Britbike forum Forums British Motorcycle specific Brands Triumph bulletin board Motor dies just as I take up throttle cable slack

Hi All, Been having a puzzling situation on my bike. This started suddenly. Motor will intermittently die as you try to take off. Also will occasionally die on deceleration at lower speeds when rolling to a stop. Again intermittent.

At the same time I was having clutch dragging problems. Clutch is now corrected. Clutch is not part of this any longer.

Bike '73 Tiger 24k miles from new. Original carb, all stock as from new. Bike never dies or stutters at any other time. Idle set to 1100rpm now. If I lower idle is much worse.

After stalling will restart on 1st kick easily ever time.

After much road testing I found I can tease throttle & make it die at will. But not always...

Checked power to coils etc while it dies & that is all good.

As I wanted to change slide from 3.5 to 3 to make richer I installed new #3 slide. Needle clip on bottom groove. Changing to new slide may have improved, but only a trace if at all.

I road tested 70 miles yesterday. Was good for 30 miles, then after 5 stop lights started in. I went to parking lot to test.

I found just as I remove throttle cable slack by twisting throttle the motor will often die. No stutter, just like you turned off key. As I said lowering RPM makes worse as might be expected.

Can be in neutral or 1st gear as stand still with clutch pulled no difference so clutch is not effecting problem anymore.

Removing air filter & feeling, looking at slide the problem happens with only .001-.002" inch lift. Maybe less actually when the cable just starts to take slide off stop it will happen.

Moving throttle very slowly like you just want to move up 2' to the stop line puts it in the danger zone. If moving throttle past this point it never ever dies. At the same time I found on deceleration when it dies throttle is in this same zone. Like you slow to stop, but are ready to move up so just teasing the throttle slightly. I've learned to ride around this but I need to fix it.

The motor must be well warmed or never dies.

Cheating the mixture too lean will make it much worse. Cheating mixture screw richer makes it better. Too rich to the point idle is really slowing really helps. That tells me the mixture is suddenly going lean during that tiny movement of slide. That's my hunch anyway. I really don't want to raise idle more if I can help it.

Again once past that tiny point all is good. If I turn even moderately fast past this point it's good. If I get past this point even just slightly like idle RPM 1400 it's good no matter how slow I turn throttle. At that point throttle is maybe .003-.004" open.

I don't want to change carb unless I'm 100% it will fix it.

Has anybody had personal experience with this?
Thanks, Don

Pilot jet clear?

Pilot jet and or its associated jet holes in the floor of the venturi fore and aft of the down stream slide lip .
Out of curiosity do you run a separate earth from motor to battery earth wire,?If you dont , its a long shot but its possible that part of the motor earth return path is via the throttle cable, as the slide lifts this circuit may be weakened so that the spark suffers momentarily. Wouldnt happen with my flaking paint, but a powder coated frame could be a good insulator at the motor mounts.
As soon as I get this sort of bother I drain float bowls into a divination vessel and run a #78 drill through the pilot jet as a first precaution.
Sometimes a glob of water will move to cover a jet as fuel demand increases.
I did spend an age trying to jet around a weak spit back that turned out to be bad HT leads, you know the old saying, 90 % of carb faults are electrics and visa versa.

Like TT says, it's probably related to the idle circuit, most likely the jet. I fight this problem constantly on my bikes especially if I don't ride them regularly. I'm convinced that it's entirely the fault of ethanol in the fuel.
I don't remember ever having these problems in the past.
I recommend and install the Premier carbs on the bikes that come to me for work. That and perhaps a Pazon and the list of niggly little problems these bikes give gets very short. The idle jet is removable which makes it possible to clean them without leaving the debris inside the carb to block things again.
In fact, the latest iterations of ignitions have idle stabilization circuits in them now, which sounds like a good move to me, especially on twin carb bikes which are frequently over-carburated. They are easy to stall if you grab the throttle too fast at low idle speeds.

Cheers,
Bill

...as mentioned to you in other thread, I had similar problem when I was in the city; included the drag on the clutch, that still persist in some situations.
And this one have Mikunis.
I am not too much help because I am waiting for a lost order of Mikuni parts from UK, the shop sent the package again; I hope that this time it would arrive.
Do not checked the clutch yet due to I am waiting for the triplex chain and I have only one gasket; plus I do not have enough time right now.

Hi All, Thanks for the replies.

I cleaned idle jet 3 times. Each time it was clear as was passage to bowl. Also each time the floor drillings were clean. I've cleaned

I don't think idle circuit is directly at fault. Idle mixture screw has the effect of lean/rich so it's passing air & gas. I'm wondering about bore wear for slide?? When the slack is taken from cable does slide move sideways slightly & make a big air leak?? Once slide actually starts to lift it's fine.

Fuel in drain plug is clean. I'm running paper fuel filters on fuel lines as well as the normal strainers. Tank was cleaned & lined with Caswell last fall. Liner still feels hard & looks perfect. Filters are clean & fuel flow is good. Cap vent is clear.

I hadn't thought about motor ground... Good idea.

The red ground wire bundle is bolted to front rocker box stay bolt. Verified it was tight. Ground on rear fender brace is tight. Charging system working good. Checked connectors to battery, coils, points. All good. The head lamp doesn't flicker or act odd while motor dies.

I will volt drop motor ground while running tomorrow. Wouldn't it be something if the throttle cable was the ground! Will volt drop the other grounds & coil power while running too.

Does anybody have an exact test for slide wear? I mean when bike does a certain thing it's going to be slide wear?
Don

Electronic ignition yes?

Are you using their best recommended coils (6V in series or dual coil in many cases)?

Anything in the HT circuit with insulation flaws can make the spark fail when any load is put on the engine. Then when the rpm goes up, the spark gets stronger and the engine can fire better.

A failing battery or bad conductivity on the low tension side could possibly do something similar and those are very common faults with these rattly switches and home-fitted Boyers.

i have had slides so worn that they rattled audibly, the bike would still run OK , idling got a little unstable, didnt die on throttle opening .

While you have the meter out check for resistance across the ignition switch contacts and the kill switch, if its still the original ignition switch I would be very suspicious of this item. For a quick dirty test jumper out the switches with a hot wire from batt neg to Boyer feed, see if the problem persists.

Throttle cable grounding out? Had that exact problem with my clutch cable. Cable when tensioned, would short out ignition.

As the slide wears, the idle adjustment screw needs to go in to enrich and compensate for the increase air. When the screw goes in all the way and there is no change in the idle, the slide is done and maybe the body, too, as they wear together.
On my Commando, before that could happen, the slides self-destructed and parts were sucked through the combustion chamber. This happened on both sides within a few miles of each other. I didn't loose the motor, but it created a lot of damage to the pistons and combustion chamber.

Maybe not so big of a problem on Triumphs unless highly tuned. Induction pulses slam the slides back and forth, weakening them eventually. I have chromed brass Mikuni slides in there now. Problem solved.

I'm dealing with a problem somewhat similar on my Triumph to yours. My bike has become a real bear to start when cold. Once started, it behaves like always.

I'm going through the list of easy fixes first. I checked all the high tension wires, changed the plugs, cleaned the points, rotor and cap(I have a distributor) and still no joy.

I'll be installing a new condenser here when it arrives from the UK. I suspect that will do it. I'll be setting the valve clearances first, though.

Speaking of which. A tight inlet valve will cause what you are describing as they very likely get tighter as they get hot exacerbating the situation.

Cheers,
Bill

When there is a little more slack in the slide/body fit, this can arise.
If you can imagine at tickover, big suction on the engine side of the slide, so the lower part of the slide is pulled in that direction. (the slide has a little slack, easily feel-able).
You set your tickover/mixture in that condition.
As the slack in the cable is taken up, it starts to pull the centre of the slide, which will initially pull the slide more vertically true.
Previously, the slide was sealing fairly well against the engine side carb body, now that it has been pulled away from it there is more air weakness avenue.
Try turning the airscrew in by 1/4 to 1/2 and see what difference it makes?
The carb isn't necessarily dead, just a sign that it's not new!

Have you checked to see if the float bowl sealing surface is warped?

Combination of slide and carb body wear?
Slide moving from a canted position in the body with throttle cabkle slack to the opposite canted position with slight tension in the throttle cable.
Perhaps?

"Motor must be well warmed or it never dies"

Any diagnosis must account for this.

Air leak would probably make it worse when the motor is cold because the motor prefers a richer mixture until warmed up then prefers a leaner mixture.

Something is changing when the motor warms up causing this. Heat soak to the carb, closing clearances on the valves, something like this.

Cheers,
Bill

Hi All, Torqued head, it wasn't loose. Adjusted valves, points, strobe set timing, new plugs (they were still fine though). Also replaced intake manifold gaskets as they were seeping, but not really leaking air. Using the thick o-ring as per factory carb to manifold. Air gap .045". Verified no leaks & marked carb nuts, they have not moved.

Volt dropped motor ground to battery + 0.05v so that was good. However putting wrench on ground nut which also holds engine stay to front rocker box it took 1/8 turn. Motor was not smoking hot... I kicked myself for not waiting until I do the full 70 mile test route now. Retested volt drops after 20 miles. Still good. 0.05v.

Volt drop battery - to coil power was 1.38v, but hot it's varying from .9-1.01v. Still too high, but it's been like this for 3-4 years. I was always going to address this but never did. I don't think it's causing my current problem though.

Recheck carb passages, bores, still good.

Took bike out only 20 miles today, all the time I had. Was good.

Mixture screw exactly in the sweet spot is 1-9/16 turns out. I usually cheat it rich from here 1/16 turn inwards. I cheated it richer, inwards further now so it's 1-3/8 turns out. Motor has slowed rpm some due to too rich. But this helped before so I left it there.

Further testing by turning throttle ever so slightly, the RPM lowers slightly, but didn't want to die. While it was in this lowered state & experimented with mixture screw. It speeds up RPM slightly when it's turned outward 1/2 turn.... Go figure, that would show was too rich. Then letting go throttle it becomes way too lean.


Holding RPM at 2000 at standstill & turning mixture screw all the way in & way out it has very dramatic effect on motor. As it does at idle.


It is my strong feeling the root problem is carb bore wear for slide. New slide helped, but not cured problem. Looking a slide most carefully at .001 lift, it rotates slightly & goes sideways slightly & makes a click. Maybe the wear has reached a threshold now & it's time for new carb??

That will have to wait for a few more retirement checks. I'll keep you all posted.

I'm doing 200 mile ride Saturday & it will be hot 100F. Late June is 900 mile ride. I have to ride downtown San Francisco to starting point. I was hoping to have this corrected, but not to be. I've learned to ride around the problem. Of course I'm very gun shy at intersections these days....

Thanks for all the input!
Don

Hi John, Thanks for the suggestion.

I flattened bowl surface when I cleaned jets the first time. It was not very warped. It did not seep gas prior & bike ran fine, never ever stalled prior to this happening.

With flat bowl, no gasket, bowl held to carb, carb body is slightly bowed. I can just pinch a .0015" feeler gauge. .001 shim stock will pull out freely. So a tad more than .001" warp. I left it like this since it was good prior.

Using genuine AMAL bowl gasket (actually all parts are genuine). Verified good float clearance to gasket & float pivot pin is not fouling gasket or keeping it from pinching tight. No fuel seeps from bowl gasket now either.

This started about 1500 miles ago. I felt it was clutch, which indeed was causing much of this. After new clutch was way better, at least ridable.


I've been fighting the carb itself now for 1000 miles. I didn't think to try carb in neutral while clutch was bad since I knew clutch was grabbing/dragging.

Wouldn't it be something if the problem was the motor ground! Time will tell.

Thanks again for the suggestions.
Don

Fuel leak is not the concern here, but when warped it creates an air leak in the fuel transport tube. The leak breaks the vacuum that draws the fuel out of the bowl for the idle carburetor.

Hi John, Thanks for the repy. I know what you mean exactly.

Looking at gasket I can see imprint on gasket where it seems to be making good seal from bowl tube to body. But I don't know of a way to actually test that. The AMAL gasket is rather soft & compresses easily. But... I don't really know.

I've heard is say you can tell something about idle fuel flow by position of air screw. Is that true? My genuine '73 shop manual states base setting for air screw is 2.5 turns out. I'm a full turn in from that to get the sweet spot on mixture. But some folks say it's really 1.5 turns. What is correct?

So maybe I'm lacking idle fuel?

Thanks, Don

1 1/2, 1 14/32, 1 7/16. The book is merley a guide, not a bible. And the carbs can be different fron side to side.

Hi desco, Where I got this was the Tuning Your Carburetor Guide put out by John Healy.

In the section "The Pilot Air Screw". Pilot air screw steady idle 1-1/2 turns from seated. I'm close to that.

If 1 turn or less, increase size of pilot jet. If 2 turns or more decrease size of pilot jet.

My bike has always wanted to be in less than the 2-1/2 turns shop manual states.

That tuning guide has been very helpful.

Don

http://victorylibrary.com/CONC.htm

This is an updated version of a book I already have. More than anybody in thier right mind needs to know about Amals.

http://vintagebikemagazine.com/technical-articles/AMAL/

A newer online version of the original articles we have been doing for 20 odd years.

AND YES 1 1/2 turns out from seated is proper for your bike.

Like many things in Triumph manuals the 2 1/2 is a hold over from a previous carburetor from a previous year.

Hi John, Thank you very much for the update. You've been most helpful to me over the last several years.

This clears up the air screw turn subject. I'm printing this & inserting into my manual.
Don

Like I tell all my friends who have these bikes. get that devil carb behind you tout suite and install the excellent Premier carb. Take it all apart, check that all the components match your specs, set the float height if needed, remove the choke and plug the hole and install with the Triumph part number O ring. You've already got way more mileage on it than it was ever intended to go.

One of the best things you'll ever do for your bike and your riding enjoyment.

Cheers,
Bill

I had my 650's 1 3/16" Monobloc carb reconditioned and it made a really big difference. The chap bored the body and sleeved the slide. It only cost £35. My trouble was the idle. If I set the idle to keep going the bike was very lumpy around town as it was too rich. I had to set it for the road. This made the idle very uneven and it would gradually peter out and stop. Once fixed I just set the pilot at 1 1/2 turns out and it would idle at 500 rpm. This is actually too slow for oiling the cylinders so I have it at 750rpm. It is funny as you would expect the worn slide to make the idle weak but not so. Someone on here explained this phenomenon.



Get the new carb if you have the money like Bill says. The carbs are so simple that you can just check it, fit it and ride. If it doesn't work straight off then the problem is elsewhere.

Dave