Triumph Rat Motorcycle Forums banner
  • Hey everyone! Enter your ride HERE to be a part of December's Bike of the Month Challenge!
21 - 27 of 27 Posts

·
Registered
Joined
·
8,604 Posts
Most of these bikes have increased idle speed when hot. First reason is the oil is warm and less oil friction. Second reason might be worn slides or carb body letting air in around the slide.
I dont use the fat O rings on mine,i use a thick gasket instead about 5mm thick. I just prefer this method.
 

·
Registered
1970 Triumph Bonneville T120
Joined
·
29 Posts
Discussion Starter · #22 ·
Well, I took the bike on a nice long ride and then tried to dial in the idle. After fiddling around for like 15 minutes, all I managed to do was make it worse.

I must have gotten them out of sync because the idle became erratic and bounced between 1k and 2k RPMs. I was better off when it was just running at a straight 2,000.

I'll come back and try again another day when I'm not so p*ssed off at it. :)

At least I had fun warming it up. This bike gets better and better the more I ride it. I'd say it's worth the hassle. :)
 

·
Registered
Joined
·
8,604 Posts
I would check the slides for wear as the first task. Worn slides and the bike will never tickover as the slides chatter away and the air passing around the sides keeps varying due to the vibration. A new pair of carbs needed then.
 

·
Registered
Joined
·
5,866 Posts
Not to argue with anyone but the carbs on my 78 are shot. Always got the machine to idle at 1000 no problem Off idle there was a small pfart but I drove thu it. At one time I considered having them machined with sleeved slides or go the mikuni slide route.

2fdots don't let it get to you. If you messed with the mixture screws go back to square one and readjust Then turn the mixture screws IN 1/8th turn. Then adjust each cylinder so they are at the same rpm say 900-1000ea. Then get both cylinders running and back off both the speed screws equally until you get to the desired speed. I suggest while you are doing this you place a box fan in front of the machine. I am also assuming you have adjusted the cables so the slides are coming off the seats at the same time.

K 😷
 

·
Registered
1970 Triumph Bonneville T120
Joined
·
29 Posts
Discussion Starter · #25 ·
Took another crack at it today. I was able to get the idle to hover between around 900 and 1,500. The fluctuation is annoying, but at least it stays running and isn't screaming at 3k when I come to a stop.

Maybe I need new carbs... But I'm going to let that decision wait until next spring. Today is likely the last riding day of the year here in Iowa. The snow is coming tomorrow, and with that, salt on the roads.

I'm grateful for all of the help and advice everyone here has provided. I'm hoping to be able to pay it forward as I continue to learn.
 

·
Registered
Joined
·
2,636 Posts
Hi 2fdots,

I have been watching this thread but nothing jumped out at me, then you said idle was hunting between 900 and 1500.
If you still have points fitted, I would be suspicious that the advance control springs are stretched and the ignition advance is fluctuating between 14 degrees and 38degrees (if set correctly) at random. This would give you fast and slow idle depending on where the spark fires.
I would either check by seeing if the points cam feels slack to turn, or by checking the stability of the spark with a strobe lamp at tickover (it should be steady at 14 degrees before tdc, not at the 38 degrees mark). The important thing is that the spark advance point is steady.
If there is an advance/retard problem, you do not have a chance of successfully setting the carbs with a spark stability problem.

if the carbs are worn out, it is very hard to set them up. The original carbs wear out very quickly due to poor materials (consider 10,000 miles high mileage) The modern premier amal replacements should last a long time as they are made from decent metal, only the needle jets will need replacing every 10,000 miles.

There are 4 things that I would check before I contemplated attempting idle adjustment.
A) That the carburettor float heights were set correctly
B) That the carburettor pilot jet was not partially blocked, Amal mk1 carbs are prone to these jets blocking, they are located in the carb body and are not removable.
C) That the ignition timing was set correctly on each cylinder and the advance mechanism was working as expected.
D) That the valve clearances were set correctly.
Once these four items were correct, only then would I attempt a carb tuneup.

When adjusting Amal carbs, you will find that it is difficult to get a reference point to work from.
There are three adjustments to make,
1) Setting the fuel/air mix on the idle circuit.
2) Setting the balance of the slides when they are held on the throttle cables.
3) setting the balance of the slides when they are sitting on the tickover screws.

I use a mechanical/visual method to get a base setting for the carbs, this method is pretty accurate for setting up the carbs.
Below I have pasted this method from a previous post


The problem you have is that you now have no reference point for the carb settings, to balance between left and right carb once you have turned the screws.
1) On the back of the carb there are two extension trumpets or air filters if you gently unscrew these you will be able to see into the back of the carburettors.
2) If you twist the throttle you can see the two air slides lift, make sure that you have about 1/16" free play on the cables before the slides move.
3) Get two pieces of thin stiff wire (gas welding rod is ideal) about 5-6" long, and wedge one of them under the cutout in the back of each carburettor slide. So that the rods balance on the edge of the carb, but are stopped from falling by the end tucked under the slide.
4) Now unscrew the idle speed screw the end of the rod should lift as the slide lowers. You will reach a point where the rod stops lifting because the slide is fully down.
5) Turn the screw very gently backwards and forwards, until you find the point that the rod starts to lift/drop.
Do this for both carbs.
You now have a lower reference point where the setting is the same for each carb. Make a note, or mark on the carb of where the screw slot is, as you will use these references to set the idle speed.

Before you set the idle speed it might be a good idea to check the balance between the cable settings.
very gently open the throttle a tiny amount (1/64" or less) and watch both metal rods, if one rod twitches before the other then slacken the throttle cable adjuster for that side, until both rods start to move at exactly the same time. The cables are now set.
Remove the metal rods and screw the trumpet extensions or filters back on.

Go back to the adjuster screws, making a note where the screwdriver slots are, turn the screw on each carb in exactly one turn. From now on when you turn the screw on one carb make exactly the same adjustment to the other carburettor, make a note each time you move the screw as it is easy to get confused where you are in the adjustment.

There are two other adjuster screws, this is the air/fuel mixture at tickover, if you have been playing with them, then screw them very gently in until they stop, then screw them out 1 1/2 turns each.
If you have not previously moved them then leave them alone, as the bike was running fine before.

Now these old engines are not good at idling when cold, so you will need to make a basic adjustment now and another fine adjustment when the engine is hot.

The golden rule is: If you move one adjuster screw, you must move the other adjuster screw exactly the same amount.
5) Start the engine and rev it with the throttle for a minute or two to get it warm, then lower the throttle. If the engine dies, screw in each adjuster 1/4" of a turn.
Start the engine and try again, if it dies, screw the adjusters another 1/4 turn. If the engine revs to high back them off 1/8 of a turn.
You will get to a point where the engine will sit at idle ticking over nicely. About 1000rpm
With Triumphs I like to have the tickover speed on the high side, as this helps maintain good oil pressure at idle.

6) check your throttle clearance on the cables, after all of the adjustments you should have about 1/16", if you have to adjust the cables again, make sure that if you adjust one cable, the other has to be adjusted exactly the same amount.
Here I like to put the rods back in, and just double check if they start to move at exactly the same time , the opening of the throttles together is key to smooth running around town, also if they are out one cylinder is doing the majority of the work pulling away, this is not good

7) Go for a 10 mile ride and get the engine hot, when you get back home if the tickover has speeded up, back each adjuster screw off by exactly the same amount until you are back to a good tickover speed.

It is often a balancing act between too high speed hot and stalling cold, On these old machines.

this is quite a good way to get a base setup, you can fine tune further by tweaking the carb balance using vacuum gauges and fine tuning the idle mixture screws. But that is a whole new post.

the reason I prefer this as a basic set up compared to other methods, is not only do you set the tickover screws, you set the throttle cables as well, so it is a double win.
I always fine tune with vacuum gauges afterwards, if I am able.


Good luck
Regards
Peg
 
  • Like
Reactions: slofut
21 - 27 of 27 Posts
Top