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by Mal Marsden - 06/16/22 7:00 pm
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Original Post (Thread Starter)
by Peter Williams
Peter Williams
Hi all,

I am at a loss with trying to sort out the idling revs on my 1968 T120. Whatever I do, I cannot get them below 2000 RPM.

For background, the bike was off the road for the winter whilst I fixed (replaced) the wiring loom. I had some dodgy wiring, including the Boyer EI being wired in reverse and then butchered to try to get the bike to run (previous owner). I have replaced the butchered part and built a custom loom that only has what is needed for the Boyer MKIII EI and Podtronics Reg/Rec. As well as sorting the wiring, I removed both carbs and gave them a good clean and blow through with compressed air, plus new seals / gaskets. To be honest they looked pretty clean anyway. Finally, I have checked / adjusted the valve timing.

The bike now starts first kick with no choke, which is a vast improvement over last year when I would often wear myself out trying to start it. Also, last year the bike, once running, would tick over ok (1000 rpm maybe).

So, to the idling problem. I started with the throttle cable(s) and checked that both sliders lift simultaneously - they do. I then set the throttle stops to 1/4 turn in after the point they first touching the slider. Then 1 1/2 turns out on the air pilot screw. Set like this the idle is about 3000 rpm!

Next I tried the approach suggested in the workshop manual for synchronizing the carbs. Removed the left plug cap, started the bike and tried to get it to run smoothly. With no throttle stop at all and 2 turns out on the air pilot screw (RH carb) I could get it to run smoothly at about 1200 rpm.
Repeating this with the LH carb. The best I could get on this side was 2000 rpm with no throttle stop and the air pilot screw wound out about 4 turn (I don't think anything after 3 turns is supposed to make any further adjustment?)

I have checked for an air leak by spraying the inlet manifold side of the carb with Easy Start spray. This has no impact on the revs.

I have replaced the spark plugs. Having just checked them again, they are slightly "oily / shiny" looking I would say, but certainly not sooty and not looking like the engine it running lean either. Difficult to read too much into this though, I think, as they have only just been fitted and the bike has only been idled for a couple of minutes whilst fiddling with stuff!

If anyone can suggest where I need to start looking next I would be very grateful.

Many thanks
Liked Replies
by Peter Williams
Peter Williams
Bit of an update for anyone who is interested.

Whilst I managed to get the bike to run ok with the original cable setup, I was not particularly happy with the overall reliability or the aesthetics.

I came across a local specialist (Surrey Cycles) that supply things called "Vincent Swept Bends". These go straight into the top of the carb with a "top hat" type fitting and sweep 90 degs. and then have an adjuster on the end. I originally planned to add these to the 2 into 1 setup I had but, after a really helpful chat with Michael at Surrey Cycles) he suggested going twin cables to the twist grip with only one adjuster on each cable (at the swept bend).

Purchased twin cable twist grip, two universal cables, two swept bends and also some lighter carb springs.

Have completed the install and I am really pleased with the results.

I have no affiliation with Surrey Cycles, but Michael is a really knowledgeable chap (BSA's are more his thing but I'll forgive him that) and he knows a lot about Amal Carbs. Worth a look at his site for carb related bits and pieces.
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3 members like this
by quinten
( as a lower order of potential problem )
have you checked the timing at idle .
the boyer may not be a retarding or retarding enough .
1 member likes this
by triton thrasher
triton thrasher
Another possibility is an air leak into the intake between carburettor and cylinder head.

Spray the joints with WD40 while it’s idling. If the engine reacts, there is a leak.
1 member likes this
by quinten
bodge alert :

"splint" the cable sheathing
into the radius that needs to meet the carb .

Source a piece of Rod about twice the diameter of a coat hanger
and almost as long as the first piece of cable sheathing at the carbburetor .
form it into the small radius needed at the carb end
, like one of those cable end " noodles" , just not hollow .

Nylon-tie this splint to the side of the sheathing
This will supply a solid exoskeleton
to the flexi- sheath-radius
without having to make up new custom cables .
1 member likes this
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