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Hello all,

When riding, at first it has a normal idle and runs well. Once warmed up, the idle doesn't properly drop down when throttle is backed off. Obviously it is a problem when shifting to have the neutral rpm around 3-3500. Not only that, coasting to a stop light with such an idle results in fighting the bike's speed with the brakes. Have to do all the down shifting at a stop with clutch lever pulled in.

Points and timing are properly set. Throttle slides are synchronized and it will idle around 1200 rpm when not fully hot. Yes, I have to keep the throttle slightly increased when cold to keep from stalling, but it only lasts a little while. The air bleed circuit is properly cleaned with a wire and I removed the screw and shot brake cleaner into the screw hole to force out any debris. I installed the screw and set it to 1 1/8. Seems good.

It doesn't appear to be a gasket leak at carb or the intake manifold. I verified by spraying starting fluid around carb gaskets and there's no reaction.

I'm thinking the only thing left is one or both slides are hanging up when hot. I loosened one nut on each carb without positive result. Loosened it only one flat. I thought the carbs might be over tightened but they weren't. I put the nut back into place using the Goldilocks method.

Anyone care to give opinions re slides hanging up? I did clean the carbs when I received the bike, but didn't pay attention to the slides moving sweetly and only paid attention to the cleaning of the units.

Thanks very much. Steve

Last edited by steve-d; 05/12/21 4:18 pm.

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You set the idle speed and mixture when the engine is fully warm.

It should not idle reliably when cold.


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Be sure there is some slack in the throttle cable so the slides are sitting on the stops. When the engine is warmed and the RPM stays high try backing out the slide stop screws, then you know it it is the slides. If it is not then put a timing light on it. You did not mention what ignition system you are using. If points, the advance could be hanging up.

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As an experiment when its warm and idling at 3K turn the engine off and open the throttle with the grip. Then with your head next to the carbs release the throttle grip, if the slides stick going down you should hear nothing, if not sticking you should hear a click as the slides hit the throttle stops.

If you hear the click then drop the throttle stops when the bike is fully warm to get 1100 revs, then when its cold you will have to give it more throttle to stop it dying.

No click then either fix the warped bodies or replace bodies. Slides will also need attention if marked from the warped body.

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Thanks for your comments.

**With a strobe, I verified the points advance mechanism is functioning correctly.
** There is slack in the cables. When warm I set the idle speed to approximately 1200 rpm. When on the road, it frequently wouldn't drop down to this level. Sometimes yes, but most times it would continue at approximately 3-3500 rpm.

Last edited by steve-d; 05/12/21 4:03 pm.

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Worn carb slides and bodies?

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Hi steve-d, A few thoughts. First are you still running chokes? Not sure. is it the '77 or your '73. The earlier had choke lever on left intake manifold attached to underside with 1/4" bolt. The hole goes through to inside manifold. So if bolt is missing it makes a big air leak. I believe the '77 has bar mounted choke lever. Later manifolds weren't drilled, but check to be sure.

Cold motor will often not idle reliably at all. Once warm same exact adjustments will bring idle to about 950-1100. It takes about 4-5 miles depending on how cold it is to bring motor to reliable idle. High speed riding like 10 miles of freeway 60-65mph will bring idle up 50-150 rpm. Again the exact same bike. Now you slow through easy riding roads. Idle goes back to normal in a mile or so. Suppose you are on same freeway ride. Bottom of off ramp your at 1200. Now you go down long hill on compression braking, say 3/4-1 mile. More is not too cold. Again motor cold idle is lower now about 925. Sometimes you think it may try to stall. Getting onto level or a few red lights, motor warms back up, idle goes back to normal. This is normal for a correct running bike.

In any case cold motor should not idle high. In fact cold you'll have to hold the rpm up as there is no provision for cold fast idle. Also if bike runs really well cold with no choke, it often is too rich when warm. However if you idle is stuck at near 1500 cold, & you have no air leaks the slides are too far open.

There has been mention of throttle cable play at idle. That is most important. Do you have one or 2 cables to twist grip? Normally would have 1 & a splitter to rear cables. Some get fed up with splitter & go to 2 cables & twin cable twist grip. No matter both work fine.

So.... right now with motor off reach down & lift the cable at top of carb. Gently at first. It should lift about 1/16" ish. But should have some play.

Then pull harder on cable, so hard you can feel sides lift. Now lower cable slowly, can you feel the slide bind. Moving cable by hand you may or may not feel slide hit stop screw.

Now open throttle a lot & close very quickly. You should hear decided click as slides hit the stop screw. You can feel it with finger on side of carb once you know what to feel for. If you don't hear the click something is binding or you don't have enough play in cable at carb top.

With air filter cover off you can stick finger in slide & feel for binding & feel & hear the click when you let it drop.

My hunch is you don't have enough play at cables. You have 3 cables. If front cable is too short, it will hold splitter up in junction box. Then it's like the twist grip is being held open some.

At this time, I'd like you to feel for play. Give it more play like 3/16" at least. Now hold one cable gently, back off rpm screw 2 turns. Does cable go down following the slide? Do this both sides. Now start bike & see where rpm is.

When syncing cables you may or may not know where to start with RPM screw. So you make a best guess so to speak. If rpm is too high you back out rpm screws in attempt to lower. This removes play from cable. So you always have to keep the cable play in mind when syncing carbs. You may/will have to go back & adjust cable play as you find the rpm & mixture on new build or new carbs etc.

I personally like to check slide opening at idle with drill bits to make them even. Sort of using them like feeler gauges. I first get lots & lots of cable play. I start from zero then tighten screw upwards until slide lifts a little. Measure with bit. Then set other side the same. Do click test with finger or throttle to verify slides drop to adjuster screw. Then set cable play to 1/8" or so both sides. Now open throttle to about 3/8". Stick drill bit in both sides. Just get them close for now.

Mixture screw base setting is 1.5 turns out. At 1-1/8 you're biasing idle mixture fairly rich. No matter you'll trim mixture to best hot idle later.

Now I start bike & ride it. Usually runs pretty well, maybe not perfect. Remember won't idle right for first few miles. After 5 miles I pull over & evaluate rpm & mixture. If rpm is close to what I like I do mixture screws first one side, then the other. Out & in feeling for best/smoothest/fastest idle. You do one side, then the other. Trim rpm, always thinking about running out of cable play... If rpm is too high or too low change rpm first, then do mixture screws.

Remember you set slides even in garage. So move the mixture screws same amount each side. Tip, if going up (inwards) with rpm screw hold throttle open slightly when you turn screw so slide is not touching screw as you turn it. Let throttle go. Blip throttle a few times & let rpm settle down. Turn rpm screws as needed evenly.

Then go back to mixture screws. So you end up in a loop. Rpm, mixture. Round & round until motor runs best at desired rpm. Ride bike another 5 miles. Pull over & recheck idle rpm & mixture. Trim as needed. These two 5 mile road tests will get you close, not perfect, but close. It takes a good 20+ miles to really warm the motor. It can change then, but not too much. Final adjustments will be done after 20 miles. You'll find the happy medium idle rpm by how bike runs. I tend to like about 1050 as it idles very reliably in all conditions. Some like it a bit slower. Personal choice. It's 100f+ often in my summers so faster idle is better in hot stop & go traffic. Yet again keep eye on cable play. For now just make play even by eye.

Now back in garage you'll adjust the true lift of slides. Again I like drill bit about 3/8". Very steadily hold throttle open. You can tighten the friction lock screw or have steady assistant hold throttle. Feel the slide opening with drill bits. Adjust both sides even at rear cable adjusters, or carb top adjuster as case may be. Always thinking about the cable play at idle. We don't want to end up with no play... If you don't have enough play loosen both adjusters more. Then use the 3/8 bit again. But don't touch the rpm screws.

Finally you have play at idle, the slides lift evenly. Now go back & check gap at idle. Using drill bits (set is by 1/64s is what you need to 3/8. To 1/2 is better). How different are slides? The 3/8 bit showed slides lifted evenly. That is very important. Idle the slides should be close, but sometimes motor runs better with one slide a little lower or higher at idle, just like one side may want a little richer or leaner mixture. It is how the bike runs best.

Seems this takes a lot of warm up road test time & fiddling. Yes it does. When you get right the motor will run so good you'll be delighted. It will stay adjusted a long time. With practice & experience this is not hard at all & not too time consuming. There is no short cutting the warming of motor. If you short cut warming & adjustments you cannot achieve best running.

Back to binding slides. After you get good cable play & sides are hitting stop screws as they should cold. Heat soak the motor. Again 20+ miles. Even at 100f it takes a good 15+ miles. 20 really. On 50f day it may take 50 miles or maybe really never heat soak fully.

So you have idle rpm good. Memorize or measure cable play both sides. Remember at idle play may not be the same as one side may have wanted a little more or less rpm screw. So look at each side in it's own right.

Now is starts hanging up. Idle rpm obviously too high. To check for binding slide if it hangs up blip throttle wide open for an instant. Let it go. The slide drops fast & hard from the spring pressure. Now rpm is normal. Or not... If rpm won't drop check the cable play. If slide(s) is sticking the cable will have additional play by amount slide is sticking.

I've only experienced sticking slides a few times. Each time after installing new slides in old carbs. The slide felt perfectly free on install & cold. But heat soaked on rare occasions the slide would stick at about 3/16-1/4 throttle. (grip was marked). Throttle grip I could feel lots of free play/lost motion so I know slide was partly up. Scare the pants off you in town or slowing for sharp curve. Blipping throttle would drop slide to normal. Twice I hit kill switch just in case as I blipped throttle. On a friends bike put old slides back in, was again ok. Got new carbs. On my bike I pulled new slide. Looked like it was catching on front of bore of throttle due to wear. I put a tiny chamfer on bottom of new slide. Kept riding it. It wanted to catch slightly after but very rarely & even a tiny blip would drop it. Finally wore into old carb & never stuck again. About 5k miles later Premier casting were back in stock. Got new carb. New premier carb has covered 10k with basically zero slide or bore wear. Worth the $$! Not surprisingly the new slide I put in old carb quickly wore to same as old slide.

Sticking slide is unnerving at best, kills you at worst. Only sticks at the worst time. Kill switch is a valid safety feature.


I don't know why you are sticking. Now you can feel cable play. If it's sticking & you don't have cable play the cables and/or junction block is at fault.

If your slides/carb bore are worn out, even if not causing sticking, I mean just worn out, there is a test that has shown reliable. Heat soak motor well, I mean fully heat soaked. Now pull over. Let motor idle 15-20 seconds. Normal idle 950-1050. If idle is too fast test is not valid. Now open throttle slowly as possible. I mean slowly as you can. If motor falters bad or actually dies before rpm picks up, that's worn out carb. However a quicker turn of throttle will raise rpm as expected, no dying. Do this test several times. Get the feel for it. This was told to me by member here. I've used this several times. Hasn't failed yet. I do same test over a few road tests on different days. If carb is worn the tests will be repeatable. Once you get the hang of it you can feel it right off the bat. New slide may improve short term. Don't even think about sleeving. Get new premiers . They were on back order recently. Waiting for castings.

In the mean time waiting for new carb, raise idle rpm to a bit faster than we'd like. Play with mixture screw. At best idle it will die opening slowly. You can change/cheat mixture & experiment with slow opening. Sounds counter intuitive, but in each case I found cheating mixture about 1/4 lean(out) , then raising rpm helped. Took me several thousand miles to get a full diagnosis & learn to believe the slow opening test was valid. Then a good 6 months for carbs to come off back order. I got really good at working around the dying, but certainly no fun to ride.

A good, not worn carb will raise rpm with no trace of faltering or dying. No matter how slow you turn grip motor will follow the grip & run well the entire time.

A thought about points... I run points, I like the reliability of points & you can start bike with dead battery with points. I don't mind servicing points at all. The bad part of points is advance curve is not desirable with modern fuel. Especially California fuel.

Sometimes (often!) the advance springs are not quite tight or strong enough. This allows the advance unit to advance too rapidly. Also the slot in weights wear flat spot & even a dip where weight can sort of catch & stick slightly, then advance suddenly. This can start right about 1100 ish rpm. You feel rpm is a bit slow, so you raise it. Then it does some odd things like my creep up for no reason, but it's the weights advancing timing. Timing change has huge effect on idle rpm.

Triumph sent out a Tech bulletin about stronger/different springs. No longer made it seems. Lots of sellers sell the correct part #, but new springs prove worse than old... A quick & dirty test is put points cam where both rubbing blocks are putting pressure on cam best you can. Move cam to advance & let go. Springs should reliably pull cam back to zero every time. Again new springs can be worse than old. I shortened the spring loops both ends, both springs, by about half. They barely fit on pegs of AAU & weights. Looking in old posts by Pete here & RAT group he shortened them to change curves.

If loose/long/weak springs is part of problem, shortening will help. If not, it's never bad to shorten springs, but won't cure current problem.

Take you time, go one step at a time. This may prove simple or not. It certainly is not normal & can be fixed.
Don


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Originally Posted by TR7RVMan
I believe the '77 has bar mounted choke lever.

No. (or 1978 T140V).

http://www.classicbike.biz/Triumph/Brochures/1970/77ModelsCanadaTri.pdf

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Hi L.A.B., Thanks for pointing that out. Looking like it was all the way in 81??
A guy in the club has bike with choke lever half way up left bar. I just couldn't remember. Was thinking earlier.
Don


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Originally Posted by TR7RVMan
Hi L.A.B., Thanks for pointing that out. Looking like it was all the way in 81??
A guy in the club has bike with choke lever half way up left bar. I just couldn't remember. Was thinking earlier.
Don

Those models with AMAL Mk2s ('78 T140E and later) didn't have the manifold-mounted choke lever (no separate manifolds to mount it on the 'E' head) so had the carb-mounted linkage but the '81 brochure shows handlebar-mounted levers, and, of course, the 'V' choke lever can be handlebar-mounted if required.
http://www.classicbike.biz/Triumph/Brochures/1970/81Bonneville.pdf

Edit: Also, models with Bing carbs had a handlebar-mounted choke lever.

Last edited by L.A.B.; 05/13/21 8:28 am.
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Thanks so much for all your detailed comments. I'll get to examining the details noted. I've read another reason for slide hanging up is someone in the past 40+ years may have overtightened the carb mounting nuts and distorted the carb body. I'll pull the carbs and examine the interior and the slides for wear and anything else.

Steve


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Hi Steve, I wouldn’t pull carbs unless you have to. Slides come out with carbs on. When my slide was catching it felt perfect cold carb in hand. Manifold vacuum pulls slide forwards. This most of wear is front of slide & bore. The Tiny chamfer I did was an educated guess. It helped.
I bought bike new. Carb never ever over tightened. The front flange still likes to bow. You can flatten it if you convert to thin Oring & phenolic insulator. With fat oring per parts book the bow doesn’t seem to matter. The fat oring or phenolic spacer is a wash. Both keep carb same amount of coolness.

Clogged idle passage won’t cause high idle. So don’t bother cleaning it for that fault.

I don’t know clearance of slide. If I get chance I’ll measure good premier tonight.

Old carb very hard to measure bore as the wear that seems to matter most is very deep & also around groove bottom of throat.

If you Mark grip you’ll be shocked how little throttle is used. Most riding between 1/16&1/8. So wear is concentrated.
Needle wear is big deal. It wears a “groove” at where we ride most. .0005 wear effects mixture. .001” wear is already to next jet size. At the same the needle jet wears oval. Again making unwanted enrichment. This won’t effect idle rpm at all. But onc you correct idle problems mixture problems may now be un masked. This carbs are simple & complicated at the same time. They were built & jetted for 98-100 octane leaded fuel, no ethanol.

If you want to ride bike far from home you must tune it to the gas available. You may not find ethanol free fuel. Also modern gas is made to work under pressure in injectors. Not mix with air from carb.

Keep an open mind when tuning. Use jets & slides that work best. Where I live about 5% richer across the board. Our best gas is 91, E10. Make it work or stay home!
Most EI ignition gives full advance at about 3500, where Points AAU is 2000. With modern fuel that is huge advantage.
I recently put over 200 miles on ‘73 Bonnie with Trispark. Trispark has idle stabilization. Works by altering timing at low rpm to keep idle speed consistent. Works outstanding good!
Actually will mask many carb problems at low rpm. Makes starting much better. This bikes start easy anyway. But hot here starting can be a problem & we tickle carbs hot. Experimenting with Trispark you kick hot no tickle. Sounds like it not starting, then it picks up. Would take some time with timing light to see how they do this. But starting cold timing advances several degrees to keep cold motor running. As motor warms timing begins to retard to normal. I found setting idle I needed to always go from too fast down. Then do mixture. Mixture screw rpm drop was compensated for so you had to really listen & feel.
Low idle is compensated for. That’s why it’s better to start fast & go down.
Watching timing marks this adjustment puts timing in optimal position. This gives more room for compensation to prevent stalling. Super easy to install. All electronics in points cavity. No box to mount.
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Originally Posted by steve-d
Thanks so much for all your detailed comments. I'll get to examining the details noted. I've read another reason for slide hanging up is someone in the past 40+ years may have overtightened the carb mounting nuts and distorted the carb body. I'll pull the carbs and examine the interior and the slides for wear and anything else.
Steve
this relates back to kommandos "clump" sound test .
the air-slides make a distinct Clump sound when the throttle cable is released
and the slides drop onto the throttle stop . ( yes this is how they are supposed to work )
( the throttle cable adjustment should have ... just enough Slack ... to allow the throttle stop to do its job )

the sound is subtle enough that you may have never noticed it until you are clued-in to listen for it .
if there is no clump even when cold ,
the throttle cable adjustment is wrong and the slide is hanging off the throttle cable ... or the carb body is totally warped .

if the clump-stop sound goes away , but only when the the carb body is heat soaked
... the warpage is only bad enough to show up in warm conditions .

slide bore warpage is a little different then slide wear .
Thankfully in the world of AMAL carburetors ,
you can bore warpage , ... or slide wear ... or both

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Hi Don,
I’m fascinated by this “idle stabilisation”.
I wonder how the electronics can know what to do, and when to do it, without having any temperature sensing for the cylinder head/inlet manifold.

Anyone have any ideas? NickL or others perhaps?

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Originally Posted by koan58
I’m fascinated by this “idle stabilisation”.
I wonder how the electronics can know what to do, and when to do it, without having any temperature sensing for the cylinder head/inlet manifold.

If the revs drop below idle then ignitions with idle stabilisation (Tri-Spark, Boyer Micro Digital, Pazon Altair) are programmed to advance.
https://www.accessnorton.com/attachments/rick-in-seattle-advance-curves-jpg.3613/
https://www.accessnorton.com/attachments/rick-in-seattle-advance-curve-update-jpg.3614/

Last edited by L.A.B.; 05/13/21 9:46 pm.
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Thanks LAB,
That makes sense, that the electronics are only aware of RPM, nothing to do with temperature. It seems more like “stall-prevention” than “idle-stabilisation”.

It seems those systems, Pazon Altair, Boyer MD and Trispark have chosen ~1200 rpm as the idling speed aimed for. This is possibly a safe default for dependable (non-stall) idling, but may be a little high for some folk.
If someone preferred say a 900-1000 rpm idle, I can imagine such systems “arguing” with this, and producing hunting of the idle speed. This is only a thought, I’ve yet to play with these more expensive EI’s.

1200 rpm is above where I like my 650 to idle. If a twin can’t manage a steady 1200 rpm without risk of stalling (with any of the ordinary systems), I’d suggest there are matters to investigate.
Perhaps “idle stabilisation” helps in situations where things aren’t in the best state of tune, to at least provide a reasonable riding experience, but can be masking (sometimes) minor adjustments that would be better attended to.

I am minded of a funny incident in the early 80’s. A pal had bought a Z1100 fuel injected. At a soggy Glastonbury Festival he wanted to move it a little away from a quagmire. He intended to just walk it a few yards by his side, under its power. So he started it, tapped it into first gear, and the bike bolted out of his hands and carried on under its own steam (anti-stall technology) for quite some yards till it fell over. Luckily it didn’t collide with any people or tents etc, and just lay there motoring away to itself until we slitheringly caught up with it.

Semi-jokingly I suggest a portent of driverless vehicles!

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Don says, "If you want to ride bike far from home you must tune it to the gas available. You may not find ethanol free fuel. Also modern gas is made to work under pressure in injectors. Not mix with air from carb. "
John Healy has said several times here the EPA requires gasoline to function acceptably in older engines with carburetors... I was unable to find info supporting this...


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Originally Posted by koan58
If someone preferred say a 900-1000 rpm idle, I can imagine such systems “arguing” with this, and producing hunting of the idle speed. This is only a thought, I’ve yet to play with these more expensive EI’s.

Having used Tri-Spark and Boyer Micro Digital myself then the hunting doesn't happen if the idle RPM is set lower, or not that I've noticed.

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Thanks LAB, that's good enough for me!

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Hi Koan58, Trispark idle is maybe around 975 the way I set the idle screw on carbs. I’ll just say very comfortable. Slow enough for easy good selection of neutral & not much bang into first hot even with synthetic trans oil.

My problem is my timing light doesn’t read rpm or degrees. Just a light. I marked TDC on rotor using tdc tool in crank.
Looks like it can modify timing maybe 15-20 deg?? That’s a guess. Working by myself. If I had assistant & full function timing light I’d do some real testing instead of quick observations.

I can say it never ever even once gave a stumble hiccup or hunted for rpm. If call it perfect. I was most impressed. Trust me the road testing was very critical.

I was at Audi dealer when they introduced DIS. Digital Idle Stabilization. A small black box between Hall effect distributor and coil. Did not have electronic throttle or tie into lamda computer. CIS injection. Advance was centrifugal. If you unplugged box & bypassed it ignition was normal like the prior year.
Idle speed was set with air bypass around butterfly so it was measured by air flow sensor plate of fuel injection. Cold fast idle was separate air valve.

To set base timing you bypassed unit. Factory set air bypass to some nebulous value they couldn’t tell us. The system worked oddly. Customers complained it felt sluggish & idle too slow at times.
Cure was heat soak motor. Set base timing bypassed. Reconnect DIS. Turn air screw out until too fast. Then using timing light turn are screw until stabilized timing was on Mark. Remember DIS was trying to retard to slow motor. Bringing rpm down to just that point where timing got to base setting was the sweet spot. So basically DIS could not slow idle, only make it idle faster.

The same concept it was shooting for with the Trispark. I think I found it. I can say bike ran like a watch. Starting was unreal easy. I was truly surprised.
Don


1973 Tiger 750
Joined: Feb 2007
Posts: 2,748
Likes: 55
Britbike forum member
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Britbike forum member
Joined: Feb 2007
Posts: 2,748
Likes: 55
I bet the idle circuit is blocked and the slide(s) are opened more than normal to compensate.


Moderated by  John Healy 

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