-net.com>, Paul Matthews
I don't know if this is relevant, from a non-biker....
My old MGB was very sensitive to getting the carbs balanced.
I ended up with a vacuum-gauge, T-piece, and twin connections,
so that I could look at each carb in turn and get the vacuum
balanced over the rev range. Also handy was a set of those
pyrex spark plugs, so that you could see the colour of the flame
and get the mixture right, also over the rev range. Took time,
but well worth the effort.
--
[Tony Williams, Ledbury, Herefordshire, UK.---Pagewidth=64-----]
Paul Matthews
> I don't know if this is relevant, from a non-biker....
>
> My old MGB was very sensitive to getting the carbs balanced.
>
> I ended up with a vacuum-gauge, T-piece, and twin connections,
> so that I could look at each carb in turn and get the vacuum
> balanced over the rev range.
Vacuum dial gauges are available, in sets of 2,3&4 - anyone going to the NEC
look in at the Davida stand - Ł35 for two. I KNEW there was something I had
forgotten to go back for....
> Also handy was a set of those
> pyrex spark plugs, so that you could see the colour of the flame
> and get the mixture right, also over the rev range. Took time,
> but well worth the effort.
Gunson's Colortune. Even came with a black pipe with a mirror at the top so you
could still see even if at an awkward angle.
john tisi
More info about your problem !
Set up on these carbs is by the following method, I presume they are AMAL
carbs .
For twin setup, set first the following with the bike stopped.
- loosen off all adjustment on the cables so both pistons , proprly called
slides can return by their spring pressure to the fully closed position.
- ensure the air slides, the block things at the in let side are fully
retracted to the top of their travel by the cables keep them up out of the
way fully.
In fact if you live in a warm climate you can ditch them altogether as they
are only used when starting from very cold i.e. sub zero.
- check the needle clip is on the middle groove of the needle as a start
point this only affects full throttle mixture nothing to do with idle speed
or mixture.
- set the throttle stop screws to be 1 1/2 turns open from where they touch
the slide and it commences to open.
- set the mixture screws open 1 1/4 turns from fully closed.
start the bike and warm it up best if you ride it but allow to fast idle
for about 5 mins.
Now any adjustments must be made to BOTH carbs identically
If the idle is rough try a little move ment of one mixture screw in or out
to get best idle on that cylinder then do the same to the other one.keep
tweaking a little at a time to get the engine running even on both pots.
Now the idle speed will be wrong so set it down on both carbs the same
ammount to get a nice idle. I usually set my Bonnie to 1000 revs or a bit
under.
Now try the mixture setting again as the two are inter dependant,
Keep repeating the two adjustments until all is ok.
TIP #1 if you cant get any mixture adjustment try blowing out the hole in
the idle system with an air line by removeing the mixture screw and
pressurising thru the hole.
Now stop the engine and adjust the three cable length adjusters to first
get the two slides to move at exactly the same time, put one finger of the
same hand in the carb mouth and "feel " the movement. This is very
omportant as one carb open more than the other will cause the engine to run
rough and over heat on the "high " carb as that cylinder is doing more
work.
Now adjust the single adjuster to give about 1/8 " move ment of the cable
at the twist grip before the carbs open up and check on both locks that the
cable pull does not use up theis clearance and cause the engine to rev up.
TIP #2 when starting the bike from cold flood both carbs till fuel appears
and don't touch the air slides (chke ) thigs at all.
Hope this helps and best of luck it's a delicate process but worth getting
right cos the improvement is un believable.
Rubberboy.
Colin Richmond <colin.R...@dial.pipex.com~nospam> wrote in article
<649a2k$qu0$1...@plug.news.pipex.net>...
>
> Jeff Williams wrote in message <647pnr$i...@piolet.mxim.com>...
> >
>
> clip
>
> However, it's idling waaaay too high. Not a good idea for a
> >zero hour motor which was lovingly rebuilt. So, how do you adjust the
> >idle? There's a throttle stop screw, which goes in diagonally from the
> bottom,
> >the mixture(?) screw, which comes in right on the side, and that's it.
> >I saw on the blow up drawing that there's a clip that can set the
throttle
> >cable
> >in one of 3 positions, I think. Would you set it on the lowest and then
> >tighten up the adjustment on the top of the carb to open up the throttle
> piston
> >to get it to idle? What's the role of the trottle stop screw? Does it
> limit
> >max throttle or min? I'd suspect min, based on where it looks like it
> comes
> >out at. What is the clearance of the opening formed by the piston and
the
> >bottom of the trottle supposed to be at idle? What's the tolerance
s'posed
> >to be between the two carbs?
> >
>
Bill Leeney
Try this:
Start with the needle clips in the middle position.
Set the cable adjusters (at top of carbs) so that both carb slide bottoms
are level with the top of their respective bores (ie the engine is at full
throttle on both cyls) with the throttle held open. The carbs are now
mechanically synched.
Set the throttle stop screws to a fast idle. Screw in the mixture screw all
the way on both sides and back out, in synch, until you get the fastest idle
possible (so now the mixture is right). If the motor stalls just back the
screws out a bit more before restarting.
Adjust the throttle stops in synch until you get the idle speed you want.
Do a plug chop by cutting the motor at full revs and remove and check the
plugs. Light sandy brown is ok, dark brown or black is too rich, pale too
lean (sloppy - too many curries). Richen the mixture by raising the needle
and readjust all the other settings again (if necessary).
Fun for a wet Saturday afternoon......
Cheers
Bill Leeney
Jeff Williams wrote in message <647pnr$i...@piolet.mxim.com>...
>
>I was helping a friend solve the electronic gremlins on his 66 Triumph
>Thunderbird 6T (it's freakin' beautiful!). It is not an original, and
>he insisted on the dual carbs. (not a nice fit between tank and where
>throttle cables go into carb). But, then again, just about nothing is a
>nice fit on that bike... Ask me about the nacelle....
>
>Anyhoo, after replacing a shorted zener, the thing fired up on first kick
>(what's all this about them being hard to start? Man, this thing worked
>great!). However, it's idling waaaay too high. Not a good idea for a