Triumphrat

rrunk

I'm hoping someone can help me figure out what's going on with my bike. It is a '66 T120R custom bobber that I've had for about 3 weeks. Here are some symptoms.

Starts fine - first or second kick all the time. It won't tickover right away, but will after about 2-3 minutes.

When warm, if I smoothly open the throttle up to 3/4, everything is fine.

When warm, if I quickly open the throttle up to 3/4, it will sputter and die.

The plugs (Champion N3) are dark, sooty and dry. The plugs are new (less than 20 miles).

I've taken the carbs (dual amal 930s) apart and they look to have been rebuilt by the PO. The jets, float, needle and gaskets are all new. 190 main, 106 needle, #3 cutaway. There is no choke on the bike.

I tested for air leaks and I didn't notice any.

If I remove intake filters, the bike won't run at all - any throttle level kills it.

I'm confused with all of these facts whether I'm running lean or rich (or some other problem?). The plugs seems to me like they have signs I'm running rich, but the other symptoms I *think* point to running lean.

Any thoughts?

psasak

Try a plug chop? When running, open up the throttle, then kill the ignition (this takes some planning and coordination, I found out) then coast to a stop and check the plugs then... you should have a better idea what's going on inside the engine above the idle mixture setting.

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thirdbike

I agree with psasack that you will find a difference when you check the plugs above idle.
If it won't run at all with the air filters off that tells you right there that you are way too lean. With custom pipes ie less back pressure you will definitely run leaner. You don't indicate needle type and position. You may want to double check that your mains and throttle valves are what was originally spec'd for that year. However, being a bobber there may have been other custom engine work that negates the need to go back to original carb specs.

GNPsandman

On this topic: my bike is running GREAT above 3k rpms, pulls strong and no hesitation whatsoever. My plugs are white as a ghost and for some reason, I just think the bike is hotter than it should be. Will the plugs color themselves over a short period of time?

I went from 125's to 130's and think I should go up to 135's. Still tinkering with idle mixture, some burps around 2k every now and then even though I'm 3 3/4 turns out.


Sandy-
05' T100, (thrux needle), open exhaust, AI removed, desnorkled, K&N, 130 main, 40 pilot, sea level.

TigerDave

White plugs and hot running are signs of a very lean condition.

Engine damage could follow if the situation is not corrected.

GNPsandman

It doesn't seem I should be this lean though. Given all the databases, jetting calculators, guides, etc, I'm not far off if at all. I'll try a shim or two next before going up in size.

I've read so many mixed opinions on the throttle chop. If not done precisely, it will read incorrectly. I have not bothered.

S-

Beezer

Black sooty plugs mean it is running lean at some point or that the plugs are too cold.
Try this. Roll on the throttle until the engine starts to loose power. Then slowly roll back. If it is lean the engine will pick back up. If its rich it will die. I am not familiar enough with these particular carbs on this kind of a setup to suggest a jet combination.
The throttle chop as described should give you an idea wether your main jet is about right. Shut off should be done between 3/4 and full throttle. Can be tricky. Once the main jet is set than you can work on the needle position. You can use the same throttle roll on technique to help determine the needle position. If the power increases as you roll off the throttle slowly than you are too lean. If it stumbles as you roll on the throttle slowly and will recover if you open more quickly than it is to rich. Your plugs will always be the best indicator as to how you are doing.

If it were mine I would drop the needle one slot and see what effect it has. Getting the main right and working backwards through the needle position, pilot jet and then mixture screw would be the proper order for things.