Triumphrat

Ken Reece · 09-07-2009, 12:41 PM

I recently bought a set of concentrics off fleabay for my 1967 T120. I had to fully rebuild the concentrics with new slides, jets and air screws. The carbs have transformed the bike, which tells me my monoblocks were really in bad shape. I have the jetting and carb slide sorted and have settled on a really sweet combination. the odd thing is that the idle air screw on one carb does not seem to have any affect, but the other one acts normally. Bike idles fine. even exhaust pressure from both cylinders. Is it possible that one of these carbs has a fixed idle circuit?

Thanks in advance for the help,

Ken

Caulky · 09-07-2009, 12:54 PM

If they are MK1 concentrics, the air/mixture jet maybe the pressed in type. The (horizontal) screw is tapered and controls the amount of air to the idle mixture.
The jet is likely to be partially blocked.
The jet size is .016" and can be cleared using a #78 drill.
http://www.jba.bc.ca/Bushmans%20Carb%20Tuning.html

Ken Reece · 09-07-2009, 01:58 PM

I originally thought this, but have cleared the jet hole with a piece of wire. I would think if it was blocked that it would not idle correctly. It idles fine and is balanced like the other side.

I may pull it off and subject it to another bath in the ultrasonic cleaner and more probing, but am hesitant since it is running so nice.

Ken

nhwally · 09-07-2009, 04:57 PM

Ken,
Also check the 'straw' in the float bowl. There are two of them going from holes in the gasket area to the bottom of the bowl. One of mine was so clogged, that I thought it was intentionally plugged. Exact same symptoms. (Sometimes its surprising how well it can run while screwed up!).

Ken Reece · 09-09-2009, 10:38 PM

Just wanted to follow up and say that I have solved the problem. As was suggested in an earlier post, the problem was a plugged passage. Even though I thought I had cleared them all out, there are two very small holes in the throat of the carb. One of these was still blocked even though it looked to be clear. Once I had run a fine piece of wire through and blew everything out again, problem solved. Man does the bike run nice. No hesitation just off idle and great looking plugs. Thanks for the advice!!

Ken